U.S. patent number 3,892,058 [Application Number 05/399,346] was granted by the patent office on 1975-07-01 for process for the preparation of high-temperature short-time sterilized packaged articles.
This patent grant is currently assigned to Toyo Seikan Kaisha Limited. Invention is credited to Yoshihiro Komatsu, Kanemichi Yamaguchi.
United States Patent |
3,892,058 |
Komatsu , et al. |
July 1, 1975 |
Process for the preparation of high-temperature short-time
sterilized packaged articles
Abstract
A process for the preparation of high-temperature shorttime
sterilized packaged articles which comprises filling an article to
be sterilized in a flexible container having a heat-sealable resin
film at least on an area to be heat sealed of the inner surface
therof, removing non-condensable gases present in interior voids of
the flexible container, hermetically sealing the flexible container
by application of heat in such a manner that the coefficient of
flatness in the resulting sealed packaged article is maintained at
a level not exceeding 0.25, said coefficient of flatness being
defined by the following formula ##EQU1## in which l indicates a
maximum thickness (cm) in the sealed packaged article, L disignates
a maximum distance (cm) across the surface of the container, and
K.sub.F stands for the coefficient of flatness. Maintaining the
sealed packaged article for a short time in a heating medium of a
temperature exceeding 130.degree.C. to thereby sterilize the
article packaged in the container, and cooling the so heat-treated
packaged article under a pressure hegher than the saturated vapor
pressure corresponding to the sterilization temperature.
Inventors: |
Komatsu; Yoshihiro (Yokohama,
JA), Yamaguchi; Kanemichi (Yokohama, JA) |
Assignee: |
Toyo Seikan Kaisha Limited
(Tokyo, JA)
|
Family
ID: |
26436562 |
Appl.
No.: |
05/399,346 |
Filed: |
September 20, 1973 |
Foreign Application Priority Data
|
|
|
|
|
Sep 22, 1972 [JA] |
|
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47-95314 |
Dec 13, 1972 [JA] |
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47-124393 |
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Current U.S.
Class: |
53/425;
428/475.5; 53/434; 422/40; 428/337; 428/461; 428/480; 53/440;
53/479; 426/412; 428/458 |
Current CPC
Class: |
A23L
3/00 (20130101); A23L 3/10 (20130101); Y10T
428/31681 (20150401); Y10T 428/31786 (20150401); Y10T
428/31739 (20150401); Y10T 428/31692 (20150401); Y10T
428/266 (20150115) |
Current International
Class: |
A23L
3/00 (20060101); A23L 3/10 (20060101); B65b
055/02 () |
Field of
Search: |
;53/21FC,22B ;21/56
;426/407,401,412 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: McGehee; Travis S.
Attorney, Agent or Firm: Diller, Brown, Ramik &
Wight
Claims
What we claim is:
1. A process for the preparation of high-temperature short-time
sterilized packaged articles which comprises the steps of providing
a flexible container having a heat-sealable resin film at least on
an area to be heat sealed of the inner surface thereof with the
heat sealable resin being a crystalline thermoplastic resin having
a softening or melting point of from 130.degree. to 250.degree. C.,
filling an article to be sterilized in the flexible container,
removing non-condensable gases present in interior voids of the
flexible container, hermetically sealing the flexible container by
application of heat in such a manner that the coefficient of
flatness in the resulting sealed packaged article is maintained at
a level not exceeding 0.25, said coefficient of flatness being
defined by the following formula ##EQU5## in which 1 indicates a
maximum thickness (cm) in the sealed packaged article, L designates
a maximum distance (cm) across the surface of the container, and
K.sub.F stands for the coefficient of flatness,
maintaining the sealed packaged article for 1 to 8 minutes in a
heating medium of a temperature of 130.degree. to 160.degree.C. to
thereby sterilize the article packaged in the container, and
cooling the so heat-treated packaged article under a pressure
higher than the saturated vapor pressure corresponding to the
sterilization temperature.
2. A process according to claim 1 wherein when said flexible
container is supplied it is assured that said flexible container
has two inner surfaces confronting each other and is composed of
one or two flexible sheets with said container comprising a space
for receiving the article to be sterilized between said two inner
surfaces and an area to be heat sealed which is provided around the
periphery of said space, and said inner surfaces having at least on
said area a film of a heat sealable resin having a softening or
melting point of 130.degree. to 250.degree.C.
3. A process according to claim 1 wherein when said flexible
container is supplied it is assured that said flexible container is
a bag-like container formed by piling two flexible laminate sheets
with each sheet including an inner layer of a heat sealable resin
having a thickness of 5 to 200 .mu., an intermediate layer of a
metal foil having a thickness of 5 to 100 .mu. and an outer layer
of a heat resistant resin having a thickness of 2 to 100 .mu., so
that the heat sealable resin layers confront each other, and
providing an area to be heat sealed around a space for receiving
the article to be sterilized.
4. A process according to claim 1 wherein when said flexible
container is supplied it is assured that said flexible container is
a bag-like container formed by piling two flexible laminate sheets
with each sheet including an inner layer of a heat sealable resin
having 5 to 100 .mu., an intermediate layer of a heat resistant
resin having a thickness of 5 to 100 .mu., a layer of a metal foil
having a thickness of 2 to 100 .mu. and an outer layer of a heat
resistant resin having a thickness of 2 to 100 .mu., so that the
heat sealable resin layers confront each other, and providing an
area to be heat sealed around the periphery of a space for
receiving the article to be sterilized.
5. A process according to claim 1 wherein when said flexible
container is supplied it is assured that said flexible container
comprises a container proper which is formed by press forming a
metal foil having a thickness of 25 to 150 .mu. and has a space for
receiving the article to be sterilized and a flange portion, and a
lid composed of a metal foil or a laminate of a metal foil and a
resin film, a layer of a heat sealable resin being formed between
said lid and said flange portion of the container proper.
6. A process according to claim 1 wherein it is assured that the
heat sealable resin is poly-.omega.-aminodecanoic acid.
7. A process according to claim 1 wherein it is assured that the
heat sealable resin is polypropylene.
8. A process according to claim 1 wherein it is assured that the
heat sealable resin is poly-4-methylpentene-1.
9. A process according to claim 1 wherein it is assured that the
heat sealable resin is polytetramethylene terephthalate including a
polyalkylene ether structure in the molecular chain.
10. A process according to claim 1 wherein it is assured that the
heat resistant resin is biaxially stretched polyethylene
terephthalate.
11. A process according to claim 4 wherein it is assured that the
intermediate layer-constituting heat resistant resin is biaxially
stretched polycapramide.
12. A process according to claim 1 wherein removal of
noncondensable gases is accomplished by reducing the pressure in
the interior of the container to a level lower by at least 70 cmHg
than the atmospheric pressure.
13. A process according to claim 1 wherein removal of
non-condensable gases is accomplished by substituting
non-condensable gases present in the interior voids of the
container by steam.
14. A process according to claim 1 wherein removal of
non-condensable gases is accomplished by applying a pressure from
the outside to the article-filled flexible container to thereby
change the shape of the container.
15. A process according to claim 1 wherein the hermetic sealing is
accomplished by applying to the area to be heat sealed of the
flexible container heat sealing temperature and pressure necessary
for melt bonding the heat sealable resin film.
16. A process according to claim 1 wherein the coefficient K.sub.F
of flatness of the sealed packaged article is maintained at a level
not exceeding 0.15.
17. A process according to claim 1 wherein the sterilization is
accomplished by maintaining the sealed packaged article in a
heating medium of a temperature of 130.degree. to 160.degree.C. for
1 to 8 minutes under a pressure of 1.7 to 8 Kg/cm.sup.2 gauge.
18. A process according to claim 1 wherein the heating medium is
selected from saturated steam and a mixture of saturated steam with
10 to 30 % of compressed air.
19. A process according to claim 1 wherein the heating medium is
compressed hot water maintained at a pressure of 1.7 to 8
Kg/cm.sup.2 gauge and a temperature of 130.degree. to
160.degree.C.
20. A process according to claim 1 wherein the sealed packaged
article is maintained at a temperature of 85.degree. to
98.degree.C. prior to the heat sterilization.
21. A process according to claim 1 wherein the heat sterilized
sealed packaged article is cooled under a pressure higher by 0.1 to
1 Kg/cm.sup.2 than the saturated vapor pressure corresponding to
the sterilization temperature.
22. A process according to claim 1 wherein the heat sterilized
sealed packaged article is cooled by introducing it directly into
cooling water from the heating medium.
23. A process for the preparation of high-temperature short-time
sterilized packaged articles which comprises the steps of providing
a flexible container having a heatsealable resin film at least on
an area to be heat sealed of the inner surface thereof with said
heat sealable resin being a crystalline thermoplastic resin having
a softening of melting point of from 130.degree. to 250.degree.C.,
filling an article to be sterilized in the flexible container,
removing noncondensable gases present in interior voids of the
flexible container, hermetically sealing the flexible container by
application of heat in such a manner that the coefficient of
flatness in the resulting sealed packaged article is maintained at
a level not exceeding 0.25, said coefficient of flatness being
defined by the following formula ##EQU6## in which l indicates a
maximum thickness (cm) in the sealed packaged article, L designates
a maximum distance (cm) across the surface of the container, and
K.sub.F stands for the coefficient of flatness,
charging the sealed packaged article into a sterilizer, raising a
temperature of a heating medium in said sterilizer to a
sterilization temperature of 130.degree. to 160.degree.C. within
180 seconds, maintaining the sealed packaged article for 1 to 8
minutes in said heating medium of said sterilization temperature to
thereby sterilize the article packaged in the container, and
cooling the so heat-treated packaged article under a pressure
higher than the saturated vapor pressure corresponding to the
sterilization temperature.
24. A process for the preparation of high-temperature short-time
sterilized packaged articles which comprises the steps of providing
a flexible container having a heat-sealable resin film at least on
an area to be heat sealed of the inner surface thereof with said
heat sealable resin being a crystalline thermoplastic resin having
a softening or melting point of from 130.degree. to 250.degree.C.,
filling an article to be sterilized in the flexible container,
removing non-condensable gases present in interior voids of the
flexible container, hermetically sealing the flexible container by
application of heat in such a manner that the coefficient of
flatness in the resulting sealed packaged article is maintained at
a level not exceeding 0.25, said coefficient of flatness being
defined by the following formula ##EQU7## in which l indicates a
maximum thickness (cm) in the sealed packaged article, L designates
a maximum distance (cm) across the surface of the container, and
K.sub.F stands for the coefficient of flatness, charging the sealed
packaged article into a sterilizer in which a heating medium of a
temperature of from 130.degree. to 160.degree.C. is present,
maintaining the sealed packaged article for 1 to 8 minutes in said
heating medium to thereby sterilize the article packaged in the
container, and cooling the so heat-treated packaged article under a
pressure higher than the saturated vapor pressure corresponding to
the sterilization temperature.
Description
This invention relates to a process for the preparation of
high-temperature short-time sterilized packaged articles. More
particularly, the invention relates to a heat sterilization process
comprising subjecting a food filled in a flexible packaging
container to high-temperature short-time sterilization in the
in-package state.
Conventional processes for the preparation of canned or bottled
foods comprise, in principle, the step of hermetically sealing a
food into a container capable of keeping the food in the air-tight
state and the step of subjecting the sealed packaged articles to
heat sterilization at a high temperature for a certain period of
time so that the food can be preserved with certainty.
In such in-package heat sterilization process practised in the
manufacture of canned or bottled foods, however, the heating time
corresponding to a fixed heating temperature is determined so that
a necessary but minimum heat history is given to a portion in the
package to which heat is most difficultly transferable, and
therefore, there should naturally be brought about a great
difference to the heat history between such portion to which heat
is most difficultly transeferable and a portion to which heat is
most easily transferable. For instance, in the case of heat
sterilization of cylindrical canned foods, it is known that the
quantity of heat given to the surface portion of the content in the
vicinity of the inner wall face of the container throughout the
entire steps of heat sterilization from the come-up time to the
cooling step is several tens to several hundred times as large as
the quantity of heat given to the central portion of the content in
the vicinity of the center of the container, and that the surface
portion of the content is excessively heated. Further, this
non-uniformity of the heat history is also brought about by the
configuration and food-containing capacity of the container,
physical properties of the packaged foods, the amount of
non-condensable gas sealed in the package or occluded in the
packaged food and sterilization conditions such as the
sterilization temperature and sterilization time.
More specifically, as the food-containing capacity of one container
is small and the distance between the container wall and the center
of the container is small, the non-uniformity of the heat history
is, in general, small. In case the article to be packaged is
uniform and rich in flowability and has such a property that the
heat transfer is effected in the article not only by conduction but
also by convection, non-uniformity of the heat history is small.
Furthermore, if a large amount of non-condensable gas is present in
the packaged article, the transfer of heat is obstructed by such
gas interposing between the inner wall surface of the container and
the surface of the content. Therefore, as the amount of
non-condensable gas is small, non-uniformity of the heat history is
small. As regards the strelization conditions, it is considered
that the low sterilization temperature and gradual heating will
result in a low degree of nonuniformity of the heat history.
Therefore, in the conventional inpackage heat sterilization
processes, the sterilization temperature is usually maintained
within a range of from 105.degree.to 121.degree.C.
In heat sterilization of articles contaminated with microorganisms
including bacterial spores, the relation between the time required
for annihilation of micrrorganisms and the heating temperature is
in comformity with the equation of arrhenius as the
time-temperature relation in a chemical reaction. The annihilation
rates depends greatly on the temperature, and at heating
temperatures exceeding 100.degree.C., the annihilation rate
increases by 5 to 10 times when the temperature is elevated by
10.degree.C. and theoretically, the heating time is shortened to
1/5 to 1/10. In case an article to be heat sterilized is a food,
degradation of quality by heating is due mainly to such phenomena
caused by chemical reactions as reduction of vitamin contents,
decrease of amounts of flavor constituents, accumulation of
browning substances and hydrolysis. As is known in the art,
reaction rates in these chemical changes are also in conformity
with the equation of Arrenius, and they depend on the temperature
to such a degree that when the temperature is elevated by
10.degree.C., the reaction rates increase by 2 to 3 times. When
this value is compared with the degree of increase in the
microorganism annihilation rate attained when the temperature is
elevated by 10.degree.C., it is seen that the rate of annihilation
of microorganisms including bacterial spores by heat is several
times as high as the reaction rates in the above chemical changes.
Therefore, in sterilzation of articles which tend to undergo
chemical changes under heating, the quality of such article can be
kept in better conditions by effecting the heating at a high
temperature for a short time than by heating the article at a
relatively low temperature for a long time, when compared based on
the same degree of annihilation of microorganisms. For this reason,
there has been broadly adopted a so-called high temperature
short-time sterilization process (HTST sterilization process).
In the heat sterilization operation, however, since the heat energy
is not directly applied to objective microorganisms but through an
article to be sterilized, the majority of the heat energy given at
the sterilization step is consumed for elevating the temperature of
the article to be sterilized and accumulating the heat in the
article and therefore, the heat given to the article is much larger
than the heat necessary for annihilating the objective
microorganisms by heating. In the case of an article having a low
heat conductivity and an article in which a long time is required
to transfer heat to the central portion, especially in the case of
an article filled in a sealed package, complete sterilization
cannot be attained unless a much excessive heat history is given to
the article.
As in the manufacture of canned or bottled foods, in the inpackage
sterilization process (post-packaging sterilization process) in
which an article to be sterilized is filled and sealed in a
container having a certain capacity and a cylindrical form and then
the sterilization is conducted, the reduction of quality
represented by browning (called burning) is caused to occur in the
packaged article at the surface portion of content having a contact
with the inner wall face of the container or at the portion having
a contact with interior voids, and this quality reduction surpasses
the improvement of the quality attained by the shortening of the
heating time owing to elevation of the sterilization temperature.
Accordingly, it has been considered that the upper limit of the
temperature to be adopted for sterilization is about 121.degree.C.
(250.degree.F.) and elevation of the temperature beyond this upper
limit is of no significance. Further, in the case of a rigid
container such as can or bottle, when the outer or inner pressure
is too excessive under heat sterilization conditions, permanent
deformation or breakage is caused in the container, which results
in damage or release of the sealing compound layer and in breakage
of the air-tight seal. Accordingly, in the art of the in-package
heat-sterilization, it has been considered that in the case of such
rigid container the upper limit of the heat sterilization
temperature is about 125.degree.C. even in extreme cases.
Therefore, rigid containers cannot be used with good reliability
under such sterilization temperature condition as will cause the
pressure difference of 2 Kg/cm.sup.2 or higher between the inner
pressure and outer pressure at the temperature elevation or
cooling, namely at sterilization temperatures exceeding about
130.degree.C.
In the art of manufacture of canned and bottled foods, therefore,
various proposals have heretofore been made in order to obtain
sterilized packaged articles according to the high-temperature
short-time sterilization technique. As a typical instance of such
proposals on the in-package sterilization process, there is known a
so called agitating sterilization process according to which the
sterilization of canned or bottled foods is carried out while they
are being agitated and rotated. More specifically, according to
this process, canned or bottled foods are passed through a heating
medium such as high temperature steam, hot water or flame while
they are being agitated and rotated. In this method, by movement
and rolling agitation of solids or bubbles contained in the article
to be sterilized, the temperature elevation in the article is
accelerated and the heat history is uniformalized in the article.
Indeed, in case this agitating sterilization process is applied to
a fluid or semifluid article, attainment of such effects can be
fully expected, and and undesired phenomenon of burning can be
effectively obviated. However, in case this process is applied to a
non-fluid or highly viscous article, a solid article or a composite
article containing solids extremely irregular in the size, any
effects can hardly be expected. Therefore, this process should be
distinguished from the true in-package high-temperature short-time
sterilization process.
As the process to be used mainly for sterilizing a fluid article,
there has been broadly adopted a process comprising subjecting
continuously such fluid article to the high-temperature short-time
sterilization by passing it through a plate-type heat exchanger, a
tubular heat exchanger or a surface-scraping heat exchanger,
optionally cooling the sterilized article, and then filling and
sealing it in a pre-sterilized can or glass jar in an aseptic
atmosphere. This process is well known as a so-called aseptic
packaging process and is used broadly for sterilizing fluid
articles, mainly liquid articles such as milk products and soups.
As an improvement of this aseptic packaging process, there is known
a process in which a sterilizied article is filled and sealed in a
container without cooling it, the sealed article is allowed to
stand still for a while after the sealing, and then the cooling of
the article is performed. In each of these aseptic packaging
processes, the article to be sterilized and the container therefore
are sterilized separately, a heat exchanger is employed instead of
a retort as the sterilizing means, and use of an aseptic atmosphere
particularly shielded from the outer atmosphere is indispensable.
Accordingly, each of these aseptic packaging processes is an
out-package sterilization process (pre-packaging sterilization
process) and therefore, it should be distinguished in the
in-package sterilization process (post-packaging sterilization
process) in which the filling and sealing of an article into a
container is at first conducted and then the sterilization is
carried out. In other words, such aseptic packaging process is an
out-package high-temperature short-time sterilization process in
which articles to be treated are substantially limited to fluid
articles that can pass through a heat exchanger.
It is a primary object of this invention to provide a process for
the heat sterilization of sealed and packaged articles in which a
variety of articles to be preserved, inclusive of solid articles
which can hardly be sterilized according to the above-mentioned
out-package (pre-packaging) high-temperature short-time
sterilization process, fluid or hardly fluid articles containing
uniform or non-unform solids, can be subjected to high-temperature
short-time sterilization to shorten greatly the time required for
the heat sterilization treatment while lessening degradation of
quality of packaged articles by heating, whereby completely
sterilized packaged articles having a good combination of long-time
preservability and high quality can be obtained.
We have made extensive research works with a view to developing a
process for the in-package high-temperature short-time
sterilization which can provide a sterilized packaged article in
which degradation of quality of the contained food is much reduced
and long-time preservability is much improved, and as a result, we
have found that (i) flexible containers are much more suitable for
high-temperature short-time sterilization than rigid containers
heretofore used in this field, such as cans and bottles; (ii) when
such flexible containers are employed, in order to obtain good heat
transfer at the high temperature heating step, it is important that
non-condensable gases are removed from food-filled containers to
thereby exclude voids from the interiors of the containers; (iii)
when such flexible container is used and non-condensable gases are
removed from the interior thereof, in order to give such a heat
history to the content of the package that spores of microorganisms
present in the content can be completely annihilated even at the
central portion of the interior of the package without causing
substantial thermal degradation of quality of the content even at
the surface portion having a contact with the inner surface of the
container, it is important that the coefficient of flatness
(hereinafter detailed) should be maintained at a level not
exceeding 0.25 in the sealed packaged article; and that (iv) in
order to prevent occurrence of breakages and other troubles in the
sealed area of the packaged article, it is important to cool under
pressure the packaged article heated at a high temperature for
sterilization of the content. Based on these findings, we have now
arrived at this invention.
In accordance with this invention, there is provided a process for
the preparation of high-temperature short-time sterilized packaged
articles which comprises filling an article to be sterilized in a
flexible container having a heat-sealable resin film at least on an
area to be heat sealed of the inner surface thereof, removing
non-condensable gases present in interior voids of the flexible
container, hermetically sealing the flexible container by
application of heat in such a manner that the coefficient of
flatness in the resulting sealed packaged article is maintained at
a level not exceeding 0.25, said coefficient of flatness being
defined by the following formula ##EQU2## in which l indicates a
maximum thickness (cm) in the sealed packaged article, L designates
a maximum distance across the surface of the container, and K.sub.F
stands for the coefficient of flatness,
maintaining the sealed packaged article for a short time in a
heating medium of a temperature exceeding 130.degree.C. to thereby
sterilize the article packaged in the container, and cooling the so
heat-treated packaged article under a pressure higher than the
saturated vapor pressure corresponding to the sterization
temperature.
This invention will now be detailed.
CONTAINER
It is important that the container to be used in this invention
should have heat resistance and flexibility. In case such flexible
container is employed, non-condensable gases contained in voids of
the container can be easily removed, for instance, by applying a
pressure from the outside. Further, while a sealed packaged article
from the interior of which non-condensable gases have thus been
removed is heat sterilized and cooled, the void-free, flat state
can be maintained in the packaged article. Namely, the coefficient
of flatness K.sub.F can be maintained at a level specified in this
invention, i.e., at a level not exceeding 0.25. In addition, in
this flexible container it is important that at least an area to be
heat sealed of the inner surface of the container is composed of a
heat-sealable resin film. If this requirement is not satisfied, it
is impossible to hermetically seal the container in the
content-filled and non-condensable gas-excluded state.
By the flexible container referred to in the instant specification
is meant a container having such flexibility and deformability that
when an article to be sterilized is filled in the container in the
state where substantial amounts of non-condensable gases are still
present in interior voids of the container, the container is
hermetically sealed while excluding such non-condensable gases by
application of an optional pressure from the outside or in a
reduced pressure lower by at least 70 cmHg than the atmospheric
pressure and than the sealed container is taken out into the
atmosphere, the container can retain its shape regardless of the
change in the pressure and no voids of a reduced pressure are
formed in the interior of the container. Whether the container is
flexible or not is determined by such factors as the kind of the
container-constituting material, the thickness and configuration of
the container and the ambient temperature. The container to be used
can take an optional configuration, as far as it is composed of a
flexible sheet having two inner surfaces confronting each other and
it has such a structure that a space for an article to be contained
is formed between said two inner surfaces and an area to be heat
sealed is provided around the periphery of said space or around the
periphery of an inlet for filling such article into said space. In
order for the container to be hermetically sealed under heating, it
is important that the entire of the inner surface of the container
or an area to be heat sealed of the inner surface of the container
should be composed of a heatsealable resin film. Such container may
take a bag-like form. Namely, a so-called pouch can be used in this
invention.
More specifically, a bag-like container or pouch prepared by piling
laminate materials composed of a heat sealable resin film as the
inner layer and a metal foil such as aluminum foil or a layer of a
heat resistant resin, e.g., polyethylene terephthalate as the outer
layer, or laminate materials composed of a heat sealable resin film
as the inner layer, an intermediate layer of a metal foil or a heat
resistant resin and an outer layer of a heat resistant layer, and
heat sealing the peripheral portion of the resulting laminate
structure, can be used as a flexible container in this
invention.
For a better understanding of the invention, reference is made to
the accompanying drawing, in which:
FIG. 1 is a sectional view illustrating diagrammatically a
packaging container in the pouch form to be used in this invention;
and
FIG. 2 is a sectional view illustrating diagrammatically a flanged
flexible container to be used in this invention.
As is illustrated in FIG. 1, in the case of a bag-like container or
pouch, flexible laminate sheet materials 4 and 4', each consisting
of an inner layer 1 composed of a heat sealable resin such as a
polyamide film, an inner layer 2 composed of a metal foil such as
an aluminum foil and an outer layer 3 composed of a heat resistant
resin such as a polyester film, are piled so that heat sealable
resin layers confront and an area 6 to be heat sealed is formed
around a space 5 for containing an article therein. The resulting
bag-like structure can be used as a packaging container. One
flexible sheet can be used instead of two sheets 4 and 4' to be
heat sealed at the peripheral portions. Namely, a bag-like
container formed by bending one flexible sheet and heat sealing the
bent sheet at three open side portions can be used as a container.
In the case of such bag-like container or pouch, provision of the
intermediate layer composed of a metal foil can be omitted if not
required. It is also possible to employ a laminate structure of
four or more layers. For instance, a laminate structure comprising
a heat resistant layer as another intermediate layer in addition to
the metal foil layer.
A container including a container proper having a space for
containing an article therein and a flange portion and a lid can be
used as a flexible container (sometimes called "semi-rigid
container"). This container is so constructed that heat sealing is
performed between the flange portion of the container proper and
the lid. A container of an optional form having a space for
containing a food therein is used as the container proper. For
instance, there can be employed a sucer or plate such as a dish, a
roasting platter, a cake tray and the like, a pan or pot such as a
frying pan, a saucepan, a cooking pot, a sukiyaki pan, a stew pan
and the like, a cup a lunchbox, etc. The size and configuration of
the lid are determined depending on the size, configuration and
structure of the container proper, so that it can seal completely
an open portion of the container proper together with the flange
portion of the container proper. The container proper is shaped to
have a desirable form by molding a metal foil by pressing or other
molding methods, and it has a heat sealable resin film on the
entire inner surface thereof or only on the flange portion to be
heat sealed with the lid. As the lid constituting material, there
is employed a material having no substantial gas permeability, such
as a metal foil and a laminate of a metal foil and a resin
film.
An instance of such semi-rigid container is illustrated in FIG. 2.
A container proper 11 is composed of a metal foil 12, and it is
shaped by press molding so that it has a space 13 for containing
food therein and a flange portion 14 disposed around the periphery
of the space 13. A heat sealable resin film 15 is formed on the
inside of the flange portion 14. A lid member 16 comprises a metal
foil 17 and a heat sealable resin film 18 formed on the inner face
of the metal foil 17. When the flange portion 14 and lid 16 are
pressed under heating, a hermetic sealing is formed between
them.
In each of the foregoing containers, when two flexible sheet
materials are piled and a heat sealed area is formed around the
periphery of a food-containing space by bonding these two flexible
sheet materials with a heat sealble resin, it is possible to obtain
a sealed interface which can resist fully the heating sterilization
conducted at a temperature exceeding 130.degree.C. In case a heat
sealable resin layer is formed on the entire inner face of the
sheet material, this resin layer acts also as a protective coating
layer preventing the content from having a direct contact with the
metal foil.
Heat sealable resins heretofore used for formation of heat sealed
structures in this field can also be used in this invention as the
heat sealable resin for forming an inner face of an area to be heat
sealed of a packaging container. It is desired that such heat
sealable resin is softened or molten at a temperature not exceeding
250.degree.C., especially at a temperature approximating
200.degree.C. In view of the strength of the heat sealed area, it
is preferred that the heat sealable resin to be used is
crystalline.
In this invention it is desired to employ (1) polyamides, (2)
polyesters of a relatively low melting point and (3) polyolefins,
which are selected from resins having a heat resistance
characterized by a softening or melting point exceeding
130.degree.C.
As such polyamide there can be used polyamides or interpolyamides
composed of recurring units expressed by the following formula
##SPC1##
wherein n is an integer of from 3 to 13 and m is an integer of 1 to
4.
Specific examples of such polyamide include
poly-.omega.-aminocaproic acid, poly-.omega.-heptanoic acid,
poly-.omega.-aminopelargonic acid, poly-.omega.-aminodecanoic acid,
poly-.omega.-aminododecanoic acid, poly-.omega.-aminotridecanoic
acid, polyhexamethylene adipamide, polyhexamethylene sebacamide,
polyhexamethylene dodecamide, polyhexamethylenetridecamide,
polydecamethylene dodecamide, polydecamethylene tridecamide,
polydodecamethylene adipamide, polydodecamethylene sebacamide,
polydodecamethylene dodecamide, polydodecamethylene tridecamide,
polytridecamethylene adipamide, polytridecamethylene sebacamide,
polytridecamethylene dodecamide, polytridecamethylene tridecamide,
polyhexamethylene azelamide, polydecamethylene azelamide,
polydodecamethylene azelamide, polytridecamethylene azelamide, and
interpolyamides thereof.
As the polyester having a relatively low melting point, there are
used homopolyesters and copolyesters having a melting or softening
point not exceeding 250.degree.C., such as polytetramethylene
terephthalate, polytetramethylene terephthalate/isophthalate,
polyethylene terephthalate/isophthalate.
Instead of the foregoing polyesters, there can be used polyesters
having a structure expressed by the following formula ##EQU3##
As the polyolefin, there are employed homopolymers and copolymers
of olefins expressed by the following formula
CH=CH .vertline. R
wherein R is an alkyl group of up to 4 carbon atoms, which have a
melting point exceeding 130.degree.C.
Specific examples of such polyolefin include isotactic
polypropylene, crystalline ethylene-propylene copolymers,
poly-4-methyl-pentene-1, etc. Of course, polyolefins to be used in
this invention are not limited to homopolymers of olefins and
copolymers of two or more olefins, and there can be used copolymers
comprising such small amounts of other comonomer components as will
not damage the heat resistance and properties inherent of
polyolefins.
It is desired that the inner layer of such heat sealable resin has
a thickness of 0.005 to 0.2 mm (5 to 200 .mu.). In case the
thickness of the heat sealable resin layer is smaller than 0.005
mm, it is sometimes difficult to ensure hermetic sealing in the
packaged article. In case the thickness of the heat sealable resin
layer is greater than 0.2 mm, it is difficult to conduct the heat
sealing step at a high speed.
As the heat resistant resin constituting the outer surface layer of
the flexible container, there are employed resins having a
softening point or decomposition point higher than that of the heat
sealable resin used. In general, resins having a melting point
exceeding 150.degree.C., such as polyamides, polyesters,
polycarbonates and polypropylene, and thermosetting coating films
are employed. Suitable resins are selected so that the foregoing
requirements are satisfied.
Several examples of the heat resistant resin to be used in this
invention are now mentioned.
1. Polyamides having a relatively high softening point, which are
selected from polyamides exemplified above with respect to the heat
sealable resin. It is preferred that these polyamides are used in
the form of stretched and oriented films.
2. Polyesters having recurring units expressed by the following
formula ##SPC2##
where in R.sub.1 is an alkylene or cycloalkylene group having 2 to
8 carbon atoms and R.sub.2 is an arylene group.
Specific examples of the polyester of this type are polyethylene
sebacate, polyethylene terephthalate, polytetramethylene
isophthalate, polyethylene terephthalate/isophthalate and poly-1,4
-cyclohexylene dimethylene terephthalate. It is preferred that
these polyesters are used in the form of stretched and oriented
films.
3. Polycarbonates having recurring units expressed by the following
formula ##SPC3##
wherein R.sub.3 is a hydrocarbon group having 8 to 15 carbon
atoms.
Specific examples of the polycarbonate of this type are
poly-p-xyleneglycol biscarbonate, poly-dihydroxydiphenyl-methane
carbonate, poly-dihydroxydiphenyl-ethane carbonate,
poly-dihydroxy-2,2 -propane carbonate and
poly-dihydroxydiphenyl-1-ethane carbonate.
4. Isotactic polypropylene, especially in the form of a biaxially
stretched film.
5. When a metal foil is used as an intermediate layer, a
thermosetting coating film is sometimes employed. The thermosetting
coating film (baked coating film) referred to herein includes films
obtained by coating a lacquer containing a reactive macromolecular
compound or an intermediate leading to a macromolecular compound,
which can cause polymerization or condensation under heating to
form a cross-linked structure in the molecule, uniformly on the
surface of a metal, and drying, baking and curing the coated
lacquer at 140.degree. to 230.degree.C. for 5 to 20 minutes. If
necessary, such lacquer may contain a curing promotor, a coloring
agent, a pigment, a filler and the like. As the resin constititing
such coating film, there can be mentioned, for instance,
phenol-epoxy resins, phenol-melamine resins, urea-epoxy resins,
melamine-epoxy resins, acrylic-epoxy resins, urea-epoxy resins,
melamine-epoxy resins, acrylic-epoxy resins, polyamide resins and
polyimidazols resins.
In general, it is desired that such heat resistant resin layer has
a thickness of 0.002 to 0.1 mm (2 to 100 .mu.). In case the
thickness of the heat resistant resin layer is smaller than 0.002
mm, such undesired phenomena such as reduction of gas permeation
resistance or reduction of strength and impact resistance of the
laminate material are brought about at the step of heat sealing the
laminate material. In case the thickness of the heat resistant
resin layer is greater than 0.1 mm, flexibility is lost in the
laminate material.
When an intermediate layer of a metal foil is provided in a
flexible container or simi-rigid (form-retaining) container, as the
metal foil there are used a steel foil, an aluminum foil, a steel
foil plated with such a metal as tin, chromium and aluminum, and a
steel foil chemically treated with chromic acid, phosphoric acid or
the like or an electrolytically treated steel foil.
As pointed above, the container to be used in this invention should
have not only heat resistance but also flexibility. Therefore, the
thickness of the metal foil to be used as the intermediate layer
should naturally be limited. This upper limit of the thickness
varies depending on the kind of the foil-constituting metal and the
size and shape of the container. It is especially preferred that in
the case of a bag-like container or pouch, the thickness of the
intermediate layer is 0.005 to 0.1 mm (5 to 100 .mu.) in the case
of an aluminum foil, and 0.005 to 0.05 mm (5 to 50 .mu. ) in the
case of a steel foil, and that in the case of a semi-rigid,
form-retaining container the thickness of the intermediate layer is
0.05 to 0.15 mm (50 to 150 .mu. ) in the case of an aluminum foil
and 0.025 to 0.1 mm (25 to 100 .mu. ).
Bonding of the metal foil to the heat sealable resin or heat
resistant resin can be performed with use of known adhesives such
as an epoxy adhesive and an isocyanate adhesive. Further, the
bonding may be performed according to the so-called extrusiion
coating method without use of an adhesive.
According to this invention, by employing such flexible container,
it is made possible to maintain the coefficient (K.sub.f ) of
flatness at a level not exceeding 0.25 in the packaged article
formed by filling ann article into such container and hermetically
sealing the container. Further, when such flexible container is
employed, at the time of in-package (post-packaging)
high-temperature short-time sterilization, the packaged article can
be heat sealed without formation of an interfacial barrier by
non-condensable gases left in the container, such as residual air.
Accordingly, uniform and complete sterilization effect can be
attained in the packaged article by conducting heat sterilization
only for a short time.
FILING OF CONTENT AND DEGASIFICATION
According to this invention, an article to be sterilized is filled
in the above-mentioned flexible container and non-condensable gases
left in voids of the flexible container are excluded. Filling of
the content and exclusion of non-condensable gases may be conducted
in this order or simultaneously. In some cases, exclusion of
noncondensable gases is carried out prior to filling of the
content. In this invention, in connection with the feature that a
flexible container is employed, non-condensable gases are excluded
from the container, which makes it possible to maintain the
coefficient (K.sub.F ) of flatness to a level not exceeding 0.25
and to prevent formation of an interfacial barrier. Furthermore, if
non-condensable gases are present in the hermetically sealed
container, the following defects and disadvantages are brought
about.
a. When amounts of non-condensable gases differ in packaged
articles, there are formed excessively heated products and
insufficiently sterilized products. Therefore, it is impossible to
obtain final products uniform in quality.
b. Vitamins, colorants, fat and oil, flavor constituents and the
like contained in a packaged food are oxidized by oxygen remaining
in the sealed container, with the result that quality of the food
is degraded and appearance is worsened by browning or the like.
c. When the packaged article is heated before drinking or eating,
breakage of the container is sometimes caused to occur.
In this invention, since non-condensable gases are excluded,
occurrence of such undesired phenomena is effectively
prevented.
As the article to be sterilized according to the in-package
high-temperature short-time sterilization process of this
invention, there can be mentioned a variety of cooked and
semi-cooked foods which are subjected to retort sterilization prior
to preservation. Foods in which such undesired phenomena as
browning, decrease or loss of natural colors, loss of flavor
constituents, deterioration of the texture, thermal denaturation of
proteins and change of the water-holding capacity are readily
brought about when they are heated for a long time, are especially
preferably treated according to the process of this invention.
As foods in which browning or change of the flavor is readily
caused, there can be mentioned, for instance, milk products such as
fresh cream, evaporated milk and condensed milk, soups and desserts
containing such dairy products, e.g., pudding. As foods in which
change of the color is readily caused, there can be mentioned, for
instance, boiled crustaceans, boiled green vegetables and cooked
foods containing boiled crustaneans or green vegetables. As foods
in which the texture is readily changed, there can be mentioned,
for instance, pasty marine products such as boiled fish-pastes,
fish fillets, processed meat products such as ham, beaf steak,
roast beef, port saute and hamburger, chicken or egg processed
foods such as roast chicken, fried egg and ham egg, meat-like
vegetable products (artificial meats, and processed and pre-cooked
products of petroleum proteins). These food products can be
effectively sterilized according to the process of this
invention.
Furthermore, cooked foods containing the foregoing food products as
additive components can be sterilized according to the process of
this invention. As such cooked food, there can be mentioned, for
example, stews such as cooked curry, cooked hash, borsch and beef
stew, gravys such as meat sauce, and boiled and cooked vegetables,
fishes, shellfishes and meats such as vinegared port, sukiyaki,
Chinese kedgeree, Chinese kickshaw, cream-boiled chiken and
cream-boiled tuna.
A packaged article formed by filling such food as mentioned above
into the flexible container is subjected to degasification
according to optional means to thereby exclude non-condensable
gases from the interior of the container. Degasification can be
accomplished, for instance, by the following methods.
a. A method comprising reducing the pressure in the interior of the
packaged article to a level lower by at least 70 cmHg (gauge) than
the atmospheric pressure, to thereby exclude non-condensable gases,
e.g., air.
b. A method comprising substituting non-condensable gases, e.g.,
air, present in voids in the interior of the container by a
condensable gas such as steam, to thereby accomplish removal of
non-condensable gases.
For instance, when a food containing a large quantity of water,
which is in the boiled or almost boiled state, is filled under
heating, air contained in the interior of the container is readily
substituted by stream.
Further, air present in the interior of the container can be
substituted by steam by heating a food-filled
container(exhausting).
Still further, when steam projected into a food-filled container,
air present in the interior voids can be readily replaced by steam
(steam flashing).
c. A method comprising applying a pressure from the outside to a
food-filled flexible container to thereby deform the container and
exclude non-condensable gases from the container.
d. A method in which non-condensable gases are removed by a
combination of two or more of the foregoing techniques (a) to
(c).
SEALING
According to this invention, heat sealing is performed by applying
heat sealing temperature and pressure necessary for melting and
bonding the heat sealable resin film to an area to be heat sealed
of the flexible container in which the content has been filled and
from which non-condensable gases have been excluded. The heat
sealing temperature and pressure to be applied vary greatly
depending on the kind of the heat sealable resin of the flexible
container, and it is impossible to define such temperature and
pressure conditions sweepingly. In general, however, sufficient
results can be obtained by conducting the heat sealing at a
temperature higher than the softening or melting point of the heat
sealable resin and under a pressure not causing substantial flow of
the heat sealable resin at the temperature adopted for the heat
sealing.
Suitable heat sealing temperatures and pressures are as shown in
Table 1 given below.
Table 1 ______________________________________ Seal Bar Seal Bar
Heat Sealable Resin Pressure Temperature (Kg/cm.sup.2) (.degree.C.)
______________________________________ Polypropylene 2 - 5 180 -
280 Nylon-12 2 - 7 200 - 300 Polytetramethylene 2 - 7 220 - 320
Terephthalate ______________________________________
The time required for heat sealing varies depending on the kind of
the container and the thickness of the heat sealable resin layer.
In general, the heat sealing is conducted for a time sufficient to
perform melting and bonding of the heat sealable resin, for
instance, 0.1 to 5 seconds.
The heat sealing operation can be carried out at one stage or at
two or more stages. In the latter case, the same or different
temperature and pressure conditions can be adopted at these
stages.
The so formed sealed area is then cooled, if necessary, under
application of a pressure by optional means, to thereby form a
sealed area with good sealing credibility. For instance,
immediately after completion of the heat sealing operation, the
heat sealed area in which the resin is still in the softened or
molten state is pressed by two positively cooled press bars,
whereby the resin is solidified.
A heat sealing process especially suitable for practising this
invention is described in the specification of our copending
application Ser. No. 354,121.
In this invention, it is important that the flexible container is
so selected and steps of content-filling, degasification and heat
sealing are so conducted that the coefficient (K.sub.F ) of
flatness is maintained at a level not exceeding 0.25, preferably
not exceeding 0.15, in the sealed packaged article. The criticality
of this condition will now be described by reference to
experimental results.
In the in-package heat sterilization, it is indispensable that the
central portion of the sealed package should receive a heat history
sufficient to annihilate spores of microorganisms with which the
article to be sterilized is contaminated. Since heat is transferred
from the outer wall of the container to the central portion of the
package through the article to be sterilized, a temperature
gradient should naturally formed in the container in correspondence
to the distance from the wall surface of the container. As the
sterilization temperature is high and as the heat conductivity of
the article to be sterilized is small, this temperature gradient is
great, and the temperature difference becomes greater as the
distance between the container wall and the central portion is
greater.
From results of experiments detailed hereinafter, we found that the
in-package (post-packaging) high-temperature shot-time
sterilization can be applied effectively while controlling the
thermal deterioration of the content, only when the distance
between the wall surface of the container and the central portion
of the package does not exceed a certain limit. More specifically,
it was found that only when the coefficient of flatness defined by
the following formula ##EQU4## in which l indicates a maximum
thickness (cm) in the sealed packaged article, L designates a
maximum distance (cm) across the surface of the container, and
K.sub.F stands for a coefficient of flatness,
does not exceed 0.25, intended effects can be obtained by the
inpackage high-temperature short-time sterilization, and that if
this value exceeds 0.25, the effects attained by conducting the
high temperature short time sterilization are rather inferior to
those attained by ordinary heat sterilization.
EXPERIMENT
Commercially available starch powder (rice starch was sufficiently
kneaded with a 1 : 1 mixture of an aqueous solution containing 1/50
mole per liter of glycine and an aqueous solution containing 1/50
mole per liter of glucose, to thereby form a relatively hard paste
having a water content of about 35 to about 40 %. Then, this paste
was shaped into a plane plate, from which square samples having a
thickness of 1 cm, 2 cm, 3 cm or 4 cm were prepared. The sample was
filled in a flexible pouch (having a size of 135 .times. 200 mm)
formed by piling two laminate films, each of which was prepared by
bonding an outer layer of polyethylene terephthalate (having a
thickness of 12 .mu. ), an aluminum foil intermediate layer (having
a thickness of 9 .mu. ) and nylon-12 film inner layer (having a
thickness of 40 .mu. ) with use of an isocyanate type adhesive, so
that the inner nylone-12 films confronted each other, and heat
sealing three sides of the piled laminate structure while leaving
the remaining one side open.
In the physical central portion of the sample paste filled in the
pouch there was positioned a thermocouple for determination of the
sterilization value (Fo value) included in a measurement assembly
attached to the side wall of the pouch. Then, the pouch was
subjected to degasification and hermetic sealing under a pressure
lower by 72 cmHg than the atmospheric pressure by means of a vacuum
sealer.
The resulting packaged article was put into a retort (autoclave)
and terminals of the thermocouple were connected to a sterilization
value integrator (Fo value computor of model ZQCTF manufactured by
Erab Compnay, Denmark). Then, high temperature steam and compressed
air were introduced into the retort, and the temperature of the
retort was elevated to 150.degree.C. over a period of 2 minutes.
The heating was continued until the Fo value integrator indicated
the value of 5.0, and immediately thereafter, the cooling operation
was carried out. The foregoing operations were conducted with
respect to each of the foregoing samples. In the case of packaged
articles shown in Table 2 given below, the time required for the
above heating treatment was within a range of from 3 to 20
minutes.
After the cooling operation, the package was opened, the spectral
reflectance at each portion of the sample was determined at a
wavelength of 450 m.mu. by means of a spectrophotometer (model
EPS-3T manufactured by Hitachi Electronics, Japan) by employing a
standard plate of magnesium oxide as a control, and the difference
of browning among the determined portions was examined.
The same test was conducted with respect to samples sterilized at
115.degree.C. According to an ordinary heat sterilization
method.
Results of the foregoing tests as well as data of the coefficient
of flatness are shown in Table 2.
The sterilization value used herein is expressed in terms of the
heating time (minutes) necessary for annihilating a certain
concentration of bacterial spores. When this value is indicated as
F or Fo value, it means that the value is one obtained when the
heating was conducted at 121.degree.C. (250.degree.F.).
Table 2
__________________________________________________________________________
Relation Between Coefficient of Flatness of Sealed Packaged Article
and Effects Attained by High-Temperature Short-Time Sterilization
Coefficient Size of Packaged Article Degree of Browning (- logT,
450 m.mu.) of Flatness (thickness .times. width .times. length)
(l/L) (cm) Process of This Invention.sup.1 Conventional
Process.sup.2
__________________________________________________________________________
Surface Central Difference Surface Central Difference Portion
Portion Portion Portion
__________________________________________________________________________
0.05 1 .times. 10 .times. 18 0.0159 0.0092 0.0067 0.0742 0.0325
0.0417 0.15 2 .times. 10 .times. 9 0.0410 0.0146 0.0264 0.1129
0.0467 0.0662 0.25 3 .times. 10 .times. 6 0.1493 0.0306 0.1187
0.1302 0.0501 0.0801 0.36 4 .times. 10 .times. 4.5 0.2518 0.0550
0.1968 0.1844 0.0595 0.1249
__________________________________________________________________________
Notes: .sup.1 sterilized at 150.degree.C. .sup.2 sterilized at
115.degree.C.
From the results shown in Table 2, it is seen that conspicuous
effects are attained by the high-temperature short-time
sterilization only when the coefficient of flatness is 0.15 or
lower, and that when the coefficient of flatness of 0.25 or higher,
better effects can be obtained by ordinary heat sterilization.
In the foregoing tests, the dgree of browning at the central
portion of the packaged article was a little higher with increase
of the coefficient of flatness and this fact is not in conformity
with the concept that the same heat history should result in the
same sterilization value. However, this is due to the fact that
since the sterilization was conducted batchwise, the cooling rate
could not be kept constant.
In view of the results obtained in the above experiment, it will
readily be understood that in this invention it is specified that
the coefficient of flatness is maintained at a level not exceeding
0.25, preferably not exceeding 0.15, in the content-filled flexible
container to be used in this invention.
In this invention, by employing a container having such heat
resistance and flexibility and giving a packaged article having a
coefficient of flatness not exceeding the specific limit and by
conducting degasification under such conditions as giving such
specific coefficient of flatness, it is made possible to apply the
high-temperature short-time sterilization technique effectively,
with the result that complete sterilization of a packaged article
can be accomplished very effectively.
HIGH-TEMPERATURE SHORT-TIME STERILIZATION
According to this invention, the above-mentioned sealed packaged
article is maintained in a heating medium of a temperature of at
least 130.degree.C., especially within a range of from 130.degree.
to 160.degree.C., for a short time, for instance, 0.5 to 15
minutes, especially 1 to 8 minutes, to thereby sterilize the filled
food. Thus, the content food can be sterilized with good
reliability without undesired thermal deterioration of the food
quality.
In this invention, the foregoing high-temperature short-time
sterilization is accomplished by the following procedures.
a. Sterilizer:
A heating pressure vessel called a retort (autoclave) is used for
the sterilization, a batchprocess is adopted, and a so called
sterilizing shelf on which packaged articles are arranged in a
certain order is employed. As the sterilizer for use in the
continuous sterilization, there can be mentioned, for instance, a
continuous retort equipped with a hydro-seal valve or a continuous
retort equipped with a rotary pressure valve. In this invention,
the kind of the sterilizer to be used is not particularly critical,
and any of sterilizers that can provide the following
temperature-elevating rate, sterilization temperature,
sterilization pressure and cooling rate can be used in this
invention.
b. Heating Medium:
Saturated steam, superheated steam, high temperature high pressure
hot water and the like are used as the heating medium. In view of
prevention of breakage of the flexible container, it is preferred
that a heating medium comprising saturated steam and 10 to 30 % of
compressed air is employed. In this invention, it is unnecessary to
be the temperature of the heating medium to a level same as the
final sterilization temperature, and therefore, it is possible to
employ saturated steam in the non-supercompressed state.
c. Sterilization Temperature:
Effects by the high-temperature short-time sterilization can be
obtained when the high-temperature short-time sterilization is
conducted at temperatures ranging from 125.degree. to 200.degree.C.
However, temperatures within a range of from 130.degree.to
160.degree.C. are preferably employed because (1) complete
sterilization can be attained while maintaining the thermal
deterioration of the quality of the article to be sterilized at a
minimum level, (2) uniform heat history can be obtained in lots of
packaged articles and (3) the sterilization procedure can be
performed most efficiently.
when the sterilization temperature is lower than 130.degree.C., the
effects attained by the high-temperature short-time sterilization
are not substantially different from those attained by the
conventional sterilization conducted at a temperature of up to
121.degree.C. In case the sterilization temperature exceeds
160.degree.C., control of the sterilization time is very difficult,
and a very small variation of the sterilization time (1 to 10
seconds) results in a great change of the sterilization value (Fo
value), with the result that it is difficult to obtain sterilized
packaged articles uniform in the quality.
d. Pre-heating of Packaged Article:
In order to obtain uniform high-temperature short-time
sterilization history in sealed packaged articles, it is
indispensable that the temperature difference among packaged
articles forwarded to the sterilization step should be within a
range of .+-. 10.degree.C., preferably .+-. 5.degree.C.
Further, in order to obtain sufficient effects by the
high-temperature short-time sterilization, it is desired that the
initial temperature is maintained above 40.degree.C., especially
80.degree. to 100.degree.C., in packaged articles forwarded to the
sterilization step. Since the packaged article maintained at about
100.degree.C. under atmospheric pressure, it sometimes happens that
the package is broken by the steam pressure generated in the
packaged article. Therefore, it is especially preferred that the
sealed packaged article to be sterilized is preheated at a
temperature ranging from 85.degree. to 98.degree.C. Of course, when
the above-mentioned hot filling method or exhausting degasification
method is adopted for exclusion of non-condensable gases, such
pre-heating can be omitted. The degree of degradation of the
quality of a packaged content such as a food brought about by such
pre-heating conducted within the above temperature range is
extremely low as far as the pre-heating is not conducted for too
long a time, though the influence of the treating time in the
high-temperature short-time sterilization and the heating time on
the difference of the heat history among portions of the packaged
article is very great. When the packaged article is pre-heated
within the above temperature range, the sterilization temperature
can be correspondingly shortened. For instance, when the
sterilization is conducted at 150.degree.C., the sterilization time
can be saved by about 70 % by such pre-heating, as compared with
the case where no pre-heating is conducted.
e. Temperature-Elevating Rate at Sterilization Step:
In the process of this invention, unlike the conventional
sterilization process, it is indispensable that the time from the
point of charging of a package article into the sterilizer to the
point of elevation of the sterilizer temperature to be prescribed
high temperature for sterilization should be as short as possible,
preferably within 180 seconds, especially within 0 to 30 seconds,
if the sterilization temperature is 130.degree. to 160.degree.C.,
which was confirmed from results of experiments made by us. More
specifically, in this invention, even a batchwise retort can be
used as far as the time required for the above temperature
elevation is within 180 seconds, but in general, there is
preferably employed a method in which packaged articles are
directly introduced into high temperature atmosphere maintained at
130.degree. to 160.degree.C. through a pressure-tight valve.
f. Sterilization Pressure and Cooling Pressure:
The flexible container of the sealed packaged article is not broken
at the sterilization step as far as the foregoing conditions are
satisfied. However, at the cooling step, breakage of the flexible
container is sometimes caused to occur by the difference between
the temperature in the container and the ambient temperature. In
order to prevent such breakage of the flexible container, it is
necessary to apply a pressure higher by 0.1 to 1 Kg/cm.sup.2 than
the saturated vapor pressure at the high temperature adopted at the
high-temperature short-time sterilization by employing air or other
non-condensable gas. A preferred pressure applied at the
sterilization step is within a range of 5 to 10 Kg/cm.sup.2 if the
sterilization is conducted at a temperature of 130.degree. to
160.degree.C.
g. Cooling Rate:
The total heat history given to the packaged article is more
influenced by the cooling rate than the temperature-elevating rate
at the sterilizing step. Accordingly, it is indispensable that when
the temperature is lowered from the high temperature sterilization
range to the ordinary sterilization range (110.degree. to
121.degree.C.), the cooling is accomplished within 120 seconds,
preferably within 60 seconds, and that when the temperature is
lowered from the high temperature sterilization range from the
range approximating 100.degree.C., the cooling is accomplished
within 250 seconds, preferably within 200 seconds. Such cooling is
performed preferably by introducing the packaged article present in
the high-temperature short-time sterilization atmosphere directly
into cooling water through a pressure-tight valve.
As described above, in this invention the sterilization of the
packaged article can be efficiently accomplished by maintaining the
packaged article in an atmosphere of a heating medium of a
temperature exceeding 130.degree.C. for a short time and then
cooling the packaged article under application of a pressure.
UTILITY
In the sterilized packaged article obtained according to this
invention, each of (1) browning due to Mailard reaction of
sugar-amino acid by high temperature heating, (2) loss and decrease
of natural colors such as carotenoid and chlorophyl, (3) loss of
flavor constituents by heating and (4) texture deterioration,
thermal denaturation of proteins and change of the water holding
capacity can be maintained at a much lower level than packaged
articles sterilized by the conventional heat sterilization process
(conducted at a temperature of up to 121.degree.C.). Further, the
packaged article obtained according to this invention is completely
sterilized, and therefore, it can be preserved for a very long time
at the room temperature.
Accordingly, foods, which have heretofore been formed into
sterilized packaged articles obtained by the conventional retort
sterilization process (conducted at a temperature of up to
121.degree.C.) or the out-package (pre-packing) high-temperature
short-time sterilization process (aseptic packing process, can be
effectively formed into sterilized packaged articles while
maintaining the food quality at such a high level as not at all
attainable by such conventional processes. Further, foods which are
so sensitive to heat that they cannot be formed into sterilized
packaged articles by such conventional processes can be
conveniently formed into completely sterilized packaged articles
without substantial degradation of the food quality.
Still in addition, according to this invention, articles which are
difficult to apply to the conventional out-package high-temperature
short-time sterilization process, can be subjected to the
high-temperature short-time sterilization treatment in the stage
packaged in a flexible container without substantial degradation of
the food quality of the packaged article with the result that
sterilized packaged articles, especially sterilized packaged foods,
having a high preservability can be obtained. Accordingly, this
invention makes great contribution to the art.
This invention will now be detailed by reference to Examples.
EXAMPLE 1
In this Example, a bentonite-water mixture was chosen as a model
food and the relation between the sterilization temperature and the
degree of browning due to Maillard reaction of sugar-amino acid as
a criterion for determination of the quality retention.
A phosphate buffer solution containing 1/20 mole per liter of
glycine and 1/20 mole per liter of glucose was added to bentonite
of the chemical grade (manufactured by Kugita Chemicals, Japan) so
that the bentonite concentration was 7 % by weight, and the mixture
was kneaded and allowed to stand still overnight to obtain a
soup-like mixture. 180 g of this soup-like mixture was filled in a
flexible container (size of 130 .times. 170 mm) formed by piling
two laminate films, each being prepared by bonding an inner layer
of unstretched polypropylene (having a thickness of 50 .mu. ), an
intermediate resin layer of a polycapramide (Nylon 6, ) having a
thickness of 15 .mu. ), an intermediate layer of an aluminum foil
(having a thickness of 7 .mu. ) and an outer layer of boaxially
stretched polyethylene terephthalate (having a thickness of 12 .mu.
) with use of an isocyanate type adhesive, so that the
polypropylene layers confronted each other, and heat sealing three
sides of the resulting laminate structure while leaving the
remaining one side open. Then, the container was degasified and
sealed under a pressure lower by 74 cmHg than the atmospheric
pressure by means of a vacuum sealer. The coefficient of flatness
was below about 0.1 in the so obtained sealed packaged article. The
packaged article was pre-heated for 3 minutes in a hot water bath
maintained at 95.degree.C., charged into a retort and subjected to
the sterilization treatment under conditions indicated in Table
3.
At this sterilization step, the time required for the temperature
elevation ws 120 to 180 seconds. Then, the sterilized packaged
article was cooled below 90.degree.C. in 200 seconds. At each
sterilization experiment the Fo value was within a range of from
about 3.5 to about 4.5.
After the cooling treatment, the packaged article was opened, and
the content was mixed with 100 ml of 90 % ethanol. The mixture was
homogenized, and a part of the homogenized mixture was passed
through a filter. With respect to the filtrate, transmittance at
430 m.mu. was determined by means of a photoelectric
spectrophotometer of model EPS-3T manufactured by Hitachi
Electronics, Japan, and the obtained transmittance was expressed in
terms of the absorbance so as to examine the degree of browning.
Results are shown in Table 3.
Table 3 ______________________________________ Comparison of
Process of This Invention and Conventional Process with Respect to
Degree of Browning Sterilization Conditions Degree of Browning (-
log T, 430 m.mu.) ______________________________________
Temperature Time Pressure (.degree.C.) (min.) (Kg/cm.sup.2 gauge)
______________________________________ Conventional Process 110 50
0.9 0.7077 120 22 1.5 0.4318 Process of This Invention 130 8.4 2.5
0.2182 140 5.8 4.0 0.1681 150 4.5 5.0 0.1612
______________________________________
In the foregoing experiments, in each run according to the
conventional process the ethanol extract exhibited a concentrated
color, and it was apparent that browning proceeded greatly under
the heating conditions, whereas in each run according to this
invention, the ethanol extract was colored only slightly and it was
apparent that the degree of browning was extremely low.
EXAMPLE 2
15 g of milk concentrated to 1/3 of the original volume was hot
filled at 85.degree.C., in a flexible pouch (having a size of 70
.times. 50 mm) formed by piling two laminate films, each being
prepared by bonding an inner nylon-12 layer (having a thickness of
40 .mu. ), an intermediate aluminum foil layer (having a thickness
of 9 .mu. ) and an outer layer of polyethylene terephthalate
(having a thickness of 12 .mu. ), with use of an isocyanate type
adhesive, so that the inner nylon-12 layers confronted each other,
and heat sealing three sides of the resulting laminate structure
while leaving one side open. The coefficient of flatness was about
0.08 in the so obtained sealed packaged article. The sterilization
was conducted according to the conventional process (115.degree.C.,
1.2 Kg/cm.sup.2 gauge, 25 minutes) and according to the process of
this invention (145.degree.C., 4.7 Kg/cm.sup.2 gauge, 1.5 minutes),
respectively. With respect to the flavor of the content taken out
of the sterilized packaged article, the sensory test was conducted
according to the comparison method with a panel of 25 men. As a
result, a significant difference was observed at a significance
level of 1 % and it was found that the sterilized packaged article
according to the process of this invention was superior to the
sterilized packaged article according to the conventional process
with respect to either the taste or the color.
EXAMPLE 3
70 g of seasoned spitchcock was filled in a flexible pouch (having
a size of 130 .times. 170 mm) formed by piling two laminate films,
each being prepared by bonding an inner layer of instretched
polypropylene (having a thickness of 50 .mu. ), and intermediate
layer of a biaxially stretched polypropylene film (having a
thickness of 22 .mu. ) and an aluminum foil (having a thickness of
9 .mu. ) and an outer layer of a biaxially stretched polyethylene
terephthalate film (having a thickness of 12 .mu. ) with use of an
isocyanate type adhesive, so that the inner polypropylene layers
confronted each other, and heat sealing three sides of the
resulting laminate structure while leaving the remaining one side
open. Then, the packaged article was degasified and heat sealed
under a pressure lower by 75 cmHg than the atmospheric pressure.
The coefficient of flatness in the so obtained sealed packaged
article was below 0.06 (maximum thickness being below 12 mm).
Then, the sealed packaged article was sterilized according to the
method in which the sealed packaged article was introduced directly
in a high temperature and high pressure chambef through a
pressure-tight valve. In this method, the come-up time (time
required for temperature elevation) was 2 to 3 seconds. The cooling
was similarly conducted according to the method in which the
sterilized packaged article was directly from the high temperature
and high pressure chamber into a cooling atmosphere through a
pressure-tight valve. The come-down time (time required for
temperature reduction) was 20 to 30 seconds, and hence, the cooling
was accomplished at a very high rate.
The sterilization was conducted under such conditions as giving the
Fo value of about 3, 4 as measured by the same Fo value computor as
described in the text of the specification at a Z value of
18.degree.F.
By the Z value is meant the change (.degree.F.) of the heating
temperature corresponding to the change of the time for
annihilation of bactrial spores by heating or the annihilation
ratio to 1/10 or 10 times. In general, the Z value is defined to be
a temperature (.degree.F.) required for the heating
annihilation-time curve to cross one logarithmic cycle, and each
bacterial spore has a specific inherent Z value.
The sterilization effects were evaluated by (1) the measurement of
the texture by a texturometer (Zenken GTX-2), which can physically
express chewing movement in the mouth cavity, (2) the sensory test
by a panel of 25 men and (3) the quantitative determination of
Vitamin B.sub.1 (thiamine) according the the fluorescent thiochrome
method. Results are shown in Tables 4 and 5.
Table 4 ______________________________________ Texture Retention at
High Temperature Short Time Sterilization of Seasoned Spitchcock
Sterilization Hardness Sensory Test Conditions (cm) (description)
______________________________________ unsterilized 10.6 good
155.degree.C., 1.1 minutes 10.6 good 150.degree.C., 1.4 minutes
10.8 good 145.degree.C., 1.9 minutes 10.2 good 130.degree.C., 3.5
minutes 8.5 acceptable 115.degree.C., 23.0 minutes 6.1 unacceptable
(softened, off-flavor) ______________________________________
Table 5 ______________________________________ Vitamin Retention at
High-Temperature Short- Time Sterilization of Seasoned Spitchcock
Vitamin B.sub.1 Sterilization Content Rention Ratio Conditions
(.gamma./g) (%) ______________________________________ unsterilized
32.6 100 155.degree.C., 1.1 minutes 32.4 99.4 150.degree.C., 1.4
minutes 32.0 98.2 145.degree.C., 1.9 minutes 31.9 98.1
130.degree.C., 3.5 minutes 27.8 85.3 115.degree.C., 23.0 minutes
5.6 17.2 ______________________________________
EXAMPLE 3
120 g of beaf steak (medium treated) having a finished thickness of
15 mm was filled in a flexible container (having a size of 110
.times. 160 mm) formed by piling two laminate films, each being
prepared by bonding an inner layer of poly-.omega.-aminododecanoic
acid (nylon-12 ) (having a thickness of 40 .mu. ), an intermediate
aluminum foil layer (having a thickness of 9 .mu. ) and an outer
layer of biaxially stretched polyethylene terephthalate (having a
thickness of 12 .mu. ) with use of an isocyanate type adhesive, so
that the inner nylon-12 layes confronted each other, and heat
sealing three sides of the laminate structure while leaving the
remaining one side open. The packaged article was degasified and
sealed under a pressure lower by 74 cmHg than the atmospheric
pressure by means of a vacuum sealer. The coefficient of flatness
of the so obtained sealed packaged article was below 0.08.
According to the method described in Example 3, the sealed packaged
article was subject to the high-temperature short-time
sterilization under such conditions as giving the Fo value of about
3.5 at a Z value of 18.degree.F. The texture factor of the so
sterilized beef steak was tested by means of the same texturometer
as described in Example 3 and the sensory evaluation was conducted
in the same manner as in Example 3, to thereby evaluate the effects
by the high-temperature short-time sterilization. Results are shown
in Table 6.
Table 6 ______________________________________ Texture Retention at
High Temperature Short Time Sterilization of Beef Steak
Sterilization Hardness Chewiness Gumminess Sensory Test Conditions
(cm/v) (T.U.) (T.U.) (description)
______________________________________ unsterilized 15.2 12.8 11.6
good (control) 145.degree.C., 15.1 13.9 11.8 good 4.5 minutes
140.degree.C., 14.3 12.0 11.0 good 5.3 minutes 135.degree.C., 13.4
11.2 10.4 fairly 6.2 minutes acceptable 130.degree.C., 13.8 11.0
10.1 acceptable 7.1 minutes 125.degree.C., 11.7 10.8 9.0
unacceptable 8.2 minutes 120.degree.C., 9.6 8.8 7.4 no good 10.5
minutes 115.degree.C., 8.4 6.6 6.0 no good 23.0 minutes
______________________________________
EXAMPLE 5
Cream soup, potage soup and corn soup were prepared according to
conventional procedures. With respect to each soup, 180 g of the
sample was hot, packed at 85.degree.C. into a flexible container
(having a size of 130 .times. 170 mm) formed by piling two laminate
films, each being prepared by bonding an inner layer of an
unstretched polypropylene film (having a thickness of 40 .mu.), an
intermediate layer of a biaxially stretched polycapramide (nylon-6)
film (having a thickness of 15 .mu.) and an aluminum foil (having a
thickness of 9 .mu.) and an outer layer of a biaxially stretched
polyethylene terephthalate film (having a thickness of 12 .mu.)
with use of an isocyanate type adhesive, so that the inner
polypropylene film layers confronted each other, and heat sealing
three sides of the resulting laminate structure while leaving the
remaining one side open. Then, the package was sealed while
excluding air contained in the voids in the upper portion of the
interior of the container. The coefficient of flatness of the so
obtained sealed packaged article was below 0.1.
The so obtained sealed packaged articles were subjected to the
high-temperature short-time sterilization according to the same
method as described in Example 3. The viscosity of the soup
contained in the heat sterilized article was measured by means of
Brookfield rotary viscometer, and the sensory test was conducted in
the same manner as in Example 3. Results are shown in Table 7, from
which it is apparent that the effect of preventing the change of
the viscosity can be attained by the high-temperature short-time
sterilization process according to this invention.
Table 7 ______________________________________ Viscosity Retention
at In-Package High-Temperature Short- Time Sterilization of Soups
(Vismetron VS-Al; shear rate=0.463 sec.sup.-.sup.1) Sterilization
Viscosity, cps. Conditions cream soup potage soup corn soup
______________________________________ unsterilized 14.8 (3) 16.0
(3) 7.4 (3) 155.degree.C., 5.1 minutes 14.5 (3) 16.2 (3) 7.0 (3)
150.degree.C., 5.8 minutes 14.8 (3) 15.8 (3) 7.2 (3) 145.degree.C.,
6.3 minutes 15.0 (3) 15.9 (3) 7.0 (3) 140.degree.C., 7.0 minutes
17.9 (2) 18.8 (3) 8.8 (2) 135.degree.C., 8.0 minutes 17.3 (2) 21.5
(2) 9.8 (2) 130.degree.C., 9.4 minutes 17.7 (2) 20.7 (2) 8.5 (2)
125.degree.C., 11.8 minutes 20.2 (1) 21.9 (2) 11.0 (1)
120.degree.C., 15.5 minutes 20.0 (1) 24.2 (1) 10.4 (1)
115.degree.C., 32.0 minutes 24.0 (1) 24.0 (1) 18.0 (1)
______________________________________ Notes: Parenthesized values
indicate results of the sensory test (3: good, 2: acceptable, 1:
unacceptable).
EXAMPLE 6
140 g of 3 % potato starch solution was hot packed at 87.degree.C.
into a flanged vessel (having a maximum size of 150 mm and a depth
of 10 mm) obtained by press molding a laminate sheet comprising an
inner layer of unstretched polypropylene (having a thickness of 50
.mu.), an intermediate aluminum foil layer (having a thickness of
120 .mu.) and an outer layer of a baked coating of an epoxy-phenol
resin (having a thickness of 3 .mu.), the inner layer and
intermediate layer being bonded to each other with use of an
isocyanate type adhesive. Immediately, a lid composed of the same
laminate sheet as mentioned above was fitted on the container so
that the inner polypropylene layer of the lid confronted the inner
polypropylene layer of the container, and the assembly was heat
sealed. The coefficient of flatness of the so obtained sealed
packaged article was below 0.07.
The so obtained sealed packaged article was subjected to the
high-temperature short-time sterilization according to the method
described in Example 3 so as to control the Fo value to about 3.5
at a Z value of 18.degree.F. The viscosity of the content was
determined by means of Brookfield rotary viscometer (Vismeteron
model VS-Al) at a shear rate of 1.478 sec.sup.-1, to evaluate the
effects by the high-temperature short-time sterilization. Results
are shown in Table 8.
Table 8 ______________________________________ Viscosity Retention
at High Temperature Short Time Sterilization of 3% Potato Starch
Solution Sterilization Conditions Viscosity (cps)
______________________________________ unsterilized 21.2
155.degree.C., 2.9 minutes 20.0 150.degree.C., 3.1 minutes 20.4
145.degree.C., 3.3 minutes 19.0 140.degree.C., 3.8 minutes 20.3
135.degree.C., 4.1 minutes 28.3 130.degree.C., 5.2 minutes 32.3
125.degree.C., 6.3 minutes 33.4 120.degree.C., 10.0 minutes 35.9
115.degree.C., 21.7 minutes 37.4
______________________________________
EXAMPLE 7
Green bean, Spanish paprika and stone leek were cut into suitable
sizes and blanched in boiling water to deactivate enzymes. Then, 45
g of each sample was packed, together with 25 g of 2 % saline
water, into a flexible container (having a size of 110 .times. 70
mm) formed by piling two transparent laminate films, each being
prepared by bonding an inner layer (having a thickness of 50 .mu.)
of a polyester-ether composed mainly of polytetramethylene
terephthalate and an outer layer (having a thickness of 12 .mu.) of
a biaxially stretched polyethylene terephthalate film with use of
an isocyanate type adhesive, so that the inner polyester-ether
layers confronted each other, and heat sealing three sides of the
laminate structure while leaving the remaining one side open. Then,
the container was degasified and sealed under a pressure lower by
73 cmHg than the atmospheric pressure by means of a vacuum sealer.
In each of the so obtained sealed packaged articles, the
coefficient of flatness was below 0.05 (maximum thickness being
below 6 mm).
Each sealed packaged article was subjected to the high-temperature
short-time sterilization according to the method described in
Example 3 under such conditions as giving the Fo value of from 3.3
to 3.5, and then it was cooled. With respect to each sterilized
packaged article, the color (hue) was determined without opening of
the package by employing a Hunter type photoelectric differential
color meter (Color Machine of model CM-20 manufactured by Nippon
Denshoku, Japan) to evaluate the effects by the high-temperature
short-time sterilization.
A white plate having an L value of 92.12, on a value of -0.17 and a
b value of 4.62 was employed as a standard color plate. Results are
shown in Table 9, from which it is seen that a higher color
retention effect can be attained by the high-temperature short-time
sterilization process according to this invention than by the
conventional sterilization process.
Table 9
__________________________________________________________________________
Natural Color Retention at In-Package High-Temperature Short-Time
Sterilization of Green Vegetables (Green Bean, Spanish Paprika and
Stone Leek) Sterilization Conditions Hue (a/b) Green Bean Spanish
Paprika Stone Leek
__________________________________________________________________________
unsterilized 98.4 78.5 80.4 155.degree.C., 0.4 minute 98.0 77.1
80.1 150.degree.C., 0.5 minute 95.5 74.0 78.2 145.degree.C., 0.5
minute 87.0 69.8 76.0 140.degree.C., 0.8 minute 80.5 65.5 75.1
135.degree.C., 1.0 minute 76.6 58.4 66.0 130.degree.C., 1.6 minutes
72.9 54.3 57.0 125.degree.C., 2.3 minutes 52.1 46.5 52.4
120.degree.C., 5.6 minutes 32.4 31.2 39.5 115.degree.C., 16.3
minutes 21.3 34.6 36.7
__________________________________________________________________________
* * * * *