U.S. patent number RE30,780 [Application Number 05/929,668] was granted by the patent office on 1981-10-20 for heat treatment of heat-sensitive products.
This patent grant is currently assigned to Alfa-Laval AB. Invention is credited to Lennart A. Stenstrom.
United States Patent |
RE30,780 |
Stenstrom |
* October 20, 1981 |
Heat treatment of heat-sensitive products
Abstract
Separate units of a heat-sensitive product are each pre-treated
by heating at least the main part of the unit to a pre-heating
temperature of at least 50.degree. C., a cooling medium being
brought into direct or indirect contact with the unit so that its
surface layer is prevented from acquiring a substantially higher
preheating temperature than its inner parts; and each product unit
is then heated to substantially sterilizing or pasteurizing
temperature by electro-magnetic energy of at least microwave
frequency.
Inventors: |
Stenstrom; Lennart A.
(Huddinge, SE) |
Assignee: |
Alfa-Laval AB (Tumba,
SE)
|
[*] Notice: |
The portion of the term of this patent
subsequent to May 7, 1991 has been disclaimed. |
Family
ID: |
20295240 |
Appl.
No.: |
05/929,668 |
Filed: |
July 31, 1978 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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Reissue of: |
176966 |
Sep 1, 1971 |
03814889 |
Jun 4, 1974 |
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Foreign Application Priority Data
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Sep 8, 1970 [SE] |
|
|
12153/70 |
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Current U.S.
Class: |
219/731; 219/700;
219/759; 422/21; 426/237 |
Current CPC
Class: |
A23L
3/045 (20130101); A23L 3/01 (20130101) |
Current International
Class: |
A23L
3/02 (20060101); A23L 3/04 (20060101); A23L
3/005 (20060101); A23L 3/01 (20060101); H05B
006/78 () |
Field of
Search: |
;219/1.55M,1.55A,1.55R
;422/21,25,22,24 ;99/451,485,517 ;426/234,235,236,237 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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731272 |
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Feb 1943 |
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DE2 |
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831490 |
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Feb 1952 |
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DE |
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957179 |
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Jan 1957 |
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DE |
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1642031 |
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Aug 1966 |
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DE |
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1571833 |
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Mar 1969 |
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FR |
|
2004819 |
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Mar 1969 |
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FR |
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6809297 |
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Jul 1969 |
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NL |
|
610520 |
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Nov 1948 |
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GB |
|
954258 |
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Apr 1964 |
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GB |
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1063877 |
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Mar 1967 |
|
GB |
|
1154752 |
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Jun 1969 |
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GB |
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Primary Examiner: Reynolds; B. A.
Assistant Examiner: Leung; Philip H.
Attorney, Agent or Firm: Davis, Hoxie, Faithfull &
Hapgood
Claims
I claim:
1. In the processing of a heat-sensitive product by subjecting it
to a sterilizing or pasteurizing treatment temperature, the method
which comprises pre-treating units of the product by heating at
least the main part of each said unit to a pre-heating temperature
of at least 50.degree. C. .Iadd.interrupting said pretreatment
heating.Iaddend., and subjecting each unit to the influence of a
cooling medium to prevent the surface layer of said unit from
acquiring a substantially higher pre-heating temperature than the
inner parts of said unit, and then heating each product unit to
substantially said treatment temperature by subjecting the unit to
electromagnetic energy of at least microwave frequency.
2. The method of claim 1, in which said pre-heating temperature is
between 60.degree. and 80.degree. C.
3. The method of claim 1, in which said pre-heating is effected by
bringing first a heating medium and then a cooling medium into
contact with the surface of the product unit.
4. The method of claim 3, in which the temperature of said cooling
medium is higher than the temperature of the product unit before
said heating medium was brought into contact with the product
unit.
5. The method of claim 1, in which said pre-heating of the product
unit is effected by an electro-magnetic field.
6. The method of claim 1, wherein portions of the product unit are
heated to a temperature above said pre-heating temperature for a
period of time harmless to said portions, the product unit then
being kept in contact with a medium having a temperature no higher
than said pre-heating temperature.
7. The method of claim 1, wherein said pre-treatment gives a
surface layer of the product unit a lower temperature than its
inner portions when said heating by electro-magnetic energy is
initiated.
8. The method of claim 1, wherein the product unit is packaged
before said heating thereof to substantially said treatment
temperature.
9. The method of claim 1, in which the product unit is surrounded
with an aqueous liquid while effecting said heating thereof to
substantially said treatment temperature.
10. The method of claim 9, wherein the temperature of said aqueous
liquid is controlled to give it substantially said treatment
temperature when said electro-magnetic heating is completed. .Iadd.
11. In the processing of a heat sensitive product by subjecting it
to a sterilizing or pasteurizing treatment temperature, the method
which comprises pretreating units of the product by heating at
least the main part of each said unit to a preheating temperature
of at least 50.degree. C., interrupting said pretreatment heating,
subjecting each preheated unit to a medium having a temperature
less than the temperature of the surface of said unit to prevent
the surface layer of said unit from acquiring a substantially
higher preheating temperature than the inner parts of said unit and
then heating each product unit to substantially said treatment
temperature by subjecting the unit to electro-magnetic energy of at
least microwave frequency, while said unit is surrounded with an
aqueous liquid having a temperature less than the surface
temperature of the units..Iaddend.
Description
This invention relates to the treatment of heat-sensitive products
by electro-magnetic heating thereof, and more particularly to a
novel pre-treatment of the products to prepare them for such
heating.
Heat sterilization of a nutritive product means in practice that
the product is subjected to a heat treatment such that the spores
of the poison-producing bacteria Clostridium botulinum cannot
survive. However, a heat treatment of such products in this manner
normally leads to an impaired taste of the products, due to complex
chemical reactions.
While the sterilization effect of a heat treatment depends
substantially upon the temperature during the heat treatment, the
chemical effects (many of which are developed to a harmful extent
when a critical temperature range typical for the product is
obtained) depend to a higher degree than the sterilization effect
upon the length of time during which the heat treatment is
effected. Therefore, it is desirable in connection with heat
sterilization or pasteurization of foodstuffs to use so called
HTST-processes, i.e., heat treatment at high temperature for a
short time. (HTST is short for High Temperature Short Time).
HTST-processes have long been available for treating fluid and
semi-fluid products. For these products, a rapid and effective heat
transfer in a conventional manner, together with a subsequent
aseptic filling into packages, have been rather easily performed.
For solid products, however, HTST-processes have not been carried
out satisfactorily so far.
It was understood for many years that electromagnetic energy, due
to its special capability of heating the inner parts of a product,
should enable a heat treatment of solid food-stuffs substantially
in the manner of the HTST-processes as to rapidity and intensity.
However, in spite of intensive development work in the field, no
satisfactory method has been developed heretofore. Several test
runs have been made for heating solid product units to sterilizing
or pasteurizing temperature in an electro-magnetic field, and
although it was eventually possible to obtain a substantially
uniform field strength across a considerable area of a field, it
was recognized that product units treated in the field are still
unevenly heated. It was found that the edge portions of the product
units, as viewed in the main direction of the application of the
electro-magnetic field against the product units in a practical
frequency range for economic heating, obtained quite a different
temperature than other portions of the product units. Further, it
has proved that the effect of the electro-magnetic field was damped
gradually when penetrating into a product unit, so that product
units treated in the field were less heated in their central parts
than at their surfaces. This has made a sufficient heat treatment
effect in a product unit unachievable, when attempting to sterilize
or pasteurize it, unless certain portions of the product unit were
so strongly heated that they were spoiled in one respect or
another.
In order to solve these problems, it has been attempted to provide
means for applying an electro-magnetic field such that an even
energy conversion is obtained in product units inserted in the
field. So far, this has not given the desired result, however. It
has also been attempted to pre-heat product units in a conventional
manner prior to heating them in an electro-magnetic field. This has
led to an effect contrary to the desired one, however, probably
because conventional heating (to an even higher degree than heating
in an electro-magnetic field) leads to a greater temperature rise
in the edges and surface of the product unit than in the inner
parts of it.
The principal object of the present invention is to solve the
above-mentioned problems.
According to the invention, each product unit is pretreated by
heating at least the main part of it to a preheating temperature of
at least 50.degree. C., a cooling medium being brought into direct
or indirect contact with the product unit during at least one
period, so that the surface layer of the product unit is prevented
from acquiring a substantially higher pre-heating temperature than
the inner parts of it, and then heating the product unit to
substantially sterilizing or pasteurizing temperature by means of
electro-magnetic energy of at least microwave frequency.
By a pre-treatment of this kind, an uneven heating of the product
unit in the electro-magnwetic field can be tolerated. That is, the
temperature range over which the product unit is to be heated in
this field has deliberately been reduced, so that the uneven
heating in the field will not result in excessive temperature
differences within the product unit (for instance, too high a
temperature in the edges and surface of the product unit when the
inner portions of it have reached the predetermined heat treatment
temperature).
In spite of a pre-treatment of the product unit in accordance with
the invention, a small temperature difference may occur, of course,
in the electro-magnetic field between the outer and inner portions
of the product unit. A small temperature difference of this kind
may be advantageous, however, because of the fact that it will take
a longer time to cool the inner portions of the product unit than
the outer portions of it. A uniform sterilizing or pasteurizing
effect in the product unit may be achieved by keeping the inner
portions of the product unit at a certain temperature during a
certain period of time, while keeping the surface of the product
unit at a slightly higher temperature during a shorter period of
time.
In some cases, as when the product unit is thick or when the
electro-magnetic field is damped substantially in the product unit,
a very uneven heating of the product unit in the electro-magnetic
field can be predicted. In these cases, the advantage of the
present invention is especially apparent. The pre-treatment
according to the invention is then performed so that outer portions
of the product unit will have a lower temperature than inner
portions of it, when the final electro-magnetic heating is
initiated.
So far, the cheapest way of pre-treating a product unit according
to the invention is to bring first a heating medium and then a less
heating, surface-cooling medium into direct or indirect contact
with the surface of the product unit. The effect of this is that a
wall of heat will move inward in the product unit from its surface,
even after the point of time when the surface has been brought into
contact with the less heating medium and has acquired substantially
the temperature of the latter. In certan cases it may be suitable,
after having contacted the product unit with the less heating
medium, to contact the product unit with another medium that heats
its surface during a short time and then, if necessary, with a
further medium that cools it again during an even shorter time,
etc. In this way, an especially rapid heating of the product unit
to a substantially even and high temperature is possible, without
the necessity of having the said media at an excessive temperature
which is harmful to the product unit.
An even faster but somewhat more expensive way of pre-treating a
product unit according to the invention is to pre-heat it by means
of electro-magnetic energy, such as microwaves, while keeping down
the temperature of the surface of the product unit by means of a
cooling medium. The cooling may then be performed before, after or
in connection with the pre-heating operation by means of
electro-magnetic energy. In the two last-mentioned cases, the
cooling medium may be a medium that has been used before the
electro-magnetic pre-heating for a certain pre-heating of the
product unit in a conventional manner.
In connection with a pre-treatment of the product unit according to
the invention by means of electro-magnetic energy, as well as in
connection with a pretreatment by means of heating and cooling
media, portions of the product unit preferably are heated to a
temperature higher than the pre-heating temperature during a period
of time harmless to these portions, after which the product is kept
into contact with a medium having a temperature slightly below this
pre-heating temperature, until the desired even and high preheating
temperature has been obtained in the product unit before the
electro-magnetic heating. By the expression "harmless period of
time" is meant a period of time which is so short that no
substantial quality deterioration of the product unit is caused,
especially as to its taste.
The present invention also relates to an apparatus for performing
the method described above. This apparatus comprises a number of
adjacent vertical chambers communicating with each other at their
upper and lower portions so that they form a continuous closed
sinuous channel through which the product unit is movable to a zone
wherein an electro-magnetic field of at least microwave frequency
is applied. Chambers through which the product unit moves
downwardly on its way to said electro-magnetic zone are filled with
liquid, while the rest of the chambers are filled with gas. The
pressure in each gas-filled chamber amounts to the sum of the
pressure of the liquid in the chamber with which the gas-filled
chamber communicates at its lower end, and the pressure acting on
the liquid in this chamber. Finally, the temperatures of the media
in the different chambers are such that a product unit moved
through the channel to said zone will first pass through at least
one chamber containing a heating medium, and then will be brought
into contact with a less heating medium, so that the product unit
when reaching said zone will have a relatively even and high
temperature before the electro-magnetic heating.
In the apparatus according to the invention, the product unit may
be heated and cooled by means of either liquid or gas. It is
possible to heat the product unit by means of one kind of medium
and to cool it by means of the other kind of medium. It is also
possible to syringe the product units with liquid of a proper
temperature in the gas-filled chambers. In a preferred embodiment
of the new apparatus, the arrangement is such that the product unit
on its way to said electro-magnetic zone will pass first through a
number of hot liquid baths and then through a number of colder
liquid baths. The aforementioned zone in this embodiment is
situated in a liquid-filled chamber. Preferably, a part of the
liquid in this chamber is used for cooling the surface layer of the
product unit, before the product unit enters the heating zone.
The invention is described more in detail below with reference to
the accompanying drawings, in which FIG. 1 is a schematic view
showing how a cross-section of a product unit is influenced, as to
temperature, during the different steps of a pre-treating operation
according to the invention, and FIG. 2 is a side elevational view,
with parts broken away, of an apparatus according to the invention
for sterilizing or pasteurizing a product unit.
Referring to FIG. 1, the horizontal extension of rectangle A
represents the thickness of a product unit P, and the vertical
extension of rectangle A represents the temperature of the product
unit in a starting condition. As can be seen, the product unit in
the chosen example has the same temperature all over its
cross-section.
According to one embodiment of the invention, the product unit P is
first subjected to a heating medium during a certain time. This is
illustrated in FIG. 1 by a rectangle B, the vertical extension of
which represents the temperature of the heating medium, and the
horizontal extension of which represents the period of time during
which the medium is kept in contact with the product unit P. The
result of this heating can be seen from a figure C showing that the
surface layer of the product unit has acquired a substantially
higher temperature than the central part of the product unit.
In this state (C), the product unit is then subjected to a less
heating medium, which is "felt" by the product unit as a cooling
medium, as its temperature is lower than the temperature of the
product unit surface at this moment. This cooling (or in fact
continued heating at a lower temperature) is illustrated in FIG. 1
by a rectangle D, and its influence on the product unit P can be
seen from a figure E. The product unit has now acquired a high and
relatively even temperature.
In FIG. 1 is also shown a dotted line K representing the lower
boundary of a temperature range critical to the product in
question. For most of the nutritive products, the critical
temperature range begins at about 70.degree.-80.degree. C. It is
desired, before the subsequent electro-magnetic heating of the
product unit to sterilizing or pasteurizing temperature, to have
obtained a temperature in the product unit as even as possible
immediately below the said critical temperature range. For this
reason, the above-mentioned less heating medium is preferably
maintained at a temperature of at least 50.degree. C. or more,
preferably 60.degree.-70.degree. C.
If the product unit is pre-heated by means of electro-magnetic
energy, substantially the temperature distribution illustrated by
the figure E in FIG. 1 can be achieved much faster than if the
product unit is preheated in the above-described way. The product
unit may then be surrounded in an electro-magnetic field by a
medium which, to a small extent or not at all, absorbs
electro-magnetic energy, the medium being maintained at such a
lower temperature that the temperature of the surface of the
product unit is prevented from rising over a certain value.
It is also possible, however, to cool a surface layer of the
product unit before or after the electro-magnetic pre-heating of
it. Cooling of the surface layer after the electro-magnetic
pre-heating is permissible due to the short time required by
electro-magnetic heating.
An apparatus for performing the method according to the present
invention is illustrated in FIG. 2. As there shown, the apparatus
comprises a housing 1 which contains a large number of vertical,
parallel partitions 2 through 30. The even-numbered partitions 2,
4, 6 . . . 30 are fixed to the top 1a of the housing 1 and extend
downward to a point slightly above the bottom 1b of the housing,
while the odd-numbered partitions 3, 5, 7 . . . 29 are fixed to the
bottom 1b of the housing and extend between the other partitions
upward to a point slightly below the top 1a of the housing. The
partitions 2-30 extend from a front wall 1c of the housing to a
rear wall 1d, the housing also having opposite end walls 1e and
1f.
Thus, the housing and its vertical partitions constitute means
forming a series of vertically elongated chambers C arranged in a
row, with adjacent chambers communicating with each other
alternately at their upper and lower portions along the row.
Consequently, the chambers C form vertical parts of a continuous
and sinuous channel, and this channel is closed except at its inlet
end 32 and its outlet end 34.
Either supported by a special conveyor or interconnected only by
means of packing material, products to be heat treated in the
apparatus are movable through this channel from the left to the
right with reference to FIG. 2. One row 31 of interconnected
product units extends along the path which the product units are
intended to follow through the apparatus. At the free end of each
partition within the housing 1, a so-called deflector roll is
arranged. The deflector roll opposite to the partition 2 is
designated 2R, the deflector roll opposite to the partition 3 is
designated 3R, etc. At the inlet 32 of the apparatus, there is
arranged a deflector roll 33, and in the outlet 34 of the apparatus
is a deflector roll 35. Before entering the apparatus, the products
pass a number of deflector rolls 36-39; and after having moved
through the apparatus, they pass other deflector rolls 40 and 41.
The upper deflector rolls 3R, 5R . . . 29R, 33 and 35 are arranged
to be driven synchronously by suitable means (not shown) whereby
the product units 31 are transported through the sinuous channel of
the apparatus.
The apparatus in FIG. 2 may be divided into three sections, one
pretreating section comprising substantially what is to the left of
the partition 15, one heating section comprising the chamber
between the partitions 15 and 16, and one cooling section
comprising substantially what is to the right of the partition 16.
Product units to be treated in the apparatus will be pre-heated and
heated to the desired treatment temperature in the pre-treating and
heating sections and then cooled in the cooling section.
The product units in the heating section (i.e., between the
partitions 15 and 16) are surrounded by a pressure which at least
substantially corresponds to the vapor pressure arising within the
products at the actual heat treatment temperature. For this
purpose, the following arrangements are provided.
In the pretreatment section, all of those vertical chambers C
through which products move downward within the housing are filled
with liquid. The other chambers are filled with gas. In the chamber
between the partitions 2 and 3, the gas pressure amounts to a value
corresponding to the sum of the atmospheric pressure surrounding
the apparatus and the pressure of the liquid column between the end
wall 1e of the housing 1 and the partition 2. In the next
gas-filled chamber (i.e., between the partitions 4 and 5), the
pressure amounts to the sum of the gas pressure in the chamber
between the partitions 2 and 3 and the pressure of the liquid
between the partitions 3 and 4. In this way the pressure within the
gas-filled chambers increases in the direction toward the heating
section. Correspondingly, all those chambers C of the cooling
section through which the products move upward in the housing 1 are
filled with liquid, while the other chambers are filled with gas.
According to the same principle as in the pretreatment section, the
pressure in the gas-filled chambers of the cooling section
increases in the direction from the outlet 34 of the housing toward
the centrally situated heating section. Thus, products treated in
the apparatus will meet a gradual pressure drop in the cooling
section on their way to the outlet 34.
The centrally situated heating section C1, in which the total
pressure amounts to about 3.8 bars, may be filled either with gas
or with liquid. As shown in FIG. 2, it is filled with water. In the
heating section C1, means 42 are arranged for applying an
electro-magnetic field, such as microwaves, between the partitions
15 and 16. The product units pass through this field, as shown in
FIG. 2. The means 42 for applying this field may be conventional
and therefore are shown only schematically.
The desired gas pressures in the different chambers C of the
housing 1 can be provided in any suitable manner known to those
skilled in the art. An example of an arrangement for this purpose
is disclosed in a copending application Ser. No. 176,969, filed
Sept. 1, 1971, of Lennart A. Stenstrom and Borje R. Rosenberg
entitled "Rapid Heating of Products," such disclosure being
incorporated herein by reference.
To perform the method of the invention, the temperatures in the
liquid-filled chambers C are kept at predetermined values. For this
purpose, inlets and outlets for liquid, valves, thermostat means,
etc., may be arranged in any suitable manner, as will be understood
by those skilled in the art. An example of such an arrangement is
also disclosed in said copending application.
In the following, different temperature programs for the apparatus
according to the invention are described, which have been used for
sterilizing and pasteurizing products consisting of minced meat
packed under vacuum in a thin material that is transparent to
microwaves. The outer measurements of the product units were
85.times.100.times.20 mm, and the product units were moved while
interconnected only by means of the packing material along the bath
31, the units being moved through the apparatus at a speed of about
6 cm. per second. Along this path, the product units passed under a
gradually increasing pressure from the surrounding medium through
seven separate liquid baths. In one test run, when the product
units were heated to sterility, the Centigrade temperatures in the
seven liquid baths were kept at the following values: 90.degree.;
90.degree.; 90.degree.; 80.degree.; 80.degree.; 70.degree.;
70.degree.. Each product unit, when leaving the seventh liquid bath
was heated to the following temperatures (.degree.C.) measured at
regular intervals along the line M--M in FIG. 1: 69.8.degree.;
69.5.degree.; 69.3.degree.; 69.2.degree.; 69.1.degree.;
69.2.degree.; 69.3.degree.; 69.5.degree. ; 69.8.degree.. After
having passed through the gas-filled chamber between the partitions
14 and 15, the temperatures in the product units were equalized to
the following values (.degree.C.): 69.5.degree.; 69.5.degree.;
69.4.degree.; 69.4.degree.; 69.3.degree.; 69.4.degree.;
69.4.degree.; 69.5.degree.; 69.5.degree.;. (If desired, the
temperature of the product unit surface can be prevented from
rising over a predetermined value, or even be lowered further, in
the gas-filled chamber by syringing the product units with liquid
of a proper temperature.)
In this state, the product units entered the electro-magnetic field
provided by the means 42 in the upper part of the liquid-filled
chamber between the partitions 15 and 16. Through an inlet 43, new
liquid having a temperature of 65.degree. C. was continuously
supplied to this chamber, while simultaneously heated liquid was
discharged through an outlet 44 below the inlet in the same
chamber. When the product units left the electro-magnetic field,
the Centigrade temperatures in them along the line M--M were:
138.0.degree.; 145.1.degree.; 139.0.degree.; 134.1.degree.;
132.3.degree.; 134.1.degree.; 139.0.degree.; 145.1.degree.;
138.0.degree.. The temperature of the liquid situated in the field
was gradually increasing in the moving direction of the product
units, and was 135.degree. C. where the product units left the
field. By means of the liquid in the chamber between the partitions
15 and 16, the obtained temperature in the product units was then
maintained until the latter had reached the lower part of this
chamber, where the Centigrade temperatures along the line M--M
were: 136.3.degree.; 137.6.degree.; 137.5.degree.; 136.5.degree.;
136.0.degree.; 136.5.degree.; 137.5.degree.; 137.6.degree.;
136.3.degree..
The product units then entered the cooling section of the
apparatus, where their temperature was rapidly lowered to about
10.degree. C., the pressure of the surrounding medium being
gradually decreasing.
In one test run, where similar products were preheated exactly as
during the above-described sterilizing operation and were then
heated only to pasteurizing temperature, the following Centigrade
temperature distribution was obtained in the product units when
they left the electro-magnetic field: 97.4.degree.; 101.6.degree.;
99.3.degree.; 97.2.degree.; 96.4.degree.; 97.2.degree.;
99.3.degree.; 101.6.degree.; 97.4.degree.. When the product units
had reached the lower part of the chamber between the partitions 15
and 16, the temperature distribution in the product units was the
following (.degree.C.): 96.2.degree.; 97.6.degree.; 98.1.degree.;
98.0.degree.; 97.9.degree.; 98.0.degree.; 98.1.degree.;
97.6.degree.; 96.2.degree..
FIG. 2 shows in dotted lines further means 45 for applying an
electro-magnetic field in the heating section of the apparatus.
This arrangement of two means for applying electro-magnetic fields
illustrates how product units, by means of an apparatus of this
kind, can be both pre-heated and then heated to pasteurizing or
sterilizing temperature by means of electro-magnetic energy. In
this case, the product units are pre-heated by the means 42 to a
temperature slightly below the temperature range critical to the
product in question (for instance, 70.degree. C.), after which the
temperature rise to pasteurizing or sterilizing temperature is
effected by the means 45. The interspace between the means 42 and
45 may be suitable for the equalization of possible temperature
differences arising in the product unit in the means 42, before
initiating the final heating. Temperature differences of this kind
often can be avoided, however, by means of a proper temperature
program for the above-described seven liquid baths. Of course, the
means for electro-magnetic pre-heating of the product units may be
situated anywhere in the pretreatment section of the apparatus,
where the pressure surrounding the product units is sufficiently
high to correspond to the vapor pressure created within the product
units at the temperature to which the product units are to be
pre-heated.
During a sterilizing operation performed by means of the
arrangement according to FIG. 2 and in which the temperature in all
of the seven liquid baths in the pretreatment section of the
apparatus was maintained at 20.degree. C., and the means 42 were
used for the pre-heating of the product units, the following
Centigrade temperature distribution was obtained in the product
units before the final heating by the means 45: 70.0.degree.;
74.1.degree.; 74.0.degree.; 71.9.degree.; 70.8.degree.;
71.9.degree.; 74.0.degree.; 74.1.degree.; 70.0.degree.. During the
entire operation, water having a temperature of 6.degree. C. was
continuously supplied through the inlet 43, hot water being
discharged through the outlet 44.
* * * * *