Method for detecting bruises in fruit

Abstract

Detection of bruises in several genera of fruit including apples, peaches, and pears has been accomplished by measuring the reflectance of near infrared light from the fruit surface. The method is particularly adaptable to automation.

Description

BACKGROUND OF THE INVENTION

This invention relates to a method of detecting bruises in fruit which will lend itself to automation.

Bruises on fruits such as applies, peaches, pears, and cherries result in grade defects, higher grading cost, and the necessity for hand trimming or alternate uses of the fruit. Studies of hand fruit harvests have shown that bruises occur in as high as 16 percent of the applies, 24 percent of the peaches, and 6 percent of the pears. Increased use of mechanical pickers will increase the number of bruises and other surface defects. Most of the grading of fruit is a visual-hand operation despite the fact that much research has been done in an effort to find an automatic method of grading.

T. L. Stiefvater (M. S. Thesis, Cornell Univ. Agr. Eng. Dept., 1970) reviewed the literature for suitable methods of detecting bruises in apples and set forth three primary criteria. A suitable method must be:

1. BASED ON RELIABLY IDENTIFIABLE BRUISE EFFECTS;

2. NONDESTRUCTIVE; AND

3. ADAPTABLE TO HIGH-SPEED SORTING.

For bruise detection to be nondestructive, an apparatus used for this purpose must necessarily perform its task from outside the fruit without undue manipulation. X-ray techniques reportedly have been quite successful in detecting flaws in fruits (Diener, et al., ASAE Paper No. 69-380, 1969; and Ziegler et al., ASAE Paper No. 70-553, 1970), but X-rays have the inherent problems of expense and safety.

Rehkugler, et al., (Transactions of the ASAE 14: 1189-1194, 1971) describe a method of bruise detection in apples which relies on the fact that most bruises leave a dent such that a discontinuity is formed on the surface (i.e., skin) of the fruit. A ray of visible light reflecting from the surface is sufficiently deflected by the discontinuity to be detected.

Ingle and Hyde (Proc. Amer. Soc. Hort. Sci. 93: 738-745, 1968) determined differences in light reflectance at 600 nanometers (nm.) between bruised and unbruised apple pulp. However, this required slicing the apple to obtain samples to be tested. There have been several publications describing surface reflectance of light in the region of 400 to 2,100 nm. R. V. Lott (Proc. Amer. Soc. Hort. Sci. 43: 59-62, 1943; and ibid., 44: 157-171, 1944) and Bittner and Norris (Transactions of the ASAE 11: 534-536, 1968) recorded spectral data from reflectance measurements of several varieties of apples, peaches, and pears and related the data to maturation. However, all of these studies were silent as to the reflectance measurements of bruises. We were surprised, therefore, when we discovered a significant difference between the reflectance measurements from the unbroken surfaces of bruised and unbruised portions of fruit.

Bruises on apples, peaches, pears, and the like can be easily detected by the following steps:

a. illuminating a fruit surface with diffuse light at wavelengths of from 700 to 2,200 nm.;

b. detecting the light reflected from the surface with a photoemissive detector;

c. determining the amount of light reflected from the surface of an unbruised portion of the fruit;

d. determining the amount of light reflected from each portion of the entire fruit surface having an area equal to the area of the portion disclosed in (c);

e. comparing the amount of light determined in (c) to the amount of light determined in (d); and

f. detecting a bruise in the fruit when the amount of light determined in (d) is significantly lower than the amount of light determined in (c).

The drawings consist of 17 figures which depict graphs comparing wavelength versus reflectance measurements of bruised and unbruised portions of peaches, pears, and several varieties of apples.

DETAILED DESCRIPTION OF THE INVENTION

In order to accomplish its object, the invention relies on the discovery that the dry surfaces over bruised portions of certain fleshy fruits (e.g., apples, peaches, pears, and cherries) reflect less infrared light than the surfaces of unbruised portions. The object of the invention is the provision of a quick, reliable method of detecting bruises so that bruised fruit can be sorted from unbruised fruit, preferably by automated mechanical means.

Sorting methods of the type in which the invention is used begin by positioning the fruit in front of the infrared light source and detector. This can be done by hand or by a suitable mechanical means. The light source is such that the portion of the fruit surface to be measured is evenly illuminated with diffuse light having wavelengths of at least from 700 to 2,200 nm., and the photoemissive detector is positioned so that it measures light reflected from the specified portions of the fruit surface. It is first necessary to measure the light reflected from a portion of fruit surface which is known to be free of any sort of bruise or other impairment. This is used as the control. Since at any given wavelength the unbruised surface of each member of a specific fruit variety at the same level of maturity reflects essentially the same amount of light, the control measurement need only be taken once for each batch of fruit to be tested. However, it is preferred that the control measurement be rechecked at frequent intervals. After the control measurement has been established, reflectance measurements are taken over the entire surface of each fruit to be inspected in the same manner as the control. Changes in position of the fruit in relation to the detector port can be accomplished by hand or by some suitable mechanical means. All reflectance measurements subsequent to the control measurement are then compared to the control. A reflectance measurement which is significantly lower than the control indicates a bruise or other similar damage. Bruises inflicted on the fruit in the examples consistently had reflectance measurements which were from 0.02 to 0.32 reflectance units lower than the controls. Reflectance units are expressed as percent of the total amount of light reflected from a white standard. Measurements taken a few seconds after bruising showed that reflectance is lowered at the instant the bruise was made. At most wavelengths, more than half of the decrease in bruise reflectance for apples occurred within the first day of the 28- to 42-day test period.

Output of photoemissive detectors is an electric current so that reflectance can be read directly from an ammeter calibrated to read "bruised" and "unbruised" or the like. An operator is then able to reject the bruised fruit either by hand or by some suitable mechanical means. The electric output of the detector is also useful for activating mechanical rejection devices in completely automated systems.

For optimum measurement accuracy it is preferred that the diffuse infrared light evenly illuminate that portion of the fruit surface from which the measurement is being taken and that the detection area be the same for all measurements including those taken for controls. It is preferred that the detection area be equal to or less than the bruises to be measured. It is also preferred that the range of wavelengths being detected at any one time be relatively narrow. This range of wavelength or band width can be controlled by use of a narrow "slit." In the examples a constant slit width of 0.2 mm. was used, which resulted in band widths of from 20 nm. at a wavelength of 700 nm. to 28 nm. at a wavelength of 2,200 nm.

It will be understood that infrared photometers are manufactured by many companies and that the parameters of slit width, band width, detection area, methods of illumination, etc., may vary with each different manufacturer. Therefore, it will be further understood that the instant invention should not be limited to the exact parameters described above or in the following examples.

GENERAL PROCEDURE

Spectral Measurements

The diffuse reflectance spectrum of the various fruit surfaces was recorded using a Zeiss PMQ II spectrophotometer and M4Q III monochromator fitted with an integrating sphere reflectance accessory with a 20-mm. sample port in front of which each fruit was positioned by hand. A constant 0.2-mm. slit width was used resulting in a spectral band width of 20 to 40 nm. depending on wavelength. Because this is a single-beam instrument with a constant long-term drift, it was necessary to record calibration spectra of black and white standards both before and after fruit spectra in order to obtain corrected fruit spectra. Reflectance was calculated with the aid of a computer using the relation

R = (R.sub.f - R.sub.b)/(R.sub.w - R.sub.b)

where R is the fruit reflectance, R.sub.f the recorded fruit reflectance, and R.sub.w and R.sub.b are the corrected reflectances of the white and black standards, respectively.

Fruit and Test Conditions

Three varieties of apple, one variety of peach, and one variety of pear were included in the tests. The apples were picked from the Michigan State University orchards at East Lansing, the peaches, grown in Pennsylvania, were purchased in East Lansing supermarket, and the pears were picked from a commercial orchard near Fennville, Mich. All fruit were 21/2-inch diameter minimum.

Flesh firmness values were determined using a Chatillon motorized universal test stand (model HCTM) at a rate of 15 cm. per minute, with appropriate Magness-Taylor probe. Six readings were taken on unbruised portions of each of six fruit for apples and pears, see Table 1.

Three fruit of each type were selected randomly for the reflectance measurements which were averaged and R calculated as above and plotted against wavelength (see FIG. 1-17). A uniform bruise was produced on each fruit by dropping a 263-g. flat steel plate on the fruit as it rested on a flat steel

Table 1 __________________________________________________________________________ Number Number Impact Avg. energy, Test of Force,.sup.1 of energy, diameter, period, Fruit fruit lb. fruit g.-cm. mm. days __________________________________________________________________________ Red Globe peach -- -- 3 30000 32 4 Bartlett pear 6 18.4 .+-. 1.5 3 30000 25 18 McIntosh apple 6 13.9 .+-. 0.6 3 10000 26 42 Jonathan apple 6 14.4 .+-. 1.0 3 10000 26 28 3 5000 22 28 3 2500 18 28 Golden Delicious 6 16.8 .+-. 1.4 3 10000 24 28 apple 3 2500 16 28 Stored 4 months McIntosh apple -- -- 1 10000 25 7 Jonathan apple -- -- 1 10000 27 7 Golden Delicious -- -- 1 10000 28 7 apple __________________________________________________________________________ .sup.1 Mean force .+-. 1 standard deviation for all measurements. Pears: 5/16 diameter probe, fruit skin removed. Apples: 7/16 diameter probe, fruit skin removed.

table. The drop height was varied to give different impact energies, see Table 1. These energies generally produced a bruise larger than the sample port of the integrating sphere.

The first reflectance measurements were taken within 2 hours after bruising, then the fruit were placed in cold storage at recommended conditions of temperature and relative humidity (Wright, U.S. Dept. Agr., Agriculture Handbook No. 66, 77 pp., 1954).

EXAMPLE 1

A single variety of peach was selected and treated as described above. Reflectance measurements were taken at 2 hours after bruising and 4 days after bruising; R was calculated and plotted against wavelength, FIGS. 1 and 2. Standard deviation of reflectance was 0.02 and 0.09 for the bruise compared with 0.03 and 0.03 for the control at 800 and 1,200 nm., respectively.

After the reflectance measurements were taken on the 4th day, fruit firmness was determined (Table 1) and each fruit was cut open to observe the nature of the bruise. A shatter cone (V shape) bruise and 1/8-inch layer of unbruised flesh just beneath the skin, previously reported by Fridley and Adrian (Transactions of the ASAE 9: 135-138, 142, 1966), were observed.

EXAMPLE 2

A single variety of pear was selected and treated as described above. Reflectance measurements were taken at 2 hours after bruising, 14 days after bruising (before the fruit was ripened), and 18 days after bruising (after the fruit was ripened). Reflectance, R, was calculated and plotted against wavelength. The reflectance curves were similar at 2 hours after bruising, 14 days after bruising, or 18 days after bruising. In FIG. 3 are shown reflectance at 14 days.

After the reflectance readings were taken on the 18th day, fruit firmness was determined (Table 1), and each fruit was cut open to observe the nature of the bruise. Bruises were of similar size, and a shatter cone bruise and 1/8-inch layer of unbruised flesh just beneath the skin were observed.

EXAMPLE 3

Three varieties of apple were selected and treated as described above. Reflectance measurements were taken and R calculated and plotted against wavelength for McIntosh, Jonathan, and Golden Delicious apples having 10,000 g.-cm. impact bruises 1 day after bruising (FIGS. 4, 6, and 8), 28 or 42 days after bruising (FIGS. 5, 7, and 9), and 7 days after bruising of apples held 4 months in storage (FIGS. 15, 16, and 17). Jonathan apples were also bruised with 5,000 and 2,500 g.-cm. impacts and the reflectance of the bruises measured at 1 and 28 days after bruising (FIGS. 10, 11, 12, and 13). Reflectance measurements of 2,500 g.-cm. impact bruises were taken from Golden Delicious variety apples at 1 day after bruising, FIG. 14. In the wavelength regions of 800, 1,200, and 1,700 nm. the standard deviation of reflectance for the controls were 0.01, 0.02, and 0.02 nm., respectively; and for the bruises were 0.02, 0.03, and 0.02, respectively. After reflectance measurements were taken, each fruit was cut open to observe the nature of the bruise. The bruises for a given variety and impact energy were of similar size, no shatter cone was evident, and the bruises began just under the skin.

Claims

1. A method of detecting bruises in apples, peaches, pears, and the like comprising the steps of:

a. illuminating the outer surface of a fruit, said surface being unbroken, with diffuse light at wavelengths of from 700 to 2,200 nm.;

b. detecting the light reflected from the fruit surface with a photoemissive detector;

c. determining the amount of light reflected from the surface of an unbruised portion of said fruit;

d. determining the amount of light reflected from each portion of the entire fruit surface having an area equal to the area of the portion disclosed in (c);

e. comparing the amount of light determined in (c) to the amount of light determined in (d); and

f. detecting a bruise in the fruit when the amount of light determined in

2. The method described in claim 1 wherein the area of the portions of fruit surface disclosed in (c) and (d) is equal to or less than the area

3. The method described in claim 1 wherein the amount of light determined in (d) is from 0.02 to 0.32 reflectance units lower than the amount of

6. The method described in claim 1 wherein the fruit are pears.