ELECTRONIC COLOR SORTING OFSTRAWBERRIES

N.R. Bulley

Member CSAE

Agricultural Engineering DepartmentUniversity of British Columbia

Vancouver, B.C.

INTRODUCTION

The handpicking and sorting of strawberries necessitates a seasonal labor force.The management and salaries for theseworkers represents a major expenditure intime and money to the strawberrygrower. One of the solutions to thisproblem would be the automation ofstrawberry picking and sorting.

There are two main approaches to themechanical picking of strawberries:

(1) the once-over harvester, whichwould harvest all of the straw

berries at one time;(2) the selective harvester, which

would select only those berriesthat are ripe and leave the unripeberries to be harvested at a later

date.

At the moment the once-over harves

ter seems to be feasible, but it would beincapable of selecting only the ready-to-pick fruit.

Normally the picking and sorting operations are carried out by a visual colorjudgement of fruit ripeness, which isevaluated on the basis of surface redness.

If berries are to be picked mechanicallyand then processed in a continuous operation, some suitable criterion that correlates with the ripeness of the berry isneeded for quick and accurate sorting. Aninstrument capable of distinguishing between a no. 1 grade of berry and apacking berry could then be used inconjunction with the mechanical onceover harvester for final sorting of thefruit. Several researchers have reported onthe reflectance properties of a variety offruits and correlated these results withdifferent physical properties of the fruitthat might indicate the degree of maturity of the fruitb, c, d, e (i? 4? 6, 7).

Some of this information has been

used to develop instruments capable of

RECEIVED FOR PUBLICATION JULY 17,1973

L.M. Staley

Member CSAE

Agricultural Engineering DepartmentUniversity of British Columbia

Vancouver B.C.

grading lemons, tomatoes, and prunesbased on colora> c (1, 5).

This report describes the design andtesting of an instrument for the colorsorting of strawberries.

DESIGN OF PROTOTYPE SORTER

Basis of Design

The recent report on the physical andrheological properties of strawberries (4)contains the information needed to design a system capable of sorting ripe fromunripe berries. Strawberries were irradiated with different wavelengths of lightof equal intensities, and the ratio of thereflected light intensities at different pairsof wavelengths determined. The magnitude of the ratio of the reflected lightintensities at 650 and 525 nm correlated

most favorably with the subjective evaluation of the percentage of the berry'ssurface that was red (Figure 1). This ratioof the relative light reflectances at thetwo wavelengths was called the maturityratio.

a Burkhardt, T.H. and R.F. Mrozek. 1971.Light reflectance as a criterion for sortingdried prunes. Paper No. 71-313. Amer. Soc.Agr. Eng., St. Joseph, Michigan.

b Goddard, W.B., M. O'Brian, C. Lorenzen,and D.W. Williams. 1970. Development ofcriteria for mechanization of gradingprocessing tomatoes. Paper No. 70-389.Amer. Soc. Agr. Eng., St. Joseph, Michigan.

c Heron, J.R., K.H. Kromer, and G.L.Zachariah. 1971. Variation of tomato re

flectance properties in maturity evaluation.Paper No. 71-329. Amer. Soc. Agr. Eng., St.Joseph, Michigan.

d Hood, C.E., B.K. Webb, and L.O. Drew.1968. Correlations of certain physical properties of tomatoes. Paper No. 68-120.Amer. Soc. Agr. Eng., St. Joseph, Michigan.

e Long, J.F. and B.W. Webb. 1972. Correlations of reflectance ratios to maturity forwhole peaches. Paper No. 72-312. Amer.Soc. Agr. Eng., St. Joseph, Michigan.

CANADIAN AGRICULTURAL ENGINEERING, VOL. 15, NO. 2, DECEMBER 1973

H.K. Mehra

Agricultural Engineering DepartmentUniversity of British Columbia

Vancouver, B.C.

Because handpicking and sorting ofstrawberries are also based on the overallredness of the berries, the maturity ratiocan be used in place of the subjectivecolor evaluation.

Machine Design

Based on the correlation between ma

turity ratio and subjective berry ripeness,a system for the sorting of strawberrieswould include the following components:

(1) a light source to illuminate theberry;

(2) light sensors and filters capableof measuring the intensity ofreflected light in two narrowregions of the spectrum;

(3) a comparator capable of determining the magnitude of theratio of the two signals; and

(4) a transducer that will physicallyseparate berries based on themagnitude of this maturity ratio.

Source of Light

Because the analysis of the reflectancespectrum indicated that the maturity ofthe berry can be correlated with the lightreflectances at 650 and 525 nm, it isevident that the reflectances at these

wavelengths should be measured simultaneously and from the same spot on theberry. This was done by illuminating thestrawberry with one light source andmeasuring the reflectances with twophoto-diodes of the same sensitivity. Theoptical apparatus for this purpose isshown in Figure 2.

Illumination was provided by a 12-Vtungsten lamp (color temperature2854°K). The lamp was of fixed intensityand could be operated on AC or DCvoltage. A focussing lens was mounted infront of the light to focus the light on thesubject.

Sensors and Filters

Two available Balser Interference Fil

ters with peak transmissions of 640 and540 nm (9 nm half-band widths) wereused in the prototype sorter. The inten-

103

120r

Mr = -1.76 + 0.11Rp

n = 118

Sy= 29.05

r2 = 0.493

2 3 4 5 6

MATURITY RATIO

Figure 1. Least square simple regression percent surface red on maturity ratio.

focussing

intef erence

filter

Figure 2. Optical apparatus for color measuringsystem.

metal tube foam cushion

photodiode/

Figure 3. Cross-sectional view of theprobe assembly (not to scale).

filter-

in a rubber holder at one end of the

probe and the photo-diode was heldbehind the filter on a Plexiglas mount.Foam padding was used to cushion thephoto-diodes. The probe circuit was obtained from Hewlett Packard applicationnotes (3).

Signals from the photo-diode wereamplified using a circuit suggested byHewlett Packard (2). The amplifier wasmounted on a perforated circuit boardand enclosed in a metal casing.

Voltage Comparator

The purpose of the comparator was toproduce a signal to activate a solenoidthat would be used in the actual sortingof berries. When the numerical value ofmaturity ratio was greater than somepredetermined value, the solenoid was toclose. When the value dropped below thispredetermined value, the solenoid wouldopen.

TESTING PROCEDURE

The experimental setup shown (Figure4) was used to test the electronic system.A circular disc painted with black mattepaint on its periphery was rotated by avariable speed motor. Strawberries wereplaced in 12 equally spaced holes alongthe periphery of the disc. Each berry inthe hole was illuminated from the top bya 12-V tungsten lamp as it passed underthe viewing field of the two photo-diodes.As the berry passed under the light thereflectances were measured by thephoto-diodes.

For these trials the separation betweenacceptable fruit and unacceptable fruitwas arbitrarily set at 80% surface redness.

From the previous report (4) based onthe reflectances at 650 and 525 nm amaturity ratio of 7 would give the bestseparation of ripe from unripe berries.Those berries having a maturity ratiogreater than 7 should have more than80% surface redness (Figure 1), and thosehaving a maturity ratio of less than 7would be too green and classified asunacceptable.

Because the filters being used in ourprototype were 640 and 540 nm and not650 and 525 nm, the value of thematurity ratio actually used in the testingof the equipment was set at 5. This meantthat when the irradiance upon the640-nm sensor was equal to or greaterthan five times that received by the540-nm sensor, a solenoid would betriggered to close and select the berry,but if the value of this maturity ratio wasless than five, the solenoid would remainopen and reject the berry. To do this, thesignal from the 640-nm probe was reduced to one-fifth using a variable voltagedivider and the resulting voltage wassubtracted from the signal from the540-nm probe. If the resulting voltagewas greater than zero, the signal activateda current flow in a zener diode, which, inturn, closed the solenoid. If the signalfrom the comparator was less than zero,the zener diode remained open and thesolenoid remained in the open position.As the voltage divider on the signal fromthe red filter could be altered, any desiredratio could be set and any effectivenumerical value of the maturity ratioused for the sorting of the berries.

The maturity ratio was initially set at5 for the test trials to give a separation atthe mature or 80% surface redness, butsince only one point on the berries wasbeing observed and the exact percentredness is difficult to determine, a sampleof 118 berries was analyzed to determinewhat effect changing the value of thematurity ratio being used for separationwould have on the final yield of usableberries.

The berries were electronically separated using different values of the maturity ratio in the comparator.

These results were then compared witha similar separation based on a visualcolor judgement and the maturity ratiothat would give the minimum loss ofberries was found (Figure 5). The lostberries included the immature berries that

the system had selected and the matureberries that the system had rejected.

RESULTS AND DISCUSSION

The percentage of lost berries de-

sity of the reflected light that passedthrough the filters was measured by twoHewlett Packard HP-5082-4205 pinphoto-diodes. The photo-diodes had aresponse time of less than 1 ns. Becauseof the directional sensitivity of thephoto-diodes, the axis of the sensors hadto be within ±10° of the direction of thereflected light.

The filters and photo-diodes weremounted in a probe (Figure 3), whichalso contained a field effect transistor and10 megohm resistor. The three leads thatare separated from ground by a highimpedance were kept short to minimizestray electrical pickup. The filter was held

104 CANADIAN AGRICULTURAL ENGINEERING, VOL. 15, NO. 2, DECEMBER 1973

power

supply

photocell

ill uminant

ampli f ier

voltagecompara

torH-O^'photocell

digital voltmeter

Figure4. Schematic diagram of experimental arrangement for testing color discriminatorequipment.

Figure 5. Theoretical loss distribution by selecting different values of maturity coefficient.

creased as the value of the maturity ratioincreased to about 4.5 and then steadilyincreased for higher maturity ratios(Figure 5). The sample of 118 berriesgave a minimum loss of 10% at thematurity ratio of 4.5.

A second larger sampling of berrieswas also handled in the same way inwhich one group was deemed not readyto pick, i.e., the average berry did nothave 80% surface redness; and the secondgroup was deemed ready to pick becausein this group the average berry had 80%or more surface redness. In these two

instances, using a maturity ratio of 5, theelectronic sorter gave percent losses of 25and 14.4, respectively, when comparedwith careful hand sorting assumed to givezero loss. The results are shown in Table

I.

The losses from unripe berries beingplaced with the mature berries is reallynot a loss but represents a lessening of theoverall standard of the mature gradeberries. The acceptability of this problemin quality control and the acceptability ofwhat percentage of ripe berries that couldbe rejected would depend on the individual operation and can be controlled tosome extent by changes in the maturityratio used as the bases for separation.

The maximum speed of response ofthe system was measured at the solenoidby increasing the rotational speed of thedisc until the solenoid could no longerrespond to the signal change. Maximumresponse obtained from the componentsused was 0.166 s per strawberry. Such aresponse is slow for economical performance for the sorting of strawberries. The

CANADIAN AGRICULTURAL ENGINEERING, VOL. 15, NO. 2, DECEMBER 1973

TABLE I RESULTS OF THE TESTS CONDUCTED ON THE ELECTRONIC COLOR DETECTIONSYSTEM FOR PERFORMANCE

EFFICIENCY USING NORTH

WEST CULTIVAR OF STRAW

BERRIES

No. of

lost berries

Total unripe accepted1" %

berries or ripe rejected* loss

Immature 207 52+ 25

(<80% red)Mature 177 25* 14.4

(>80% red)Total 384 77 20

speed could be greatly increased by usinga faster response solenoid or a completelydifferent transducer. The shape and surface conditions of the strawberries had nomeasurable effect upon the performanceof the system. The signal produced byeach berry was found to be independentof the surface conditions within the limits

of the accuracy of the measurements. Theoverhead fluorescent laboratory lightsand other stray light appeared to have noeffect on the system's performance dueto the directional sensitivity of thephoto-diodes, but direct sunlight caused ashift in the value of the measured maturi

ty ratio. This might have been due toglare problems but was not investigatedsince elimination of interference from

direct sunlight was easily controlled.

The angle of viewing of the sensors tothe berry had no effect on the sortingwhen the measurements were confined

between 10° and 60° from the incidentillumination on the berry surface. Themaximum output signal was obtainedwhen the reflectance was measured at an

angle from 40° to 45° from the incidentillumination.

The system was tested for its stabilityby comparing the results performed onthe same samples at two different times.A sample of 24 berries was tested justafter switching on the system and thesame sample was tested after severalhours. There was no appreciable shift inthe signals.

CONCLUSIONS

(1) The maturity ratio for the Northwestcultivar of strawberry increases aspercent surface redness increases.

(2) The maturity ratio can be used inplace of the subjective color evaluation as an indicator of the percentsurface redness.

105

(3) Strawberries can be sorted into separate grades based on their maturityratio. The grades will be determinedby the variety of strawberry, wavelengths of reflected light selected forthe maturity ratio, and the numericalvalue of the maturity ratio selectedas the criteria for ripeness.

SUMMARY

An instrument has been developed andtested that is capable of sorting ripe(greater than 80 percent surface redness)from unripe (less then 80 percent surfaceredness) strawberries. The berry is irradiated with light and the ratio of thereflected light intensities at 650 and 525nanometers determined. The magnitudeof this ratio (maturity ratio) correlatedmost favorably with the subjective evaluation of the percentage of the berry's

106

surface that was red. This ratio wasdetermined electronically and differentvalues of this ratio could be used toseparate strawberries into different gradesbased on color. Problems in color sortingdue to fruit size, shape, orientation, andsurface moisture have been minimized bythe use of the maturity ratio as the basisof sorting.

The basic system should be adaptableto the sorting of any fruit or vegetablethat undergoes a color change duringripeness and for which a correlationbetween this color change and maturityratio can be established.

REFERENCES

Gaffrey, J.J. and O.L. John. 1970. Photoelectric color sorting of vine ripened tomatoes. Marketing Res. Rep. No. 868, U.S.

6.

Dep. Agr., Washington, D.C.

Hewlett Packard. 1967. Application NotesNo. 915. Hewlett-Packard, Palo Alto,California 94304.

Hewlett Packard. 1968. Technical data,pin diodes. HP 5082-4200 series. Hewlett-Packard, Palo Alto, California 94304.

Mehra, H.K., N.R. Bulley, and L.M.Staley. 1972. Some physical and Theological properties of strawberries. Can. Agr.Eng. 14(2): 85-88.

Powers, J.B., J.T. Gunn, and F.C. Jacob.1953. Electronic color sorting of fruitsand vegetables. Agr. Eng. 34: 149-154,158.

Rood, P. 1957. Development and evaluation of objective maturity indices forCalifornia freestone peaches. Proc. Amer.Soc. Hort. Sci. 70: 104-11.

Stephenson, K.Q. 1964. Selective fruitseparation for mechanical tomato harvester. Agr. Eng. 45: 250-253.

CANADIAN AGRICULTURAL ENGINEERING, VOL. 15, NO. 2, DECEMBER 1973