A REVIEW OF TECHNIQUES FOR DETECTING HOLLOW HEART INPOTATOES

K.C. Watts1 and L.T. Russell2

1Agricultural Engineering, and 2Mechanical Engineering, Technical University ofNova Scotia, Halifax,Nova Scotia B3J 2X4

Received 6 March 1985, accepted 13 June 1985

Watts, K.C. and L.T Russell. 1985. A reviewof techniques for detecting hollow heart in potatoes. Can Agric. Eng.27: 85-90.

The presence of hollow hearts in potatoes can occasionally lead to considerable financial loss to a farmer. For example,a Prince Edward Island farmer lost over $250 000 in 1year when his crop destined for a market out of the province wasfound to have more than 10% hollow hearts. Although there are some external physiological symptoms that are indicativeof the internal hollow heart defect, reliable detection is not possible. Size sorting and specific gravity methods do not leadto sufficiently consistent results to be commercially viable. Optical methods can detect discolored hollow hearts withreasonable accuracy. Other defects are also detected. X-ray devices detect hollow hearts but, to automate, signalprocessing is necessary. Preliminary results of work being undertaken at the Technical University of Nova Scotia showedthat acoustic detection of hollow hearts in potatoes has possibilities.

INTRODUCTION

The formation of a hollow approximately in the center of a potato during thegrowing season (subsequently called hollow heart) is undesirable both to the consumer and to the farmer. To the consumer,a potato with a hollow heart is estheticallyunpleasant especially if it is encountered ata connoisseur restaurant where a highprice is paid for the meal. To the farmer, itcould mean severe loss of market potentialand with it financial loss. For example, if abin of potatoes on a P.E.I, farm destinedfor an out-of-province market is found tocontain more than 10%of the potatoes withhollow hearts, the bin is condemned andcannot be shipped. The U.S. regulationonly allows 5% hollow hearts. Exact dataon the frequency of occurrence of hollowheart are not available from government orprivate sources. However, the average percentage of hollow hearts in processedpotatoes is estimated by one company (inNova Scotia) as varying from 3 to 15%,with 1 year in every 5 or 6 in which thefrequency of hollow hearts exceeds 10%.If it were possible to nondestructivelydetect defective potatoes, the loss to farmers and to the market could be significantlyreduced. Also, if large potatoes could beguaranteed to be free of hollow hearts,then the farmer should be able to obtain ahigher price from the connoisseur market.

Although the primary objective of thispaper is to evaluate previous methods ofdetection, the cause and condition andcultural control of hollow heart in potatoesis first briefly examined, since detection ofhollow hearts becomes irrelevant if the

incidence can be reduced or eliminated byproper cultural practice.

HOLLOW HEART — CAUSE,

CONDITION, AND CULTURAL

CONTROL

Hollow heart is a physiological disorder (Levitt 1942) and hence no causalorganism has been found associated withit; transmission of the disorder has notbeen found possible and plants fromaffected tubers produce just as healthytubers as plants from unaffected tubers,under favorable conditions. It appears thathollow heart formation is preceded by thedeath of a patch of cells either in the centeror nearer the apical end of the potato withsubsequent development of wound cambium around them. Later uniform tubergrowth outside of the dead cells cannotaccount for either the ultimate size or

shape of the hollow heart. Since tubergrowth is primarily due to cell enlargement, non-uniform growth rates outsidethe affected hollow heart must be responsible for the changes of the hollow heartrelative to the rest of the tubers.

The cause of death of the cells in thepith is attributed to several factors. Mineral difficiencies (Levitt 1942) in the cellssurrounding the hollow heart indicate theoccurrence, at some point in time, of somestress on the plant (Kranz and Lana 1942).Addition of extra potash is sometimesadvocated (Nelson 1975). It is also thoughtthat a period of drought shortly after tuberset is the primary cause (Kallio 1960). Theincidence of hollow heart is a function ofvariety, some varietiesbeing less susceptible to mineral deficiencies.

Hollow hearts in potatoes can be of anysize or shape at harvest. Normally the hollow begins as a tear in the pith area and

CANADIAN AGRICULTURAL ENGINEERING, VOL. 27, NO. 2, FALL 1985

rapidly progresses into an irregular cavitythat may extend over a considerable lengthof the tuber. The walls of the cavity areinitially white, but usually soon turnbrown.

The first most obvious action necessaryto reduce the incidences of hollow heart is

appropriate selection of variety (Nelson1975). Cultural practices to control theincidence of hollow hearts have includedremoval of foliage and shading (Kranz andLana 1942), addition of nutrients (Kallio1960; Nelson 1970), planting date (Nelson1970), plant spacing (Nelson 1970; Nelsonet al. 1979b), seed tuber and seed piecesize (Nelson and Thoreson 1982), irrigation (Nelson 1975), and root pruning andspraying (Nelson and Thoreson 1974). Theconclusion from the above views is that

practices which give rise to slow steadytuber growth without excessive stress onthe tuber will reduce the incidence of hol

low heart. For example, foliage removalearly in the season gives rise to rapidfoliage regrowth which produces a mineraldeficiency in the tubers leading to higherincidence of hollow heart. Heavy application of nitrogen enhances rapid vinegrowth which leads to mineral deficiencyin the tubers (Kallio 1960). Growing alarger number of smaller tubers by usingcloser plant spacing and larger seed pieceswas effective in slowing down the growthof large tubers. Root pruning and sublethalspraying with Dinoseb reduced the incidence of hollow heart (and the yield) byreducing the rate of tuber growth.

If weather conditions are optimal, it isdifficult to induce hollow heart, no matterhow poor the cultural practices (Nelson etal. 1982). With the variability in the

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Figure1. Percenthollowheart indifferentclasssizesandforallpotatoessampled, (a) Nelsonet al. (1979a), (b) Finney and Norris (1978), (c) Nylund and Lutz (1950).

weather it is sometimes hard to judge whencertain practices should be implemented.

In summary, it is impossible to guarantee freedom from hollow hearts in potatoesthrough implementation of cultural practices only.

METHODS OF HOLLOW HEART

DETECTION

Size SortingBecause hollow hearts usually occur

during a period of rapid growth, a predominance of larger potatoes have hollowhearts. Removing all the larger potatoesshould reduce the incidence of hollow

hearts. This has been studied by Nylundand Lutz (1950), Finney and Norris (1978)and Nelson et al. (1979a). Figure 1 hasbeen derived from the data in three papersof both the incidence in each size class and

the distribution among all classes.It can be seen that the larger potatoes

have more incidence of hollow heart; but itis also evident that a large proportion of thecrop would have to be discarded to ensurethat less than 10% of potatoes were freefrom hollow hearts. Therefore, this is notan economically effective means ofremoving hollow heart potatoes from afield lot of potatoes.

Specific GravityIntuitively it could be concluded that if

a hollow heart exists in a potato, its specific gravity should be reduced. Two of theabove workers also investigated the variation of specific gravity of potatoes with andwithout hollow hearts. Nylund and Lutz(1950) obtained the specific gravity of eachpotato by passing the potatoes through aseries of salt solutions whose specific gravity was varied. Finney and Norris (1978)weighed the potatoes in air and in water toobtain the specific gravity. The percentdistribution of the hollow heart potatoes inseveral specific gravity classes is shown inFig. 2 where the total percentage of hollowhearts in the study of Nylund and Lutz(1950) was 24.3% by weight, and in that ofFinney and Norris (1978) was 21.4%.

It is obvious that no well-defined value

of specific gravity is apparent at whichhollow hearts begin or cease to exist,although there are more hollow heartpotatoes in lower specific gravity classes.The reason for this poor definition is thatthere is considerable variation in the specific gravity of various parts of the potato(Sharma et al. 1958). Moreover, the practical difficulties of making specific gravitymeasurements without immersing thepotatoes in water would make this methodunacceptable for commercial use.

86 CANADIAN AGRICULTURAL ENGINEERING, VOL. 27, NO. 2, FALL 1985

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Optical MethodsTransmittance and reflectance methods

have both been used to detect imperfections in potatoes. The reflectance methodsused by Muir et al. (1982) and Porteous etal. (1981) are only useful for detectingsurface bruising and not for detecting hollow hearts since it is an internal defect that

does not give much exterior indication.Harvey (1937) attempted to use the can

dling process employed in egg production,but with little success.

Birth (1960) used a wide-range spectrophotometer specifically designed for recording spectral absorption curves of biological materials on a linear or logarithmicenergy scale. The apparatus is shown inFig. 3. The potato is oriented so that thelight passes through the shortest dimension of the potato. The telescopic housingencloses the potato to exclude all ambientlight.

An image of the void cannot be seen

when visible or near infrared energy isused, but there is selective absorption ofenergy in the near infrared region which isindicative of hollow heart when there isdiscoloration in the vicinity of the void. Toobtain the absorption it is necessary tocompare the shape of the absorption curvesfor a sound potato with that of a defectivepotato (Fig. 4). In recording the twocurves, the optical density (OD) isarbitrarily set equal to zero at the minimumabsorption (~ 800 \im). All other pointsare plotted relative to this value. It can beseen from the graph that the potato with thehollow heart absorbs considerably moreenergy in the 650-|xm to 750-|xm rangethan the sound potato. The optical densitydifference between 800 and 710 |xm wasused by Birth (1960) as an indication of theshape of the curve in this part of the spectrum because both these wavelengths wereat absorption minima and thus the measurements are least affected by other com

CANADIAN AGRICULTURAL ENGINEERING, VOL. 27, NO. 2, FALL 1985

positional factors such as skin defects.Because this method detects dif

ferences in color, black spot and greeningare also detected. However, if the hollowheart is not discolored to any great extent,the instrument will not detect the defect.

The absorption characteristics of thepotato skin are similar to the absorptioncharacteristics of the discoloration associ

ated with hollow heart. Therefore the size

of the potato is found to be a factor in thedetection of hollow hearts (i.e. the pathlength of the light through the skin constitutes a larger proportion of the total pathlength for small potatoes than largepotatoes).

Other variables that affect the measure

ment and may cause errors are; (1) variation in the spectral absorption of the skin,(2) presence of scarred tissues, and (3) soilon the tubers. Yellowing of potatoes withage did not appear to be a factor.

It is claimed that this method could be

automated to process about two potatoesper second. In the tests carried out, 83% ofpotatoes with hollow hearts were rejectedalong with 10% of potatoes which weresound. Some potatoes with other defectssuch as blackspot and greening were alsorejected which is an additional advantageof this method.

X-rayThree workers have reported using X-

rays to detect hollow hearts in potatoes:Harvey (1937), Nylund and Lutz (1950),and Finney and Norris (1973, 1978).

Harvey (1937) reported that a commercial machine employing an X-ray tubewith a fluoroscopic screen had been constructed that could process 7 500 poundsan hour. The accuracy and difficulties indetection of hollow hearts were not discussed.

Nylund and Lutz (1950) used three varieties of potatoes to compare the results ofX-ray detection versus manual inspectionby cutting open the tubers. The samplesize for each variety was over 1200 tubers.The percentage of hollow heart detectedby the X-ray machine and visually recognized by the operator varied from 75 to84%. Part of the problem in detection ofhollow hearts in potatoes using X-rays isthe presence of "eyes", which are displayed as lighter areas on the X-ray imagesimilar to the image generated by smallinternal hollows. Hence detection of smallholes is difficult. Sufficient hollow heartswere removed to reduce the incidence of

hollow hearts to an acceptable level.Finney and Norris (1973) chose to

examine potatoes in water using X-rays to

87

BLACK SPONGE RUBBER

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MULTIPLIER-TYPE PHOTOTUBE

POTATO WITH HOLLOW HEART

MONOCHROMATIC LIGHT

Figure 3. Spectrophotometer used for detectionof hollow hearts (Birth 1960).

100

POTATO WITH \HOLLOW HEART i

are shown in Fig. 6 for a tuber in air. Onanalyzing two cartons of tubers, it wasfound that the second derivatives of sound

potatoes clustered around zero, with a distinct separation between those positivevalues of the second derivative for hollow

heart potatoes. The magnitude of the second derivative of the X-ray density curvewas proportional to the size of the hollowwith a correlation coefficient of 0.91. All

hollow heart potatoes were detected by thismeans, with no whole tubers being rejected.

These workers also retested these

tubers using a water medium as in theirearlier work but the detection efficiency ofhollow hearts was lower than when usingX-rays in air, contrary to their previousconclusions.

The potential danger and cost of an X-ray machine with the attendant computerhas not, up to this point, permitteddevelopment of a fully automated system.Two farms on Prince Edward Island use X-

ray systems similar to those found at airports for baggage security scans. Howeverdefective potatoes have to be visually identified.

Ultrasonic DetectionThe conventional means of detecting

voids and flaws in metals employs ultrasonics. The pulse-echo mode involves

600 700

WAVELENGTH m>iFigure 4. Output of spectrophotometer (Birth 1960).

800 9oo

enhance the contrast of the hollow heart.The X-ray image of sound potatoes underwater was entirely masked, while the hollow heart areas appeared as a bright spotunder the water. This modification cer

tainly does give rise to the advantage ofreducing or eliminating the effect of theexternal eyes, although the practicality ofsubmerging potatoes on a commercialbasis is very questionable.

Finney and Norris (1978) later sought toexamine the X-ray scan to determine someobjective criterion by which automatic

88

sorting of defective potatoes could beobtained. A potato was placed in an X-rayfield and a scanning detector traversed thelength of the potato (Fig. 5). The output ofthe detector was amplified, digitized andfed to a digital computer for analysis andrecording. The X-ray transmission characteristics were analyzed in terms of a non-dimensional X-ray density parameter, log(1/7) where T is the ratio of the X radiationtransmitted with and without the potato.The first and second derivatives of this

parameter were then obtained. The results

CANADIAN AGRICULTURAL ENGINEERING, VOL. 27, NO. 2, FALL 1985

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Figure 6. Output of X-ray machine (Finney and Norris 1978).

CANADIAN AGRICULTURAL ENGINEERING, VOL. 27, NO. 2, FALL 1985

insonifying the piece of material andobserving the time for reflected waves tobe received by the transmitting/receivingcrystal or transducer. The time delay isthen proportional to the distance of theflaw or boundary from the crystal. Thetransmission mode, with two separatecrystals, one for transmitting the sound,and one for receiving the sound which isplaced on opposite sides of a sample, is notcommonly employed for metals but is usedfor wood (McDonald 1982).

Several workers (Porteous, 1981 pers.commun.; Parker, 1982 pers. commun.)including the authors, have attempted theuse of conventional ultrasonic test equipment on potatoes to evaluate the potentialof using ultrasonics to detect hollow heartsin potatoes. The initial conclusion was thatpotatoes do not transmit sound and thatultrasonics therefore could not be used to

detect hollow hearts in potatoes.However, workers at the Scottish

Institute of Agricultural Engineering (Porteous 1981, pers. commun.) have reportedsome initial studies on the use of sound to

detect hollow hearts in potatoes andexpressed confidence that the method isfeasible and can be made to work. The

difficulties they encountered dealt with thedesign of an appropriate transducer. Theyreport that the attenuation coefficient isquite high, 1 db/mm, and that the velocityof sound in potatoes is about two-thirds ofthat in water (900 m/sec). It is thought thatthe attenuation coefficient is high becauseof the scattering of the sound both by thecellular structure of the potato and thepresence of 1-2% of interconnecting airchannels in the potato (Cargill 1976).

We too have been able to transmit sound

through a slice of a potato using the apparatus shown in Fig. 7. The transducer(crystal) resonates at 175 kHz and is drivenby a gated sine wave (three complete sinewaves) at the resonant frequency with apeak voltage of about 60 V. The oscilloscope trace is also noted on the figure(rotated through 90°). Furtherwork is proceeding on this method at Technical University of Nova Scotia.

The advantages of using ultrasonicsinclude its freedom from radiation hazards

and the possible ease of automation ofpotato rejection, i.e. the electrical signalgenerated by acoustic detection methodscan be more readily translated into animpulse to drive a solenoid to eject defective potatoes.

There are several challenges to be overcome before ultrasonics can be used com

mercially. It is essential that good acousticcoupling be obtained between the transducer and the potato. To do this, water

89

OSCILLOSCOPE

VOLTAGE

WATER SURFACE OF

LARGE MATER TANK

SLICE

TRANSDUCER

Figure 7. Schematic of apparatus and results of acoustic transducer insonifying a potato slicesubmerged in water. The oscilloscope trace of voltage (proportional to signal echoreceived) vs. time (proportional to distance) is rotated 90 degrees for ease of comparison with the physical apparatus.

(preferably with a wetting agent) must beplaced between the transducer and thepotato. Initial enquiries of a potato growerindicated that one spot of water on one sideof a potato at the location of the transducerwould be acceptable. Drying of that smallarea is also feasible.

Soil from the surface of the potatowould also have to be removed by brushing. The decision as to the optimum frequency of insonification is somewhatdependent on the attenuation sinceattenuation is greater at higher frequencies. However, the resolution at 175 kHz isthought to be barely sufficient to obtain agood indication of hollow hearts inpotatoes. Therefore, the optimum frequency to obtain reproducible results withmaximum resolution must be sought.

CONCLUSIONS

Prevention of hollow hearts cannot be

attained by cultural practices. Detection isnot feasible using either size or specificgravity as a sole criterion. Detection bylight transmission has been demonstratedto be reasonably efficient if browning ofthe hollow heart has occurred. X-rays

work well if the second derivative of the

sensing signal is used as the criterion forthe presence of hollow heart. The expenseand potential safety hazard are the twoweaknesses of X-ray detection. Ultrasonicdetection of hollow hearts has potential ifthe attendant problems can be overcome.

REFERENCES

BIRTH, G.S. 1960. A nondestructive technique for detecting internal discolorations inpotatoes. Am. Potato J. 37: 53-60.

CARGILL, B.F. (ed.) 1976. The potato storage. Michigan State University, ASAE, St.Joseph, Mich.

FINNEY, E.E. and K.H. NORRIS. 1973. X-Ray images of hollow heart potatoes inwater. Am. Potato J. 50:1-8.

FINNEY, E.E. and K.H. NORRIS. 1978. X-ray scans for detecting hollow heart inpotatoes. Am. Potato J. 55: 95-105.

HARVEY, R.B. 1937. The X-ray inspection ofinternal defects of fruit and vegetables. Am.Potato J. 35:156-157.

KALLIO, A. 1960. Effect of fertility level onthe incidence of hollow heart. Am. Potato J.

37: 338-343.

KRANZ, FA. and E.P. LANA. 1942. Incidence of hollow heart in potatoes as influ

enced by removal of foliage and shading.Am. Potato J. 19: 144-149.

LEVITT, J. 1942. A histological study of hollow heart in potatoes. Am. Potato J. 19:134-143.

MCDONALD, K.A. 1982. Application ofultrasonics in the wood industry. J. Can.Soc. Nondestructive Testing 4: 12-14.

MUIR, A.Y., R.L. PORTEOUS and R.L.WASTIE. 1982. Experiments in the detection of incipient diseases in potato tubers byoptical methods. J. Agric. Eng. Res. 27:131-138.

NELSON, D.C. 1970. Effect of planting date,spacing, and potassium on hollow heart inNorgold Russet potatoes. Am. Potato J. 47:130-135.

NELSON, D.C. and M.C. THORESON.1974. Effect of root pruning and sub-lethalrates of dinoseb on hollow heart of potatoes.Am. Potato J. 51:132-138.

NELSON, D.C. 1975. Hollow heart ofpotatoes. Red River Valley Potato Facts #2,University of Minnesota, North DakotaState University.

NELSON, D.C, M.C. THORESON, andD.AJONES. 1979a. The relationship betweentuber size and hollow heart in Norgold Russet potatoes. Am. Potato J. 56: 329-337.

NELSON, D.C, D.A JONES, and M.CTHORESON. 1979b. Relationshipsbetween weather, plant spacing and the incidence of hollow heart in Norgold Russetpotatoes. Am. Potato J. 56: 581-586.

NELSON, D.C. and M.C. THORESON.1982. Effect of seed tuber and seed piecesize on growth and incidence of hollowheart in Norgold Russet potatoes. Am.Potato J. 59: 367-373.

NYLUND, R.E. and J.M. LUTZ. 1950. Separation of hollow heart potatoes by means ofsize, grading, specific gravity and X-rayexamination. Am. Potato J. 27: 214-222.

PORTEOUS, R.L., A.Y. MUIR, and R.L.WASTIE. 1981. The identification of dis

eases and defects in potato tubers from measurements of optical spectral reflectance. J.Agric. Eng. Res. 26: 151-160.

SHARMA, M.K., D.R. ISLEIB, and S.T.DEXTER. 1958. Specific gravity of different zones within potato tubers. Am.Potato J. 35:784-788.

90 CANADIAN AGRICULTURAL ENGINEERING, VOL. 27, NO. 2, FALL 1985