prediction of susceptibility of potato tubers to internal bruising by using a pendulum-mounted...

Potato Research 28 (1985) 285-294

Prediction of susceptibility of potato tubers to internal bruising by using a pendulum-mounted accelerometer

R. NOBLE

Silsoe College, Department of Agricultural Engineering, Silsoe, Bedford MK45 4DT, England

Accepted for publication: 25 October 1984

Zusammenfassung, Rbsumb p. 292

Summary

Impact tests were conducted on two potato cultivars: Record, which is susceptible to blackspot, and King Edward, which is susceptible to internal crushing and shatter bruising. The type and size of each bruise resulting from each impact were recorded. A pendulum impact tester which had a piezo-electric accelerometer attached to the impacting head so that the acceleration-time curve of an impact could be recorded on a storage oscilloscope, was used. No difference was found between the dimensions of acceleration-time curves obtained from bruised and undamaged tubers. The occurrence of internal crushing and shatter bruising could be predicted with 77-87 % accuracy from the characteristic shape of the acceleration-time curve produced by impacting sJasceptible tubers. The possibility of identifying the susceptibility of tubers to internal crushing and shatter bruising by using a pendulum impact tester with an accelerometer attached, is discussed.

Introduction

The methods for testing the susceptibility of potato tubers to internal bruising which involve inflicting a standard damage treatment followed by visual examination are extremely t ime-consuming because of the time which is necessary for the tissue discolour- ation to develop and for the subsequent peeling or slicing of tubers. Several workers have therefore at tempted to relate the susceptibility of tubers to internal bruising to measurements obtained from less t ime-consuming mechanical tests.

Blight & Hamil ton (1974), using a quasi-static penetrometer test, found that the ratio of surface hardness of a tuber to hardness at a depth of 2 mm represented only an approximate guide to the susceptibility of a variety to internal bruising. Umaerus & Umaerus (1976) used a pendulum impact tester for testing the susceptibility of seventy cultivars and breeding clones to internal bruising; the bruising was classified into blackspot, bruising, i.e. internal crushing or 'corky ' bruise, and shatter bruising. They found a significant, positive relationship between the resistance of cultivars or clones to shatter bruising and the rebound height of the pendulum (r ~ = 0.39). Hughes & Grant (personal communicat ion), also using a pendulum test, found a significant, positive relationship between the combined resistance of samples of tubers to internal crushing and shatter bruising and the rebound height of the pendulum (r 2 = 0.45). However, in neither case was the relationship sufficiently close to enable the pendulum rebound test to be used alone as a selection criterion in a breeding programme.

Fluck & Ahmed (1973) developed an impact test whereby a falling mass with a piezoelectric accelerometer attached was allowed to fall on to test specimens. The

285

R. NOBLE

instrument provided an acceleration-time curve of an impact on a storage oscilloscope. The curve normally resembled an inverted U or V. They found that impact curves for undamaged apples were usually smooth and rounded at the peak whereas many of the impact curves for bruised apples had a flattened or extended peak. Peterson & Hall (1975) used a similar device to test the susceptibility of potato tubers to internal bruising. They found that the presence of a discontinuity in the acceleration-time curves of impacted tubers predicted severe bruising with 93 % accuracy. However, mainly shatter bruising was produced and the occurrence of internal crushing was not reported.

It was decided to investigate the predictability of susceptibility of potato tubers to black spot and internal crushing as well as to shatter bruising by using an accelerometer in an impact test.

Materials and methods

Tubers Two contrasting cultivars of potato were used for the tests: 1. King Edward, which is susceptible to internal crushing and shatter bruising, i.e. cell wall damage, but relatively resistant to black-spot-type bruising; 2. Record, which is susceptible to black-spot-type bruising but relatively resistant to cell wall damage. These differences were obtained from the 1981-82 NIAB List of Potato Varieties (Anon., 1981) and from Hughes (personal communication).

Both cultivars were grown at the college trials grounds at Flitton, Bedfordshire. Tubers were harvested with an elevator digger and selUcted for uniformity in shape and size.

Impact tester A pendulum impact test was considered to be more suitable than a falling bolt test because Peterson (1973) noted serious variations in frictional losses in the guide tube of a falling bolt test due to the accumulation of soil.

A pendulum impact tester, similar to that shown by Hughes (1980) and that described in detail by Finney & Massie (1975), was built for the experiments. The mass and drop angle of the pendulum could be altered to vary the impact and it was fitted with a hemispherical impacting head of radius 12.5 ram.

A Bruel and Kjaer miniature piezoelectric accelerometer, type 4375, was fitted to the pendulum head so that the acceleration-time curve of an impact could be obtained on a storage oscilloscope. The acceleration-time curve could then be plotted on an x-y recorder for subsequent analysis.

Procedure Tubers were halved longitudinally before impact to reduce the variability which would have been caused by different surfaces resting on the backplate of the impact tester. The more rounded-halfof each tuber was then impacted as shown in Fig. 1. This technique was justified by the results of Finney (1963), who showed that there was no difference in the mechanical properties of whole and halved tubers which were loaded at the point of impact shown in Fig. 1.

All tubers were impacted at a tuber temperature of I0 o C.

286 Potato Research 28 (1985)

PREDICTION OF SUSCEPTIBILITY TO INTERNAL BRUISING

Fig. 1. Method of halving and impacting a tuber.

point of impact

stem ~ bud end end

Point of impact - S c h l a g s t e l l e - Point d'hnpact: Stem end - S t o l o n e n d e - talon; Bud end -

K r o n e n e n d e - couronne

A bb. 1. Methodenfiir Halbieren und Beschiidigen ehTer Knolle. Fig. I. Mbthode de coupe d'un tubercule h tester.

The following measurements from the acceleration-time curve were taken for each impact: 1. Impact duration: the width of the curve. 2. Peak acceleration: the maximum height of the curve. 3. Total change in velocity: the area under the curve. 4. Initial slope of the curve. 5. Time to peak acceleration. 6. Time tO zero velocity, obtained by integrating the curve with respect to time to determine where velocity equalled zero.

Following impact, all tubers were left at 20 o C for 10 days to allow full development of the bruises. The mean of the length, width and depth of each bruise was calculated and used as a measure of bruise size.

Experiment 1 Tubers of cv. King Edward were used for the impact tests two days after harvest. Four pendulum masses, 445,487, 526 and 577 g, were used in combination with four drop angles, 50, 55, 60 and 65 ~ to give 16 treatments selected to cover a range which resulted in internal bruising. For each impact treatment, a sample of 10 tubers was used.

Experiment 2 Two cultivars, King Edward and Record, were used for impact tests on tubers two days after harvest and after six months storage at 5 oc . One impact treatment, which consisted of dropping a pendulum of mass 526 g through an angle of 65 ~ , was used. This resulted in internal bruising in some of the tubers of all four samples; there were 20 tubers per sample.

Potato Research 28 (1985) 287

R. NOBLE

Results

Experiment 1

Bruise types Little or no internal bruising was produced when the smallest pendulum mass, 445 g, was dropped from the three smallest drop angles, 50 ~ , 55 ~ and 60 ~ . Progressively.greater proportions of tubers were bruised as the pendulum mass or drop angle was increased; the bruising consisted of internal crushing and shatter bruising which resembled that shown by Umaerus & Umaerus (1976) and Noble (1985). The internal crushing was brownish with distinct edges; the centre of the bruise often shrunk, leaving a hollow cavity.

Some external splitting occurred when the largest mass, 577 g, was dropped from the greatest drop angle, 65 ~ . This treatment was therefore omitted from the main analysis.

Relationship between susceptibility to internal bruising and acceleration-time curve measurements The acceleration-time curve measurements, listed 1-6 in the Materials and methods section, of tubers which developed internal crushing, shatter bruising or no damage were compared. There were significant differences (P ~< 0.05) between the acceleration-time curve measurements 1-6 obtained under different pendulum masses and drop angles. However, there was no significant difference between the acceleration-time curve measurements 1-6 obtained from tubers which developed internal crushing, shatter bruising or no damage under any particular impact treatment.

In regression analyses, the mean of the bruise dimensions (internal crushing and shatter bruising) was used as the dependent variable and the acceleration-time curve measurements as the independent variables. None of the regressions under any of the impact treatments produced regression coefficients which were significantly different from zero (P <~ 0.05). The coefficients of determination, r 2, were generally less than 0.20. A number of multiple regressions were attempted, using the mean of the bruise dimensions as the dependent variable and combinations of the acceleration-time curve measurements as independent variables. However, these also failed to produce coefficients of determination, r 2, greater than 0.30.

Two distinct types of acceleration-time curve were obtained from different tubers. The curves had either a single peak or multiple 'jagged' peaks (Fig. 2). Of the latter, 77 % were related to the occurrence of internal crushing or shatter bruising whereas 83 % of the single peak impact curves were related to tubers which remained undamaged (Table 1). Many of the multiple jagged peak curves had a distinct peak before the point of peak acceleration, similar to the bioyield point observed in the force-deformation curves of bruised apples under slow compression loading by Mohsenin & G~hlich (1962).

Experiment 2

Bruise types In cv. Record both stored and unstored tubers developed black spot although the proportion was greater in stored tubers (Table 2). Black spot was distinguished from internal crushing by blue-grey pigmentation and diffuse edges. Internal crushing devel-



Fig. 2. Single peak (a) and multiple peak (b) acceleration-time curves produced by an accelerometer mounted on a pendulum which impacted against tubers (cv. King Edward).

u~

E

2 0 0 ,

Z 0 - - 4 0 0 -

=E i i i . J Ud U LU 6 0 0 - C]

8 0 0 "

a

? b

4 6 ~ ~ a

T I M E 10-3s

? /

Deceleration - Verlangsamung - Dbcblbration, Time - Zeit - Temps

A bb. 2. Einzelner Gipfel (a) und muhipler Gipfel (b) als Besehleunigungsformen-Zeitkurven dureh einen Beschleunigungsmesser produziert, der an ein Pendel angebaut war, welches gegen Knollen schlug (Sorte King Edward). Fig. 2. Courbes accklbration-temps h pic simple (a) et h pies multiples (b) obtenues h raide d'un aecklbrombtre montb sur un pendule percutant des tubercules (varibtk King Edward).

oped in some of the unstored tubers but not in any of the stored tubers. In cv. King Edward, both s tored and unstored tubers developed internal c rush ingand

shat ter bruising a l though the p ropor t ion was greater in unstored tubers. Black spot was produced in some stored but not in unstored tubers.

Relat ionship between susceptibil i ty to internal bruising and acceleration-time curve measurements Within each sample of tubers, the accelera t ion- t ime curve measurements from tubers which developed black spot, internal crushing, shat ter bruising or no damage were compared . There was no significant difference between the accelerat ion-t ime curve measurements t aken f rom these different categories of tubers, There were significant differences between the accelera t ion- t ime curve measurements obta ined f rom the two cultivars and f rom stored and unstored tubers.


Table 1. Number of impacts resulting in a particular shape of acceleration-time curve and bruise type under different pendulum impact treatments (cv. King Edward).

Pendulum I Single peak 4 Multiple peak 7

mass 2 drop undamaged 5 shattering undamaged shattering (g) angle 3 or crushing 6 or crushing

445 50 o 7 1 55 ~ 7 0 60 ~ 7 0 65 ~ 5 0

487 50 ~ 6 1 55 ~ 6 I 60 ~ 4 0 65 ~ 3 0

526 50 ~ 5 I 55 ~ 4 I 60 o 4 I 65 ~ 3 I

577 50 ~ 4 2 55 ~ 2 2 60 ~ 2 2

Total s 69 12 17 52

i P e n d e l - P e n d u l e ; 2 M a s s e - M a s s e ; 3 F a l l w i n k e l - A n g l e d e c h u t e ; 4 E i n z e l g i p f e l - P i c s i m p l e ; 5 U n b e s c h i i d i g t - I n d e m n e ; 6 S c h l a g e n o d e r Q u e t s c h e n - Ec la t~ o u bcras~; 7 M u l t i p l e r G i R f e l - P ie s m u l t i p l e s ; 8 G e s a m t - T o t a l

Tabelle 1. Zahl der Schl~ge aus einer bestimmten Form der Beschleunigung hervorgehend-Zeit- kurve und Schlagtyp unter verschiedenen Pendelschlag-Behandlungen (Sorte King Edward). Tableau 1. Nombre d'impacts pour chaque forme de courbe accSlSration-temps et type d 'endommagements pour diffSrents traitements (varlet8 King Edward).

Table 2. Number of impacts resulting in a particular shape of acceleration-time curve and bruise type in four samples of tubers (pendulum mass 526 g, drop angle 65~

Cultivar j Single peak 4 Multiple peak 7

undamaged shattering undamaged shattering or black spot 5 or crushing 6 or black spot or crushing

Record unstored 2 9 I 4 6 Record stored 3 19 0 1 0 King Edward unstored 6 2 2 10 King Edward stored 12 3 0 5

Totals 46 6 7 21

I S o r t e - Varibt~; 2 N i c h t g e l a g e r t - A v a n t c o n s e r v a t i o n ; 3 Ge lager t - A p r ~ s c o n s e r v a t i o n ; 4.6.7,8 S i e h e T a b e l l e 1 - V o i r t a b l e a u 1; 5 U n b e s c h i i d i g t o d e r s c h w a r z f l e c k i g - I n d e m n e o u no i rc i s - s e m e n t i n t e r n e

Tabelle 2. Zahl der Schl~ge, die aus einer bestimmten Form der Beschleunigung hervorgehen- Zeitkurve und Schlagtyp in vier Knollenproben (Pendelmasse 526 g, Fallwinkel 65 ~ Tableau 2. Nombre d'impacts pour chaque forme de courbe acceleration-temps et type d 'endommagement pour 4 lots de tubercules (masse du pendule 526 g, angle de chute 65~


In regression analyses, the mean of the bruise dimensions was used as the dependent variable and the different acceleration-time curve measurements as independent variables. Separate regressions were conducted for each type of bruise in each of the four samples of tubers.

There was no relationship between the mean dimension of crush or shatter bruises and any of the acceleration-time curve measurements in any of the samples. There was a relationship between the mean dimension of black-spot bruises and impact duration in the unstored tubers of cv. Record (r 2 --- 0.50). None of the other regressions between mean dimensions of black-spot bruises and acceleration-time curve measurements produced regression coefficients which were significantly different from zero (P ~< 0.05).

Table 2 shows the number of impacts resulting in a particular shape of acceleration- time curve and bruise type in the stored and unstored tubers of cvs Record and King Edward. Of all the single peak impact curves produced in the four samples of tubers, 87 % were related to tubers which remained undamaged or which developed black spot. Of all the multiple peak impact curves produced, 78 % were related to tubers which developed internal crushing or shatter bruising.

Discussion

The results show that the occurrence of internal crushing and shatter bruising in potato tubers can be predicted with 77-87 % accuracy by the shape of an acceleration-time curve produced by a pendulum-mounted accelerometer. The occurrence of black spot could not be predicted from a difference in the shape of the acceleration-time curve. The relationship which was found in one sample of tubers between the size of black spot bruises and impact duration was probably due to the more flaccid tubers in the sample being more susceptible to black spot (Sawyer & Collin, 1960), rather than to a specific response of tubers susceptible to black spot. Hughes et al. (1975) showed that tuber firmness could not be used to predict susceptibility to black spot since different samples of tubers produced different amounts of black spot for the same amount of tuber deformation in a pendulum test.

Transmission electron micrographs by Mcllroy (1976) have shown that black spot may be associated with damage to the cell contents without gross cell wall damage. It is likely that the accelerometer was not sensitive enough to monitor damage to the cell contents alone, but it was sensitive enough to monitor internal crushing and shatter bruising, which are associated with cell wall damage (Hughes, 1981). This limitation of the accelerometer technique is not serious because the results of this investigation (Table 2), as well as those of others (Hughes, 1981; Specht, 1981) have shown that internal crushing and shatter bruising are the most common forms of internal bruising at or shortly after harvest. Black spot is mainly associated with stored tubers (Hughes, 1981 ; Specht, 1981) although I found it in one sample soon after harvest.

Preliminary observations have shown that the occurrence of external splitting can also be detected by the acceleration-time curve. It may therefore be possible to use the accelerometer technique for assessing the susceptibility of tubers to both external and internal structural damage.

I used halved tubers for the impact tests, but improved methods of holding tubers in a pendulum impact tester may enable whole tubers to be used instead. One method has been developed at the Food Research Institute, Norwich, whereby the static load applied


R. NOBLE

to a tuber in a pendulum impact test can be precisely controlled (Grant, 1981). The use of whole tubers would make the pendulum test quicker and non-destructive. This would be advantageous for screening tests for susceptibility to internal crushing and shatter bruising in the early stages of a breeding programme where only limited time is available for each sample.

The pendulum test may be of use in commerce when a rapid assessment of the susceptibility of a potato crop to internal bruising is required before harvest. However, it is likely that a cheaper and simplified version of my equipment will be needed for this to be practicable.

Acknowledgements

I wish to thank my supervisor, Mr R. F. A. Murfitt for his advice and assistance, and Dr J. C. Hughes and Mr A. Grant of the AFR.C Food Research Institute, Norwich, for their useful suggestions. I am also grateful to the Potato Marketing Board and to United Biscuits Ltd who provided funds for the pendulum impact tester.

Zusammenfassung

Vorhersage der Anfdlligkeit von Kartoffelknollen gegen interne Beschiidigungen durch Verwendung eines in ein Pendel eingebauten Beschleunigungsrnessers

Der Ablauf der Verlangsamung eines auf eine Kartoffelknolle schlagenden Pendels wurde als Schnelltest zur Erfassung tier Anf'~illigkeit von Knollen gegen spezifische Typen von internen Beschadigungen verwendet. Ein piezoelek- trischer Beschleunigungsmesser wurde in den Schlagkopf des Pendels derart eingebaut, dass die Beschleunigungs-Zeitkurve eines Schlages auf einem Lagerungs-Oscilloskop erfasst wet- den konnte.

FOr die Schlagtests wurden zwei Kartoffel- sorten verwendet: Record, welche gegentiber Schwarzfleckigkeit anf'~illig ist, und King Ed- ward, welche gegentiber internen Quetschun- gen und Schlagverletzungen anf'~illig ist. Die Schlagtests wurden an den KnoUen zwei Tage nach der Ernte und nach 6-monatiger Lage- rung durchgefiihrt. Typ und Grt~sse jeder Ver- letzung pro Schlag wurden erfasst. Schwarz- fleckigkeit konnte von internen Quetschungen durch die eher blaugraue als braune Verf'~ir- bung unterschieden werden, und bei tier eher diffusen als scharfen Abgrenzung der Besch~- digungen.

Keine Unterschiede ergaben sich zwischen den Dimensionen der Beschleunigungszeit yon besch~idigten und unbesch~idigten Knollen. Be- trachtung der Kurven der Beschleunigungszeit yon verschiedenen Knollen zeigte die Produk- tion zweier verschiedener Kurventypen. Die

Kurven hatten entweder einfache oder mehr- zackige Gipfel (Abb. 2). Von den Einfachgipfel- Schlagkurven konnten 83-87 % auf Knollen bezogen werden, die unbesch~idigt blieben. Von den Mehrgipfel-Schlagkurven liessen sich 77-78 % auf Knollen mit internen Quetschun- gen oder Schlagverletzungen zurtickftihren (Tabellen 1 und 2). Kein Unterschied ergab sich zwischen der Form der Beschleunigungs-Zeit- Kurven von Knollen, die Schwarzfleckigkeit entwickelten, und yon Knollen, die unbescha- digt blieben. Schwarzfleckigkeit k6nnte mit Sch~idigung der Zellinhalte ohne gr6ssere Zell- wand-Sch~idigung zusammenh~ngen (Mcllroy, 1976). Mt~glicherweise war der Beschleuni- gungsmesser nicht empfindlich genug, Schaden nur ftir die Zellinhalte aufzuzeichnen, jedoch empfindlich genug zur Aufzeichnung yon internen Quetschungen und Schlagverletzungen, die mit Zellwand-Schadigung einhergehen (Hughes, 1981). Diese Begrenzung der Accele- rometer-Technik wird als nicht schwerwiegend betrachtet, zumal die Ergebnisse dieser Unter- suchung (Tab. 2) wie auch die Ergebnisse und Beobachtungen anderer Bearbeiter (Hughes, 1981 ; S pecht, 1981) gezeigt haben, dass interne Quetschung und Schlagverletzung die vorherr- schenden Formen interner Verletzungen zur Ernte oder kurz danach sind.

Es wird vorgeschlagen, dass ein Pendel-



schlag-Testger~it mit einem angebauten Be- schleunigungsmesser als Ausleseverfahren for interne Quetschungen und Schlagbeschadi- gungen in einem frtihen Stadium eines Ztich- tungsprogrammes verwendet wird. Die Tech-

nik macht Sch~ilen oder Schneiden der Knollen nicht erforderlich, ferner wird die zeitliche Ver- z6gerung bis zur Verfarbung, die mit der Ent- wicklung interner Besch~idigungen einhergeht, vermieden.

R6sum6

Prbvision de la sensibilitb des tubercules de pommes de terre aux endommagements internes a raide d'un accblbromktre montk sur svstkme pendulaire

La mesure de la d6cbl6ration d'un pendule percutant un tubercule de pomme de terre est utili- s6e comme test instantan6 d'estimation de la sensibilit6 h diff6rents types d 'endommagements internes. Un acc~l~rom~tre pi6zo61ec- trique est fix~ au niveau de la t~te d'impact du pendule afin de pouvoir enregistrer la courbe acc~16ration - temps d'un choc sur un oscilloscope ~ m~moire.

Deux vari~t~s de pommes de terre sont utili- s6es pour les tests: la vari~t6 Record sensible au noircissement interne et la vari6t~ King Ed- ward sensible h l'~crasement interne et l'~clatement. Les tests d'impacts sont rbalis~s sur des tubercules r~colt~s depuis 2 jours on stock/:s pendant 6 mois. Le type et la taille de rendommagement sont notes pour chaque impact. Le noircissement interne est distingu~ de l'~crasement interne par sa coloration gris-bleue plut6t que brune et par des bords moins nets de la zone d'impact.

Pour les tubercules blesses ou indemnes, la dimension des courbes acceleration-temps est ~quivalente. L'examen des courbes acceleration-temps obtenues /~ partir de tubercules diff~rents montre qu'il existe deux formes dis- tinctes de courbes: des courbes h pic simple ou b. pics multiples. Parmi les courbes d'impact ~. pic simple, 83-87 % correspondent h des tubercules rest~s indemnes. Parmi les courbes b. pics multiples, 77-78 % proviennent de tubercules at-

teints d'bcrasement interne ou d'6clatement (tableau l et 2). La forme des courbes acc616ra- tion-temps est semblable pour les tubercules pr~sentant du noircissement interne ou rest~s indemnes. Le noircissement interne peut ~tre associ6 h des endommagements intracellulaires sans endommagement important des patois eel- lulaires (McIlroy, 1976). II est probable que I'acc616rom~tre ne soit pas suffisamment sensible pour enregistrer rendommagement intra- cellulaire, mais suffisamment pour d6tecter l'6crasement interne et rbclatement, qui eux sont associ6s h l 'endommagement des patois cellulaires (Hughes, 1981). Cette restriction n'est pas consid6r~e comme importante d'autant- que les r+sultats de cette 6tude (tableau 2) ainsi que des travaux d'autres chercheurs (Hughes, 1981; Specht, 1981) ont montr6 que l'~crasement interne et r6clatement sont les formes les plus communes d'endommagements internes h la r6colte ou juste aprbs rbcolte.

Un systbme pendulaire coupl+ h u n acc61+- rom~tre peut ~tre utilis6 comme test de dbpis- tage de la sensibilit6/l l'bcrasement interne et l'bclatement dans les premieres 6tapes d'un programme de s~lection. La technique bvite le pelage ou le tranchage des tubercules ainsi que les d6lais n~cessaires au d6veloppement de la coloration associbe h rendommagement interne.

References

Anonymous, 1981. Farmers leaflet No 3; Recommended varieties of potatoes. National Institute of Agricultural Botany, Cambridge.

Blight, D. P. & A. J. Hamilton, 1974. Varietal susceptibility to damage in potatoes. Potato Research 17:261-270.

Finney, E. E., 1963. The viscoelastic behaviour of the potato, Solanum tuberosum, under quasi-static loading. Ph.D. dissertation, Michigan State University (unpublished).

Finney, E. E. & D. R. Massie, 1975. Instrumentation for testing the response of fruits to mechanical impact. Transactions of the American Society of Agricultural Engineers 18: 1184-1187, 1192.


R. NOBLE

Fluck, R. C. & E. M. Ahmed, 1973. Impact testing of fruits and vegetables. Transactions of the American Society of Agricultural Engineers 16: 660-666.

Grant, A., 1981. Effect of pressure on type and amount of tuber damage. Potato Research 24: 227-228.

Hughes, J. C., 1980. Potatoes. I. Factors affecting susceptibility to damage. Span 23: 65-67. Hughes, J. C., 1981. Special report No 5. Mechanical damage in potatoes. Biennial report 1979

and 1980 of the ARC Food Research Institute, Norwich, p. 22-25. Hughes, J. C., A. Grant & R. M. Faulks, 1975. Susceptibility of tubers to internal damage (black

spot). Potato Research 18: 338-339. Mcllroy, D. C., 1976. Biochemical and physiological aspects of bruising in potato tubers. Ph.D.

dissertation, London University (unpublished). Mohsenin, N. N. & H. GiShlich, 1962. Techniques for determination of mechanical properties of

fruits and vegetables as related to design and development of harvesting and processing machinery. Journal of Agricultural Engineering Research 7:300-315.

Noble, R., 1985. The relationship between impact and internal bruising in potato tubers. Journal of Agricultural Engineering Research (accepted for publication).

Peterson, C. L., 1973. Dynamic mechanical properties and bruise susceptibility of the Russett Burbank potato as influenced by temperature. Ph.D. dissertation, Washington State Uni~cersi- ty (unpublished).

Peterson, C. L. & C. W. Hall, 1975. Dynamic mechanical properties of the Russett Burbank potato as related to temperature and bruise susceptibility. American Potato Journal 52: 289-312.

Sawyer, R. L. & G. H. Collin, 1960. Blackspot of potatoes. American Potato Journal37:115-126. Specht, A., 1981. Beschiidigungen an der Kartoffel vermeiden. Auswertungs- und Informations-

dienst ftir Ernii.hrung, Landwirtschaft und Forsten, Bonn, 28 pp. Umaerus, M., 1978. Report of survey of methods for screening for susceptibility to mechanical

tuber damage. Abstracts of the 7th Triennial Conference of the European Association for Potato Research, Warsaw, p. 115-I 17.

Umaerus, V. & M. Umaerus, 1976. Foralding for motstandskraft mot mekaniska skador i potatis. Sveriges UtsiidesfOrenings Tidskrift 86: 41-64.


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