Dynamic elastic properties of some fruits during growth and development

J. agric. Engng Res. (1967) 12 (4) 249-256

Dynamic Elastic Properties of Some Fruits during Growth and Development E. E.

FINNEY, JR*

The dynamic elastic properties of the fleshof apples, peaches and pears wereevaluated to determine whether these properties were significantlyaltered during growth, development and maturation of the fruit. Mechanical resonance within specimens of flesh removed from the fruit was measured during various stages of development, and Young's modulus of elasticity, the shear modulus, Poisson's ratio and internal friction (damping) were calculated. Young's modulus for Late Elberta peaches decreased rapidly from 1925 x 105 dynes/em- three weeks prior to the estimated date of picking maturity to 195 x 105 dynes/ems two weeks after estimated maturity. Kieffer pears had a more gradual decline from 2884 X 105 to 1151 X 105 dynes/em" over a three-month growth period. For apples, Young's modulus of elasticity generally declined during the period between 90 and 115 d after full bloom, then remained relatively unaffected and, finally, declined as the fruit ripened on the tree. Average values for apples at intermediate stages of maturation ranged from 780 X 105 dynes/ems for Mcintosh to 1300 X 105 dynes/ems for Rome Beauty. The results of dynamic measurements of Young's modulus were generally higher than those from quasi-static tests previously reported for apples. For apples and pears, internal friction (loss coefficient) decreased during the growing season. After maturation, internal friction of apples increased as the fruit ripened on the tree. The average loss coefficient for peaches increased from 0·090 to 0·143 as the fruit developed, matured and ripened. Poisson's ratio ranged from 0·020 for overmature Rome Beauty apples to 0·236 for Rome Beauty in the early stage of maturation , to 0·391 for Kieffer pears approaching maturity. Results indicate that the fruits studied have certain measurable dynamic mechanical propert ies, some of which change significantly with growth and development and hence may be of value as an index of maturity or readiness for harvest. Further exploitation of the techniques used in this study may result in a better understanding of the engineering properties of fruits and in more objective characterization of the textural attribute s of such commodities.

1. Introduction In fruits and vegetables, the mechanical properties of cell aggregates (the flesh) are often the chief determinants of textural characteristics. Young's modulus of elasticity. for example, has been suggested as a measure of the textural attribute designated as firmness I, 2 and changes in mechanical damping (internal friction) are reported 1 , 3 to be associated with viscosity changes in the cellular tissues. In addition to evaluating kinesthetic or textural characteristics.t-" the mechanical properties of fruits are also of interest from the standpoint of reducing mechanical damage during harvesting and handling," and predicting "readiness for harvest". 2 Even though considerable research has been devoted to studies of the static and quasi-static elastic and viscoelastic properties of fruits and vegetables.t- to relatively little research has been reported concerning their dynamic mechanical behaviour. The present work was undertaken to evaluate the dynamic elastic properties of the flesh of apples, peaches and pears to determine (i) whether they are comparable to previously reported quasi-static values and (ii) whether they are significantly influenced by growth, development and maturation of the fruit. Mechanical resonance within specimens of flesh removed from the fruit was measured during various stages of development, and Young's modulus of elasticity and internal frict ion were calculated. During maturation, the shear modulus also was determined and Poisson's ratio was calculated. 2. Literature review There are few published data concerning the dynamic mechanical properties of fruits compared to the extensive information available in th e literature on q uasi-static-type tests . One of the • Instrumentation Research Laborator y. Market Qual ity Research Laborator y, A.R .S., U. S.D .A., Beltsville, Maryl and 20705 A

249

250

DYNAMIC ELASTIC PROPERTIES OF SOME FRUITS DURING GROWTH AND DEVELOPMENT

earliest reported attempts to use dynamic techniques to measure the mechanical behaviour of fruits was by Clark and Mikelson 11 in 1942. They patented a technique for determining the ripeness of fruits, such as melons and pineapples, by measuring changes in the vibration characteristics of the fruits during ripening. The fruit was placed between, and in direct contact with, two electromagnetic vibratory elements. Vibration was generated by one element, transmitted by the fruit and detected by the other element. Green fruit was reported to transmit vibrations better than ripe fruit and the natural period of vibration changed as the fruit ripened. Twenty years later, Nybom" reported a similar technique for measuring the firmness of soft fruits, such as raspberries, strawberries and cherries. The fruit was placed between two electrodynamic earphones. One earphone was excited with a fixed frequency of 50 Hz, causing vibrations which were transmitted through the fruit into the coils of the other earphone. The induced current in the coils was measured and used as an indication of firmness. A high correlation between firmness and anthocyanin content and a less significant correlation between firmness and soluble solids were reported for Newburgh raspberries of varying degrees of ripeness. In 1962, Drake" explained a technique for automatic recording of the mechanical resonance curves for transversally vibrating specimens of foodstuffs. An optical arrangement was used to measure the amplitude of vibration of the specimen and an X-Y recorder plotted a continuous experimental curve of the vibration amplitude as a function of the vibration frequency. Vibrational characteristics of apples, pears, potatoes, cheese and fish pudding were evaluated and reported. Published data concerning the dynamic mechanical properties of fruits are insufficient to establish how these properties are affected by fruit development, maturation and senescense. This information is needed, however, to provide a basis for evaluating these dynamic techniques, particularly as they might be used to evaluate texture. The present paper reports some of this information. 3. Experimental techniques Fruits used in this study were grown in unirrigated orchards at the Plant Industry Station, Beltsville, Maryland, in 1966. Sampling and testing of fruit started at the end of July and continued through October or until approximately 90 % of the fruit of a particular variety had fallen from the tree. Fruits were selected trom each of four quadrants of the tree on each sampling date. A sample of a minimum of 10 fruits from each variety was taken to the laboratory, weighed in air, weighed in water, and tested for resonance within 8 h after picking. Full bloom dates were estimated as I May for apples, and 20 April for Late Elberta peaches and Kieffer pears. The techniques of Finney and Norris'" were used to determine the dynamic elastic properties. Cylindrical specimens of flesh were removed from the fruits with a cork borer parallel to the stem axis and midway between the axis and the outer equator of the fruit. The diameter of the longitudinal resonance specimens was approximately 1·0 em and of the torsional specimens, 1·5 ern, Young's modulus of elasticity was calculated, using the length of the cylindrical specimen of flesh from the fruit, its density, and its frequency of resonance while vibrating in the fundamental longitudinal mode (Fig. 1). To determine the shear modulus, the fundamental resonant frequency of the specimen resonating in torsion was measured (Fig. 2) and used in the equation for computing shear modulus.'! Assuming homogeneity, isotropy and linear elasticity, Poisson's ratio was calculated, using the shear modulus and Young's modulus values. Internal friction was evaluated by calculating the loss coefficient'" for the tissue specimens vibrating longitudinally in the fundamental mode. The band width, /if (Fig. 1), of the resonance curve was measured and divided by the resonant frequency, f This result multiplied by (1/ y3) is, by definition, the loss coefficient of the test specimen. The density of the cylindrical specimens was assumed to be the same as that of the whole intact fruit. This was estimated from specific gravity measurements based upon weights in air and in water. Errors due to differences in the densities of the flesh and the whole fruit"! were estimated

251

E. E. FINNEY, JR.

to be approximately 2 %, which is comparable to the accuracies reported for these experimental techniques. 13 m "0

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Fig. 3. Changes in the dynamic modulus ofelasticity of Kieffer pears (1), Delicious apples (2) and Late Elberta peaches (3) during growth and development (Arrows indicate estimated date of picking maturity)

4. Results and discussion Fruits such as apples, peaches, and pears have many generally well-defined stages of growth and development. These have been designated as the cell multiplication, cell enlargement, maturity, senescence and death phases." From a market-quality standpoint, the stage of development at which fruits are harvested may be more important than postharvest storage conditions. IS Fig. 3 shows that the elastic modulus of pears and peaches generally declined with advancing development. With Delicious apples, on the other hand, it declined during early tests, remained relatively unaffected by growth in the intermediate period, and then declined after the estimated

252

DYNAMIC ELASTIC PROP ERTIES OF SOME FRUITS DURING GROWTH AND DEVELOPMENT TABL E

I

Dynamic elastic modulus and internal friction (loss coefficient) for certain apple vari eties

Variety

C.Y.

=

Test, date, 1966

Elastic modulus, x 106 dyn esjcm "

Internal fr iction

M ean

C.V.

Me an

C. V.

Mcintosh

9/8 23/8 31/8 13/9· 20/9 30/9 5/10

997 725 721 775 794 618 580

6 7 6 7 6 11 7

0·068 0'068 0·069 0·061 0·062 0·058 0·063

9 7 6 6 8 4 6

Del icious

1/8 8/8 16/8 23/8 1/9 12/9 29/9· 10/10

1190 1153 1040 1016 1015 1023 1003 843

7 6 8 5 5 4 6 7

0·091 0·069 0074 0071 0·070 0·072 0·067 0 ·076

13 6 4 7 4 7 5 20

Rome Beau ty

1/8 8/8 16/8 23/8 1/9 12/9 29/9 6/10· 18/10 25/10 1/11

1482 1483 1327 1240 1280 1227 1416 1315 1331 1336 1254

8 6 3 4 8 7 9 10 6 5 8

0·087 0·081 0·074 0079 0·073 0·071 0063 0·064 0·055 0·055 0·054

19 13 8 10 8 5 4 12 6 8 8

Stayma n

8/8 16/8 23/8 1/9 12/9 29/9 16/10· 24/10

1172 1139 1150 1086 1164 1211 1047 951

9 8 13 9 11 11 9 18

0·083 0·076 0·074 0·070 0'069 0·060 0·063 0 ·069

13 3 6 7 4 5 9 14

York

1/8 8/8 16/8 23/8 1/9 12/9 29/9 4/10 18/10· 25/ 10 1/11

1664 1457 1350 1211 1145 1194 1215 1201 1319 1283 1230

11 7 7 13 9 11 12 14 7 10 10

0·083 0·075 0'072 0'074 0·069 0'070 0·062 0'063 0'054 0'058 0·058

10 4 9 7 11 9 12 8 7 12 6

coeff. of variability designated in terms of standard deviation as a

· A pproximate date of pick ing maturity based upon days from full bloom"

% of the

mean

253

E. E. FINNEY, JR.

picking maturity date. For other apple varieties (Table I) the elastic modulus fluctuated during growth, and, in some cases (McIntosh, Rome Beauty, Stayman and York), increased slightly just as the fruit approached picking maturity. In all cases, however, the elastic modulus of apples decreased after the estimated date of picking maturity even though this decrease was delayed with varieties McIntosh and Rome Beauty. If one assumes that modulus of elasticity is an important characteristic associated with firmness of fruits, 1. 2 then the trends presented in Fig. 3 and Table I may partially account for observations by Haller"? concerning the use of pressure testers to evaluate maturity and firmness of fruit. Haller stated that pressure tests are useful tor establishing standards for picking pears and peaches, but for apples they were not found to be a reliable index to maturity"... except to indicate when certain varieties were becoming too soft and overmature for storage." TABLE

II

Dynamic and quasi-static elastic modulus for apples, Ib/in 2 Quasi-static'

Variety

--

Dynamic

1961

1962

1966

McIntosh At harvest (130)* Mean of all data taken

720 630

693 698

1090 1080

Delicious At harvest (146)* Mean of all data taken

1320 1230

486 724

1465 1500

Stayman At harvest (158)* Mean of all data taken

795 1292

895 916

1522 1617

Rome Beauty At harvest (165)* Mean of all data taken

1020 1519

799 1031

1914 1937

*Approximate

number of days after full bloom, to the date of harvest or estimated picking maturity

The above dynamic elastic modulus values for apples are compared (Table II) with previously reported values," obtained in quasi-static tests and based upon the slope of the stress-strain curve during the second loading cycle. This was reported to be a better indication of the true modulus of elasticity of the apple than the slope of the first loading curve, which was used to suppress the effects of plastic deformation within the apple tissue. Differences in growing conditions in different locations and seasons might be expected to cause differences in the elastic modulus. The values obtained from dynamic and quasi-static tests, however, are comparable in order of magnitude, and the relative rankings of the varieties are the same for both techniques during all three years with respect to the means of all data taken, which include all measurements on a particular variety before, during and after maturation (Fig. 3). The internal friction (or loss coefficient) of Late Elberta peaches increased during fruit development (Table III). For apples the pattern was different (Fig. 4): internal friction generally declined during preharvest development and then increased upon completion of the maturation process. In viscoelastic commodities, such as fruits, internal friction is associated with viscous characteristics. These results suggest, therefore, that decreases in firmness of peaches during maturation and ripening are associated with the fruit becoming less elastic and more viscous. Such changes in

254

DYNAMIC ELASTIC PROPERTIES OF SOME FRUITS DURING GROWTH AND DEVELOPMENT TABLE III Some properties of Late Elberta peaches

Test date,

1966 3/8 9/8 17/8 24/8* 2/9 19/9

Specific gravity of whole fruit

Elastic modulus, X 10 5 dyneslcm"

Internal friction (loss coefficient)

1·023 1·022 1·004 0·994 0·982 0·967

1715 1926 1060 948 567 195

0·090 0·093 0·092 0·108 0·143

*Estimated date of picking maturity

mechanical properties of peaches may be identified with the sensory characteristics of softness, or yielding quality." Apples, on the other hand, showed a declining internal friction during development prior to maturity and, with the exception of Rome Beauty, an increasing internal friction after maturation (Fig. 4). [The exception may be related to the uncertainty associated 0'100

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Fig, 4. Changes in internal friction (loss coefficient) of apples (J) McIntosh (estimated 130-145 d between full bloom and picking maturity); (2) Delicious (145-160 d); (3) York (155-175 d); (4) Stayman (155-175 d); (5) Rome Beauty (150-170 d)

with establishing a reliable date of maturity.] West." has stated that the longer apples remain on the tree, the better their flavour, colour and texture. Since modulus of elasticity, as discussed earlier, changes very little prior to maturity, the implication is that the declining internal friction may be associated with the texture of the apple, ignoring, of course, possible relationships between this mechanical property and flavour or colour. High internal friction values, for example, may indicate toughness in an immature fruit or mealiness in an overmature apple. Conversely, a low internal friction may be indicative of a crisp apple. These are points which need to be explored further.

255

E. E. FINNEY, JR. TABLE

IV

Some properties of Kieffer pears Test date, 1966

2/8 9/8 17/8 24/8 2/9 19/9 30/9 4/10 17/10* 25/10 2/11

Specific gravity of whole fruit

1-041 1-038 1·031 1-033 1-034 1·028 1·020 1-022 1·020 1·017 1-015

Elastic modulus, x 105 dyneslcm"

Internal friction (loss coefficient)

Shear modulus, 10 5 dynesjcm"

X

0-087 0-099 0·095 0·098 0·084 0·076 0·080 0-077 0·073 0-072 0-075

2884 2447 2174 2040 2015 1721 1415 1578 1534 1442 1151

583 552 522 459

*Approximate date of picking maturity TABLE

V

Poisson's ratio as calculated from Young's modulus and shear modulus measurements on fruit tissues Fruit

Test dute, 19661 Poisson's ratio_

Golden Delicious \

Apples 3/10

0-068

Delicious

10/10

0·046

Mcintosh

5/10

0-031

Stayman

6/10 24/10

0-125 0-031

Rome Beauty

6/10 18/10 25/10 1/11 2/11*

0·236 0·162 0-151 0-136 0-020

Turley

10/10 24/10 1/11 2/11*

0-134 0-144 0-17\ 0-081

York

4/10 18/10 25/10 1/1 q 2/11*

0·105 0·083 0-169 0·144 0-040

Pears 4/10 17/10 25/10 2/11

0-391

Kieffer

0-354 0·382 0-252

*Fruit selected which had previously dropped from the tree

256

DYNAMIC ELASTIC PROPERTIES OF SOME FRUITS DURING GROWTH AND DEVELOPMENT

During the latter stage of sampling and testing, the shear modulus of Kieffer pears (Table IV) was found to decline in a manner similar to that of Young's modulus of elasticity. From these two moduli, Poisson's ratio was calculated. Theoretical values for Poisson's ratio generally range between 0 and 0·5. Morrow and Mohsenin? reported values of 0'22 and 0·37 for apples using two different loading conditions. Results from the present study indicate a variation in Poisson's ratio from 0·236 to 0-020 for Rome Beauty apples (Table V); this range of values also included all other measurements on the other apple varieties. For Kieffer pears, however, the values were greater (0' 391-0,252). Even though Poisson's ratio tended towards lower values at the latter stages of fruit development, its significance in characterizing the texture of fruits is not known. White and M ohsenin 18 suggested that this parameter may be inversely related to air spaces or percentage voids within cellular tissues, but this hypothesis remains to be proven. Being a mechanical property, however, Poisson's ratio is of interest from an engineering point of view and future studies may demonstrate its relationship with fruit quality. REFERENCES I

2

3

4

5

6

7

8

9

10

II 12

13

14

15

16

17 18

Finney, E. K; Norris, K. H. Sonic resonant methods for measuring properties associated with texture of "Irish" and sweet potatoes. Proc. Amer. Soc. Hort. Sci. 90, 1967,275 Mohsenin, N. N.; Cooper, H. E.; Hammerle, J. R.; Fletcher, S. W.; Tukey, L. D. "Readiness for harvest" of apples as affected by physical and mechanical properties of the fruit. BuI. 721, Penna agric. Exp. Sta., 1965 Drake, B. Automatic recording ofvibrationalproperties offoodstuffs. J. Fd Sci., 1962,27, 182 Bourne, M. c.; Moyer, J. c.; Hand, D. B. Measurement offood texture by a Universal Testing Machine. Fd TechnoI., Champaign, 1966,20, 170 Kramer, A.; Twigg, B. A. Fundamentals of quality control for the food industry. AVI Publishing Co., Westport, Conn., 1962,81 Mohsenin, N.; Cooper, H. E.; Tukey, L. D. Engineering approach to evaluating textural factors in fruits and vegetables. Trans. ASAE 6, 1963, 85 Mohsenin, N.; Gohlich, H. Techniquesfor determination ofmechanical properties offruits and vegetables as related to design of harvesting and processing machinery. J. agric. Engng Res., 1962,7 (4) 300 Finney, E. E.; HaU, C. W.; Mase, G. E. Theory of linear viscoelasticity applied to the potato. J. agric. Engng Res., 1964, 9 (4) 307 Morrow, C. T.; Mohsenin, N. N. Consideration ofselected agricultural products as viscoelastic materials. J. Fd Sci., 1966, 31, 686 Sommers, F. G. Viscoelastic properties of storage tissues from potato, apple and pear, J. Fd Sci., 1965, 30, 922 Clark, H. L.; Mikelson, W. Fruit ripeness tester. V.S. Patent 2277 037: 1942 Nybom, N. A new principle for measuring firmness offruits. Hart. Res., 1962, 2, 1. Finney, E. K; Norris, K. H. Instrumentation for investigating dynamic mechanical properties offruits and vegetables. Pap. 67-325, ASAE, June 1967 Westwood, M. N. Seasonal changes in specific gravity and shape of apple, pear and peach fruits. Proc. Amer. Soc. Hart. Sci., 1962, 80,90 West, C. When the amateur should pick and how he should store apples and pears. JI R. Hart. Soc., 1947,72,49 HaUer, M. H.; Magness, J. R. Picking maturity ofapples. Cir. 711, V.S.D.A., 1944 HaUer, Mark H. Fruit pressure testers and their practical applications. Cir. 627, V.S.D.A., 1941 White, R. K.; Mohsenin, N. N. Apparatus for determination of bulk modulus and compressibility of materials. Pap. 66-832, ASAE, Dec. 1966