Compression characteristics, firmness, and ... - Wiley Online Library

International Journal of Food Science and Technology 1996, 31, 1–5

Compression characteristics, firmness, and texture perception of heat treated and unheated apples Susan Lurie1 & Amos Nussinovitch2 1 Department of Postharvest Science, Volcani Center, ARO, Bet Dagan 50250 and 2 Department of Biochemistry, Food Science and Nutrition, Faculty of Agriculture, The Hebrew University, Rehovot 76100 Israel

Summary

Two cultivars of apple fruits, Malus domestica cvs. ‘Golden Delicious’ and ‘Granny Smith’, were heated for 4 days at 388C and then held for a week at 208C or at 08C. At the end of this period texture, firmness and organoleptic tests were made on these apples and the results were compared with those for unheated apples. The heated apples were found by a taste panel to be crispier and sweeter than the unheated apples. Flesh firmness, tested with a Magness-Taylor penetrometer correlated well with crispiness for ‘Golden Delicious’ apples and less well with ‘Granny Smith’. Instron compression tests of strength and stiffness correlated with crispiness in both varieties. Instron measurement of brittleness was not correlated with crispiness. We suggest that the properties of apple flesh sensed during mastication are most closely related to the properties derived from the compression test of strength and stiffness.

Keywords

Malus domestica, storage, taste.

Introduction

A number of postharvest treatments of apples to maintain fruit quality during storage have been investigated. One such treatment is a postharvest hot air heat treatment. Porritt & Lidster (1978) exposed ‘Spartan’ and ‘Golden Delicious’ apples to 388C for 4–6 days before 218C storage. Fruit softening was suppressed and naturally occurring decay was reduced. Liu (1978) found that holding fruit for 2–4 days at 408C also suppressed softening of ‘Golden Delicious’ apples. More recently it was shown that holding apples above 358C will inhibit ripening processes, including softening, and allow for longer storage and shelf-life of poorly storing apple varieties, such as ‘Anna’ (Lurie & Klein, 1990; Klein & Lurie, 1990).

Correspondent: Department of Postharvest Science, Volcani Center, Agricultural Research Organization, Bet Dagan 50250 Israel. Fax: 972 3-9683622. e-mail: [email protected]. 2

© 1996 Blackwell Science Ltd

Apple firmness is traditionally measured as the maximum force to push a manually operated Magness Taylor (MT) fruit firmness probe of specified shape and with an 11 mm tip into pared flesh. The MT measurement is accepted as the standard firmness measurement in the apple industry (Abbott, 1994). In the tests conducted by marketing inspectors at harvest, texture has often been equated with quality in apples with the firm, crisp apple being the ideal. Firmness is also frequently used as a measure of maturity and ripeness. Bourne (1965) showed that, in apples, the maximum force during penetration of the MT probe can occur at any depth from bioyield (initial tissue failure) to maximum penetration. Bourne (1974) and others have used the standard MT probe in a universal testing machine (Instron Corp., Canton, MA), which controls speed and distance of probe movement and displays force vs. deformation during measurement. In addition, the Instron can be adapted to determine other physical properties of apples through the use of

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Firmness and texture of heated apples S. Lurie & A. Nussinovitch

compression, stress-relaxation, creep and energy absorbance. In this study a compression test was used to determine mechanical properties which can be related to sensory parameters. These mechanical properties included stress at failure (strength), strain at failure (brittleness) and deformability modulus (stiffness) of the apple flesh. In organoleptic tests heated apples consistently gave better results than the unheated, control apples. This was due to a combination of their being perceived as sweeter and crispier than unheated apples. Heating caused loss of organic acids and some elevation of sugars, leading to sweeter apples (Klein & Lurie, 1990). This treatment also inhibited loss of insoluble pectin from cell walls which may explain the crispy texture (Ben-Shalom et al., 1993). However, MT firmness measurements do not accurately reflect the textural components crisp, tough or mealy. This study combines compression tests with the penetrometer measurement of firmness to characterize better the textural differences between heat treated and unheated apple flesh.

scale from 0 to 10. Each cultivar was examined by 20 tasters. The cultivars were tested on different days, so as to not be compared with each other. Cylindrical specimens (1.5 3 1.5 cm diameter by height) were removed from two sides of the apples along the equator with a cork borer, and compressed to failure by an Instron Universal Testing Machine, model 1100. Since apple tissue is anisotropic the test was always along the radial axis (along the cylinder). The cylinders were prepared just before testing, since moisture loss from cut surfaces is very rapid and can change test results (Khan & Vincent, 1993). The Instron was interfaced with a 486-compatible IBM personal computer. A special program developed by the Instron corporation and modified in our laboratory enabled conversion of the Instron’s voltage vs. time measurements into digitized force–time and stress–strain files with any desired definition of stress and strain. The force vs. time data was converted to a true stress sCOR and to Hencky’s strain (eH, relationship using the following substitutions (Nussinovitch et al., 1989, 1990) sCOR 5

Materials and Methods

Apples (Malus domestica cv ‘Golden Delicious’ and ‘Granny Smith’) were obtained from a packing house at commercial harvest and held at 08C, 95% RH for two weeks. Before storage the apples were divided into four lots of 60 fruit each. One lot remained in 08C storage, and a second was placed at 208C at the beginning of the experiment. The two remaining lots were placed in a heating chamber at 388C with thermostatic control and air circulation for 4 days. The apples were in plastic trays covered by a plastic bag to reduce weight loss, but unsealed so as to prevent buildup of modified atmosphere. Weight loss during the heating period was 0.5% day21. At the end of 4 days one lot was transferred to 08C, 95% RH and a second to 208C, 85% RH. After 7 days at these temperatures the fruits were tested. Fifteen fruit were tested for firmness using a MT penetrometer with an 11 mm tip on two pared sides of each apple. The fruit were then peeled and cut in slices for taste tests. The tasters were asked to judge the fruit from the four treatments for crispness, sweetness and tartness on a

F(t) H(t) Ao Ho

(1)

1H(t)2

(2)

eH(t) 5 ln

Ho

sCOR is the corrected momentary stress reflecting tissue strength, eH is the momentary Hencky or natural strain reflecting brittleness, F(t) the momentary force at time t, Ho is the initial specimen length, H(t) the height of the deformed specimen and Ao the cross-sectional area of the original specimen. Since apples are viscoelastic rather than elastic and are usually subjected to large compression forces in testing, the strict definition of Young’s modulus as expression of the stress–strain ratio of the food was replaced by the modulus of deformability, ED, as proposed by Mohsensin & Mittal (1977). It is taken from the slope of the original linear portion of the stress–strain relationship. It has stress units and can be treated as a measure of the specimen stiffness. Further details on the conversion of voltage vs. time data from the Instron into true stress (strength), Hencky’s strain (brittleness) and




deformability modulus (stiffness) can be found in Mohsensin & Mittal (1977). All specimens were compressed at a deformation rate of 10 mm min21 between parallel plates lubricated with Squibb mineral oil (E. R. Squibb and Sons, Princeton, NJ) to avoid frictional effects. Compression was stopped manually when the sample broke or when the distance between compression plates was about 2 mm. Each result is an average of at least 30 measurements. Results and Discussion

The firmness of ‘Granny Smith’ apples was consistently higher than that of ‘Golden Delicious’ fruits (Table 1). Holding the apples for 4 days at 388C did not change the firmness compared to unheated apples. However, when placed at 208C after heating, the heated apples softened less than unheated apples held for 7 days at 208C. This ability of a heat-treatment to slow softening and maintain a firmer apple has been observed previously for a number of cultivars including ‘Golden Delicious’ and ‘Granny Smith’ (Ben-Shalom et al., 1993; Klein & Lurie, 1990; Lurie & Klein, 1990). It was found that heated apples retained more insoluble pectin during ripening (BenShalom et al., 1993), which may indicate a decrease in either synthesis or activity of the cell wall degrading enzymes. This would lead to heattreated apples remaining firmer. Tissue strength and stiffness were affected by heat-treatment and by the temperature of holding after the treatment (Table 1). Heated apples from both varieties held at either 08C or 208C had greater strength than unheated apples held at

Table 1 Firmness and mechanical properties of unheated or heat-treated ‘Granny Smith’ and ‘Golden Delicious’ apples after 7 days at 08C or 208C. The firmness was measured with a MT penetrometer on whole apples and the strength, brittleness and stiffness with an Instron on apple tissue cylinders. Fifteen fruit were tested for each treatment


these temperatures. The lowest strength was found in unheated apples held at 208C. In contrast to the firmness measurement, where heated apples at 208C were softer than those held at 08C (statistically so for ‘Golden Delicious’ though not ‘Granny Smith’), the strength of heated apple tissue was statistically higher than unheated apple tissue irrespective of the holding temperature. ‘Granny Smith’ apples had a higher strength than ‘Golden Delicious’, similar to higher firmness measurements in this cultivar. Brittleness was not consistently affected by either heat-treatment or temperature of holding, but stiffness was greatest in heated apples held at 08C. In both cultivars the stiffness of heat-treated apples held at 208C was very similar to unheated apples held at 08C, and the lowest stiffness values were found in unheated apples held at 208C. Organoleptic measurements of the apples from the different temperature regimes showed that the crispiest apples were those heated and then held at 08C (Table 2). The heated apples placed at 208C had texture similar to unheated apples from 08C storage, while the unheated apples after one week at 208C were the least crispy in both apple varieties. The heat treatment slowed the loss of crispiness from the apples which occurred during the 208C shelf-life. In this case the perception of crispiness is closest to the measurement of strength of the apple tissue, which remained high in heated apples at 208C. The heated apples lose 2 to 3% of their weight during heating and storage (Klein & Lurie, 1990). In a study which evaluated controlled weight loss of apples (Hatfield & Knee, 1988), a sensory panel rated these apples as firmer, tougher and less mealy than control

Cultivar

Treatment Temperature Firmness Strength Brittleness Stiffness (kPa) ED (8C) (N) (kPa) sCOR eH

Golden Delicious

Unheated Unheated Heated Heated LSD Unheated Unheated Heated Heated LSD

Granny Smith

20 00 20 00 20 00 20 00

41 63 55 64 03.5 65 75 72 77 04.9

132 150 173 191 014.5 148 176 203 200 004.1

0.15 0.14 0.18 0.16 0.03 0.18 0.14 0.15 0.16 0.05

0779 1192 1299 1772 0047 1253 1411 1451 1662 0081


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Table 2 Taste tests of unheated and heat-treated ‘Granny Smith’ and ‘Golden Delicious’ apples after 7 days at 08C or 208C. Tasters judged each component on a scale of 0 to 10 with 10 being the highest intensity of that component. Fifteen fruit were tested for each treatment and 20 tasters evaluated the fruit Treatment Golden Delicious Unheated Unheated Heated Heated LSD0.05 Granny Smith Unheated Unheated Heated Heated LSD0.05

Temperature

Crisp

Sweet

Tart

20 00 20 00

4.0 6.5 6.0 7.5 0.15

6.9 6.5 8.0 7.5 0.21

4.0 5.5 4.0 4.0 0.23

20 00 20 00

4.0 5.7 6.9 8.0 0.17

5.0 6.5 7.3 8.1 0.20

7.1 7.4 5.5 6.0 0.21

apples. Weight loss in that study was induced by low humidity during the initial period of storage, but in the present report it was due to initial high temperature. In both cases the sensory evaluation of the apples was higher than for control apples. Heated apples also tasted sweeter than the unheated apples. The heated apples were generally less tart than the unheated apples, although unheated ‘Golden Delicious’ apples held at 208C also lost their tartness. The differences in perception of sweetness and tartness in heated apples can be attributed to the fact that heat treatment leads to loss of organic acids in apples and an increase in the sugar:acid ratio. (Klein & Lurie, 1990). In examining force vs. deformation curves from a MT probe in an Instron, Sam et al. (1993) concluded that heat-treated apples would be perceived as tougher than unheated apples. This was borne out in this study which showed heat-treated apples to have higher strength and stiffness than unheated apples. However, in taste tests these tissue properties were perceived as leading to a crispier apple, a highly desirable trait. Therefore, the heat treatment shows promise of enhancing apple quality in apples after storage. Correlation coefficients between the measurements of mechanical properties and the crispiness rating in the taste test showed a good correlation with the MT firmness measurement and with both the strength and stiffness of the compression test (Table 3). With the ‘Golden Delicious’ culti-

Table 3 Correlation coefficients between instrument measurements and taste tests of crispiness. Degrees of freedom 5 29 Measurement Golden Delicious Firmness Strength Brittleness Stiffness Granny Smith Firmness Strength Brittleness Stiffness

Correlation Coefficient

0.852 0.813 0.128 0.854 0.722 0.904 0.223 0.914

var these three parameters were very similar in their correlation to a tasters perception of crispiness. In the ‘Granny Smith’ cultivar strength and stiffness were better related to crispiness than the firmness measured by the MT penetrometer. In neither variety did brittleness correlate well with crispiness. Brittleness is a physical property which relates only to strain magnitude, while crispy is a sensory evaluation which includes within it acoustical and textural components, the involvement of saliva, and other factors. Different mechanical properties are measured by MT puncture and by compression tests. The properties sensed by humans in biting tests are more closely related to the properties derived from the compression test of strength and stiffness, particularly in firmer apples such as ‘Granny Smith.’ In ‘Golden Delicious’ apples, which are softer than ‘Granny Smith’ (63N vs. 75N in unheated apples), firmness, strength and stiffness all correlated well with crispiness. Therefore, it may depend on the apple cultivar as to which mechanical property is best for measuring fruit texture.

References Abbott, J.A. (1994). Firmness measurement of freshly harvested ‘Delicious’ apples by sensory methods, sonic transmission, Magness-Taylor, and compression. Journal of the American Society of Horticultural Science, 19, 510–515. Ben-Shalom, N., Hanzon, J., Klein, J.D. & Lurie, S. (1993). Postharvest heat treatment inhibits cell wall degradation in apples during storage. Phytochemistry, 34, 955–958.




Bourne, M.C. (1965). Studies on punch testing of apples. Food Technology, 19, 113–115. Bourne, M.C. (1974). Comparison of results from the use of the Magness-Taylor pressure tip in hand- and machine-operation. Journal of Texture Studies, 5, 105–108. Hatfield, S. & Knee, M. (1988). Effects of water loss on apples in storage. International Journal of Food Science and Technology, 23, 575–583. Khan, A. & Vincent, J. (1993). Compressive stiffness and fracture properties of apple and potato parenchyma. Journal of Texture Studies, 24, 423–435. Klein, J.D. & Lurie, S. (1990). Prestorage heat treatment as a means of improving postharvest quality of apples. Journal of the American Society of Horticultural Science, 115, 265–269. Liu, F.W. (1978). Modification of apple quality by high temperature. Journal of the American Society of Horticultural Science, 103, 730–732. Lurie, S. & Klein, J.D. (1990). Heat treatment of apples: differential effects on physiology and biochemistry. Physiologia Plantarum, 78, 181–186.

Mohsensin, N.N. & Mittal, J.P. (1977). Use of rheological terms and correction of compatible measurements in food texture research. Journal of Texture Studies, 8, 395–398. Nussinovitch, A., Peleg, M. & Dormand, M.D. (1989). A modified Maxwell and a non-exponential model for characterization of the stress relaxation of agar and alginate gels. Journal of Food Science, 54, 1013–1016. Nussinovitch, A., Ak, M.M., Normand, M.D. & Peleg, M. (1990). Characterization of gellan gels by uniaxial compression, stress relaxation and creep. Journal of Texture Studies, 21, 37–49. Porritt, S.W. & Lidster, P.D. (1978). The effect of prestorage heating on ripening and senescence of apples during cold storage. Journal of the American Society of Horticultural Science, 103, 584–587. Sams, C.E., Conway, W.S., Abbott, J.A., Lewis, R.J. & Ben-Shalom, N. (1993). Firmness and decay of apples following postharvest pressure infiltration of calcium and heat treatment. Journal of the American Society of Horticultural Science, 118, 623–627.

Received 10 June 1995, revised and accepted 5 January 1996



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