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F.R. Harker and I.C. Hallett

The relationships between cellular characteristics of cortical tissue from `Braeburn' apple fruit (Malus domestica Borkh.) that had been harvested at two maturities and changes in texture that occurred during storage at 0C were studied. Tensile tests were used to measure adhesion between neighboring cells, and turgor pressure was manipulated to determine the pressures required to burst cells. Apples of advanced maturity became mealy during cool storage, while those of less advanced maturity did not. Mealiness was associated with low adhesion between neighboring cells, and a relatively high resistance to cell rupture.

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F.R. Harker and I.C. Hallett

Kiwifruit [Actinidia deliciosa (A. Chev) C.F. Liang et A.R. Ferguson] flesh firmness can decline by as much as 94% during fruit ripening. This phenomenon was investigated at the cellular level, with the aim of characterizing changes in the physiological condition and mechanical properties of cells. The tensile strength of kiwifruit outer pericarp tissue was measured, and low-temperature scanning electron microscopy was used to examine the mode of cell failure at fracture surfaces. The propensity with which cells ruptured was determined by incubating tissue discs in hypertonic and hypotonic solutions, and water potentials, osmotic potentials, turgor pressures, and tissue density were measured. An initial rapid reduction in flesh firmness—from 80 to 27 N during 6 weeks of storage at 0C—was related to a reduction in the adhesion between neighboring cells. Following tensile tests, an examination of fracture surfaces indicated that cells from freshly harvested fruit had ruptured, exposing the cell interior. After 6 weeks of storage, neighboring cells separated from each other without breaking open. With 23 additional weeks of storage at 0C, flesh firmness decreased from 27 to 5 N. The final softening stage was associated with an increase in the proportion of cells that separated at the middle lamella and an increase in the plasticity of the cell wall.

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F.R. Harker, J.H. Maindonald and P.J. Jackson

Flesh firmness is a characteristic used to indicate fruit quality. Experimental design and data analysis are important when comparing devices that measure fruit firmness. We compared the Effegi penetrometer operated by hand, mounted in a drill press and then operated by hand, and mounted on a motorized drive and operated remotely; the hand-operated EPT pressure tester; the Instron with an Effegi probe; and a hand-operated prototype of the twist tester. Devices varied in operator differences and precision. Comparisons between devices were at the within-fruit level of variability and, therefore, more precise than comparisons where different device-operators used different fruit. We demonstrate statistical methods that are appropriate for making the comparisons of interest and discuss the possible cause of differences between operators and between devices. We also discuss how the mechanical properties of the devices may affect results and consider implications for their practical use. In this study, we found the precision of discrimination between soft and hard apples was best using the Instron in 1992, while the Instron and hand-held Effegi penetrometer were comparable in 1991. For kiwifruit, the hand-held Effegi penetrometer consistently gave the most precise measurements of softening in 1991, while the twist test was the most precise in 1992.

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F R Harker, C B Watkins, B A Cregoe, P L Brookfield and W J Bramlage

The apple growing districts of New Zealand are spread across a wide range of latitudes. Differences in growing conditions associated with these various districts may influence the way fruit mature on the tree. In this study, the relationships between background colour and physiological maturity of Royal Gala apples have been compared in four major production areas. Royal Gala apples were strip picked from trees in three orchards during the commercial harvest period Hawkes Bay, Canterbury, Nelson and Otago. The maturity of these fruit was assessed, and fruit stored at 0°C for 12 weeks. Following removal from “storage, the quality of the fruit was assessed paying particular attention to -greasiness. Results from this trial indicate that the relationship between background colour and fruit maturity is not consistent. Indeed, the maturity of apples of a particular background colour may differ according to district and harvest date. Greasiness of fruit was related to harvest maturity in Hawkes Bay. However, fruit from Canterbury and Otago became severely greasy even when harvested at early maturities.