Factors affecting mass loss of apples : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Plant Science at Massey University, New Zealand

Abstract

Mass loss from harvested apples causes direct loss in returns to growers and marketers of fruit. This thesis characterises the process of mass loss in harvested apples, exploring the effects of various factors on water vapour permeance of the fruit, a measure of the ease with which water escapes from the fruit. Values of permeance of 'Braeburn' and 'Pacific Rose'TM apples were roughly twice those of 'Cripps Pink' and 'Granny Smith'. Permeance of 'Braeburn' and Pacific Rose'TM apples increased with later harvest date whilst values for 'Cripps Pink' and 'Granny Smith' remained relatively constant. There were small differences in mean permeance of apples from different regions. Some growers produced more fruit with high water vapour permeances than others. There was no relationship between maturity indicators tested and the water vapour permeance of the fruit. Fruit from the inner regions of trees and with high numbers of fruit in contact had high permeances. Variation in water vapour permeance around the surface of the fruit had no pattern with respect to blush or sun/shade sides, nor was there any relationship with cuticular thickness. Rather, variation in water vapour permeance of fruit was linked to the extent of cuticular micro-cracking. A model was developed which explains the water vapour permeance based on the proportion of fruit surface which is cracked. Artificial stretch applied to pieces of fruit skin increased cracking and permeance. Strain in the cuticle during growth and development of the fruit created a reticulate crack network. Micro-cracking could be important in determining susceptibility to mass loss and shrivel after harvest. Permeance of 'Braeburn' apples decreased after harvest; the extent of this decrease was greater for low relative humidity and high temperature and for fruit with high initial levels of micro-cracking. Bruising caused by impact damage on 'Braeburn' apples increased water vapour permeance of fruit only very slightly. A conceptual model is presented which summarises relationships between fruit attributes, environmental conditions and processes which contribute to overall mass loss of apples. A composite mathematical model from previous models developed in the thesis is presented which describes total water loss as determined by the level of micro-cracking in the fruit cuticle, time after harvest, relative humidity and temperature of the storage environment. A number of suggestions for minimised mass loss in the apple industry are presented based on three strategies: minimisation of permeance, reduction of driving force for water loss and segregation of lines of high risk and applying appropriate handling regimes. The composite model could be used to explore a range of alternative handling and marketing scenarios in terms of total mass loss.