Relationship between water vapour permeance of apples and micro-cracking of the cuticle

Water vapour permeance is an important determinant of the rate of water loss from fruits. Variation in water vapour permanence of apple fruit may be related to micro-cracking of the cuticle. This study used scanning electron microscopy, confocal and light microscopy to characterise variation in micro-cracking in 'Braeburn' apples While there was considerable variation in cracking around the surface of the fruit as determined by data obtained with the confocal microscope (coefficient of variation = 44%), there was no obvious pattern in relation to blush. A model based on diffusion of gases was developed and used to explain the relationship between micro-cracking determined using scanning electron microscope data and the water vapour permeance of whole fruit (r(2) = 48%). This model included terms for intact cuticle, cuticle that was cracked in the outer layer, and boundary layer effects. The model was based upon the effective permeability of air to water vapour (analogous to diffusivity), the depth of the boundary layer (0.5 mm), the depth of cracks in the outer cuticle (8 mu m), and the depth of the remaining inner cuticle (8 mu m). Permeability of the inner cuticle to water vapour was predicted with the model to be 1.2 +/- 0.35 pmol m s(-1) m(-2) Pa-1 and water vapour permeance of intact cuticle was calculated as 12.3 +/- 2.7 nmol s(-1) m(-2) Pa-1. Cracks were calculated to be 12 times more permeable than intact cuticle. The experimental data and the model presented in this paper provide evidence for, and explain, the major role of cuticular cracks in determining apple fruit water vapour permeance. (C) 1999 Elsevier Science B.V. All rights reserved.