Kate M. Maguire

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

Abstract 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 (r2=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 μm), and the depth of the remaining inner cuticle (8 μm). 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.

Sources of variation in water vapour permeance of apple fruit

Abstract There can be large fruit-to-fruit variation in water vapour permeance within a population of fruit. We quantified contributions of several factors to variation in apple fruit ( Malus domestica Borkh.). In a study on ‘Braeburn’ fruit from six growers in each of five apple growing regions of New Zealand, there were large differences between growers, and marginal regional differences. The water vapour permeance of the fruit was extremely variable within each grower line. There were no relationships between four fruit maturity indicators (starch, background colour, firmness and soluble solids) and permeance, although fruit from the inner canopy had mean water vapour permeance values on average 57% greater than those from the outer canopy. Apple fruit with large numbers of adjacent fruit (three and four contact points) had on average a 54% higher water vapour permeance than those with less contact points. The areas of contacts had a mean water vapour permeance 30% higher than other areas. There was no difference in permeance between the sun and shade sides of apple fruit nor was cuticle thickness related to fruit water vapour permeance. There was no pattern of water vapour permeance of ‘Braeburn’ fruit with respect to blush or vertical displacement (shoulder or cheek). However, there were large variations among fruit within a population. A large proportion of this variation seems to be non-systematic, random error. Further work should be undertaken to determine differences in physical and chemical composition of the cuticle that lead to this variation in water vapour permeance.