Prediction of moisture loss across the cuticle of apple (Malus sylvestris subsp mitis (Wallr.)) during storage Part 1.: Model development and determination of diffusion coefficients

'Actual' diffusion coefficients of tissue, cutin, and wax were estimated for the apple cultivars 'Jonagold' and 'Elstar'. This was done using a modelling approach with finite elements using geometrical models based on microscopic images of the cuticle. Contrary to the 'apparent' diffusion coefficients, which were previously determined experimentally, the 'actual' diffusion coefficients account for the typical structure of the cuticle and the presence of lenticels and cracks. They were, therefore, always smaller than the apparent values. The actual diffusion coefficients of wax were 0.98 x 10(-15) m(2) s(-1) for 'Jonagold' and 1.79 x 10(-14) m(2) s(-1) for 'Elstar' fruit. The actual diffusivities of cutin were 4.5 x 10(-14) m(2) s(-1) for 'Jonagold' and 6.65 x 10(-14) m(2) s(-1) for 'Elstar' and for tissue the values were 1.12 x 10(-11) and 4.33 x 10(-12) m(2) s(-1) for 'Jonagold' and 'Elstar', respectively. These diffusion coefficients and the separate geometrical features can be integrated into an overall model, which describes and predicts moisture loss of whole apples during long-term storage. (C) 2003 Elsevier B.V. All rights reserved.