Simultaneous measurement of ethane diffusivity and skin resistance of ‘Jonica’ apples by efflux experiment

An experiment was carried out to simultaneously measure the diffusivity and skin resistance of ethane in ‘Jonica’ apples by equilibrating them in an ethane–air mixture, transferring them to an ethane-free jar and monitoring the concentration of the air in the jar as the ethane is released. The concentration curve is then curve fitted by a finite element model using a realistic axisymmetric geometry to determine the diffusivity and mass transfer coefficient (inverse of skin resistance). The solubility (Henry’s law constant) of ethane in apple tissue was determined at equilibrium and was the same as that of ethane in water. The diffusivity of ethane in apple tissue was 19.4 × 10−8 m2 s−1, the skin resistance 1.26 × 106 s m−1 and the mass transfer Biot number (ratio of internal to external resistance) 0.61. The values of these parameters for O2, CO2 and neon were calculated from those for ethane using a two phase co-diffusion model. They disagree with results from previous efflux experiments but are consistent with direct measurement on cut tissue samples using the diffusion cell method. This can be explained by the hypothesis that the diffusivity determined by curve fitting is that of the inner cortex, while the resistance of the outer cortex is treated as part of skin resistance. Taken in combination with previous tests, the present results give an indication of the diffusivity profile across the apples.