Using mathematical equations that describe the 02 mass-transfer and the enzymatic oxidation of the organic substrates of apples (Malus domestica Borkh.), we developed a kinetic model to correlate fruit respiration rate with environmental oxygen partial pressure (PO2. The kinetic determinations were carried out at room temperature using apples stored at 3 to 4C for 11 to 19 weeks. Results show that: 1) the calculated value of the Michaelis-Menten constant related to the enzymatic oxidation of the respiratory substrate (Km = 2.1 ± 0.5.10-5 mol·kg-1) is close to that reported in the literature for cytochrome-c oxidase; 2) the located range of PO2 levels where 02 becomes the limiting factor in the respiration process (near 2.6 kPa at T = 20.5 ± lC) is close to those usually used on a commercial scale for controlled atmosphere storage.

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The skin permeability to O2-diffusion and the mass transfer equilibrium constant for ‘Super Golden Delicious’ spur and ‘Red Stark’ (Malus domestica Borkh.) have been studied using a kinetic approach. A direct proportionality between the O2-exchange initial rate and the environmental PO2 is shown by a linear regression line passing near the origin of the axes. This confirms the suitability of the adopted mass-transfer equations and allows the calculation of the two involved constants: the equilibrium mass-transfer constant and the kinetic constant, which represents a suitable measure of permeability of the apple skin. As apple-skin permeability to O2 increases with fruit ripening, to reach a maximum near the commercial harvesting time, this parameter could be used as a ripening index.