Determination of O2 and CO2 permeance, internal respiration and fermentation for a batch of pears (cv. Conference)

By combining an existing gas exchange model with the diffusion of respiratory gases over the barrier of the fruit skin, a new gas exchange model was developed. Parameters describing O-2 and CO2 permeance, internal respiration and fermentation for a batch of pears (Pyres communis L. cv. Conference) were estimated using this new gas exchange model. The model was calibrated using data on gas exchange, internal and external gas composition and the surface of individual pears. The surface area was calculated using a computer imaging method. The absence of internal O-2 gradients in the aqueous phase of a pear, a prerequisite for using a three-compartment model set up based on Fick's first law, was confirmed using an O-2-electrode probe. Data analysis using the combined gas exchange model resulted in estimated parameters indicating an increased internal affinity for O-2 and increased internal fermentative CO2 production compared to data analysis using a traditional gas exchange model, based on external gas composition. The O-2 and CO2 permeances were also derived using Ne efflux data. The estimated permeances using the combined gas exchange model were lower than those found with the Ne method. The lower permeances can be explained assuming that the Ne method only assesses the skin permeance, while the permeances for O-2 and CO2 from the combined gas exchange model represent all the barriers between mitochondria, where respiration actually occurs, and the external atmosphere. The lower CO2 permeance found using the new model might also be explained by the relatively high pH of the cytosol. (C) 2004 Elsevier B.V. All rights reserved.