Modelling the O2 and CO2 Permeance of Conference Pear

Gas exchange is governed by the combined action of oxygen consumption and carbon dioxide production, and the transport of these gasses by diffusion. The diffusion component is formally neglected and implicitly incorporated in respiration and fermentation parameters of the current Michealis-Menten-based respiration models. The aim was to extend existing modelling approaches by including a diffusion controlled component to quantify the permeance of O2 and CO2. Permeances were estimated using a new gas exchange model which assumes that diffusion can be described by Fick’s first Law. Fick’s first Law can be applied when only one barrier is present. This was shown to be the case for O2 by O2-electrode measurements. The model estimated the permeance per batch on the basis of external gas exchange measurements, internal and external gas conditions, weight and surface per pear. Traditionally, permeances are not estimated, but calculated directly by the Neon method, using Graham’s Law. Permeances estimated using the new model were lower for O2 and CO2 than those found with the Neon method. The lower permeances may be explained by the assumption that the Neon method only assesses the permeance of the skin, while the O2 and CO2 permeances established by using the new model represent all barriers between mitochondria and the external atmosphere. The smaller CO2 permeance found using the new model might be explained by the relatively high pH of the cytosol. INTRODUCTION Gas exchange of fruits is governed by the combined action of oxygen consumption (respiration) and carbon dioxide production (respiration and fermentation) on one hand, and the transport of these gases from and to the inside of the cell by diffusion on the other hand. Only scarce information is available on internal gas concentrations in fruits in literature. Considering only external gas conditions in modelling respiration and fermentation of fruits and vegetables implies that the diffusion part of the process is formally neglected and that the diffusion-oriented effects are implicitly incorporated in model parameters. Several barriers exist in fruits for the transport of gases from outside the product into the cells and the opposite direction (e.g. the skin, flesh, cell wall, membranes and cytoplasm), each exerting its own effect on the overall gas exchange rate. The aim of this article is twofold. Firstly, existing modelling approaches on fruit respiration are extended by including a diffusion controlled component resulting in description of the permeance of the pear for O2 and CO2. Secondly, these model-based estimation of the permeances for O2 and CO2 are compared to the traditionally used Neon method (Schotsmans et al., 2002). Proc. Postharvest Unltd Eds. B.E. Verlinden et al. Acta Hort. 599, ISHS 2003 462 MATERIAL AND METHODS The Gas Exchange/Diffusion Model Nowadays’ general models describing gas exchange of fruits and vegetables are based on external gas conditions, and use a Michaelis-Menten approach. Modelling gas exchange with internal gas conditions may be accomplished by using the existing gas exchange model and exchanging the external for the internal gas conditions. However, the effect of diffusion resistance towards gases of the skin of the fruit should be considered in the gas exchange model when significant differences between internal and external gas conditions are encountered. Under the assumption that there is only one barrier for transport of gasses, namely the skin of the pear, diffusion can be described with Fick’s first law (Burg and Burg, 1965). This Law can be rewritten to the gas exchange rate (Vd in mol kgs) due to the diffusion process by converting the mass flux to the diffusion rate per kg pear (Mp) (Eq. 1)