Modeling the effects of temperature and relative humidity on gas exchange of prickly pear cactus (Opuntia spp.) stems

A model to estimate gas profile of modified atmosphere packaged (MAP) prickly pear cactus stems was developed and calibrated. The model describes the transient gas exchange taking in consideration the effect of temperature (7) and relative humidity (RH) on film permeability (P-F(gas)), respiration rate (R-x) and tissue permeance (P-T(gas)). A closed system was used for respiration measurement, generating conditions of 65-90% RH (at intervals of 5%) at 5. 14, 20 and 25 degrees C. A diffusion cell was constructed to determine P-F(gas) to O-2 and CO2 at the different conditions of T and RH. P-F(H2O) was determined gravimetrically. For P-T(gas), the profile of gas concentration inside the tissue and the permeance coefficient were obtained at the different conditions, using a diffusion cell sealed at the surface of the cactus stem. The data simulated with the inclusive model suitably describes the changes in CO2 concentration and overestimate the O-2 concentration in passive MAP (no addition of gases). For 20 kPa CO2, the model underestimates the changes in O-2 concentration but overestimates the changes in CO2. For 40 kPa, the model describes adequately the changes in O-2 concentration and overestimates the changes in CO2. For 80 kPa, the model adequately describes the changes in O-2 and CO2 The inclusion of the T, RH and P-T(gas) to the model allowed a more precise description of the in-package gas concentration. The effect of T on P-F(gas), P-T(gas) and R-x is described by the Arrhenius equation. R-x and P-T(gas) decreased and P-F(gas) increased as RH increased. RH, in the range of 65-90%, has a marked effect on the rate of gas exchange, especially for CO2, and consequently on the gas equilibrium inside MA packages. (c) 2005 Swiss Society of Food Science and Technology. Published by Elsevier Ltd. All rights reserved.