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 (T) and relative humidity (RH) on film permeability (FPgas), respiration rate (Rx) and tissue permeance (TPgas). A closed system was used for respiration measurement, generating conditions of 65–90% RH (at intervals of 5%) at 5, 14, 20 and 25 °C. A diffusion cell was constructed to determine FPgas to O2 and CO2 at the different conditions of T   and RH. FPH2OFPH2O was determined gravimetrically. For TPgas, 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 O2 concentration in passive MAP (no addition of gases). For 20 kPa CO2, the model underestimates the changes in O2 concentration but overestimates the changes in CO2. For 40 kPa, the model describes adequately the changes in O2 concentration and overestimates the changes in CO2. For 80 kPa, the model adequately describes the changes in O2 and CO2. The inclusion of the T, RH and TPgas to the model allowed a more precise description of the in-package gas concentration. The effect of T on FPgas, TPgas and Rx is described by the Arrhenius equation. Rx and TPgas decreased and FPgas 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.