The Effect of Relative Humidity on Modified Atmosphere Packaging Gas Exchange

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 (P), respiration rate (Rx) and tissue permeance (P). A diffusion cell was constructed to determine P to O2 and CO2 at the different conditions of T and RH. The integration of the T, RH and P to the model allowed a more precise description of the in-package gas concentration. The effect of T on P, P and Rx is described by the Arrhenius equation. Rx and P decreased and P increased as RH increased. RH, in the range of 65 to 90%, has a marked effect on the rate of gas exchange, especially for CO2, and consequently on the gas equilibrium inside MA packages. INTRODUCTION The application of modified atmosphere packaging (MAP) is expanding all over the world, as it provides an additional barrier to excessive loss of quality during storage and distribution. It has been studied and used for quite some time on a large number of different products with varying success. The effect of temperature and the levels of O2 and CO2 have been reported and modelled (Emond et al., 1991; Hertog et al., 1997; 1999; Lee et al., 1991; Peppelenbos et al., 1996). For all kinds of wrappings, products and conditions, the same basic relations can be used over and over again. However, the possible effect of relative humidity (RH) on the respiration and/or gas exchange in these packages remains to be investigated. MATERIAL AND METHODS Prickly pear cactus stems (Opuntia spp.) were stored at different T-RH combinations (5, 14, 20 and 25oC, 65-90% at 5% intervals) in a closed system and the gas exchange was measured as a function of the O2 and CO2 levels. The permeance of cactus stem tissue was assessed in a small gas chamber firmly attached to the product tissue, placed inside a closed vessel system at different levels of RH. The film permeability at different RH levels was assessed according to standard ASTM methods. RESULTS AND DISCUSSION To account for the different effects of RH at different temperatures, all RH values were converted into vapour pressure deficit (VPD) using a standard equation. For the gas exchange induced by product respiration, the standard model of Lee et al. (1991); Peppelenbos et al. (1996), Hertog et al. (1997) was used. In an integral analysis comprising T, RH, O2 and CO2 levels simultaneously as explaining variables, the only parameter that was found to depend on RH was the maximal rate of gas exchange at reference T (Vm,ref). The relation with VPD is shown in eq. 1, indicating the “normal” dependence of Vm on temperature, with the additional dependence on VPD. Proc. 3 IS on Model IT Eds.: M.L.A.T.M. Hertog and B.M. Nicolaï Acta Hort. 674, ISHS 2005