EFFECT OF RELATIVE HUMIDITY ON GAS PERMEATION OF BIOPLASTIC FILMS: INDICATION ON SHELF LIFE OF STRAW MUSHROOMS STORED IN MODIFIED ATMOSPHERE PACKAGING

Abstract

This study was designed to study effects of humidity on gas transmission rate of the three films; commercial breathable low density polyethylene (bLDPE), poly(lactic acid) (PLA) and modified poly(lactic acid). These films were used in the study of Bunsiri and coworkers (2012) that extended shelf life of straw mushroom from one day to four, six and six days when bLDPE, PLA and mPLA were used respectively. With the findings that oxygen transmission rates (OTR) of bLDPE was twice higher than OTR of PLA and mPLA, a question arose why bLDPE with higher OTR had shorter shelf life. As the storage condition was at 15°C and 90%RH, an assumption was made that humidity might have increased OTR of PLA and mPLA to be higher than that of bLDPE. Our study measured OTR using steady state method at 23°C and 0, 40, 60, 80 and 90%RH and found that. OTR of bLDPE and PLA did not change when %RH increased while OTR of mPLA changed more than two folds from 1,069.3 cc/m2 day at 0%RH to 2,561.1 cc/m2 day at 40%RH. At %RH higher than 40% OTR of mPLA was quite stable. With higher polarity of mPLA than PLA and bLDPE, water was absorbed to plasticize mPLA and that had increased free volume so gas could permeate easier. Another set of equipment using unsteady state method was used to measure OTR and CO2TR at 15°C and 90%RH and found OTR and CO2TR of bLDPE also more than OTR of PLA and mPLA. Respiration rate of straw mushroom were calculated. It was also found that respiration rate of straw mushroom stored in bLDPE was higher than PLA and mPLA. Higher respiration rate resulted in higher transpiration of water. As water transmission rate (WVTR) of bLDPE was only 19.5 gm/m2 day, around six times lower than PLA and mPLA (120.9 and 133 gm/m2 day, respectively), water was accumulated in bLDPE packages and resulted in reabsorption and triggering of autolysis, which eventually caused quicker deterioration. These explained why bLDPE had shorter shelf life of straw mushrooms compared with PLA and mPLA.