Water vapor permeability of bag materials used for corn storage

Polypropylene and jute bags are widely used for grain storage across the developing world. The permeability of clear polypropylene bags (PP-C), opaque polypropylene bags (PP-O) and jute bags were determined using the ASTM E96 Standard Test Methods under three temperature and relative humidity combinations (25^°C/65%, 28^°C/75% and 30^°C/80%) that resulted in three vapor pressure deficits of 1.11, 0.95, and 0.85 kPa. The water vapor transfer rate (WVTR) and the interaction between water vapor permeability (WVP) of the materials were determined. WVTR ranged from 216 g m^-2 day^-1 for opaque polypropylene (PP-O) exposed to air conditions of 30^°C/80% to 478 g m^-2 day^-1 for jute exposed to air at 25^°C/65% WVTR decreased with vapor pressure deficit for all materials. There was no significant difference in the WVTR between the polypropylene bags (PP-C and PP-O). WVP values ranged from 4.7 × 10^-5 g (m day Pa)^-1 to 6.4 × 10^-4 g (m day Pa)^-1 at 25^°C/65% for PP-O and jute, respectively. WVP of PP-C and PP-O decreased slightly as the vapor pressure deficit increased. The permeability of jute was significantly different from both polypropylene bags under these test conditions (p<0.05). The change in corn moisture content with initial moisture at 10% and 12% (wb) were investigated using mini bags constructed from the three materials. Environmental condition, initial grain moisture, and the interaction among the parameters, had a large impact on the moisture change. There was a weak positive interaction between WVP of bag materials and change in corn moisture. This study demonstrates that environmental condition affects moisture redistribution in grain stored in woven bags, thus adequate monitoring is required to maintain grain quality during storage.