Hermetic storage of paddy rice: the impact of relative humidity on fungi infection and on rheology of the pastes

Abstract: The aim of this study was to evaluate if under hermetic conditions, paddy rice can be stored during four months at different relative humidity, without losing grain quality. Trials were carried out from December to April 2016, using GrainPro® SuperGrainbag® Farm™ to store two rice varieties japonica and indica varieties, under three different relative humidity: 67.5%, 74.5% and 85.5% RH, at average temperature of 14.2 °C, both monitored by Hobo® Data loggers, with probe, placed inside the bags. Checkpoint II Portable O2 and CO2 Gas Analyzer were used to assess gas contents on the top and bottom of each bag. At the end of the trials, paddy samples were collected to analyze fungal infection, dehusked and milled, and the flour obtained used to estimate water activity (aw), using HygroPalm HP23, Rotronic, and to carry out rheological tests, performed on a MARS III controlled-stress rheometer. The isolated fungi were identified as Alternaria, Aspergillus, Epicoccum, Fusarium, Penicillium, Rhizopus, Trichoderma and Trichothecium. From the analysis of the hermetic paddy bags one can see that there was no significant differences (p > 0.05) on aw values between the two varieties: from 0.47 aw at 67.5% RH to 0.58 aw 85.5% RH, which are secure values to avoid mycotoxins development. There were significant differences (p < 0.05) on gas content for the different RH: 67.5% (20.4% O2, 1.2% CO2), 74.5% (18.5% O2, 3.7% CO2) and 85.5% (5.9% O2, 18.9% CO2) indicating an increase of respiration rate with the increase of RH. In what the rheology of the rice flours is concerned, the increase of RH showed a decrease on the viscoelastic functions of the respective rice pastes, which is consistent with the increase on respiration rate, related to starch and protein damage by fungi growth.