Effects of drying methods and storage conditions on qualities of hulled red jasmine rice Oryza sativa L.

Abstract

The objective of this study was to investigate the effects of paddy drying methods and storage conditions on qualities of hulled red jasmine rice. This study was divided into 2 parts. Firstly, the effects of paddy drying methods were investigated. Three different paddy drying methods; shade drying (control), sun drying, and fluidized bed (FB) drying (115oC, 215 seconds, 1,800 g per batch) were employed. Differential scanning calorimetry (DSC) data revealed that FB-dried samples had significantly highest onset temperature (To) and peak temperature (Tp) of gelatinization but had significantly lower enthalpy of gelatinization (ΔHg) than that of the control samples (p≤0.05). FB-dried samples showed the significantly lowest breakdown (BD) (p≤0.05) from Rapid visco analyzer (RVA) curves. These results could be due to partial gelatinization/melting of starch granules and protein denaturation during FB drying. Sensory analysis of cooked samples showed that FB-dried samples had significantly lowest fragrance but had significantly highest rancidity (p≤0.05). Secondly, the effects of storage conditions were evaluated. Portions of the dried samples were vacuum-packed in OPP/AL/LLDPE or Nylon/LLDPE pouches and stored at ambient temperature (28-35oC) or 15oC for 12 months. For all drying methods, greater extent of changes in moisture content and water activity was observed in the samples packed in Nylon/LLDPE pouches and stored at ambient temperature. DSC and RVA results indicated that gelatinization temperature of the samples increased after 6 or 8 months of storage. ΔHg initially increased and then decreased after 10-month storage. Peak viscosity and BD increased within the first 2-4 months and then decreased, while setback gradually increased during storage. Swelling power of the samples tended to decrease after storing for 4 or 6 months. Greater extent of these changes was found in the samples stored at ambient temperature. As the DSC and RVA results of aged samples treated with 2-mercaptoethanol were comparable to those results from fresh samples, changes in thermal, pasting properties and swelling power could result from an increase in disulfide linkages of oryzenin during storage. Sensory characteristics of cooked sun-dried samples stored for 0, 6 and 12 months were investigated. The samples packed in Nylon/LLDPE pouches and stored for 6 and 12 months had significantly lowest fragrance but had significantly highest rancidity (p≤0.05). The samples packed in any packaging material and stored at ambient temperature for 12 months had significantly highest hardness (p≤0.05). An increase in storage duration led to an increase in hardness of the samples. There was no significant difference of the hedonic score for color, aroma and overall acceptance among all tested samples (p>0.05) whereas the score for hardness of the cooked hulled rice prepared from fresh samples was significantly different from that of the samples stored for 6 months (p≤0.05). However, all tested samples were accepted by consumers.