Effect of accelerated aging on physicochemical properties of rice cultivar Tarom

More than half of the world's people consume rice. Rice consumers, especially in Asia know that cooking properties of rice change with its storage. This phenomenon is called aging. The increase in hardness, decrease in adhesiveness during cooking and decrease in solid loss, are the major changes in rice which caused by aging. However, while some consumers prefer new rice like Japanese (Zhou, 2002) Iranian and many other consumers prefer aged rice. The change in physicochemical properties of aged rice is caused by the alteration and interaction in lipid, protein and starch (Juliano, 1985; Sodhi, et al. 2003; Teo et al. 2000). The conventional aging of rice takes 3-6 months (Jaisut et al. 2009; Soponronnarit et al. 2008).Yet this method needs more space for storage leading to high operating costs. It is necessary to examine other methods that can reduce the aging time and maintain the rice properties similar to those obtained by the conventional aging process. The process for accelerated aging could be developed either by dry or wet heat (Rayaguru et al. 2011; Rosniyana et al. 2004). The dry heat method was successfully tested with heating of rice in high temperatures in a closed container (Battacharya, 2013). One of the basic problems that occur during dry heating is that any moisture loss leads to cracking. Although the concept is simple in theory, it is complicated in practice. Another drying process for accelerated aging of rice employs a fluidized bed dryer. However, this may decrease head rice yield (Soponronnarit et al. 2008; Wiset et al. 2005), not to mention the fact that these dryers are rare and expensive in rice producing countries. Hence, wet heat is more practical and economical in the mentioned countries. The environmental optimum conditions for accelerated aging of Basmati paddy, at temperature 30–50 °C, relative humidity 50–90% and aging period 3–14 days, has been studied and reported to be 43 °C, 71.0% relative humidity (RH) and 11 days (Rayaguru et al., 2011). Parboiling has been implemented as a method of improving the quality of rice. Nonetheless, it is a lengthy process involving soaking, steaming and drying, which results in the complete gelatinization of starch (Gujral, and Kumar, 2003). According to the importance of accelerated aging, the aim of this study was to evaluate quality attributes and physicochemical properties improvement of Tarom variety of paddy.
In this study, Tarom variety of paddy (a local variety, with high-quality and large area cultivation in Mazandaran province) was chosen. For accelerated aging, two methods were done: Steaming (or steam curing) and controlling the environmental conditions. In steaming method, the paddy was divided into two portions. The first which had an initial moisture content of 24% (wet base) and the second was the paddy which had been rested at the ambient temperature allowing its moisture content to reach 17%. To accelerate aging of the paddy in two moisture levels i.e. 24 and 17%, the samples were treated by steaming (or steam curing) for 5, 10 and 20 min, and then both of samples were dried with fixed bed dryer at 40 ◦C until the grain moisture content reached 11%. Afterwards, the paddy samples were dehusked and polished with satake miller apparatus. Milled whole rice kernels were separated from broken rice for evaluation of physicochemical properties. In controlling the environmental conditions, the samples were dried with fixed bed dryer at 40 ◦C until the moisture content reached 12%, and then stored at 43 °C and RH=74% for 11 days. Similarly, to produce white rice, the paddy samples were dehusked and polished as mentioned above. The control sample which had an initial moisture content of 24% (wet base) was only dried at 40 ◦C to moisture contend wet basis 11%. Several parameters including head rice yield (the percentage of the mass of white rice that remained as head rice after milling), hardness of raw kernel (the maximum force for breakage of raw white rice), color (b value), chalkiness (weight of the kernel that had chalkiness over one half of the kernel surface in 5 gram of white rice), aroma (using hedonic test), pasting properties (including the peak viscosity, final viscosity, breakdown viscosity and setback viscosity), texture profile analysis (TPA) and thermal properties of rice (the onset temperature, peak temperature, conclusion temperature and enthalpy of gelatinization) were determined. The resultant data were subjected to the analysis of variance using SPSS (ver. 19) software. Duncan’s test was used to establish the differences between mean values at 95% confidence interval.
The quality attributes of the produced white rice were compared with that of the control samples. By doing the steam curing treatment, solid loss of the rice decreased, adhesiveness decreased, gelatinization temperature increased and viscosity changed. All of the mentioned changes were similar to the alterations that occur in naturally aged rice. By controlling the environmental conditions, the solid loss decreased and the elongation increased. According to sensory and apparent characteristics including aroma and chalkiness, treatment by controlling the environmental conditions could be recommended. Steaming treatment for the three levels of time led to complete loss of aroma.