COMPARATIVE STUDY BETWEEN HOT AIR AND INFRARED DRYING OF PARBOILED RICE: KINETICS AND QUALITIES ASPECTS O. BUALUANG 1 , Y. TIRAWANICHAKUL 2 and S. TIRAWANICHAKUL 1,3, * 1 Department of Chemical Engineering, Faculty of Engineering, Prince of Songkla University, Hatyai 90112, Songkhla, Thailand 2 Plasma and Energy Technology Research Laboratory, Department of Physics, Faculty of Sciences, Prince of Songkla University, Hatyai 90112, Songkhla, Thailand 3 Corresponding author. TEL: +6674287306; FAX: +6674558833; EMAIL: supawan.t@psu.ac.th *Present address: Department of Chemical Engineering, Faculty of Engineering, PO Box 15, Hatyai 90112, Songkhla, Thailand. Accepted for Publication June 18, 2012 doi:10.1111/j.1745-4549.2012.00813.x ABSTRACT The objectives of this study were to analyze the effects of drying employing three different heat sources on drying kinetics and to evaluate qualities of parboiled rice after drying. Drying temperature was varied between 60 and 100C. Power of infra- red (IR) heat source was fixed at 1,000 and 1,500 W and air flow rate was fixed at 1.0  0.2 m/s. The three drying strategies composed of hot air (HA), IR and com- bined HA + IR drying. The experimental results were simulated using various equilibrium moisture content models and the mathematical drying model for pre- diction of drying kinetics and evaluation of effective diffusion coefficient (Deff) followed by Fick’s law of diffusion. The results revealed that Deff values of HA and IR drying were in the range of 10 -12 –10 -11 m 2 /s and relatively depended on tem- perature. For quality evaluation, conclusions reached that head rice yield using HA + IR drying had the highest value, while yellowness and whiteness of the par- boiled rice are significantly affected by drying condition (P > 0.05). PRACTICAL APPLICATIONS These results suggested that combined hot air and infrared (HA + IR) drying offers a great potential for preserving Leb Nok Pattani parboiled rice. Drying kinetics and quality aspects determined that IR drying is more efficient than the other drying method. However, the color degradation in the grain kernel is an issue for quality measurement. INTRODUCTION Rough rice or paddy that is subjected to hydrothermal treat- ment prior to milling is defined as parboiled rice. Tradi- tional parboiling involves soaking the paddy in cold water, followed by steaming and drying (Bhattacharya 2004). Par- boiling is practiced in many parts of the world such as Asia, Europe and America (Pillaiyar 1981; Heinemann et al. 2005). Moreover, parboiled rice products in Thailand tend to increase, especially in health food and green organic products. Parboiled rice has beneficial grain kernel because during the parboiling process, the rice grain kernel changes its physical and physicochemical properties due to rice gela- tinization and leads to a high milling rice yield (Fan et al. 1999; Bhattacharya 2004; Tirawanichakul et al. 2004a; Sopo- nronnarit et al. 2005). This is because during the parboiling process, starch gelatinization takes place – a thermo- physical reaction between the starch granules and heat energy in the presence of water. The starch gelatinization brings about changes in the physicochemical properties of the rice (Fan et al. 1999; Islam et al. 2001, 2002). In addi- tion, parboiled rice has lower nutrient loss during milling and cooking corresponding to percentage of ash enrichment and high content of mineral and nutrient matter compared with the milled rice (Rao and Juliano 1970; Wu et al. 2002; Heinemann et al. 2005). In the southern part of Thailand located among the Andaman and Pacific oceans, the rain- falls are frequent so losses during postharvesting period are also high because of the high humidity in the surrounding (Soponronnarit et al. 1998; Sun et al. 2002; Tirawanichakul et al. 2004b; Schluterman and Siebenmorgen 2007). To strive for reduced degradation of rice grain and for Journal of Food Processing and Preservation ISSN 1745-4549 Journal of Food Processing and Preservation 37 (2013) 1119–1132 © 2012 Wiley Periodicals, Inc. 1119