| Abstract | In this study the changes in physicochemical properties of brown rice due to parboiling of brown rice, storage of parboiled brown rice and germination brown rice were investigated. Thai rice varieties with high amylose content (Chainat 1 and Supanburi 1) and low amylose content (Koa Dok Mali 105) were used to produce parboiled brown rice while parboiled paddy of Chainat 1 variety was purchased from a trader. Parboiling of brown rice is an attractive alternative to parboiling of paddy (soaking, steaming and drying). During the parboiling process, brown rice with the initial moisture content of approximately 13% wet basis (w.b.) was soaked at two different initial soaking temperatures of 70 and 80oC for four different durations as 1 h, 2 h, 3 h and 4 h, followed by steam treatment at temperature of 100oC for 10, 15 and 20 min. The samples were then dried in the shade at room temperature (approximately 30oC) and at 60%RH to the final moisture content of approximately 13% wet basis (w.b.). The effects of combination of soaking-steaming temperature and time on the physicochemical properties were determined and sensory analysis was performed for selected processing conditions. Head rice yield, yellowness (b-value), whiteness, hardness, water absorption, vitamin E and vitamin B2 were measured and compared with those of commercial parboiled paddy. Results revealed that the head rice yield, yellowness (b-value), cooking time and hardness of parboiled brown rice were lower whereas whiteness and water absorption were higher compared to those of commercial parboiled paddy. Qualitatively, parboiled brown rice showed intermediate values between milled rice (white rice) and commercial parboiled paddy. Sensory analysis revealed high acceptance for cooked parboiled brown rice from the panelists. However, the concentration of vitamin B2 was decreased and vitamin E disappeared after parboiling of brown rice. Head rice yield was found to be lower for parboiled brown rice compared to that of parboiled paddy but greater than the head rice yield of non-parboiled rice.
The variety Chainat 1 was selected to produce brown rice (BR), parboiled paddy (PP) and parboiled brown rice (PB) products. Parboiled paddy was prepared by soaking paddy at 65oC for 6 h followed by steam treatment at a temperature of 100oC for 20 min and dried in shade at room temperature to the final moisture content of approximately 13% wet basis (w.b.). Parboiled brown rice was prepared by soaking brown rice of the same variety at an initial soaking temperature of 80oC for 4 h. The soaked brown rice was steamed and dried following the same treatment as parboiling of paddy. All samples were packed in polyethylene bags. Studies were undertaken to investigate the effects of three temperatures, namely 4, 25 and 37oC on the physicochemical properties of stored samples. Samples were evaluated after each month during six months of storage and tests were run to estimate changes in terms of physicochemical qualities namely moisture content, color, lightness, hardness, water absorption, total lipid, free fatty acid, SEM, pasting properties and sensory evaluation. Storage of PB, PP and BR at low temperature (4oC) retarded the quality changes, resulting in slightly decreased moisture content, lightness, water absorption and total lipid while b-value, hardness and the free fatty acid content were slightly increased with longer storage. The higher temperature (25, 37oC) storage led to significant decrease (P<0.05) of moisture content, water absorption and total lipid. The bvalue, hardness and free fatty acid content increased significantly (P<0.05) following storage at 25 and 37oC with all rice samples. Changes in the internal structure of PB, PP and BR were also visualized using scanning electron microscopy, and the BR clearly showed the polygonal shape following storage at 4oC whereas PB and PP presented smoother surface especially at higher temperature storage. Storage of all samples produced
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changes in the pasting properties depending on the variety, time and temperature. Pasting properties of PB and PP were different from the pasting properties of BR during storage at different temperatures. A small difference was observed in the shape of RVA curves during storage of BR while, differences were more pronounced in PB and PP resulting into linear curves. Within all samples, low temperature showed less change in all sensory qualities during storage than high temperature. The panelists preferred fresh rice and gave reduced score for aroma, color, texture and taste. Storage at 37oC pushed panelists to reject aged BR, PB and PP due to overall acceptability scores of less than 5 after six months of storage. Hence, it indicated that the low temperature storage provides the best shelf life for PB, PP and BR than high temperature.
Parboiling of brown rice resulted in loss of some vitamins; hence germinating of brown rice could be amending this problem. The process of germination in Koa Dok Mali 105 was studied in terms of soaking, incubating and drying treatment. After soaking of brown rice in water at 40oC for 24 h, yellowness and total lipid decreased while lightness and free fatty acid increased. The viscosity of brown rice increased with the soaking process. The content of γ-aminobutyric acid (GABA) in germinated brown rice (GBR) (8.11 mg/100 g) remarkably increased during up to 16 hrs of soaking and decreased in the subsequent soaking until to 5.97 mg/100 g at 24 h. However, the GABA content was higher than in ungerminated brown rice (2.51 mg/100 g). Brown rice soaked at 40oC for 4 h combined with 20 hrs incubation at the same temperature and humidity maintained at 90% RH resulted in higher GABA content (12.50 mg/100 g) than that observed with only soaking process. Yellowness and total lipid decreased while lightness and free fatty acids increased. However viscosities of GBR decreased with increased incubating time.
GBR was prepared by soaking combined with incubating followed by steam drying (SD) and non steam drying (NSD). For steam drying, GBR sample was steamed at 100oC for 10 min before drying at various temperatures of 20, 40, 80 and 160oC, while in case of non steam drying GBR, sample was directly dried at aforementioned temperatures. Both, steam drying and non-steam drying of germinated brown rice affected the final quality of the products. Germination and subsequent drying at low temperature produced easier cooking and better taste after cooking compared to the normal brown rice. Total aerobic plate count (APC) increased after germination process and was eliminated after steaming and drying treatment especially at high temperature drying (160oC). However, GABA content was affected by the temperature of drying as high temperature decomposed GABA content than low temperature. However, the final GABA contents of GBR products were observed to be higher than ungerminted products. GABA contents in ungerminated brown rice and germinated brown rice after cooking were higher than those in rice samples before cooking. |