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Physical, Chemical, and Sensory Properties of Antioxidant-Enriched Raw and Cooked Rice by Vacuum-Drying Impregnation in a Semidry State

¹Department of Food Science and Biotechnology, Dongguk University–Seoul, 26 Pil-Dong 3-ga, Jung-Gu, Seoul 100-715, Republic of Korea. ²Department of Food Science and Technology, Faculty of Agro-Industry, Kasetsart University, 50 Gnamwongwan Rd. Lardyaow, Jatujak, Bangkok 10900, Thailand. ³Center of Advanced Studies for Agriculture and Food, Kasetsart University Institute for Advanced Studies, Kasetsart University, Bangkok 10900, Thailand. ⁴Corresponding author. Phone: +66-2-562-5042. Fax: +66-2-562-5021. E-mail: waraporn.b@ku.ac.th

Vacuum drying was employed with a vacuum impregnation technique in a semidry state to enrich rice with antioxidants of beetroot juice. The properties of the vacuum-dried raw and cooked rice grains were characterized. The various raw rice grains (three varieties and two storage time periods) exhibited a significant absorption of beetroot juice, which was evident from the red-violet beetroot color of the rice, as distinguished from the white color of the control. The color increase (ΔE= 20−40) was linear with the juice content (R² = 0.96−0.99). Their total phenolic (TP) contents and 2,2-diphenyl-2-picrylhydrazyl radical (DPPH) scavenging activities were enhanced (ΔTP = 21−260 mg of gallic acid equivalents/100 g of rice db and ΔDPPH = 22−64 mg of vitamin C equivalents/100 g of rice db). Their grain integrity was reduced (Δforce = −1 to −63), which was potentially associated with the formation of grain surface cracks (linear relationship of %crack and %juice with R² = 0.94−0.98). After cooking, the enriched rice grains were linearly elongated with added juice (R² = 0.88−0.97, up to 1.6-, 2.0-, and 2.0-fold for Sanpatong 1, Khao Dawk Mali 105, and Chainart 1 rice samples, respectively), and the overall volume of the cooked rice was increased (likely not linear, up to 3.2-, 4.3-, and 4.8-fold for Sanpatong 1, Khao Dawk Mali 105, and Chainart 1 rice samples, respectively). Such improvements in cooking qualities were obtained by this simple vacuum-drying technique, in comparison to existing rice-aging processes that are more time consuming. The sensorial scores of the resultant rice products were excellent. Vacuum drying is an effective tool to improve the antioxidant value of rice as well as its cooking quality, and the raw quality remains appreciable. It is a simple and rapid process that could be practical for manufacturing healthy rice products.