Effects of Dry Roasting on the Vitamin E Content and Microstructure of Peanut (Arachis hypogaea)

Abstract

Effects of roasting on vitamin E content, color, microstructure and moisture of peanuts, and vitamin E content in peanut oils prepared from the roasted peanuts were investigated. Runner-type peanuts were roasted at 140, 150, and 160℃ for 10-20 min. As roasting temperature and time increased, the CIELAB L^* value of peanuts decreased while a^* and b^* values increased, resulting in formation of the golden brown color of roasted peanuts. Moisture ratio (M/Mo) and color b^* value of peanuts roasted at 140 to 160℃ showed a correlation of b^* = 21.61 (M/Mo)^2 – 40.62 (M/Mo) + 34.12 (R^2 = 0.9123). Overall changes in the tocopherol contents of peanuts and peanut oils were significantly affected by roasting temperature and time (p<0.05). Roasting at 140℃ caused a slight increase in the levels of tocopherols of peanuts over roasting time up to 20 min (p<0.05). There was no significant change in the tocopherol levels of peanuts during roasting at 150℃ for 20 min (p>0.05). At 160℃, the levels of tocopherols significantly decreased during the initial 10 min of roasting (p<0.05) while there was no extended loss after 10 min, resulting in about 5, 12, 20, and 10% losses of α-, β-, γ- and δ-T, respectively. After 20 min, total tocopherols decreased by 18%. However, tocopherol contents of pressed peanut oils significantly decreased at all roasting temperatures (p<0.05). After roasting peanuts at 160℃ for 20 min, about 84% of initial α-T in peanut oils was retained. α-T was the most stable to roasting while γ-T was the least. Swollen epidermal cells on the inner surface and broken cell walls of parenchyma tissue of peanut cotyledon were observed in peanuts after roasting at 160℃ for 15 min. Severe changes in microstructure of peanut by roasting would contribute to vitamin E stability because of exposure of oil droplets in peanuts to oxygen.