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

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 $160^{\circ}C$ 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 $160^{\circ}C$ 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 $140^{\circ}C$ 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 $150^{\circ}C$ for 20 min (p>0.05). At $160^{\circ}C$, 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 ${\alpha}$-, ${\beta}$-, ${\gamma}$- and ${\delta}$-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 $160^{\circ}C$ for 20 min, about 84% of initial ${\alpha}$-T in peanut oils was retained. ${\alpha}$-T was the most stable to roasting while ${\gamma}$-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 $160^{\circ}C$ 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.