Abstract:To understand the effect of drying temperature on the quality of citrus peels dried using a heat pump, the colors, rehydration ratios, essential oil contents, total flavonoid contents, and antioxidant activities of the sun-dried, fresh citrus peels (control) and those dried at different temperatures (40, 50, 60, 70, and 80 ℃) using a heat pump are compared. The L value first increases and then decreases with an increase in the drying temperature. The a values of the citrus peels dried using a heat pump at 40, 50, and 60 ℃ and those of the sun-dried peels show no significant differences (p>0.05). However, the a value decreases with an increase in temperature to >60 ℃. In contrast, the b value increases with an increase in the drying temperature. The lowest ΔE value is obtained when the citrus peels are dried using heat pump at 60 ℃. At equilibrium, the rehydration ratios of sun-dried citrus peels and the peels dried using a heat pump at 40, 50, and 60 ℃ exhibit insignificant differences (p>0.05), but these are significantly higher than those of the peels dried using a heat pump at 70 and 80 ℃ (p<0.05). The essential oil and total flavonoid contents of the peels first increase and then decrease as the drying temperature increases. At 60 ℃, the highest essential oil and total flavonoid contents of 2.04% and 13.46%, respectively, are obtained. The highest DPPH and ABTS+ scavenging capacities of 54.16% and 37.13%, respectively, are also observed at 60 ℃. Therefore, drying using a heat pump at 60 ℃ can allow the preservation of the quality of citrus peels, including the color, total flavonoid content, and antioxidant activity of the peels. This finding can facilitate the practical application of heat pumps in drying citrus peels.
