The effect of hot air, vacuum and microwave drying on drying characteristics, rehydration capacity, color, total phenolic content and antioxidant capacity of Kumquat (Citrus japonica)

Abstract Sliced kumquats were dried by using three different drying methods, microwave (375 W), hot air (70 and 80 °C), and vacuum (70 and 80 °C with 100 and 300 mbar) to determine drying characteristics, antioxidant capacity and total phenolic content and color. All color parameters (L, a, b, Cab, ΔE and h° ) changed depending on the drying methods. Page and Modified Page models are the best fitted drying methods with the highest value of R2 (0.9994) and the lowest values of RMSE (0.000635-0.000735) and χ2 (0.000010-0.000013) compared to other models. Effective moisture diffusivity values for dried kumquats ranged from 1.54 × 10-8 to 8.24 × 10-8 in vacuum drying at 70 °C-300 mbar and microwave drying at 375 W, respectively. In comparison to the fresh sample, the dried samples showed an increase in both total phenolic content and antioxidant capacity. The total phenolic content (3095.71 ± 101.41 mg GA/100g d.w) and antioxidant activity (10.51 ± 0.19 µmol TE/g d.w) with DPPH assay showed the highest levels for the vacuum drying at 70 °C-100 mbar method. Microwave dried samples had the highest antioxidant activity with CUPRAC assay as (17.58 ± 0.63 µmol TE/g d.w.). This study indicated that microwave drying and vacuum drying at 70 °C-100 mbar were able to yield high-quality kumquat slices.

本研究采用三种不同干燥方式对切分金桔进行干燥处理，分别为微波（375 W）、热风（70 ℃与80 ℃）以及真空干燥（70 ℃、80 ℃分别对应100 mbar与300 mbar），旨在探究其干燥特性、抗氧化能力、总酚含量与色泽变化。所有色泽参数（L、a、b、Cab、ΔE及h°）均随干燥方式的不同发生显著改变。相较于其余模型，Page模型与修正Page模型的拟合效果最优，其决定系数（Coefficient of Determination, R²）最高可达0.9994，均方根误差（Root Mean Square Error, RMSE）低至0.000635~0.000735，卡方值（Chi-squared, χ²）仅为0.000010~0.000013。干燥后金桔的有效水分扩散率介于1.54×10^-8 至8.24×10^-8 之间，其中70 ℃、300 mbar真空干燥条件下的数值最低，375 W微波干燥条件下的数值最高。与新鲜金桔样品相比，经干燥处理的样品其总酚含量与抗氧化能力均有所提升。采用70 ℃、100 mbar真空干燥方式得到的样品，其总酚含量（3095.71 ± 101.41 mg 没食子酸（Gallic Acid, GA）/100g 干重（dry weight, d.w.））与2,2-二苯基-1-苦肼基自由基法（DPPH）测定的抗氧化活性（10.51 ± 0.19 µmol Trolox当量（Trolox Equivalent, TE）/g d.w.）均为所有处理组中最高。而采用375 W微波干燥得到的样品，其铜离子还原抗氧化能力法（CUPRAC）测定的抗氧化活性最高，达(17.58 ± 0.63 µmol TE/g d.w.)。本研究表明，微波干燥与70 ℃、100 mbar真空干燥这两种方式，均可制备得到高品质的金桔切片。