不同干燥方式对甘薯叶功能成分及抗氧化活性的影响

【目的】探讨不同干燥方式对甘薯叶功能成分含量及抗氧化能力的影响,为甘薯叶综合开发和干燥加工工艺优化提供理论依据。【方法】以甘薯品种‘台农71’和‘胜南’的叶片为材料,研究蒸干结合热风干燥、真空冷冻干燥、60℃/50℃/40℃热风干燥共5种干燥方式对甘薯叶功能成分（总酚、总黄酮、绿原酸类成分、β-胡萝卜素、维生素D₃、维生素E、抗坏血酸、维生素B₁、维生素B₂）、抗氧化能力（采用DPPH、ABTS⁺自由基清除法测定）和外观色泽（叶绿素、色值）的影响,并分析各功能成分间及功能成分与抗氧化活性的关系。【结果】甘薯叶中检测到的游离酚酸主要包括新绿原酸、绿原酸、隐绿原酸、咖啡酸、异绿原酸A、异绿原酸B和异绿原酸C。5种干燥方式对甘薯叶中功能成分有不同程度的影响。真空冷冻干燥下,‘台农71’叶片中游离酚酸总量最高,达到38.4 mg·g^-1DW,是其在60℃热风干燥下含量的25.6倍。真空冷冻干燥和蒸干结合热风干燥下,叶片中总酚、总黄酮和抗坏血酸含量差异相对较小,但都显著高于60℃/50℃/40℃ 3种热风干燥。真空冷冻干燥下,两个材料的总酚和总黄酮含量最高,分别是3种热风干燥的1.7—5.3倍和1.7—3.8倍。抗坏血酸在真空冷冻干燥下有较好的保留（175.3—441.1 mg/100 g DW）,而在热风干燥中含量极低（3.4—5.7 mg/100 g DW）。维生素D₃和α-生育酚在蒸干结合热风干燥下含量最高。抗氧化活性分析表明,不同干燥方式下甘薯叶甲醇提取物抗氧化活性差异显著（P<0.05）,其中真空冷冻干燥和蒸干结合热风干燥的自由基清除率较高,显著高于3种热风干燥。相关性分析表明,总酚、总黄酮、总绿原酸、维生素D₃、α-生育酚之间存在极显著相关性（P<0.01）;甘薯叶抗氧化能力与总酚、总黄酮及各绿原酸类成分的含量也存在极显著相关（P<0.01）。【结论】真空冷冻干燥和蒸干结合热风干燥能较好地保留甘薯叶中总酚、绿原酸及衍生物、黄酮、维生素D₃、α-生育酚、抗坏血酸等功能成分,使叶片干燥后仍具有较强的抗氧化能力。与真空冷冻干燥相比,蒸干结合热风干燥具有成本低和耗时短的优势,是实际生产应用中干燥保留甘薯叶多酚和黄酮的优选方式。

Effects of Different Drying Methods on Functional Components and Antioxidant Activity in Sweet Potato Leaves

【Objective】 This study was aimed to explore the effects of different drying methods on functional components content and antioxidant capacity of sweet potato leaves, which could provide a theoretical basis for optimize drying process and fully utilize sweet potato leaves in the future. 【Method】 The leaves of sweet potato varieties of Tainong 71 and Shengnan were dried using five drying methods, including steam drying combined with hot air drying (SD+HAD), vacuum freeze drying (VFD), hot air drying (HAD) at 60℃, 50℃ and 40℃. The effects of different drying methods on the functional components (total phenols, total flavonoids, caffeoylquinic acids, β-carotene, vitamin D₃, vitamin E, ascorbic acid, vitamin B₁, and vitamin B₂), antioxidant capacity (using DPPH and ABTS ⁺ radical scavenging methods) and appearance color of sweet potato leaves (chlorophyll and color value) was evaluated, and the correlation between the functional components and the correlation between functional components and antioxidant activity were analyzed. 【Result】 Free phenolic acids detected in sweet potato leaves mainly included 5-O-caffeoylquinic acid, 3-O-caffeoylquinic acid, 4-O-caffeoylquinic acid, caffeic acid, 3,5-di-O-caffeoylquinic acid, 3,4-di-O-caffeoylquinic acid and 4,5-di-O-caffeoylquinic acid. The effects of five drying methods on the functional components of sweet potato leaves were different. Under VFD, the total content of free phenolic acids in Tainong 71 leaves was the highest, reaching to 38.4 mg·g^-1 DW, which was 25.6 times of that under 60℃ HAD. Total phenols, total flavonoids, ascorbic acid and caffeoylquinic acids showed small differences between VFD and SD+HAD, but which were significantly higher than those of 60℃, 50℃ and 40℃ HAD. The average total phenols (64.8 mg CAE·g^-1 DW) and total flavonoids (6.5 mg RE·g^-1 DW) content of the two materials under VFD were the highest, which were 1.7-5.3 times and 1.7-3.8 times of that under the three types of HADs, respectively. Vitamin C was well retained under VFD (175.3-441.1 mg/100 g DW), but it was extremely low (3.4-5.7 mg/100 g DW) in the three types of HADs. Vitamin D₃ and α-tocopherol had the highest content in SD+HAD. The antioxidant activity of methanol extracts from sweet potato leaves was significantly different under different drying methods (P<0.05). VFD and SD+HAD had higher free radical scavenging rates, and which were significantly higher than that under HADs. Correlation analysis showed that there was a strong correlation between total phenols, total flavonoids, total caffeoylquinic acids, vitamin D₃, and α-tocopherol (P<0.01). Furthermore, a significant correlation was found between the antioxidant capacity of sweet potato leaves and the contents of total phenols, total flavonoids and each caffeoylquinic acid (P<0.01). 【Conclusion】 VFD and SD+HAD could better retain the content of polyphenols, flavonoids, vitamin D₃, α-tocopherol, ascorbic acid and other functional components in sweet potato leaves, which confered strong antioxidant capacity to dried sweet potato leaves. Compared with VFD, SD+HAD had the advantages of low cost and short time-consuming, so it was the preferred drying method to retain polyphenols and flavonoids in sweet potato during practical production.