微波加热对乳清分离蛋白功能特性的影响及机制研究

本研究采用微波加热促进美拉德反应对乳清分离蛋白（Whey Protein Isolate，WPI）进行改性。以乳化性及抗氧化活性为主要指标，系统研究了微波功率、微波加热时间、pH以及乳糖与乳清粉质量比对乳清蛋白美拉德反应产物（Maillard Reaction Products，MRPs）功能特性的影响，对其工艺进行了优化，并对反应过程中二级结构的变化及氨基酸组成进行了分析。结果表明:优化后的微波加热条件为pH8.5、乳清蛋白-乳糖质量比5:5、微波加热时间7 min；在该条件下制备所得MRPs的乳化活力指数、乳化稳定指数和DPPH自由基清除率分别为0.35 m2/g、88.39 min和6.60%；红外分析结果显示乳糖与WPI发生了共价结合，微波加热促进了该反应的进程，导致氮羟基和羰基含量增多，产生了复杂的美拉德中间产物和醛酮类化合物；同时促使蛋白质二级结构由紧密向松散“溶胀”结构转化，在转变的过程中，大部分紧密有序结构首先转变成松散有序结构，进而转变成松散无序状态，并可能伴随小部分紧密结构经长时间加热后相互转变。氨基酸分析进一步证实，微波加热降低了体系氨基酸总量，可能是WPI自由氨基酸参与美拉德反应所致。

Effect and Mechanism of Microwave Heating on the Functional Properties of Whey Protein Isolate

In this study, maillard reaction was used to modify whey protein isolate (WPI) with microwave assistance. The effects of microwave power, microwave heating time, pH, and mass ratio of lactose to whey powder on the functional properties of whey protein maillard reaction products (WPI-MRPs) were systematically investigated by using emulsification and antioxidant activity as the main indexes. The process was optimized, the secondary structure and amino acid composition during microwave treatment were also analyzed in details. The optimized microwave heating conditions are as follows: pH8.5, whey protein-lactose mass ratio 5:5, microwave heating time 7 min. The emulsification activity index, emulsifying stability index and DPPH free radical scavenging rate of the obtained MRPs was 0.35 m2/g, 88.39 min and 6.60%, respectively. The infrared spectroscopy data indicated that microwave heating could promote the maillard reaction process, during the process increases in the contents of hydroxyl and carbonyl groups were observed, and complex maillard intermediates and aldehydes and ketones were produced. Meanwhile, transformation of the protein secondary structure from a compact to a loose “swelling” structure could be also promoted. During the transformation, most of the tightly ordered structure firstly transforms into a loosely ordered structure, and then into a loosely disordered state, accompanied by a small part of the tight ordered structure changed to each other after a long time heating. In addition, amino acid analysis further confirmed that the reaction would result in a decrease in total amino acids content due to participation of free amino acids in WPI during maillard reaction.