尿素对抗坏血酸-半胱氨酸模式反应形成香味化合物的影响

Effect of Urea on Aroma Compounds Generation from Model Reaction of Ascorbic Acid and Cysteine

摘要:

以固相微萃取-气相色谱-质谱联用（SPME-GC-MS）技术对抗坏血酸与半胱氨酸（ASA-Cys）的模式反应产物进行鉴定，研究了尿素对模式反应形成香味化合物的影响。呋喃、吡嗪、噻吩、噻唑、噻吩并噻吩及脂环硫化物在内的47个香味化合物被鉴定，其中，含硫化合物（噻吩，噻吩并噻吩，脂环硫化物）是最主要的香味成分。尿素加入到模式体系中（pH8.00），一方面使一些含硫化合物的生成量显著降低，甚至消失；另一方面却促进了几个含氮化合物，如吡嗪，甲基吡嗪，乙基吡嗪，2,6-二甲基吡嗪及其它烷基吡嗪的生成。这表明，在模式反应中，尿素受热能够释放出NH3，NH3与H2S和ASA降解产物发生竞争反应形成含氮化合物如烷基吡嗪，从而导致含硫化合物的生成量降低。

Abstract:

The identification of aroma compounds, formed from the model reactions of L-ascorbic acid and L-cysteine with or without urea at two different initial pH values (5.0, 8.0) and 140?癈 for 2h, was performed using a solid-phase microextraction-gas chromatography-mass spectrometry (SPME-GC-MS) technique. The effect of urea on aroma compounds was investigated. 47 aroma compounds were identified. The results showed that sulfur-containing compounds such as alicyclic S compounds, thiophenes and thienothiophenes were the most abundant compounds. Other compounds identified were furans, thiazoles and pyrazines. The addition of urea into the reaction mixture in general caused a reduction or disappearance in content of some sulfur-containing compounds at pH8.0, but caused a increase at pH5.0. On the other hand, it facilitated the generation of several nitrogen-containing volatiles such as pyrazine, methylpyrazine, ethylpyrazine, 2,6-dimethylpyrazine and other alkyl pyrazines which are known to elicit roasty and nutty flavor notes at pH8.0. A plausible explanation for this phenomenon is that ammonia can be released from urea upon heating and the formed ammonia competes with hydrogen sulfide to react with Maillard reaction precursors to produce nitrogen-containing compounds such as alkylpyrazines.