关键词: pH值, 半胱氨酸, 甘氨酸, 美拉德反应, 肉香味, 半胱氨酸-Amadori, 甘氨酸-Amadori

Abstract: Maillard reaction products (MRPs) were prepared from a complex reaction system containing cysteine, xylose, and glycine under different initial pH values (4.5?7.5) and investigated for the measurement of absorbance at 420 nm and the final pH values, and the analysis of volatile compounds by solid phase micro extraction (SPME) and gas chromatographymass spectrometry (GC-MS). It turned out that the greater the initial pH value was, the greater the degree of browning was and the more significantly the final pH values decreased. The most predominant flavor compounds identified were sulfur-containing compounds, followed by nitrogen-containing heterocyclic compounds and oxygen-containing heterocyclic compounds. The abundant sulfur-containing compounds were 2-methyl-3-furanthiol, 3-mercapto-2-pentanone, 2-furfurylthiol, 2-thiophenethiol, and bis(2-methyl-3-furyl) disulfide, 2-methylthiophene, and 2-acetylthiazole. With the increase in initial pH value, both the total content of volatile compounds and the content of sulfur-containing compounds identified firstly increased and then decreased, reaching a peak at pH 5.5. However, the content of nitrogen-containing heterocyclic compounds increased gradually, and the content of oxygen-containing heterocyclic compounds decreased. Further, the MRPs from reaction at 90 ℃ for 1 h at an initial pH of 4.5 or 7.5 were analyzed by high performance liquid chromatography with evaporated light scattering detection (HPLC-ELSD) and liquid chromatography-mass spectrometry (LC-MS). It was concluded that the pathway to develop volatile flavors at an acidic initial pH differed from that under a basic initial pH. For the former, the pathway involved cysteine-Amadori degradation, while for the latter it involved both cysteine-Amadori degradation and the reaction of glycine-Amadori with cysteine. Since at a basic initial pH, the compounds with an amino group (such as amino acids, and ammonia) were more active, the Maillard reaction became faster, which led to greater amount of cysteine-Amadoris intermediates during the early stage of the reaction and consequently the generation of more melanoidins and pyrazine compounds during the middle and late stages of the reaction. However, the emergence of glycine-Amadori under a basic initial pH could not facilitate the production of sulfur-containing compounds, since glycine- Amadoris could react with cysteine to form stable thiazolidine derivatives, which can cause the Maillard reaction to develop sulfur flavor compounds.

Key words: pH, cysteine, glycine, Maillard reaction, meaty flavor, cysteine-Amadori, glycine-Amadori