pH值对“半胱氨酸-木糖-甘氨酸”体系肉香味形成的影响

Effect of pH on the Formation of Meaty Flavor Compounds in Cysteine-Xylose-Glycine Reaction System

设计复杂美拉德反应体系"半胱氨酸-木糖-甘氨酸",在不同初始p H值(4.5、5.5、6.6、7.5)条件下反应,反应液测定波长420 nm处吸光度及p H值,并进行固相微萃取-气相色谱-质谱联用分析。结果表明,初始p H值越大,反应液褐变程度越大,p H值下降量增多。各反应体系中鉴定出的风味物质均主要是含硫化合物,其次是含氮杂环化合物、含氧杂环化合物。含硫化合物中含量较高的为2-甲基-3-呋喃硫醇、3-巯基-2-戊酮、2-糠硫醇、2-噻吩硫醇、双(2-甲基-3-呋喃基)二硫醚、2-甲基噻吩、2-乙酰基噻唑。各类化合物总量及含硫化合物总量均随p H值的升高呈先增加后减小的趋势,在p H 5.5出现峰值。但含氮杂环化合物总量却随p H值升高而增加,而含氧杂环类总量随p H值升高而减小。采用高效液相色谱-蒸发光散射检测器及液相色谱-质谱联用分析初始p H 4.5、7.5,90℃,1 h反应液,发现酸性条件下风味物质形成通过半胱氨酸-Amadori降解途径进行,碱性条件下通过半胱氨酸-Amadori降解及甘氨酸-Amadori与半胱氨酸反应2条途径进行。碱性条件下,含胺基化合物(如氨基酸、氨)的反应活性高,反应速率快,体系内形成的半胱氨酸-Amadori初期中间体含量高,但其在中、末期阶段却更多地导致类黑精及吡嗪类物质产生;而碱性条件下出现的甘氨酸-Amadori,因可与半胱氨酸结合形成较为稳定的噻唑烷衍生物,并不能促进含硫化合物的形成。

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 at420nm 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 were2-methyl-3-furanthiol,3-mercapto-2-pentanone,2-furfurylthiol,2-thiophenethiol,and bis(2-methyl-3-furyl)disulfide,2-methylthiophene,and2-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 pH5.5.However,the content of nitrogen-containing heterocyclic compounds increased gradually,and the content of oxygen-containing heterocyclic compounds decreased.Further,the MRPs from reaction at90℃for1h at an initial pH of4.5or7.5were 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.