复杂Maillard模型反应体系半胱氨酸作用于肉香味形成的初始反应途径

Cysteine is an important precursor of meat flavor. The role of cysteine in the development of meat flavor has long been recognized, and known as an essential part of meat flavor theory. Cysteine contributes to meat flavor mainly by its participation in Maillard reaction and generation of sulfur-containing flavors. However, the theory contents are mostly built on and described by researches of simple model Mailard reaction systems, which involve only reactants of a kind of amino acid and a kind of reduced sugar, rather less than those of real meat flavor developing systems. In fact, a complex Maillard model system composed of lipid,cysteine,another kind of amino acid,and a kind of reduced sugar closely parallels a real meat flavor developing system; as far as the primary Maillard reaction stage is concerned, the reduced sugar can react with either cysteine or the other kind of amino acid, while an amino acid can react with either the reduced sugar or the carbonyl compounds generated from lipid oxidization. Under such a multi-reactants circumstance, yet how meat flavors are to be generated from cysteine has not been understood, because reactive competition present among the precusors complicates such a complex model Maillard system. In this proposal, proper systems of optimum meat flavor are to be designed with lipid,cysteine,another kind of amino acid,and a kind of reduced sugar.Key sulfur-containing odorants in the reaction product along with the intermediates resulting from the primary reactions will be identified and analyzed. Considering complications of the complex model Maillard reaction systems souce from the primary reaction stage, where several possible reaction pathways are to begin with, the research work will focus on the primary reaction stage.The primary reaction pathway towards meat flavor is to be explored by correlating structures and amount of the key sulfur-containing odorants with those of the intermediates from the primary reactions. Additionally, based on reactive competition of the precursors, key factors, causes, and rules which affect the primary reaction pathway and meat flavor formation are to be investigated. Work of this proposal will help to better the present meat flavor theory, optimize the technologies of thermal reaction meat flavoring preparation and meaty food processing.

半胱氨酸为重要肉香前体。有关半胱氨酸在肉香味形成中产生作用的机制已有大量报道，并形成基本理论。但这主要建立在简单Maillard模型反应（1+1）基础上。简单模型体系组成与实际体系相差较大。对于"脂质-半胱氨酸-其它氨基酸-还原糖"混合物，这一与实际肉香味形成体系极相近的复杂Maillard模型反应体系，初始反应阶段存在其它氨基酸与半胱氨酸、及脂质氧化成分与还原糖间的反应活性竞争, 半胱氨酸如何发生反应产生肉香味？并不清楚。项目设计科学合理反应体系，分析鉴定及研究关键含硫肉香味成分的结构、含量与初始反应中间物的结构、含量关系，从初始反应途径这一关键环节，揭示"脂质-半胱氨酸-其它氨基酸-还原糖"体系半胱氨酸在肉香味形成中的作用机制;从肉香前体间反应活性竞争角度，探讨某些关键因素对初始反应途径及肉香味形成产生影响的规律及本质。研究结果对发展肉香味理论，优化热反应肉香味制备工艺有重要指导意义。

以甘氨酸作为其他氨基酸的代表进行研究。找到了“半胱氨酸-木糖”体系中含有甘氨酸或脂质（亚油酸酯）时有利于形成肉香味的较佳反应条件。制备了2-木糖基噻唑烷-4-羧酸（XTCA）、半胱氨酸-Amadori（Cys-Amadori）、甘氨酸-Amadori（Gly-Amadori）三种初始美拉德反应中间体纯品。建立了液相色谱或液-质谱机定量分析反应液中形成的初始美拉德反应中间体的方法。设计了四类模型反应体系：（1）“半胱氨酸-木糖”中添加不同量甘氨酸；（2）“半胱氨酸-木糖-甘氨酸”中添加己醛、2-庚烯醛、2,4-癸二烯醛；（3）初始中间体与游离氨基酸结合，包括XTCA与甘氨酸（XTCA/Gly）、Gly-Amadori与半胱氨酸（Gly-Amadori/Cys）、Cys-Amadori与甘氨酸（Cys-Amadori/Gly）；（4）模拟制备热反应肉味香精，酶解液中XTCA/Gly、Gly-Amadori/Cys、Cys-Amadori/Gly进行反应。通过液相色谱、液-质联机监测木糖、半胱氨酸、三种初始中间体含量及反应液420nm和294nm吸收和pH随时间变化，比较产生的挥发性含硫风味物质的量，得出：在含有半胱氨酸和其它氨基酸的复杂美拉德反应体系中，形成半胱氨酸-Amadori中间体途径最有利，但其由XTCA转化而来，因此XTCA途径是形成含硫化合物的实际可利用途径。反应体系中形成其它氨基酸-Amadori（如Gly-Amadori）时，美拉德反应速率加快，其含量与反应液褐变正相关，但与形成肉香味物质之间不是一直呈正相关。因此，采取控制反应原料中其它氨基酸与半胱氨酸在较为合适的配比，使体系中转化形成的Cys-Amadori和其它氨基酸-Amadori近于1:1比例，既可满足较高反应速率又可最大化形成肉香味物质，是较佳的反应途径。当“半胱氨酸-木糖”体系中存在脂质时，脂质与木糖的竞争对初期美拉德反应产生抑制作用，但在大量其它氨基酸如甘氨酸存在时，由于其他氨基酸（如甘氨酸）对反应有促进作用，脂质对初期美拉德反应途径的影响相对较弱。