Cysteine thermal degradation

Two patents (42-43) claim the contribution to meatlike flavors made by thiamine when it is present in the standard pyrolytic mixture. Arnold et al. (44) have reported on the volatile flavor compounds produced by the thermal degradation of thiamine alone. It is generally agreed that the presence of methionine, the other sulfur-containing amino acid in the flavor-developing mixture, produces negative and/or undesirable results. However, one patent (45) specifies methionine in a standard Maillard procedure, and no cysteine. 

Most of the original patents referring to meat flavors utilizing Maillard technology vere claimed by Unilever (48-52 56,57). More recent patents are involved with the production of meat-like flavors. While a majority of patents are concerned vith cysteine, cystine, or methionine as the sulfur source, others claim alternatives such as mercaptoacetaldehyde, mercaptoalkamines, etc. Several patents (53,54), declare the contribution to meat-like flavors produced from thiamine in the Maillard reaction. Alternately, a technical report describes the volatile flavor compounds produced by the thermal degradation of thiamine alone (55). 

Zhang, Y., Chien, M., Ho, C. T. (1988). Comparaison of the volatile compounds obtained from thermal degradation of cysteine and glutathione. J. Agric. Food Chem., 36, 992-996. 

The exact origin of thiazoles remains a mystery. They might form through the thermal degradation of cystine or cysteine (25, 26), or by the interaction of sulfur-containing amino acids and carbonyl compounds (27, 28). Thiazoles have been identified as volatile components of thermally degraded thiamine (29). 

Heyns et al. (1966a) identified it in the products formed when heating glucose. 2,3-Pentanedione is the main volatile compound formed by thermal degradation of Furaneol (1.100) after ih at 160°C in water at pH 5.1, it represents nearly half of the volatiles (GC). It is one of the aliphatic compounds identified in the thermal interaction of glucose and cysteine (Zhang and Ho, 1991). 

It was identified in the products of thermal degradation of glucose (Heyns et al., 1966a Walter and Fagerson, 1968), in the model reaction cysteine xylose (Ledl and Severin, 1973), and in model reactions of serine and threonine with sucrose (as well as in coffee) by Baltes and Bochmann... 

It is formed in the thermal degradation of glucose (Heyns et al., 1966a Fagerson, 1969). It has been found in a heated proline/glucose system by Brule et al. (1971), in a cysteine/glucose model system by Sheldon et al. (1986), and in threonine/serine/sucrose models by Baltes and Bochmann (1987a). 

Furoic acid is formed in the thermal degradation of glucose (Fagerson, 1969). It has been found in a heated cysteine/glucose model system (Sheldon et al., 1986). 

Volatile Compounds Cencraled from Thermal Degradation or Thermal Interactions of of Alkfenjyl Cysteine Sulfoxides... 

The investigation of characteristic flavors associated with cooked meats has been the subject of much research over the past four decades but, although compounds with "meaty" aromas had been synthesized, compounds with such characteristics were not found in cooked meats until recently (1). In the search for compounds with characteristic aromas it was found that furans and thiophenes with a thiol group in the 3-position possessed meat-like aromas (2). The corresponding disulfides formed by oxidation of furan and thiophene thiols were also found to have meat-like characteristics, and exceptionally low odor threshold values (3). A number of such compounds are formed in heated model systems containing hydrogen sulfide or cysteine and pentoses or other sources of carbonyl compounds (4,5), The thermal degradation of thiamine also produces 2-methyl-3-fiiranthiol and a number of sulfides and disulfides (6J). 

Allyl alcohol was the predominant volatile compound found in the thermal degraded solution of alliin. 2-Formylthiophene, 3-formylthiophene, acetaldehyde, and 4-ethyl-6-methyl-l,2,3,5-tetrathiane were the other major volatile compoimds derived from the thermal degradation of alliin (77). The formation of allyl alcohol from alliin could be explained by the [2,3]-sigmatropic rearrangement of alliin followed by the reduction process (II-I4). In the proposed mechanisms, cysteine was also generated. Further decomposition of cysteine will lead to the formation of many compounds such as acetaldehyde, 2-formylthiophene, 3-formylthiophene, 3,5-dimethyl-1,2,4-trithiolane and 4-ethyl-6-methyl-l,2,3,5-tetrathiane identified (77-74). The presence of IMP may also contribute to the formation of 2-formylthiophene and 3-formylthiophene. 

The thermal generation of flavor is a very essential process for the "taste" of many different foodstuffs, e.g. cocoa, coffee, bread, meat. The resulting aromas are formed through non-enzymatic reactions mainly with carbohydrates, lipids, amino acids (proteins), and vitamins under the influence of heat. Thiamin (vitamin B ) and the amino acids, cysteine and methionine, belong to those food constituents which act as flavor precursors in thermal reactions. The role of thiamin as a potent flavor precursor is related to its chemical structure which consists of a thiazole as well as a pyrimidine moiety. The thermal degradation of this heterocyclic constituent leads to very reactive intermediates which are able to react directly to highly odoriferous flavor compounds or with degradation products of amino acids or carbohydrates. 

The thermal interaction between 2,4-decadienal and cysteine was selected as a model for lipid-protein interaction. 2,4-Decadienal is the major degradation product of linoleic acid and cysteine is a sulfur-containing amino acid in foods. Some heterocyclic compounds identified in the reaction mixture of 2,4-decadienal and cysteine are listed in Table I. 

Sugar degradation products were determined as benzimidazole derivatives after reaction with o-phenylenediamine. More than 120 amino-acid specific Maillard products have been isolated and identified from the reaction of L-proline, hydixn roline, < teine and methionine with monosaccharides at 150° for 1-1.5 h, in connection with studies of thermally generated aromas. Proline derived components were important constituents of bread, malt and beer, and cysteine and methionine derived components were predominant in roasted coffee and meat flavours. The effects of temperature, pH, and the relative concentration of rhamnose and proline on the quantity of specific volatiles produced in the Maillard reaction of these substrates have been studied, and the data have been analysed 1 computer methodology. The glucosylated cyclopentenone (44) was one of the products of decomposition of the Amadori product 1-deoxy-l-piperidino-maltulose in warm water. ... 

The side chains of proteins can undergo post-translational modification in the course of thermal processes. The reaction can also result in the formation of protein cross-links. A known reaction which mainly proceeds in the absence of carbohydrates, for example, is the formation of dehydroalanine from serine, cysteine or serine phosphate by the elimination of H2O, H2S or phosphate. The dehydroalanine can then lead to protein cross-links with the nucleophilic side chains of lysine or cysteine (cf. 1.4.4.11). In the presence of carbohydrates or their degradation products, especially the side chains of lysine and arginine are subject to modification, which is accompanied by a reduction in the nutritional value of the proteins. The structures of important lysine modifications are summarized in Formula 4.95. The best known compounds are the Amadori product -fructoselysine and furosine, which can be formed from the former compound via the intermediate 4-deoxyosone (Formula 4.96). To detect of the extent of heat treatment, e. g., in the case of heat treated milk products, furosine is released by acid hydrolysis of the proteins and quantitatively determined by amino acid analysis. In this process, all the intermediates which lead to furosine are degraded and an unknown portion of already existing furosine is destroyed. Therefore, the hydrolysis must occur under standardized conditions or preferably by using enzymes. Examples showing the concentrations of furosine in food are presented in Table 4.13. 

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