Thiazoles Maillard reactions

Lactones are minor constituents in pork fat compared to beef fat, but 5-decalactone was highly concentrated in the FI fraction of the 207 bar/50 C extract The furans, furanones and thiazoles were undoubtedly formed from Maillard reaction precursors and this type of volatQe would be more prevalent in the lipid fraction of cooked pork compared to pork fat alone. 

Many desirable meat flavor volatiles are synthesized by heating water-soluble precursors such as amino acids and carbohydrates. These latter constituents interact to form intermediates which are converted to meat flavor compounds by oxidation, decarboxylation, condensation and cyclization. 0-, N-, and S-heterocyclics including furans, furanones, pyrazines, thiophenes, thiazoles, thiazolines and cyclic polysulfides contribute significantly to the overall desirable aroma impression of meat. The Maillard reaction, including formation of Strecker aldehydes, hydrogen sulfide and ammonia, is important in the mechanism of formation of these compounds. 

The Maillard reaction has received much attention since the 1950 s as the source of flavor chemicals in cooked foods. Numerous compounds produced by this reaction have been reported in the last two decades. The major flavor chemicals are nitrogen- and sulfur-containing heterocyclic compounds. For example, nitrogen-containing pyrazines contribute a characteristic roasted or toasted flavor to cooked foods. Sulfur-containing thiophenes and thiazoles give a characteristic cooked meat flavor. A striking property of these compounds is their extremely low odor thresholds. 

Some pyrrole derivatives have pleasant flavor. For example, pyrrole-2-carboxaldehyde gives a sweet and corn-like odor and 2-acetylpyrrole has caramel-like flavor. However, some pyrroles have been found to contribute to off-flavor of food products (24). Pyrroles have not received as much attention as flavor components as other heterocyclic Maillard reaction products such as pyrazines and thiazoles even though the number of derivatives identified is almost the same as that of pyrazines (Figure 1). Proposed formation mechanisms of pyrroles in the Maillard reaction systems are similar to those of thiophenes (Figures 2). 

The Maillard reaction plays an important role in flavor development, especially in meat and savory flavor (Buckholz, 1988). Products of the Maillard reaction are aldehydes, acids, sulfur compounds (e.g., hydrogen sulfide and methanethiol), nitrogen compounds (e.g., ammonia and amines), and heterocyclic compounds such as furans, pyrazines, pyrroles, pyridines, imidazoles, oxazoles, thiazoles, thiophenes, di- and trithiolanes, di- and trithianes, and furanthiols (Martins et al., 2001). Higher temperature results in production of more heterocyclic compounds, among which many have a roasty, toasty, or caramel-like aroma. 

The accumulation of thiazoles during the heating of a cysteine Dimethylhydroxy-furanone (DMHF) model system decreased with increasing water content while the accumulation of thiophenones and 3,5-dimethyl-l,2,4-trithiolane had an optimum moisture content (ca. 80%) and 2,4-hexanedione and 3-hydroxy-2-pentanone increased with moisture content [23.24], There is little question that the Aw (or moisture content) influences both the rate of the Maillard reaction and influences the sensory properties of a system on heating. 

Numerous different types of heterocyclic compounds containing sulfur are produced via the Maillard reaction [61,62], These include thiophenes, dithioles, dithianes, dithiins, trithiolanes, trithanes, tetrathianes, thiazoles, thiazolines, and thiazolidines. The major heterocyclic sulfur-containing compounds produced via the Maillard reaction are thiazoles and thiophenes (Figure 5.6d and Figure 5.6e, respectively). 

These compounds, like the thiazoles and thiophenes, typically are formed via the reaction of sulfur-containing amino acids with intermediates of the Maillard reaction. An alternative mechanism involves the initial formation of H2S from the sulfur-containing amino acids and then the reaction of the H2S with browning intermediates [64]. 

Sulfur Compounds of Beef Flavor. Methional, which results from the degradation of methionine, is an important contributor to flavor in meat. Thiolanes, formed during the cooking of beef, have peculiar oniony flavors that also augment the quality of the meaty flavor. Thiophenes and thiofurans are also important to meaty flavors. Sulfides, such as methyl sulfide, are oxidized to methyl sulfoxide and methyl sulfone. Condensation reactions of Maillard browning products also result in thiazoles such as benzothiazole, an important component of meat flavor. 

Thiazoles with long alkyl chains in the 2-position provide evidence of the interaction of lipid oxidation products with Maillard intermediates. The presence of such thiazoles has been reported for fried chicken, roast beef, and fried potatoes (see Mottram152). Thiophens with long alkyl chains in the 2-position have also been encountered. They were obtained when phospholipid was added to a cysteine-ribose system.153 Lipid degradation to the 2,4-dienal, followed by reaction with H2S, was thought to be responsible.154... 

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