Thermal degradation of thiamin

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). 

Among the volatile compounds listed in Table II, only thiazole compounds are derived from the thermal degradation of thiamin. 5-(2-hydroxyethyl)-4-methylthiazole and 4-methyl-5-vinylthiazoIe are well-known thermal degradation products of thiamin. 5-(2-Chloro-ethyl)-4-methylthiazole may form through the interaction of 5-(2-hydroxyethyl)-4-methylthiazole with hydrogen chloride. However, the most abundant product, 4-methylthiazole, has never been identified as a decomposition product of thiamin. The mechanism for its formation is not clear. 

Meat flavor is due to a great number of volatiles from different chemical classes. However, most of the odorants described as meaty aroma contain sulfur. The two most important reactions which generate meaty aroma compounds are the reactions between sulfur containing amino acids and reducing sugars (Maillard reaction) and the thermal degradation of thiamin [35], Sulfur-containing furans are the basic chemicals responsible for the aroma of thermally treated meat. 

The major routes which account for the formation of many of the compounds identified in YEs are (1) interactions involving carbonyl compounds, sulfur and ammonia (often via the degradation of sulfiir-containing amino acids, with or without the intervention of reducing sugars) and, (2) the thermal degradation of thiamine. 

Some of the compounds identified in YEs which are formed either by the thermal degradation of thiamine or on the interaction of thiamine degradation products with other components are shown in Fig. 2. They include aliphatic sulfur compounds, furans, thiophenes and thiazoles. 2-Methyl-3-furanthiol and 2-methyl-3-thiophenethiol have been identified in YEs 9,13 14) and are well known thermal degradation products of thiamine (29). As well as possessing meaty aromas and low odor threshold values 34), these compounds are key precursors of several other sulfur-substituted furans and thiophenes, including the derivatives in Fig. 2. Most possess meaty aromas at low concentrations and several have been identified in YEs (see Tables I and III). 

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). 

The thermal degradation of thiamin (vitamin B ) is a very complex reaction consisting of various degradation pathways. This results in the formation of many organoleptically interesting flavor compounds. Most of them contain one or more sulfur and/or nitrogen atoms, and many of them are heterocyclic structures. 

Thiamine stability in aqueous solution is relatively low. It is influenced by pH (Fig. 6.2), temperature (Table 6.8), ionic strength and metal ions. The enzyme-bound form is less stable than free thiamine (Fig. 6.2). Strong nucleophilic reagents, such as HS03 or OH , cause rapid decomposition by forming 5-(2-hydroxyethyl)-4-methylthiazole and 2-methyl-4-amino-5(methyl-sulfonic acid)-pyrimidine, or 2-methyl-4-amino-5-hydroxymethylpyrimidine (see Reactions 6.7). Thermal degradation of thiamine, which also initially yields the thiazole and pyrimidine... 

Jhoo J.-W., Lin M.-C., Sang S., Cheng X., Zhu N., Stark R.E., Ho C.-T. Characterization of 2-methyl-4-amino-5-(2-methyl-3-furylthiomethyl)pyrimidine from thermal degradation of thiamin. Journal of Agricultural and Food Chemistry, 50 4055-4058 (2002). 

Sulfur-containing heterocyclic compounds are associated with meaty characteristics. Two compounds with the most potent meaty impact include 2-methyl-3-furanthiol (1 ppt) and the corresponding dimer, -(2-methyl-3-furyl) disulflde (0.02 ppt) (18). Both substances have been identified in cooked beef and chicken broth and have a strong meaty quality upon dilution. The disulflde has a recognizable aroma character of rich aged-beef, prime-rib (15). Interestingly, both compounds are produced from the thermal degradation of thiamin (63). A related compound, 2-methyl-3-(methylthio)furan, is the character impact compound for roast beef (18). Other potent modifiers, such as 2-acetyl-2-thiazoline, impart a potent roasty, popcorn note, which enhances the meaty and roast flavor (64). 2-Ethyl-3,5-dimethylpyrazine and 2,3-diethyl-5-methylpyrazine also contribute potent roasty notes to roast beef flavor (65). A summary of character impact compounds for meat and seafood flavors is presented in Table 6. 

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