Thiamine degradation

A similar cleavage is catalyzed by thiamin-degrading enzymes known as thiaminases which are found in a number of bacteria, marine organisms, and plants. 

For the first time the identication of 2-methyl-3-furanthiol acetate, furfuryl-methylsulfide, 2-formyl-thiophene and 4-methyl-5-vinyl-thiazol confirms the assumption that products from thiamine degradation and yeast lees are present in wines at trace level. 

The basic reactions contributing to thermal flavour generation are the Maillard reaction, Strecker degradation, lipid oxidation and thiamin degradation. Interaction of products formed by these different mechanisms additionally leads to further flavour components. 

Thiamine degradation has a good share in meat aroma formation [17,64]. Neutral and acidic conditions favour the formation of 13 [65], which is a key component in boiled meat [66, 67[. It has already earlier been identified in a meat-like process flavouring [68[, prepared from cysteine, thiamine, hydrolysed vegetable protein and water [69]. Bolton and co-workers [70] showed that in model experiments with thiamine, [ S]-cysteine, glucose and xylose, only 8% of 13 contained sulphur from cysteine. They concluded that thiamine (43) was the primary precursor for the generation of 13 in this system. 

Under basic conditions thiamine degrades to 5-(2-hydroxyethyl)-4-methylthiazole (45, sulfurol) and the pyrimidine derivative 46 [71]. Sulfurol is used in compounded flavourings in the flavour industry. It is almost odourless as such [73] however, it can decompose giving rise to thiazole and its derivatives such as 4-methylthiazole (47), 4,5-dimethylthiazole (48), 4-methyl-5-ethylthiazole (49) and 4-methyl-5-vinylthia-zole (50) ]71, 74], which possess nutty, green notes [75]. 

Fig. 3.30 Thiamine degradation formation of aroma compounds (adapted from [71])...

Besides the Maillard reaction, fat oxidation and thiamine degradation, more aroma compounds in process flavours are formed by the interaction of these different reactions, as well as from other precursors (e.g. phenol compounds, terpenes) present in the raw materials used. Three examples are given below but are far from being exhaustive. 

Figure 13 Catabolic pathway of thiamin (111) (vitamin B ). (a) Thiaminase catalyzed degradation of thiamin (111) (b) salvage of base-degraded thiamin (118) and (c) thiamin degradation by oxidation of thiazole side chain of thiamin (111).

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). 

Figure 2 Formation of selected thiamine degradation products identified in YE aroma (27, 29-33)...

Several of the entries of Table IV may form on thiamine degradation as well as resulting from amino-carbonyl interactions and are indicated by ( ). The relative importance of these two reaction pathways for the production of potent sulfur-containing aroma components has been discussed recently (29) and represents a fertile area for further research. 

There are only a few studies concerning the degradation and elimination of thiamin in animals. Most excess thiamin is eliminated as such by the kidneys. In a couple of older studies (Neal and Pearson 1964 Pearson et al. 1966), radioactive thiamin was administered in rats and the urines were analysed. Several radioactive degradation products of thiamin (2-methyl-4-amino-5-pyrimidine carboxylic acid and 4-methyl-thiazole-5-acetic acid), resulting from the cleavage between the thiazole and the pyrimidine moieties, were excreted in the urine. Other products were also detected but not identified. No enzymes specifically involved in thiamin degradation in mammals have been identified. 

Fig. 2.34. Lines of equal microbiological and chemical effects for heat-treated milk (lines BIO, Bl, and BO.l correspond to a reduction in thermophilic spores by 90, 9, and 1 power of ten compared to the initial load lines CIO, Cl, and CO.l correspond to a thiamine degradation of 30%, 3%, and 0.3% according to Kessler, 1988)...

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