Methanethiol flavor

Cheese is ripened for 6 months to 1 year or longer at 5° to 15°C and 70-75% relative humidity. Cheese ripening is a complex process involving a combination of chemical, biochemical, and physical reactions. Proteolytic enzymes, e.g., rennet and lactic starter culture enzymes, hydrolyze caseins to produce flavor compounds and proper body. Lipase and lactase enzymes also hydrolyze their respective substrates to produce a large number of characteristic flavor compounds (Reiter and Sharpe 1971 Harper 1959 Law 1981 Schmidt etal. 1976), including free fatty acids, methanethiol, methanol, dimethyl sulfide, diacetyl, acetone, and others (Moskowitz 1980). 

Free amino acids are further catabolized into several volatile flavor compounds. However, the pathways involved are not fully known. A detailed summary of the various studies on the role of the catabolism of amino acids in cheese flavor development was published by Curtin and McSweeney (2004). Two major pathways have been suggested (1) aminotransferase or lyase activity and (2) deamination or decarboxylation. Aminotransferase activity results in the formation of a-ketoacids and glutamic acid. The a-ketoacids are further degraded to flavor compounds such as hydroxy acids, aldehydes, and carboxylic acids. a-Ketoacids from methionine, branched-chain amino acids (leucine, isoleucine, and valine), or aromatic amino acids (phenylalanine, tyrosine, and tryptophan) serve as the precursors to volatile flavor compounds (Yvon and Rijnen, 2001). Volatile sulfur compounds are primarily formed from methionine. Methanethiol, which at low concentrations, contributes to the characteristic flavor of Cheddar cheese, is formed from the catabolism of methionine (Curtin and McSweeney, 2004 Weimer et al., 1999). Furthermore, bacterial lyases also metabolize methionine to a-ketobutyrate, methanethiol, and ammonia (Tanaka et al., 1985). On catabolism by aminotransferase, aromatic amino acids yield volatile flavor compounds such as benzalde-hyde, phenylacetate, phenylethanol, phenyllactate, etc. Deamination reactions also result in a-ketoacids and ammonia, which add to the flavor of... 

This important flavor compound was identified in the head-space volatiles of beef broth by Brinkman, et al. (43) and although it has the odor of fresh onions, it is believed to contribute to the flavor of meat. This compound can be formed quite easily from Strecker degradation products. Schutte and Koenders (49) concluded that the most probable precursors for its formation were etha-nal, methanethiol and hydrogen sulfide. As shown in Figure 5, these immediate precursors are generated from alanine, methionine and cysteine in the presence of a Strecker degradation dicarbonyl compound such as pyruvaldehyde. These same precursors could also interact under similar conditions to give dimethyl disulfide and 3,5-dimethyl-l,2,4-trithiolane previously discussed. 

Flavor of Products of the Addition Between Unsaturated Carbonyls and Methanethiol... 

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. 

Enzymic Generation of Methanethiol To Assist in the Flavor Development of Cheddar Cheese and Other Foods... 

Quantitative studies on the enzymatic generation of methanethiol from methionine showed that methioninase obtained from Pseudomonas putida could be used for the development of flavors. Reactions carried out under anaerobic conditions yielded only methanethiol while aerobic conditions favored conversion of substantial amounts of methanethiol to dimethyl disulfide. Incorporation of free or fat-encapsulated methionine/methioninase systems into Cheddar cheeses resulted in the formation of volatile sulfur compounds, including carbon disulfide, and accelerated rates of development of aged Cheddar-like flavors. Methanethiol, when present alone, was observed not to cause the true, Cheddar-like flavor note in experimental cheeses. 

Methanethiol has been implicated as an influential aroma and flavor compound in a variety of foods (1, 2), but its occurrence is frequently associated with overall aromas that are distinctly putrid, fecal-like, and sulfurous. Such putrid-type aromas are found, for example, in spoiling meats, poultry and fish (3, j>,... 

Although methanethiol is most noted for its role in distinctive sulfurous aromas and flavors, other more subtle involvements of methanethiol in flavors occur which deserve attention in developing concepts for controlled enzymic generation of flavors. The flavors and aromas of some fruits and fruit... 

Aside from the distinctively-flavored, washed, surface-ripened cheeses mentioned earlier ( 9, 1, 30), methanethiol has been recognized as a contributor also to the flavor of mature mold surface-ripened cheeses, including Camembert and Brie (31, 32,... 

In these cheeses Brevlbacterlum linens or related coryneforms ([7, 34 35) are responsible for the formation of methanethiol. Perhaps the most significant but least understood occurrence of methanethiol in cheeses is that of Cheddar cheese where it appears to be associated with the development of distinctive, true Cheddar-type flavors. 

Methanethiol is a very volatile (b.p. 6.2°C) compound possessing a intensely putrid, fecal-like aroma even at low concentrations. The detection threshold value for methanethiol has been reported as 0.02 ppb in air (49, 50), and it readily undergoes oxidative condensation with itself in the presence of oxygen to yield dimethyl disulfide (51) which also exhibits pronounced aroma properties (12 ppb detection threshold in air 49, 50). Besides the difficulties in handling and encapsulating methanethiol for flavor applications, its propensity to adsorb to surfaces and react with other organics makes the use of this compound in flavor concentrates very troublesome indeed. 

However, if protease activity were involved, such preparations of methloninase from putIda could provide self-destructing enzyme systems for encapsulated, flavor-producing systems (65) that would prevent over-production of methanethiol and hydrogen sulfide. Alternatively, it might be possible to use other sources of methloninase, such as Brevlbacterlum linens (7 35) or Aspergillus oryzae (56, 57), as systems with potentially less protease activity. In any event further studies will be required to determine if alpha-ketobutyrlc acid assays provide suitable data for indexing methloninase activity In flavor systems. 

Methanethiol has been found to be correlated with the development of Cheddar cheese flavor by Manning and coworkers (37, 39, 40), and both nonenzymic (22) and enzymic generation of methanethiol have been proposed as the source of this compound in Cheddar cheese (46). Although the correlation of methanethiol to Cheddar flavor appears statistically valid, difficulties have been encountered in explaining the nature of its flavor-conferring properties in cheese. In addition, uniform production of methanethiol is difficult to achieve commercially, and the rate of its natural formation in accelerated-ripening may not be suitable for achieving typical Cheddar flavors (47, 48). 

Table I, and reflect the Cheddar flavor intensity correlation reported by Manning and coworkers (37). In addition to methanethiol, peaks for dimethyl disulfide were quite large in the aged Cheddar and the Colby cheese headspace profiles, but very little of this compound was found in the mild Cheddar sample.

Based on the concentration of methanethiol in the aged sample, conditions were established for the methioninase system which would approximate what might be encountered naturally, i.e., about 2 orders of magnitude greater than the target figure that was defined as the aged, full-flavored Cheddar cheese. 

Of these, Manning (40) has reported that pentan-2-one, acetone, methanol along with methanethiol correlate best with Cheddar flavor intensity and Cheddar quality. 

In conclusion, these investigations have shown that methanethiol generation by methioninase has potential applications in the development of cheese flavors as well as other for other foods. The use of fat encapsulated enzyme systems functioned well in experimental cheeses, and their use should provide assistance in controlled delivery of methanethiol into food systems during further efforts to elucidate the complex nature of Cheddar cheese flavor. 

Methyl-2-furyl)methanethiol has a burnt, onion, metallic flavor (Chemisis, 1961). It is a little less potent odorant than furanmethanethiol, detectable at a level of 0.050 ppb in water, developing a meaty flavor between 0.5 and 1 ppb and exhibiting a sulfury-mercaptan note at higher concentrations (Tressl and Silwar, 1981). 

In addition to the products of lipid oxidation, methanethiol and dimethyl trisulfide were shown to contribute to the complex odor characteristic of soy protein products such as SPI and soy protein concentrates (Boatright Lei, 2000 Lei Boatright, 2001) and soymilk (Lozano et al., 2007) at concentrations comparable to hexanal. Since the threshold in water for methanethiol was reported at 0.02 ppb compared to hexanal at 4.5 ppb (MacLeod C Ames, 1988), these sulfur compounds are intense flavor notes in soy protein products. Lei and Boatright (2007) provided evidence that methanethiol is generated in aqueous slurries of SPI or defatted soy flake from methionine by a free radical mechanism involving manganese, sulfite, and... 

Volatile sulfur compounds are found in most cheeses and can be important flavor constituents. The origin of sulfur-containing compounds is generally thought to be the sulfur-containing amino acids methionine and cysteine (Law, 1987). As Cys is rare in the caseins (occurring at low levels only in Os2- and K-caseins, which are not extensively hydrolyzed in cheese), the origin of sulfur compounds must be primarily Met. Sulfur compounds formed from Met include H2S, dimethylsulfide, and methanethiol. The importance of methanethiol and related compounds in cheese aroma is discussed by Law (1987). 

These molecules include sulfur amino acids, which play an essential role in sunlight flavor . This phenomenon is directly linked to the appearance of methanethiol and dimethyldisulfide in wines exposed to light, which give them cooked cauliflower or wet wool smells. 

Besides charcoal, ancient enology treatises mention other products likely to eliminate unpleasant smells and off-flavors toasted barley or wheat, mustard flour, oil, milk, etc. All these have practically disappeared from use. Fresh yeast lees are permitted in treating wine and are effective in eliminating a number of olfactory defects. This has already been mentioned in connection with fixing certain thiols, such as methanethiol (Section 8.6.2). This treatment is also recommended for adsorbing chloroanisoles in moldy wines (Section 8.5.2). 

Traditionally fermented dairy products have been used as beverages, meal components, and ingredients for many new products [60], The formation of flavor in fermented dairy products is a result of reactions of milk components lactose, fat, and casein. Particularly, the enzymatic degradation of proteins leads to the formation of key-flavor components that contribute to the sensory perception of the products [55], Methyl ketones are responsible for the fruity, musty, and blue cheese flavors of cheese and other dairy products. Aromatic amino acids, branched-chain amino acids, and methionine are the most relevant substrates for cheese flavor development [55]. Volatile sulfur compounds derived from methionine, such as methanethiol, dimethylsulflde, and dimethyltrisul-fide, are regarded as essential components in many cheese varieties [61], Conversion of tryptophan or phenylalanine can also lead to benzaldehyde formation. This compound, which is found in various hard- and soft-type cheeses, contributes positively to the overall flavor [57,62]. The conversion of caseins is undoubtedly the most important biochemical pathway for flavor formation in several cheese types [62,63]. A good balance between proteolysis and peptidolysis prevents the formation of bitterness in cheese [64,65],... 

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