Methanethiol cheeses

Skunks excrete 1-butanethiol and 2-methyl-1-butanethiol [1878-18-8] as a natural defense mechanism (12). Methanethiol is found in cheese, milk, coffee, and oysters (13—16). It is also found in the kuttin fmit, which is endemic to Southeast Asia. 

In soft cheeses, such as Brie, Camembert, and Limburger, the following sulfur compounds were implicated 3-(methylthio)propanol, MT, DMS, DMDS, DMTS, dimethyl tetrasulfide, methyl ethyl disulfide, diethyl disulfide, 2,4-dithiapentane, 3-methylthio-2,4-dithiapentane, methional, 2,4,5-trithiahex-ane, 1,1-fe-methylmercaptodisulfide, methyl thioacetate (=methanethiol acetate), benzothiazole, methylthiobenzothiazole, methyl ethyl sulfonate, methyl methane thiosulfonate, thiophene 2-aldehyde, and H2S.34 Many of these were only present in small amounts Limburger cheese was notable for 13.2% of DMDS, 0.5% of methyl thioacetate, and 0.8% of DMTS. 

Owing to very low thresholds, volatile sulfur compounds (VSCs) usually have prime impact on food aromas they are found in lots of natural sources, including fermented foods (e.g. wine, beer, cheese), and act as both flavours and off-flavours [249, 250]. Although their biogenetic formation has been elucidated in detail, only few biotechnological processes with potential for commercial application have been reported. The sulfur-containing amino acids L-methionine and L-cysteine are the natural precursors of a wide variety of VSCs. Methanethiol is the most frequently found VSC in cheese and can be readily oxidised to other VSCs, such as dimethyl suMde and dimethyl disulfide, or... 

Many cheeses contain the same or similar compounds but at different concentrations and proportions chromatograms of some cheese varieties are shown in Figure 10.25. The principal classes of components present are aldehydes, ketones, acids, amines, lactones, esters, hydrocarbons and sulphur compounds the latter, e.g. H2S, methanethiol (CH3SH), dimethyl sulphide (H3C-S-CH3) and dimethyl disulphide (H3C-S-S-CH3), are considered to be particularly important in Cheddar cheese. The biogenesis of flavour compounds has been reviewed by Fox et al. (1993, 1996a) and Fox, Singh and McSweeney (1995). 

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... 

Arora et al., 1995 Engels et al., 1997). Thioesters are formed by the reaction of an FFA with a sulphydryl compound, particularly methanethiol (CH3SH Molimard and Spinnler, 1996 Collins et al., 2003a, 2004). Methylthioesters of short-chain FFAs have been associated with the characteristic aromas of Cheddar and smear-ripened cheeses (Arora et al., 1995 Lamberet et al., 1997). 

Aroma models were prepared for gruyere by applying the methods reported for emmental [57, 58], The aroma of the models was similar to that of the original cheeses. Consequently, it was concluded that 2-/3-methylbutanal, methional, dimeth-yltrisulphide, phenylacetaldehyde, 2-ethyl-3,5-dimethylpyrazine, methanethiol, as well as acetic, propionic, butyric, 3-methylbutyric and phenylacetic acids are the key odorants of gruyere. 

Among the odorants listed in Table 6.33, l-octen-3-ol (no. 4), the character impact aroma compound of mushrooms [62], is also responsible for the characteristic mush-room-like note of camembert, which is intensified by l-octen-3-one (no. 5). Although the concentration of this ketone is much lower than that of the alcohol, it can be aroma-active in cheese because its odour threshold is 100 times lower than that of the alcohol [60], Methanethiol, methional, dimethylsulphide, dimethyl trisulphide and methylene-bis(methylsulphide) generate the sulphurous odour note, whereas phenyle-thyl acetate is responsible for the floral odour note [61 ]. 

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. 

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. 

Butterfat-Encapsulated P. putida Methioninase for Methanethiol Production in Cheddar Cheese... 

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). 

While the relationship of headspace concentrations of methanethiol and other volatiles to their actual concentrations in cheese remains to be established, headspace concentrations of volatile sulfur compounds appear appropriate to assess relative effects of various treatments (64). A baseline of information on methanethiol concentrations found in cheeses manufactured in the University of Wisconsin Dairy Plant was initially developed to serve as a magnitude guide for the current trials as well as for comparative purposes with those published for Cheddar cheese by others (37, 40, 48). Concentrations of methanethiol are shown in... 

Table I. Calculated concentrations of methanethiol in the headspace of selected Cheddar-type cheeses.

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. 

Concentrations of methanethiol measured in headspace samples of the experimental cheeses are summarized in Table II for the analysis times of 1 day, 21 days and 4 months for each of the two ripening conditions employed. Notably, the cheese made with only encapsulated buffer did not contain methanethiol after 1 day at either temperature. However, the encapsulated methioninase system yielded significant amounts of methanethiol at 1 day, and continued to increase through 4 months. Generally, the final concentration of methanethiol in the encapsulated-buffer control... 

However, encapsulation provided more rapid and extensive generation of methanethiol than was observed for the unencapsulated system. Thus, the advantages of encapsulation which include close proximity of enzyme and substrate, stability of microenvironments within capsules, and better retention of enzyme in cheese appear to be borne out by the current experiments. 

In the current study, special attention was given to confirming the presence of carbon disulfide in the headspace of Cheddar cheese, and conclusive mass spectral evidence was developed for this compound. The concentration of carbon disulfide varied widely in the samples analyzed for the methanethiol studies, and became quite abundant in the most-aged sample of cheese that contained the encapsulated methioninase system (Figure 6). This sample also contained a lower molecular weight sulfur compound which remains unidentified, but it was observed only in the cheese prepared with encapsulated methioninase. 

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. 

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). 

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],... 

Methanethiol is the characteristic odor of oysters, Cheddar cheese, onions, and garlic. Garlic also contains 2-propene-l-thiol. The odor of onions is due to 1-propanethiol, which is also a lachrymator, a substance that makes eyes tear. 

Sulfur compounds are generated by sulfur amino acid catabolism and are potent odorants that contribute flavor to many fermented foods. Methionine catabolism produces various volatile sulfur compounds (VSCs) such as H S, methanethiol, dimethyl sulfide (DMS), dimethyl disulfide (DMDS), and dimethyl trisulfide (DMTS) (Fernandez et al. 2000). The enzymes in LAB strains from raw goats milk cheeses crucial for VSC formation from L-methionine have very diverse enzyme capabilities. 

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