Aliphatic sulfur compounds

FIGURE 11.9 Elimination reactions during metabolism of aliphatic sulfur compounds (a) cysteine, (b) methionine, and (c) 2-dimethylsulfoniopropionate. 

Other Compounds. Other types of compounds formed under the proper conditions include, but are not limited to, N-alkylidenealkenylamines (35,37) and various aliphatic sulfur compounds (e.g., sulfides, disulfides, etc.) (33,35). 

Aliphatic sulfur compounds, in particular disulfides such as methyl propyl disulfide, dipropyl disulfide, and especially cis- and /ran -propenyl propenyl disulfide are mainly responsible for the typical odor of onion oil [235]. The presence of 2-hexyl-5-methyl-3(2H)-furanone [33922-66-6] is a characteristic of authenticity [235a]. 

The root of parsnip Pastinaca sativa) is eaten boiled or baked. The major classes of compounds identified in raw and cooked parsnip are monoterpenoids, aliphatic sulfur compounds, and 3-alkyl-2-methoxypyrazines [35]. To the best of our knowledge, no investigations have been performed to elucidate the character-impact compounds in parsnip by modern GC-O techniques however, it has been suggested that volatile compounds such as terpinolene, myristicin and 3-sec-butyl-2-methoxypyrazine maybe important contributors to the flavour of parsnip owing to either their high concentrations or their low threshold values, or both [35]. 

Sugioka and Aomura (133) provide kinetic evidence indicating that the rate-determining step in the hydrocracking of aliphatic sulfur compounds over silica-alumina is catalyzed by Brdnsted acid sites. Conversions of reactants were measured by use of a pulse reactor hydrogen was used as the carrier gas. They found that reactivities of mercaptans are in the following order ... 

Alternatively, if reduction of sulfate in associated waters occurred, the active sulfur (H2S) would have preferentially reacted with the saturates producing thiols and sulfides. This mechanism, too, should result in an increased ratio of aliphatic sulfur compounds over aromatic in the biodegraded oil. 

Heterocyclic compounds are dominant among the aroma compounds produced in the Maillard reaction, and sulfur-containing heterocyclics have been shown to be particularly important in meat-like flavors. In a recent review, MacLeod (6) listed 78 compounds which have been reported in the literature as possessing meaty aromas seven are aliphatic sulfur compounds, the other 71 are heterocyclic of which 65 contain sulfur. The Strecker degradation of cysteine by dicarbonyls is an extremely important route for the formation of many heterocyclic sulfur compounds hydrogen sulfide and mercaptoacetaldehyde are formed by the decarboxylation and deamination of cysteine and provide reactive intermediates for interaction with other Maillard products. 

Food, flavors consist of numerous compounds, none of which alone is characteristic of specific food. Classes of compounds which emcompass food flavors are - hydrocarbons (aliphatic, ali-cyclic, aromatic) carbonyls (aldehydes, ketones) carboxylic acids, esters, imides, anhydrides alcohols, phenols, ethers alkylamines, alkylimines aliphatic sulfur compounds (thiols, mono-, di- and tri-sulfides) nitrogen heterocyclics (pyrroles, pyrazines, pyridines) sulfur heterocylics (thiophenes, thiazoles, trithiolane, thialidine) and oxygen-heterocyclics (lactone, pyrone, furan). Discussion will be limited to striking developments in heterocyclics. 

Yeast extracts represent an important source of volatile sulfur compounds, many of which possess low odor threshold values. They are used as sources of flavor for a range of savory foods, especially when a meaty note is required. In spite of the usefulness of yeast extracts, there are very few reports of their volatile flavor components. The production of yeast extracts is briefly reviewed, and the volatile sulfur compounds which have been identified are discussed. A recent study is presented in which the aroma components of some yeast extracts were analyzed. A total of 268 compounds were identified, including 67 sulfur compounds. The 34 sulfur compounds reported for the first time comprised 3 aliphatic sulfur compounds, one sulfur-substituted benzene derivative, 10 thiophenes, 18 thiazoles and 2 alicyclic sulfur compounds. Their importance as components of flavors and routes to their formation are considered. 

Acetaldehyde can undergo various interactions with butanedione, hydrogen sulfide, methanethiol and ammonia to give various aliphatic sulfur compounds, thiazoles, thiadiazines and dithiazines as shown in Fig. 1. Compounds identified in YEs are indicated by ( ) (29). The involvement of alternative aldehydes, such as formaldehyde in place of acetaldehyde or mixtures of aldehydes, will lead to the corresponding homologues, some of which have also been reported in YEs. Another aldehyde, 3-methylbutanal, reacts with hydrogen sulfide and methanethiol to give the hemidithioacetal, l-methylthio-3-methyl-l-butanethiol which has been reported in YEs (14). Compounds of this class possess characterisitic meaty aromas with an onion note (14). 

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). 

Significant aliphatic sulfur compounds are methional, 3-methyl-but-2-ene-1-thiol, 3-mercapto-3-methylbutan-l-ol (8-124), its ester 3-mercapto-3-methylbutyl formate, methanethiol and dimethyltrisulfide. 3-Mercapto-3-methyl-l-ol also occurs in passion fruit and blackcurrant, and as a putative cat pheromone in cat urine, where it is formed as a degradation product of amino acid L-felinine (see Section 2.2.1.2.2). Of more than 70 known pyrazines, the most important compounds in roasted coffee are isopropylpyrazine, 2-isobutyl-3-methoxypyrazine, 2-ethyl-3,5-dimethylpyrazine, 2,3-diethyl-5-methylpyrazine, 2,6-dimethyl-3-vinylpyrazine and 2-ethyl-6-methyl-3-vinylpyrazine. Pyridine and its alkyl derivatives and bicyclic pyridines have a negative impact on the quality of coffee aroma. Important aromatic... 

Furfural and other Furan Compounds. By F. N. Peters, Jr., and H. J. Brownlee. Aliphatic Sulfur Compounds. By E. Emmet Reid. 

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