Volatile sulfur compounds origin

Information about the biochemical precursors of volatile sulfur compounds has been obtained from artificial culture stiidies, reviewed by Kadota and Ishida (1972), and from incubation studies of natural and amended soils, reviewed by Bremner and Steele (1978). Table 10-5 summarizes our knowledge about the origin of sulfides, which have been observed to emanate from soils as degradation products under aerobic as well as anaerobic conditions. As indicated above, H2S is formed not only by the degradation of organic compounds, but also by anaerobic reduction of sulfate. 

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). 

The presence of snlfnr derivatives in mnst and wine is always a matter of concern for winemakers, who are fully aware of the risk that they may cause unpleasant smells. Mnch research is in progress on this topic as these phenomena are not clearly understood. Indeed, nntil certain modem analysis techniques were recently perfected, it was impossible to assay these snbstances as they are only present in trace amonnts. Another reason for concern about these volatile sulfur compounds is the wide range of possible origins and the diversity of preventive actions. 

Volatile sulfur compounds formed in wine and beer production originate from methionine and are by-products of the microorganism s metabolism. The compounds formed are methional (I), methionol (II) and acetic acid-3-(methylthio)-propyl ester (III, cf. Reaction 5.33). 

Charentais cantaloupe melon Cucumismelo L. var. cantalupensis Naud.) was characterised by abundant sweetness and a good aromatic flavour [68]. The aroma volatiles of Charentais-type cantaloupe melons, as with other cantaloupes, comprise a complex mixture of compounds including esters, saturated and unsaturated aldehydes and alcohols, as well as sulfur compounds [26, 65]. Among these compounds, volatile esters were quantitatively the most important and therefore represent key contributors to the aroma [68]. The linear saturated and unsaturated aldehydes seem to originate from the degradation of linolenic and linoleic acids [26, 32, 33, 67]. 

In meats, of course, there are components which arise from the protein which cannot be present in the products from pure fat. Table III shows some of the sulfur compounds and aromatic compounds which are also found in irradiated meats. Many of these can be postulated as arising from direct bond cleavage of amino acid moieties. Benzene and toluene may come from phenylalanine and phenol and p-cresol from tyrosine. Recent studies have been directed to considering the origin of some of the compounds from proteinaceous substances. Some of the sulfides, disulfides, and mercaptans can derive directly from cysteine or methionine, but those containing more than two carbon atoms in a chain require more than a superficial explanation. In order to evaluate the contribution of the volatiles from the protein as well as the lipid constituents of meat, volatile components produced in various protein substances have also been analyzed. 

The major components identified among the volatiles produced in haddock upon irradiation are benzene and toluene and the sulfur compounds. These compounds may be expected from the radiation-induced degradation of protein. The only carbonyl compounds found are acetone and methyl ethyl ketone, and these are present in only moderate amount. Trace quantities of low molecular weight hydrocarbons were also found. The detection of hydrocarbons, even in trace amounts, led us to question whether their origin was in the protein or in the small amount of fat present in the haddock. 

It thus seemed that the origin of the various components in meat volatiles could best be established by analyzing irradiation-induced compounds in meat protein and meat fat separately. Accordingly, a 500-gram sample of meat, the same size of sample normally used in irradiation studies of whole meat, was separated into a protein, a lipid, and a lipoprotein fraction by means of a methanol-chloroform extraction of the fat. The dry, air-free, fractions were then irradiated separately with 6 megarads of gamma radiation in the manner used for whole meat. The analytical results (Table V) show clearly that mainly sulfur compounds and aromatic hydrocarbons are formed in the protein fraction, whereas mainly aliphatic hydrocarbons are formed from the lipid. The lipoprotein fraction produced, as expected, both aliphatic hydrocarbons and sulfur compounds. Only the lipoprotein fraction had a characteristic irradiation odor. 

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). 

Additional minor volatile compounds are found in both frying oils and in fried foods, including cis,trans- and trans,trans-2,4-decaderived from oxidized linoleate and 2,4-heptadienal derived from linolenate. The isomers of 2,4-decadienal impart a desirable fried food flavor in fried potatoes when present in small amounts, but excessive amounts of this aldehyde would be expected to cause undesirable rancid flavors. Furfural compounds may be derived from interactions between food sugars and proteins. Minor amounts of sulfur compounds and nitrogen-containing heterocyclic compounds (methyl pyrazine and 2,5-dimethylpyrazine in potatoes) may originate from Maillard reactions (Chapter 11). 

The gas phase consists of hydrocarbons vaporized by distillation processes and/or formed by cracking or other decomposition of fluids. Sulfur compounds, such as hydrogen sulfide and volatile mercaptans, often present in the gas phase, may be components of the original feed to the unit of interest, e.g. the crude still they may be formed by thermal degradation of disulfides, thiophenes, etc., or they may be the result of various hydrogenation processes such as hydrodesulfurizing, hydrocracking, etc. 

The exact origin of thiazoles remains a mystery. They might form through the thermal degradation of cystine or cysteine (25, 26), or by the interaction of sulfur-containing amino acids and carbonyl compounds (27, 28). Thiazoles have been identified as volatile components of thermally degraded thiamine (29). 

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