Sulfur-containing furans

Sulfur-Containing Furans in Arabica, Robusta, and Instant Coffees, 10 Days After Roasting19... 

Figure 7 shows sulfur containing furans and their range in roasted coffee. It can be seen that Robustas... 

Figure 7 Sulfur containing furans characterized under different roasting conditions (units in ppm)...

The concentrations of furfurylsulfides and -disulfides wich possess toasted, bread- and meat-like aromas do not change during storage of roasted coffee. Sulfur containing furans are formed by heating furanaldehydes with cysteine and methionine, respectively (24). 

Meat flavor is due to a great number of volatiles from different chemical classes. However, most of the odorants described as meaty aroma contain sulfur. The two most important reactions which generate meaty aroma compounds are the reactions between sulfur containing amino acids and reducing sugars (Maillard reaction) and the thermal degradation of thiamin [35], Sulfur-containing furans are the basic chemicals responsible for the aroma of thermally treated meat. 

In the work reported in this paper, meat has been used as the substrate in which to study the effect of pH on the formation of thiols and disulfides, using acid or alkali to adjust the pH. The importance of IMP concentration in meat on the formation of sulfur-containing furans during cooking was also examined in meat containing added IMP. 

Competitive metallation experiments with IV-methylpyrrole and thiophene and with IV-methylindole and benzo[6]thiophene indicate that the sulfur-containing heterocycles react more rapidly with H-butyllithium in ether. The comparative reactivity of thiophene and furan with butyllithium depends on the metallation conditions. In hexane, furan reacts more rapidly than thiophene but in ether, in the presence of tetramethylethylenediamine (TMEDA), the order of reactivity is reversed (77JCS(P1)887). Competitive metallation experiments have established that dibenzofuran is more easily lithiated than dibenzothiophene, which in turn is more easily lithiated than A-ethylcarbazole. These compounds lose the proton bound to carbon 4 in dibenzofuran and dibenzothiophene and the equivalent proton (bound to carbon 1) in the carbazole (64JOM(2)304). 

Sometimes, hydrogen sulfide converts an oxygen-containing heterocycle into a sulfur-containing one, and as furans and polycyclic furans are very common in nature such reactions may be the origin of the polycyclic thiophenes mentioned above. A typical example is the formation of the isothiazolin-3-thione (6) from the isoxazolin-3-thione (7) on treatment with hydrogen sulfide and hydrogen bromide.8... 

Five-membered oxygen- and sulfur-containing heterocyclic ketones reveal notable reactivity (03RCB961, 03RCB1380). Benzo[fc]furan-3-one 204 with arylidenemalononitriles 30 gives dibenzo[b,d]pyrans 206 instead of expected pyran 205 as a result of Michael reaction and the exchange of methylene components (03RCB961) (Scheme 77). 

The fruit of pumpkin (Cucurbita pepo) is eaten boiled or baked. About 30 compounds have been identified in the volatile extracts of raw pumpkin, with the major classes of compounds being aliphatic alcohols and carbonyl compounds, furan derivatives and sulfur-containing compounds. Hexanal, ( )-2-hexenal, (Z)-3-hexen-l-ol and 2,3-butanedione have been identified as important for the flavour of freshly cooked pumpkins (Table 7.7) [35] however, studies using GC-O techniques are needed to get a better understanding of the character-impact compounds of pumpkins. 

There are few new authenticated examples of [ 4 + 4] photoadditions in oxygen- and sulfur-containing heterocycles. 1-Cyanonaphthalene (362), for example, undergoes an addition of this type to furan on irradiation in benzene to give adduct 363 in high yield.309 Addition of 2-cyanonaphthalene... 

The investigation of a series of model meat systems has demonstrated the important role of volatile sulfur-containing heterocyclic components substituted with sulfur in the 3-position. One of these 3-substituted sulfur compounds, 2-methy1-3-methy1thio-furan was identified recently in the volatiles from cooked beef aroma (5J and from a heated yeast extract composition (6J and is considered a meaty character impact compound. 

Of particular interest is the identification of five different types of heterocyclic sulfur-containing flavor components, the preponderance of which were furans and thiophenes substituted with sulfur in the 2- and 3-position. The bulk of these flavor compounds had not been identified in meat and had not been reported in the literature so far. These sulfur-substituted heterocyclic flavor components will be further discussed below. 

These novel sulfur alkaloids and their properties are given in Table II. Structural studies have demonstrated for all the known Nuphar alkaloids shown in Tables I and II either the sesquiterpenoid or the triterpenoid structure incorporating the quinolizidine and furan systems, with the tetrahydrothiophcne system in the sulfur-containing alkaloids. 

Dianions derived from sulfur and oxygen containing polycyclic systems have not been reported until very recently. The weak carbon-sulfur bond could discourage attempts to prepare such dianions. Sulfur removal studies by alkali metals were successful due to the properties of the carbon-sulfur bond 182,183). Reductive alkylation studies on model compounds which are relevant to sulfur containing systems demonstrated the stability of hydrocarbon dianions 182). A spectroscopic study performed under mild conditions afforded the characterization of sulfur and oxygen containing dianions 184 186). Benzo[b]thiophene, (73), 1,3-diphenylbenzo[c]thiophene (74) and l,3-diphenylbenzo[c]furan (75) form the respective dianions at low temperature. 

The flavor of fish and seafoods is composed of taste-active low molecular-weight extractives and aroma-active compounds. The taste-active compoimds are more abundant in the tissues of mollusks and crustaceans than fish. The most important non-volatile taste components are fi-ee amino acids, nucleotides, inorganic salts and quaternary ammonia bases. Alcohols, aldehydes, ketones, furans, nitrogen-containing compounds, sulfur-containing compounds, hydrocarbons, esters and phenols are the most important volatiles is shellfish. Alkyl pyrazines and sulfur-containing compounds are important contributors to the cooked aroma of crustaceans. Furans pyrazines, and Lactones have been found in heat-treated seafoods. Dimethyl disulfide, dimethyl trisulfide, heterocyclic sulfiir-containing compounds (alkylthiophenes) have been found in most thermally treated crustaceans like prawn, crab, oyster, crayfish and shrimp (52). 

Group 3 (Tressl et al., Berlin) identified mainly furans, pyrroles, phenols and sulfur-containing compounds, emphasizing particularly their formation pathways. 

Figure 4.3 compares the number of compounds identified in green and in roasted coffee beans. Green coffee contains a larger number of identified alcohols (B) and nearly the same number of identified aldehydes (C) and esters (F) than roasted beans. On the contrary, the latter are richer in furans (I), pyrazines (O), ketones (D) and phenols (H). Thiophenes (J), oxazoles (L) and thiazoles (M) have only been identified in roasted coffee. The roasting effect is also revealed by the increased number of pyrroles (K) and sulfur-containing compounds. The distribution of aroma volatiles is shown in Figure 4.4. 

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