Methanethiol, 31 Table

Thiols range over the gamut of physical properties. Most of the important thiols ate Hquids, however methanethiol is a gas, and 1-hexadecanethiol and 1-octadecanethiol ate waxy soflds. Tables 2 and 3 Hst a variety of physical properties for the mote important thiols. 

Sulfonium salt 103a also underwent ring-opening reactions with nucleophiles, such as NaBH4, sodium azide, and sodium methanethiolate, to afford trithiocins 107 (see Equation (30) and Table 18) <1996BCJ2349> and <1998BCJ1187>. 

The reaction of 6-aminouracils 157 with dimethyl Ai-cyanodithioimidocarbonate, shown in Scheme 27, gave the intermediates 158 which on coupling to phenyldiazonium salt, followed by elimination of methanethiol, gave the 5,7-disubstituted-2-phenylpyrimido[4,5- ]-l,2,4-triazines 159 (Ar = Ph) in the isolated yields shown in Table 10 (entries... 

Sulphurous and fatty odour notes predominated in the odour profile of boiled pork. It is assumed [37, 38] that the sulphurous (e.g. methanethiol) and fatty odorants (e.g. octanal, nonanal) are clearly perceptible in pork due to the much lower concentration of furaneol (Table 6.29). Although when compared to pork, the levels of octanal and nonanal were twice as high in beef (Table 6.29) the fatty odour note was weak. In particular the fourfold higher concentration of furaneol might have reduced the intensity of the fatty odour. 

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 ]. 

The volatiles that have been established in dilution experiments [76] as important for the aroma of boiled potatoes are listed in Table 6.42. The potato aroma note can be reproduced with an aqueous solution (pH 6) of pyrazines nos. 4 and 5, methional (no. 2), methanethiol (no. 11) and dimethylsulphide (no. 12) in the concentrations given in Table 6.42 [77], Despite its smell of boiled potatoes, methional (no. 11) is not important for the aroma (see below). In the drying process applied for producing a granulate, the concentration of the pyrazines nos. 4 and 5 decrease, and therefore, the intensity of the potato note also decreases [77],... 

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.

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... 

It is true that light sulfur compounds have particularly unpleasant smells (rotten eggs, garlic, etc.). Even at low concentrations (on the order of p-g/1) these odors are likely to ruin a wine s aroma. Among the sulfur compounds identified and assayed in wine (Tables 8.14 and 8.15), mercaptans (H2S, methanethiol and sometimes ethanethiol) play a decisive role in reduction defects. They are always present in reduced wine at concentrations much higher than their perception thresholds. 

GC-FPD analysis of freshly-opened commercial caraway-spiced and comparable unspiced sauerkraut samples (70 g drained sauerkraut) were conducted. These samples were contained in 120-mL serum-type vials (Supelco Co., Bellefonte, PA) closed with Mininert valves (Supelco). Samples were warmed to 30°C in a water bath for 10 min before withdrawing headspace gases (4 mL) for GC-FPD analysis. Data in Table II show that caraway-spiced sauerkraut had lower quantities of dimethyl disulfide than the unspiced sauerkraut, whereas dimethyl sulfide was present in comparable amounts in both samples. Methanethiol and dimethyl tiisulfide were not detected in the caraway-spiced sauerkraut, but they were present in significant amounts in the unspiced samples. These results showed that caraway seed modified the flavor of unspiced sauerkraut by suppression of the formation of methanethiol-related volatile sulfur compounds rather than simply masking the undesirable flavors in sauerkraut with spice flavoring. 

However, unheated and heat-treated crude caraway seed extracts did not affect the concentrations of dimethyl disulfide and dimethyl trisulfide in model systems over an extended period (5 hr). Thus, it appeared that the overall suppression of methanethiol-related compounds in sauerkraut by caraway seed (Table I) was dependent on methanethiol which has been shown to be a precursor for both dimethyl disulfide and dimethyl trisulfide under oxidative conditions (18,25). Recently, Chin and Lindsay (25) have shown that methanethiol was readily converted to dimethyl disulfide and dimethyl trisulfide in an aerobic cabbage model system (pH 6.3) consisting of 1 ppm hydrogen sulfide, 450 ppm ascorbic acid and 4 ppm Fe(m). 

Nature of Caraway Seed Components Responsible for Methanethiol Depletion. The nature of the methanethiol-depleting heat-stable and heat-labile entities in caraway seed were further studied. Flavonoid compounds, which are naturally-occurring plant phenolics, appeared to comprise the heat-stable methanethiol-depleting entity based on evidence obtained from absorption spectra and color changes at different pH values (21). The caraway seed flavonoid components removed methanethiol from the headspace of closed model systems, and the formation of oxidized forms of methanethiol (dimethyl disulfide and dimethyl trisulfide) did not account for its disappearance (Table m). 

Tertiary butylhydroquinone (TBHQ), a commonly-employed synthetic antioxidant in foods, was also included in the studies as a model phenolic compound, and it had the same effect on depletion of methanethiol as caraway seed fiavonoids (Table ni). These results indicated that TBHQ could be used also for suppressing unpleasant flavors in foods that are caused by methanethiol and its oxidative derivatives. Because the methanethiol-depleting effect of caraway seed fiavonoids and TBHQ increased with increasing oxygen tensions and pH values in the model systems... 

Table IIL Effects of isolated caraway seed flavonoid components and TBHQ on methanethiol concentrations in closed model systems after 5 hr at 30°C...

Data from GC-FPD analysis of the broccoli florets are shown in Table V, and these data corresponded directionally with the sensory data (Table IV). Although TBHQ and crude caraway seed extracts showed suppression effects on the formation of methanethiol-related compounds, the performance of TBHQ and caraway seed extracts in depletion of methanethiol within the packages was not as high as expected from studies employing model systems. This was probably caused by the anaerobic... 

Analysis of samples showed that concentrations of methanethiol and dimethyl disulfide were lower in caraway- and TBHQ-treated broccoli florets than in control samples (Table V). Dimethyl tiisulfide was not detected in the refrigerated samples which indicated that its contribution to the objectionable odors of modified atmosphere stored broccoli florets was minimal. Dimethyl sulfide which has a precursor (Sr methyl methionine sulfonium salt 34) different from that for methanethiol-related compounds was present in all samples. 

The influence of dimethyl disulfide on flavor quality of modified atmosphere stored broccoli florets was further demonstrated by data shown in Table VI which revealed that aroma assessments of undesirable sulfurous aromas in broccoli florets more closely corresponded with concentrations of dimethyl disulfide than with methanethiol. In this experiment broccoli florets were treated by dipping in various solutions containing either ascorbic acid (500 ppm), sodium hy oxide (0.01 M), or phosphoric acid (0.1 M). Broccoli florets dipp in distilled water were used as the control samples. Samples held in sealed Curlon 850 pouches were analyzed after storage for 4 days at 10°C. Ascorbic acid was included in this study because it acts as a reducing agent for methanethiol under anaerobic conditions in model systems (25). 

Alkaline and acidic solutions were employed in the experiment because Marks et al. (20) reported that the formation of -methyl-L-cysteine sulfoxide breakdown products was dependent on pH of the vegetable tissues. Table YU shows that the alkaline treatment had the most pronounced effect among the treatments on suppressing the formation of dimethyl disulfide which appears to contribute strongly to the unpleasant volatile sulfur compound aroma in stored broccoli. This effect probably results from the destruction of methyl methanethiosulfinate by the hydroxide ion (33). Thus, strategies for suppressing undesirable volatile sulfur compounds in cruciferous vegetables and other foods need to incorporate means to minimize contributions by oxidized forms of methanethiol as well as methanethiol itself. 

Application of the tables - to calculate thermodynamic contributions requires knowledge of molecular geometry to determine the appropriate reduced moment of inertia. In cases where the potential barrier height is known from spectroscopic measurements, the tables may be used directly to calculate thermodynamic properties. Examples of molecules for which this procedure has been used include methanol, methanethiol, fluoroethane, 1,1-difluoroethane, 1,1,1-trifluoroethane, pentafluoro-chloroethane, chloroethane, 1,1,1,2-tetrachloroethane, pentachloro-... 

An example are the AVs of the odorants of French fries based on their odor thresholds in an oil (Table 5.14). Methanethiol, methional, methyl-propanal and 2-methylbutanal exhibit the highest aroma values. Consequently, they should belong to the most important odorants of French fries. 

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