Yeast extracts, volatile

A study of three commercially available extracts of yeast (Saccharomyces cerevisiae) identified 268 volatile components of which 67 contained sulfur (34 of these were identified for the first time as yeast extract components). The identified substances ranged from CS2 to complex thiophenes and thiazoles.28... 

In a continuous-flow mixed-film methanogenic column study, tetrachloroethylene degraded to trichloroethylene with traces of vinyl chloride, dichloroethylene isomers, and carbon dioxide (Vogel and McCarty, 1985). In a static-culture-flask screening test, tetrachloroethylene (5 and 10 mg/L) was statically incubated in the dark at 25 °C with yeast extract and settled domestic wastewater inoculum. Significant degradation with gradual adaptation was observed after 28 d of incubation. The amount lost due to volatilization after 10 d was 16 to 23% (Tabak et al., 1981). 

In a static-culture-flask screening test, 1,1,2-trichloroethane was statically incubated in the dark at 25 °C with yeast extract and settled domestic wastewater inoculum. Biodegradative activity was slow to moderate, concomitant with a significant rate of volatilization (Tabak et al., 1981). 

The butanol-acetone fermentation, using whey supplemented with yeast extract, is substantially a butanol fermentation. Volatile solvents from the fermentation consist of 80% butanol, 13% acetone, and 5% ethanol. Approximately 30% of the lactose that is fermented is converted to butanol, 5% to acetone, 2% to ethanol, and the balance largely to CO2 and small quantities of butyric and acetic acids, acetyl-methylcarbinol, and hydrogen. 

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. 

Rumen Strain. In studies on strain Ml of M. ruminantium isolated from bovine rumen contents, we confirmed the results of Smith and Hungate (17), showing that this species requires factors present in rumen fiuid (23). It would not grow in medium containing H2-CO2, bicarbonate, sulfide, cysteine, yeast extract, trypticase, and minerals unless rumen fiuid was also added. In further experiments (23) we assayed fractions of rumen fiuid using the above medium and showed that growth factors could be separated into two components by acidification and ether extraction. One factor was not extractable with ether, and others were shown to be volatile acids. 

In the reaction of 2,3-butanedione and ammonium sulfide at 25 °C, quantitation of volatile compoimds identified in the reaction is summarized in Table II. A pair of interesting intermediate compounds, 5-hydroxy-3-thiazolines, were tentatively identified by GC/MS-EI. 5-Hydroxy-2,4,5-trimethyl-3-thiazolines have been separated and identified by GC/MS-EI and NMR in the early study of volatile flavor conq)ounds of yeast extracts (S). They were also found in the reaction of 2,3-butanedione or 2,3-pentanedione with acetaldehyde, hydrogen sulfide and ammonia (2). 

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. 

Yeast extracts (YEs) are sources of natural flavor compounds, and their exact composition varies according to the type of yeast and the conditions used for its propagation and the production of the final YE. This chapter is devoted to a review of the volatile sulfur compounds of YEs (many of which play a crucial role in determining the overall flavor) and to a report of some sulfor compounds which have recently been identified as components of selected YEs. 

AMES Volatile Sulfur Compounds in Yeast Extracts... 

Table I. Volatile sulfur components of yeast autolysates and yeast extracts...

Table IV. Selected volatile sulfur compounds reported in cysteine (or cystine)-reducing sugar (or carbonyl compound) model systems which have been identified in yeast extracts...

Another study by Izzo and Ho (1992) involved the addition of glucose and/or ammonium bicarbonate to autolyzed yeast extract prior to extrusion. These researchers measured the pyrazine formation and brown color development when the yeast extract mixtures were extruded on a single screw extruder at 115°C, and 140 rpm screw speed. Liquid-solid extraction was used to extract the volatiles pyrazines were identified and quantified using... 

We have been involved in the production of SF and SW by fermentation of leguminicola ATCC 26280 for animal studies for several years. To our knowledge, no systematic comparison of strains for relative yields of SF and SW had been conducted. We report a comparison of strains for their abilities to produce SF and SW in pure culture on yeast extract-supplemented Czapek-Dox medium. Since SF has been reported to be unstable or volatile, we have examined the short-term (2 hr) stability of freebase SF. 

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