Alkali extraction, yeast

Legend crude protein prepared by precipitation of an alkali extract at pH 4.0 ( ) yeast protein obtained following activation of endogenous ribonuclease (82) (O), and yeast protein prepared by the succinylation procedure (O). 

Other chemicals of possible concern for health and safety foimd in yeast proteins include tyramine (0—2.25 mg/g) and histamine (0.2—2.8 mg/g), formed by decarboxjiation of the corresponding amino acids (38). These compoimds are also found in other fermented (including piclded) foods. Their presence in yeast extracts used as condiments contributes very Htde to human intake. Likewise, the nephrotoxic compoimd lysinoalanine has been identified in alkali-treated yeast extracts, at a level of 0.12 mg/g. However, the chemical occurs at similar low concentrations in almost all heat- and alkaU-treated foods. 

Many years ago, Liss and Langen (1960a,b) showed that the most highly polymerized yeast PolyP fraction, extractable only with strong alkali (0.05 M) or when kept for a long period with diluted CaCl2 solution, is apparently firmly bound to some cell components other than RNA. The removal of RNA by RNAase had no effect on the extraction rate of this PolyP fraction. It was considered that in this case PolyP was bound to a certain protein. 

Partial hydrolysis of proteins using acid, alkali or enzymes is commonly employed to improve functionality and usefulness of novel proteins. Acid hydrolysis is the most common method for preparing hydrolysates of soy, zein, casein, yeast and gluten. Hydrolysates are used in formulated foods, soups, sauces, gravies, canned meats, and beverages as flavorants and thickeners (2,3,6). Alkaline treatments have been employed to solubilize and facilitate protein extraction from soy, single cells, and leaves. 

Alkali treatment has been used to improve the functional properties of the insoluble protein prepared by heat precipitation of an alkaline extract of broken yeast cells (63). Heating yeast protein at pH 11.8 followed by acid precipitation (pH 4.5) yielded a preparation composed of polypeptides with increased aqueous solubility. It also increased foaming capacity of the protein 20-fold. The emulsifying capacity of the modified protein was good whereas the original insoluble protein was incapable of forming an emulsion. Alkali treatment must be carefully controlled to avoid its possible deleterious effects (24,75), e.g. alkaline treatment of yeast protein resulted in a loss (60%) of cysteine (63). 

The absorption spectra of three yeast protein preparations prepared by different procedures were compared (Fig. 8). The presence of nucleic acid which has a X maximum at 260 nm tend to shift the absorption spectrum of yeast protein to lower wavelengths. The ratio of absorption at 280 to 260 nm is indicative of NA contamination in protein samples a ratio of more than one indicates pure protein devoid of nucleic acid whereas a ratio of 0.65 indicates approximately 30% contamination with NA. The yeast protein extracted with alkali and directly acid precipitated showed a X max at 260, a 280/260 ratio of 0.67 and contained 28%, NA determined chemically. Protein extracted in alkali, adjusted to pH 6 and incubated at 55°C for 3-5 hours, to reduce NA with endogenous ribonuclease, had a X max at 260, a 280/260 ratio of 0.8 and a NA content of 3.3% while yeast protein prepared by the succinylation procedure and precipitated at pH 4.5 showed a X max at 275 nm, a 280/260 ratio of 1.0 and nucleic acid content of 1.8. 

The cell wall of the yeast Candida albicans contains glucan and mannan extractable with hot, aqueous alkali. Bishop and associates ... 

Yeast mannan, commonly known as yeast gum by early workers, was first isolated in a pure form by Salkowski. It was extracted from pressed bakers yeast (Saccharomyces cerevisiae) with hot, aqueous alkali, and separated from such contaminants as glycogen through the insoluble, copper complex formed with Fehling solution. Mundkur has presented evidence indicating that a layer of mannan... 

Fractions having different solubility characteristics were isolated by Kessler and Nickerson, following alkaline extraction of cell walls of strains of bakers yeast and Candida albicans. The alkali-insoluble... 

The most frequently applied technique for cell disintegration of yeast is autolysis. The cell wall is digested by yeast-bome proteases. The process may be supported by externally added proteolytic enzymes from other sources and/or acid or alkali. Consecutively biomass is separated and the aqueous filtrate further processed. In many cases simple concentration by evaporation and drying is sufficient for the production of the yeast extracts or autolysates. 

Alkali-soluble Glucan. Several authors have reported the presence of alkali-soluble glucan in yeast cell wall preparations (14,15). This material is extracted together with mannan from whole cells or cell walls with hot alkali (see Scheme 1). It is not precipitated with Fehling s solution and can thus be separated from the mannan fraction (1). Recent work from Manners laboratory (139) shows that the alkaline extract neu-... 

The mode of action of sulfonamides was greatly clarified by Woods in 1940. It had been shown that tissue extracts, pus, bacteria, and particularly yeast extract contained a heat-stable substance of low molecular weight which would inhibit the action of sulfonamides on bacteria (Stamp, 1939). Woods, recalling that enzymes are inhibited by substances which chemically and sterically resemble their substrates (see Section 9.3.1), adopted this hypothesis that the inhibitory substance in yeast is the substrate of an enzyme widely distributed in nature, and that it resembles sulfanilamide chemically. He found activity was concentrated in an alkali-soluble fraction of yeast, and that it ran parallel to a colour test for an aromatic amino-group. Activity was lost on esterification or acetylation, recovered on hydrolysis, and lost again on treatment with nitrous acid (Woods, 1940). Thus he made it clear that the active substance was an aromatic amino-acid. Because -aminobenzoic acid (/ AB) 2,12, p. 31) is the aromatic amino acid that most resembles sulfanilamide 2.13) he tried it as an inhibitor of bacteriostasis, and found that one molecule could prevent 5000 to 25 000 molecules of sulfanilamide from functioning. 

Viable yeasts in pressed or slurried form may be autolysed or plasmolysed to yield extracts. Surplus brewers yeast is usually debittered (the adsorbed hop bitter substances removed) by treatment with alkali before processing. 

Note YPG media (0.2% yeast extract, 1.0% peptone, and 2.0% glucose) MSM, molasses salt medium (0.2% yeast extract and molasses as carbon source) PDB, potato dextrose broth (20% potato extract and 2% dextrose) YM media (0.3% yeast extract, 0.3% malt extract, 0.5% polypeptone, and 1.0% glucose) AIF, alkali insoluble fraction DA, degree of acetylation DDA, degree of deacetylation DP, degree of polymerization M, molecular weight SmF, submerged fermentation SSF, solid-state fermentation. 

As in higher animals, the Bu requirement is spared rather than fully satisfied by methionine. Methionine in turn was not spared by homocystine plus choline (or other methyl donors) cystine had some sparing activity (Hutner et al., 1953). DNA and acid and alkali hydrolysates were inert. The indifference to deoxyribosides is in accord with the failure of high concentrations of yeast autolyzate or plant extracts to affect the slope of the cyanocobalamin response curves. 

With certain materials, however, the nature of the tissue or of the results required have necessitated changes in the method. In the analysis of reticulocytes, acetone rather than alcohol extraction of the lipid is necessary to eliminate gelation of the tissue (51). Extractions with hot trichloroacetic acid (93) have been found inadequate to liberate ail the RNA from baker s yeast (11), the method of alkali hydrolysis being preferred. 

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