The yeast plasma membrane regulates the movement of compounds into the yeast celj and of yeast metabolites from the cell into the medium. The rate of penetration of organic acids into the yeast cell depends on their lipophilic nature, and on their molecular size and degree of branching. During fermentation yeast synthesizes a vast number of aroma compounds. The numerically and quantitatively largest groups of aroma compounds include fusel alcohols, fatty acids and fatty acid esters. The yeast used and the fermentation conditions can influence the formation of aroma com pounds. The yeast also has a profound effect on the formation of other aroma compounds, such as sulphur compounds and phenols. In addition to fermentation, the maturing of a beverage can also influence the aroma. During the maturing lactones, phenols and other compounds are extracted from the oak casks in which the beverage is aged. The presence of the so-called "whisky lactone", p-methyl-Y-octalactone, is characteristic of a beverage that has been matured in oak.
It has been shown that the proportion of fatty acid ethyl esters retained by the yeast cell increases with increasing acyl chain length as the ester becomes more lipid soluble. The distribution of esters depends on the yeast strain and on the fermentation temperature; larger amounts of esters were found to transfer from the cells into the medium at higher temperature. It was shown that esterase activity is located both inside and outside the yeast cell plasma membrane. Intact yeast was capable of hydrolysing the ethyl esters of caproic, caprylic and capric acid. Acetate esters, were hydrolysed only very slowly or not at all.The hydrolytic activity of baker's yeast was studied with ethyl caprylate as substrate. The hydro lysis was very fast at the beginning. The equilibrium attained depended not only on the con centration of ester and alcohol but also on the pH, a higher amount of ester remaining in solutions of lower pH. It was also shown that the esterases possess appreciable ester synthesizing ability and an equilibrium was attained by incubating yeast with caprylic acid and ethanol.The experiments described show that the ester level in an alcoholic beverage, such as beer, is not dependent solely on the ester concentration formed during fermentation: in the presence of yeast the level can be shifted in either direction by changing temperature, pH or alcohol con centration -or the amount and type of yeast.
A study was made of the enzyme content of the isolated cell walls and of a plasma-membrane preparation obtained by centrifugation after enzymic digestion of the cell walls of baker's yeast. The isolated cell walls showed no hexokinase, alkaline phosphatase, esterase or NADH oxidase activity. It was concluded that these enzymes exist only in the interior of the cell. Further, only a negligible activity of deamidase was detectable in the cell walls. Noticeable amounts of saccharase, phosphatases hydrolysing p-nitrophenyl phosphate, ATP, ADP, thiamin pyrophosphate and PP(i), with optimum activity at pH3-4, and an activity of Mg(2+)-dependent adenosine triphosphatase at neutral pH, were found in the isolated cell walls. During enzymic digestion, the other activities appearing in the cell walls were mostly released into the medium, but the bulk of the Mg(2+)-dependent adenosine triphosphatase remained in the plasma-membrane preparation. Accordingly, it may be assumed that the enzymes released into the medium during digestion are located in the cell wall outside the plasma membrane, whereas the Mg(2+)-dependent adenosine triphosphatase is an enzyme of the plasma membrane. This enzyme differs from the phosphatases with pH optima in the range pH3-4 with regard to location, pH optimum, substrate specificity and different requirement of activators.
Sensory examination of an aroma model consisting of alcohols, fatty acids, esters and aldehydes, in concentrations estimated in whisky, has proved thatin addition to vicinal diketones-esters predominate as components influencing the aroma, but also some fatty acids exercise a most marked influence. Further more, some people possess a good ability to smell certain compounds whereas others cannot smell them as well, but detect other compounds more easily.Within the group of more than 100 identified aroma components, mainly the same substances appear in the volatile aroma fraction of beer, wine and distilled beverages. In view of this, it seems evident that the raw materials utilized for the production of the beverages contribute to no more than a limited extent towards the aroma composition. The aroma components which are most notice able are produced by the yeast during fermentation, and the nature of the final aroma first and foremost depends upon the kind of yeast used, and upon the fermentation conditions. Although the yeast for the most part produces the same aroma components also in different beverages, considerable variation may occur in the quantities of aroma components even in beverages of the same type.The structure and function of the plasma membrane selectively regulate the uptake of different compounds from the medium into the yeast cell, and the release of the metabolites from the cell into the medium. In this sense, the lipid composition of the plasma membrane and its lipolytic enzymes may possess a decisive importance. The activity of the lipolytic enzymes present in the plasma membrane must be controlled by some regulatory mechanism. If this is the case, the permeability of the plasma membrane can be modified, e.g., by activation of the phospholipases, with consequent degradation of membrane phospholipids.Consequently, it is likely that one of the functions of the lipolytic enzymes present in the plasma membrane is that of participation in the transport processes by changing the permeability properties of the plasma membrane.
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