Fermentation Enzymes, Yeast

Yeast Enzymes. Yeast has a special relationship with enzymes as enzymes were first discovered in yeast. The name enzyme is derived from the German for in yeast (i.e. Zym). It was found that an extract produced from the yeast could undertake a fermentation. This at the time was a philosophically important point since it showed that there... 

Starch derived from maize, potatoes, barley, cassava or other somces must be pretreated with hydrolytic enzymes (amylases, amyloglucosidase, proteases), which carry out liquefaction, saccharification and protein hydrolysis, respectively, before it can be fermented by yeasts and other microorganisms into potable or non-potable alcohol. Enzymes can be added in the form of malt (germinated barley) or koji (germinated rice), but this is expensive. Therefore, industrial enzymes have nearly totally replaced malt and koji as enzyme sources, thereby not only improving the economics but also the predictability of the process. 

The brewer next prepares the wort, the nutrient medium required for fermentation by yeast cells. The malt is mixed with water and then mashed or crushed. This allows the enzymes formed in the malting process to act on the cereal polysaccharides to form maltose, glucose, and other simple sugars, which are soluble in the aqueous medium. The remaining cell matter is then separated, and the liquid wort is boiled with hops to give flavor. The wort is cooled and then aerated. 

The toxic action of white arsenic has been attributed to its inhibitory action on the oxidative processes,9 partly owing to the effect of the change of pH on the enzyme concerned. Small quantities of arsenious acid reduce the power of suitably prepared extracts of animal tissues to oxidise reduced phenolphthalein. The oxidation of tartaric acid at the ordinary temperature and at 37-5° C. is inhibited by arsenious acid, as also is the respiration and fermentation of yeast,10 but the latter... 

In 1896, Fischer developed the phenylhydrazine test" for the detection of hydrolytic scission of disaccharides, especially by enzymes this depends on the fact that the phenylosazones of disaccharides are soluble in hot water, whereas those of the monosaccharides are not. Lactose is hydrolyzed by emulsin (1894) and by lactase it is not fermentable by yeast, and is unaffected by invertase (1894). An extract of the small intestine of horses and cattle, especially from young animals, hydrolyzes lactose (1896). The action of enzymes on lactose allowed it to be classified, along with cellobiose and maltose, with the normal (and not the y-type of) methyl glucoside (1914). In the discussion of maltose, the relationship of lactose to the /9-series will be mentioned later. 

In the early days of enzyme study attention was focused mainly on glycolysis in muscle, fermentation in yeast and oxidative phosphorylation, all of which provided the basis for vast and rapid development of biochemistry. Dixon and Webb estimated the number of enzymes to be about 650 in their book, Enzyme, in 1957. The number has grown to about 3000 at the present time and continues to increase (Fig. I. 1). 

Variables in alcoholic fermentation, the yeast-enzyme conversion of grape sugar to ethanol and carbon dioxide, have a major impact on the character, composition, and quality of North Coast white table wines. Type of yeast, juice solids content, juice S02 content, juice protein content, fermentation temperature, and fermentation rate are factors the enologist may consider and control. 

The overpressure only inhibits fermentation by yeasts, but does not inactivate enzymes and other microorganisms. These can cause changes in the colour, odour and taste of the juice. 

P. stipitis and C. shehatae are Crabtree-negative yeasts. Glucose (or xylose) does not induce fermentation in these organisms. Rather, fermentation is activated in response to anaerobiosis or oxygen Hmitation. The induction of fermentative enzymes results from a reduction in the oxygen tension rather than from an increase in the glycolytic flux, as in the case of S. cerevisiae. 

Pyruvate decarboxylase is also part of the pathway involved in alcoholic fermentation in yeast. In the reaction, pyruvate is converted to acetaldehyde, which is subsequently converted to ethanol by action of the enzyme alcohol dehydrogenase ... 

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