Yeast juice

Interpretation of the process of fermentation by yeast was one of the most controversial issues for vitalists. Its resolution was fundamental for the future development of biochemistry. In the early nineteenth century fermentation was believed to be related to putrefaction and decay. Liebig considered it to result from the breakdown of a substance (sugar) following the admission of air to the nitrogenous components in yeast juices. After the must of grape juice had fermented, the liquid cleared and the yellow sediment, yeast, was deposited. 

In 1878, the "fragments" identified by Pasteur were named enzymes by the German physiologist Wilhelm Kuhne. In 1897, Eduard Buchner, a German chemist, accidentally discovered that a yeast juice could convert sucrose to ethanol. He was able to show that the sugar was fermented even in the absence of living yeast cells in the mixture, and named the factor responsible for the fermentation of sucrose zymase. In 1907, he received the Nobel Prize in Chemistry. The 40 years of biochemical research that followed yielded the details of the chemical reactions of fermentation. 

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. 

Harden and Young, J. Ch. Soc. (1908) Abs. i, 590 were the first to show that the addition of phosphates (disodium phosphate) to a mixture of yeast juice and dextrose resulted in both an initial acceleration and in an increased total fermentation. They also showed that there was an optimum concentration of phosphate, deviations from which in either direction resulted in a diminished rate of fermentation. It is assumed that a hexose-di-phosphate is formed as follows ... 

It has been found by further investigation that there is apparently another step in the fermentation reaction in which the dextrose di-phosphate splits into two moles of triose mono-phosphate. It should be particularly noted that the immediately preceding study of the mechanism of the action of yeast juice is not directly applicable to the action of yeast. E.gSlator (loc. cit.) found that phosphates are without accelerating effect when living yeast cells are employed. 

Abderhalden and Teruuchi used glycyl-l-tyrosine to determine the nature of the enzymes in yeast juice, i.e. endotryptase, in papain and in the juice of nepenthes. The two former hydrolysed it, and consequently they contain tryptic enzymes the last had no action upon it, and the enzyme of nepenthes is therefore like pepsin in its action. These results confirm the observations of other investigators, and the confusion concerning the nature of these enzymes would appear to be now settled with certainty. 

The ca e is quite otherwise when yeast juice is left for a long time to itself. Here, not only does the co-enzyme deteriorate, but even the zymase itself is digested or destroyed by the endotryptase. Under these conditions, the addition of co-... 

It is seen that the optimum temperature for yeast juice is 55°, while pancreatic juice gives the maximum effect at 45°. Hydrolysis of polypeptids takes place in neutral reaction. The presence of free acid is very injurious. A very slight alkalinity a q>ears to be favorable, but we note here a much greater sensitiveness to alkali than in the action of tiypsin. The presence of small quantities of sodium cyanide, 0.03 per cent for example, favors hydrolysis of glycyl-l-tyro. A larger quantity, or o.r5 per cent, almost completely arrests action. 0.20% NaCl has no effect. CaCli is, on the contrary, often very favorable. The presence of glycin as well as of other products of hydrolysis is... 

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