Fermentation by yeasts

There are interesting examples of enantiomers that not only are found separately but also have different chemical properties when reacting with some reagent which is itself an enantiomer. For example (+ )-glucose is metabolized by animals and can be fermented by yeasts, but (—)-glucose has neither of these properties. The enantiomer ( + )-carvone smells of caraway whereas (—)-carvone smells of spearmint. 

Despite the similarities of their structures, cellobiose and maltose have dramatically different biological properties. Cellobiose can t be digested by humans and can t be fermented by yeast. Maltose, however, is digested without difficulty and is fermented readily. 

Although Fischer2 stated that D-glucosone is not fermented by yeast it has recently been reported157 to be fermented by yeast extracts fortified with adenosine-5-triphosphoric acid and codehydrogenase I. 

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. 

The third mode of energy conversion, fermentation, was discovered by Pasteur. After examining alcoholic fermentation by yeast, he studied several bacterial fermentations, including butyric acid fermentation and its... 

When sugar that is produced by the Scholler process is fermented by the acid-producing organism, the yield is only 35% of the total sugar. If the solution is first fermented by yeast, however, a 37 % yield of alcohol is produced, and from the residual liquor fermented by the butyric acid organism, a 15.5% yield of acids is produced, making a total recovery of 52.5%. 

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. 

Table 10.1 gives a summary of the main by-products of fermentation by yeasts and other microbiological activities which can be identified in distilled spirits from different raw materials, like fruits, wine, grain, sugar cane, or other carbohydrate-containing plants. Since the sensory relevance of a flavour compound is related to its odour thresholds and odour quality. Table 10.1 presents also odour qualities and a review of threshold values of the fermentation by-products in ethanol solutions [9-10] and/or water [11-14] (Christoph and Bauer-Christoph 2006, unpublished results). 

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 growth of malo-lactic bacteria in wines is favored by moderate temperatures, low acidity, very low levels of S02, and the presence of small amounts of sugar undergoing fermentation by yeast. It is frequently possible to inoculate a wine with a pure culture of a desirable strain of bacteria and obtain the malo-lactic fermentation under controlled conditions. The pure-culture multiplication of the selected strain of bacteria is difficult, however. It is also difficult to control the time of the malo-lactic fermentation—sometimes it occurs when not wanted, and at other times will not go when very much desired. For the home winemaker it is probably most satisfactory to accept the malo-lactic fermentation if it occurs immediately following the alcoholic fermentation. The wines should then be siphoned away from deposits, stored in completely filled containers at cool temperatures, and have added to them about 50 ppm S02. If the malo-lactic fermentation does not take place spontaneously and the wine is reasonably tart, the above described regime of preservation will likely prevent its occurrence. When the malo-lactic transformation takes place in wines in bottles, the results are nearly always bad. The wine becomes slightly carbonated, and the spoiled sauerkraut flavors are emphasized. 

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. 

Pradeep, P., Goud, G. K., and Reddy, O. V. S. (2010). Optimization of very high gravity (VHG) finger millet (ragi) medium for ethanolic fermentation by yeast. ChiangMlai ]. Sci. 37,116-123. 

Purves and Hudson76 regarded the positive, Raybin77 diazouracil reaction given by raffinose as being a more specific test for the presence of a sucrose linkage than is fermentability by yeast or action by yeast invertase. 

Glucose syrups are easily fermented by yeast to ethanol. While beverage ethanol has been produced from many sources of sugar and starch for countless centuries, large-scale production of fuel-grade ethanol by fermentation is attributed to a demand for combustible motor fuel additives. 

Isbell and Pigman17 have shown that the rapid and anomalous mutarotation involves pyranose—furanose interconversion. On the basis that only D-fructofuranose (Ic) is fermented by yeast, Gottschalk18 has shown that the equilibrium mixture in aqueous solution at 0° contains 12% of D-fructofuranose. Gottschalk has calculated, from the kinetics of the mutarotation, that the aqueous solution at 20° contains about 20 % of the sugar in the furanose form. 

Possible toxic reactions of sulfur dioxide are also indicated in Table I. The reaction of bisulfite with aldehydes has a classic position in biochemistry since Neuberg demonstrated in 1918 that the products of fermentation by yeast were altered by the addition of sodium sulfite, which caused the production of equal amounts of the bisulfite addition compound of acetaldehyde and of glycerol. This was concomitant with the blockage of conversion of acetaldehyde to ethanol. Addition compounds can also be formed with quinones and with ,/ -unsaturated compounds. None of these reactions has been adequately assessed as a possible contributor to toxicity. 

The pH affects the ethanol production rate, yeast growth, byproduct formation and bacterial contamination. Sugar fermentation by yeast is relatively insensitive... 

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