Alcoholic fermentation yeast development during

Bacteria do not transform all of the malic acid contained in the grape. From the start, during alcoholic fermentation, yeasts metabolize a maximum of 30% of the malic acid. The product, pyruvate, then enters one of many yeast metabolic pathways—notably leading to the formation of ethanol. This malo-alcoholic fermentation is catalyzed at the first stage by the malic enzyme. The bacteria must develop a sufficient population before malolactic fermentation can truly start. The production of L-lactic acid is coupled with the decrease in malic acid (Figure 6.3). 

The role of yeast in fermenting dough maturation is even less clear. The alcohol and carbon dioxide developed during fermentation must influence the elastic properties of the protein matrix. However, experimental procedures that would permit this to be checked in the absence of yeast have not been developed. 

Unpleasant odors may sometimes develop in wine during alcoholic fermentation, due to the formation of sulfur compounds by yeast. In view of the complexity of yeast s sulfur metabolism, there are many biochemical mechanisms capable of producing these malodorous molecules. For this reason, theories that attempt to explain the appearance of reduction defects in fermenting wines are often contradictory, and of practically no use to winemakers wishing to implement reliable preventive measures (Rankine, 1963 Eschenbruch, 1974). 

Thus, the development of reduction defects in wine during alcoholic fermentation is mainly due to the yeast producing abnormally high concentrations of a small number of malodorous sulfur compounds. The most important of these... 

Figure 9.14 Potential targets for wine yeast strain development. The primary role of wine yeast is to conduct the alcoholic fermentation during which grape sugars (mainly glucose and fructose) are converted into ethanol, carbon dioxide and other minor, but important, metabolites. The main emphasis in strain development programmes is on the development of Saccharomyces cerevisiae strains with improved fermentation, processing and biopreservation abilities, and the capacity to enhance the wholesomeness and sensory quality of wine. It is hoped that such genetically improved yeast strains could enhance cost-effective production of wine with minimised resource inputs, improved quality and low environmental impact. Adapted from Pretorius (2000).

Like some native yeast and acetic acid bacteria, native LAB may not be killed during primary processing and alcoholic fermentation. As some of these may be spoilage strains, they may represent a significant carry-through threat, in that if future conditions permit, they may develop into dense populations. 

At least 2-5 years are needed to make dessert wines. In the production of sherry the wine is stored in partially filled butts, i.e. in the presence of excess air. Ror yeasts develop on the wine surface in the form of a continuous film or wine cover (sherry yeast). The typical sherry flavor is derived from the aerobic conditions of maturation. During this time the concentrations of the following compounds increase at the expense of alcohol and volatile acids ethanal, acetals, esters, sotolon (cf. 5.3.1.3) and 2,3-butylene glycol. In port wine production the wine is drawn off to casks before the end of fermentation and is fortified with wine distillates. The fortifying procedure is repeated several times ( multiple addition ) until the desired alcohol content is reached. Sotolon is the key aroma substance of Port wine. Its odor threshold in this wine is 19 pg/1. Its concentration increases linearly during storage. Port stored for one year and for 60 years contained 5 and 958 pg/1 sotolon respectively. 

However, quantities formed are considerably lower than those originating from the nrea released by yeasts during alcoholic fermentation. There is no need to worry about the development of strains of 0. oeni that degrade arginine dnring malolactic fermentation. It is, however, advisable to prevent these strains, as well as heterofermentative lactobacilli, from proliferating after malolactic fermentation. It is advisable not to risk the formation of these precnrsors of ethyl carbamate. 

The must from the press readily enters into spontaneous fermentation from the development of wine yeasts which are always present on the skins and stalks of the grapes, or which fall into it from the air. These yeasts are varieties of Saccharomyces ellipsoidcus, and are capable of withstanding a high acidity and alcoholic content. White wines are fermented in barrels with the bung-hole left open to allow of the escape of carbon dioxide. Valuable red wines are fermented in conical wooden vats open above, since the skins or husks require to be stirred up during the fermentation. 

Beer, on the other hand, is produced by more complex biochemical and technological processes, which all affect its flavor. Yeast amino acid metabolism, a key to the development of beer flavor as described earlier, is affected by process temperature and use of cell immobilization techniqnes. Therefore, technologies based on these features as well as other process conditions and strain selection have been developed to control beer flavor. The combination of immobilized yeast and low-temperature primary fermentation was found to produce beers with low diacetyl amounts, therefore indicating potential of low-cost industrial application since maturation is a high-energy-consuming process. Finally, Perpete and Collin showed that during alcohol-free beer production, the enzymatic reduction of worty flavor (caused by Strecker aldehydes) by brewer s yeast was improved by cold contact fermentation. 

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