Alcohols winemakers

In enzymes, this folding process is crucial to their activity as catalysts, with part of the structure as the center of reactivity. Heating enzymes (or other treatments) destroys their three-dimensional structure so stops further action. For example, in winemaking, the rising alcohol content eventually denatures the enzymes responsible for turning sugar into alcohol, and fermentation stops. 

Port-type ted dessert wines require skin contact time to extract the anthocyanins, but the fermentation must be short to retain the sugar level neat the 6—10% level desired. The winemaker cannot always achieve desired composition in individual lots. In order to teach the desired standard, it is necessary to make new lots to enable blending to that standard. The right volume of a tedder, less sweet wine will need to be made to bring to standard a lot with low color and mote sugar, for example, while keeping the alcohol also within the desired limits. 

The next step of the biotechnical sequence, yeast fermentation, is of the utmost importance to the chemistry of winemaking as well as to the formation of flavor substances. We have investigated this previously using 14C-tagged compounds (16). Amino acids, for example, enter the yeast fermentation with a quasi biochemical valence with regard to the formation of metabolic side products like alcohols and esters. In that respect, the composition of the fermentation substrate, the grape must, is highly important to the formation of aroma substances by yeasts. 

Fumaric Aero Inhibition. Another means of preventing malo-lactic fermentation is to add fumaric acid after alcoholic fermentation is complete (45, 46, 47,48). The inhibition is relative and its extent is dependent on the amount added. The susceptibility to fumaric acid is also dependent on the strain of malo-lactic bacteria tested (49). However, we know of no case where fumaric acid addition at the levels suggested by Cofran and Meyer (45) (about 0.05%) did not delay malo-lactic fermentation under normal winemaking conditions. This includes several experiments from our pilot winery (50). Nevertheless, we have not been hasty to recommend the use of fumaric acid as an inhibitor because 1) of the difficulty in solubilizing the acid in wine 2) we do not know the mechanism of action of its inhibition [Pilone (47, 48) has shown that the bacteria metabolize low levels of fumaric acid to lactic acid but, at inhibitory levels at wine pH, the acid is bactericidal] and 3) of the desirability of minimizing the use of chemical additives. 

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. 

FIGURE 1.1 Scheme of carbonic maceration winemaking. AM, anaerobic metabolism of grape berries YAF, yeast alcoholic fermentation M, maceration qd = pair temperature (q°C) action duration (days). (Figure from CEnologie—fondements scientifi-ques et techniques. Flanzy et al. collection Sciences Techniques Agroalimentaires. Technique Documentation, 1998, p. 780. Reproduced with the permission of the Editor.)... 

The fermentation usually is complete within 30 or 40 days. During this period additional anthocyanins are extracted from pomace and the color stabilizes due to polymerization between the anthocyanins and tannins. Winemakers call this process pomace "maturation". Moreover, due to the increasing alcohol concentration, many other metabolites are extracted from the pomace. In addition, yeast metabolites contribute to the aromatic complexity of the wine. 

At the end of alcoholic and malolactic fermentation, the wine undergoes clarification and stabilization prior to bottling the phase inappropriately called "aging." This is the final step in winemaking, which involved those operations required to permit the expression of features considered necessary to the wine s overall quality. Nuances that distinguish wine styles also develop. 

As described above, astringency is a very complex sensation that occurs in several foodstuff and beverages, involving different kinds of compounds (phenolic compounds, proteins), and that is affected by medium conditions (pH, alcohol, presence of other molecules in solution, etc.). This sensation constitutes one of the main organoleptic properties of red wine and has been a matter of research over the years for wine chemists. Winemakers believe that polysaccharides contribute to the mouthfeel volume of a wine, but this is a rather empirical observation. 

In the alcoholic fermentation of juice to wine, SO2 is added at the crushers at the rate of about one pound per one thousand gallons. Pectic enzymes may be added to increase juice yields and clarity. The fermentation is conducted with the native yeast of the grape or various pure culture strains of Saccaromyces cerevisiae. Whether or not yeast cultures are added depends on the quality of the grapes and the winemaker s preferences. Dehydrated wine yeast is in general use at the beginning of the crushing season. 

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