Overview of Grape Juice Fermentation
Grape juice contains a rich source of substrates derived from plant photosynthesis. The major sugar substrates in grape are the two hexoses, glucose and fructose, but other hexoses and pentoses may be formed as well. Fermentation of grape sugar is largely conducted by the yeast Saccharomyces. Through the process of glycolysis, the 6-carbon hexoses are broken down into 2 molecules of the 3 carbon pyruvate. Since this process generates excess electrons, the cell must eliminate those electrons by transferring them to another molecule via a process that does not consume energy. In the case of the alcoholic fermentation, the yeast cell decarboxylates pyruvate yielding acetaldehyde, in order to generate an electron acceptor. Acetaldehyde has a high affinity for electrons, becoming reduced to ethanol. Thus, one molecule of a 6 carbon sugar yields 2 molecules of carbon dioxide and 2 molecules of the 2 carbon compound ethanol.
The goal of wine fermentation is to convert all of the available sugar to ethanol and carbon dioxide and to not leave residual sugar in the wine. There are several factors that determine whether or notSaccharomyces will be able to completely utilize the available glucose and fructose.· Adequate consumption of sugar requires a metabolically healthy population of sufficiently high numbers of cells that sustain substrate conversion rates. Sugar consumption also requires that conditions, chemical, physical and biological, allow metabolic activity to continue. The goal of fermentation management is the maintenance of permissive metabolic conditions and healthy cell populations to allow complete sugar conversion. An aligned goal is the ability to control the other metabolic activities of yeast to generate desired flavor impacts in the wine.