Sugar in Grape Berry and Wine---Sugar Fermentation

The companion blog to this was called Sugar in Grape Berry and Wine---Sugar Transport and Accumulation in Berry, this blog picks up the fate of sugar after harvest.
One secret to making great wine is to find/grow high quality berries and ensuring that they are harvested in optimal condition.
During the fermentation process, glucose and fructose, the two major sugars of berries, are converted into alcohol by the action of the yeast Saccharomyces cerevisiae, releasing carbon dioxide and heat.

Winemakers control the fermentation through several parameters:

• temperature
• skin contact time
• pressing technique
• other techniques

The fermenting yeast S. cerevisiae has several hexose transporters (encoded by HXT1-HXT17 and GAL2), six of which (Hxt1-4p and Hxt6-7p) have been characterized as the most physiologically relevant. During must fermentation they perform different roles, and have different expression patterns according to their regulatory and kinetic properties:

• HXT2 is expressed essentially in the lag phase
• HXT1 is then expressed from the beginning of fermentation to the stationary phase
• HXT3 is present throughout fermentation, with a maximal expression when growth stops
• HXT6 and HXT7 are induced at the entry of stationary phase and remain expressed till the end of fermentation

These hexose transporters have a preference for glucose so towards the end of fermentation there is a prevalence of fructose. In addition to fructose, wines also contain small amounts of other sugars, such as arabinose, xylose, ribose, rhamnose, galactose, etc. The residual sugar in dry wines is enough to add mouth-feel without a perceivable sweet taste, and can balance the wine, and increase viscosity.
References:
1. Carlos Conde, Paulo Silva, Natacha Fontes, Alberto C. P. Dias, Rui M. Tavares, Maria J. Sousa, Alice Agasse, Serge Delrot, Hernâni Gerós, Biochemical changes throughout Grape Berry development and fruit and wine quality, Food, 1(1), 1-22 ©2007 Global Science Books.