Factors Affecting Sugar Utilization and Rate of Fermentation During Vinification
The HXT Display technique has been successfully applied to cells isolated from white grape juice, and a preliminary analysis of the HXT genes expressed during grape juice fermentation has been completed. Of the 14 HXT genes that we were able to evaluate, 11 were found to be expressed in grape juice. This technique did not allow quantitation of the relative abundance of the messages for the expressed HXT genes, so we are developing an alternative method that will allow quantitation, based upon the unique sequences identified in the untranslated regions of the messages identified in the original HXT display analysis. The four genes which were not amplified in this analysis are being cloned in order to characterize the 3′ untranslated regions. The pathway for the internalization and turnover of the HXT2 protein has been evaluated. It was originally thought that if this pathway could be blocked, fermentations would not arrest prematurely. This does not appear to be a viable option for the elimination of fermentation problems. Cells not turning over Hxt2p appear to be sluggish in conducting the fermentation from the beginning. Further, blocking the pathway for internalization blocks internalization of other factors as well and seems to make the cells more sensitive to ethanol. Finally, the role of fatty acids in ethanol tolerance and stuck and sluggish fermentations has been evaluated. The requirement for unsaturated fatty acids has been demonstrated under enological conditions. Saturated fatty acids offer little benefit to the organisms. Fatty acid limitation appears to affect consumption of fructose more severely than that of glucose for reasons that are as yet obscure. This may be related to the type of HXT genes that are expressed upon fatty acid limitation. Surprisingly, fatty acid limited cultures maintained viability while stuck as long as sufficient glucose was available.