Impact of Oxygen Alterations in Chardonnay Juice on Fermentation Behavior and Bound Sulfur Formation

Book excerpt: In an attempt to determine the impact of various oxidative conditions on fermentations inoculated with strains of Saccharomyces cerevisiae containing the MET10-932 allele, a pilot experiment was carried out. A total of twenty-five small-scale fermentations were carried out in a commercial setting using Alexander Valley Chardonnay juice in the 2012 vintage. A total of five different yeast strains and five different juice and wine treatments were performed. The yeast strains included three MET10-932, Phyterra strains: P1Y0, P2Y3, and P7Y9, as well as two strains of the MET10 wildtype allele; CY3079 and EC1118. The MET10 allele is known to be responsible in Saccharomyces cerevisiae strains for low basal levels of sulfite reductase activity. Thus, the MET10-932 yeast strains do not produce hydrogen sulfide in primary fermentation. Experimentation with these yeast strains in various juice conditions is beneficial to understand optimization of fermentative behavior. The oxidative conditions of the juice were altered at various points in the fermentation to determine optimal production conditions as well as to determine fermentative behavior of the MET10-932 and MET10 wildtype yeast strains given various production approaches and techniques. The oxidative treatments ranged from aeration prior to fermentation, sparging with nitrogen pre-fermentation, and aerations at specific time points in the fermentation. In this pilot experiment, no noticeable impacts of aeration treatments were observed on the fermentation behavior. Additionally, the MET10-932 yeast strains illustrated a significantly higher bound SO2 content upon completion of fermentation in the large-scale fermentations. Potential causes for the higher yields of bound SO2 in the MET10-932 yeast strains include use of specific vineyard treatments, nutrients utilized during the fermentation and other fermentative conditions. In order to investigate the source of the higher bound SO2 in the MET10-932 yeast strains, small-scale fermentations were carried out in MMM medium using a treatment of Fermaid K and no nutrient addition. Results indicate that nutrient addition in the form of Fermaid K did not impact the total sulfite production as compared to the fermentation that did not receive nutrient addition. However, the MET10-932 strains did express a higher bound sulfite production as compared to the MET10 wildtype strains. It can be concluded that although various aeration impacts did not greatly impact the fermentation behavior there are still certainly benefits in understanding the optimization of these strains for production purposes. Ruling out Fermaid K nutrient addition as the source of high bound SO2 helps in learning what might be responsible. Further, it can be determined that the high bound SO2 is not solely linked to varietal characteristic of Chardonnay as the high levels were also observed in MMM.