The goal of this project is to uncover the causes of chronically difficult to ferment juices. These juices often derive from the same vineyard or block of a vineyard and other similarly managed vineyards and blocks display normal fermentation kinetics. These difficult to ferment juices do not appear to respond to nitrogen or other commercial nutrient addition, occur regardless of strain used, and are challenging to restart. Our aims for the first year of this project were to evaluate the nutritional content of these juices to determine if a nutritional deficiency or presence of toxic compound was the cause of inhibition of fermentation. In this first year we were able to narrow down the possible issues with these juices. Although low nitrogen is a factor, addition of nitrogen to the fermentation seems to partially address the nutritional limitation. In addition these juices appear to impose a high vitamin demand on the yeast which we will explore in detail in this second year. This past year we were able to demonstrate that the accumulation of mannitol observed from the metabolomics analysis of the yeast strains grown in difficult to ferment juices is an indicator of oxidative stress in the juice.
It is not clear what juice compositional factors are leading to the oxidative stress and why this is not alleviated by use of Sulfur dioxide, and this will be explored further in this coming year. The low arginine and high proline of these juices suggests a problem with development, activity or destruction of the fine roots of the vine. Addressing this problem by adjustment of either yeast strain or nutrient supplementation would prevent the need for more invasive vineyard management strategies.
In this past year several commercial wineries sent us examples of chronic to ferment juices and we discovered that these juices either resembled the J. Lohr juice in having a deficient nutrient profile or were characterized by the presence of four rare (for grape) acetic acid bacteria species that seem to retain viability throughout the winemaking process and that appear to be inhibitory in ways not due to simple acetic acid production. An added goal for this coming year’s grant is to evaluate the role(s) of these bacteria in fermentation progression and to assess their sulfite sensitivities and persistence during fermentation.