An arrested or a sluggish fermentation is expensive for a winery in multiple ways. Careful management of nutrients, strain selection, and fermentation conditions dramatically reduce this risk. However, instances of problematic fermentations arise periodically in spite of prudent management. Our study focused on defining the causes underlying the yeast strain’s poor fermentation performance in reference to identified stress factors found in the difficult-to-ferment juices. The initial microbial load of grape must is known to impact yeast fermentation. The inhibitory role of previously identified lactic acid bacteria in yeast fermentation was confirmed and these bacteria are efficient at inducing establishment of the [GAR⁺] prion in wine strains. This prion is a protein conformational change, inherited by the progeny cells, which alters the metabolic state of the yeast which has a reduced fermentation capacity. Several lactic acid and acetic acid bacteria isolated from stuck wines produced varying levels of acetate and lactate and were also found to be capable of inducing the [GAR⁺] prion in many wine yeast. Oxidative stress is another factor that can affect fermentation efficiency. Simulating oxidative stress by adding the oxidizing agent hydrogen peroxide (H₂O₂) leads to difficulty in fermentation progression and the accumulation of mannitol by the yeast. The level of mannitol produced during fermentation peaks seven to nine days after oxidative stress exposure, though the level is juice- and strain-dependent. Samples were collected for exo-metabolome analysis of H₂O₂-treated and untreated juices using juices known to induce mannitol accumulation when oxidized to identify the components of the juice that change under these conditions. The goal of this work is to identify biomarkers in the juice that predict stress issues.

Funding Support: American Vineyard Foundation