Assessments of Difficult to Ferment Juices
The goal of this project is to uncover the causes of chronically difficult to ferment juices. These juices are often derived from the same vineyard or block of a vineyard and othersimilarly managed vineyards and blocks displaying normal fermentation kinetics. We haveconfirmed the inhibitory role of previously identified lactic acid bacteria in yeast fermentation buthave also discovered that these bacteria are efficient at inducing the establishment of the GAR+prion in wine strains. This prion is a protein conformational change that is inherited by progenycells during cell division, thus once cells in the population have changed to establish the prion subsequent generations will also be in the prion state without need for continued induction. We have also discovered that commercial wine strains that rapidly induce the prion as this induction occurred at a rapid rate in five of the 11 genetically unrelated commercial wine strains evaluated. The prevalence of this ability to rapidly induce this state suggests the prion state plays an important role in survival during wine fermentation. We are continuing to work out the metabolic changes that occur under these conditions to help identify or genetically construct via breeding strains that would be insensitive to the bacteria but also able to grow and ferment normally. In addition to inhibitory lactic acid bacteria, we discovered three species of acetic acid bacteria that can be found on grape berries at harvest that lead to arrest of fermentation. One of these bacteria, Gluconobacter cerinus, is as efficient as the lactic acid bacteria in inducing formation of the GAR+ prion. The other two acetic acid bacteria, Acetobacter malorum and Acetobacter ghanensis are inhibitory to yeast growth, showing similar levels of inhibition as Acetobacter aceti but without making the high concentrations of acetic acid found with A. aceti infection of wine. The inhibitor in this case is as yet unknown. Under certain metabolite conditions of low nitrogen or low vitamins we have shown that the high proline can be inhibitory to yeast metabolism.
The metabolomics analyses performed this past year confirmed the presence of mannitol in sluggish fermentations from this fruit and confirmed the induction of mannitol accumulation in juices by treatment with oxidizing agents. Our working model is that the phenolic profile of these juices may have changed due to environmental conditions and the accumulation of proline in the berry is done to minimize an inhibitory aspect of this compound or compounds and that yeast cells are similarly sensitive to these inhibitors and accumulate mannitol to minimize inhibition. We further propose that high proline might interfere with the functionality of the mannitol or lead to changes in the cell membrane that decrease ability to take up key nutrients like vitamins thus requiring higher vitamin supplementation.