Assessments of Difficult to Ferment Juices

The major goal of this project is to uncover the causes of chronically difficult to ferment juices. These juices are defined as those not due to fermentation management failures and inattention to nutritional needs of the yeast and maintenance of permissive growth and fermentation conditions. These juices are often derived from the same vineyard or block of a vineyard, and other similarly managed vineyards and blocks display normal fermentation kinetics. One class of these difficult to ferment juices is characterized by a high proline to arginine ratio. We have confirmed in several yeast strains that mannitol accumulates within the yeast in these juices and that this is associated with the presence of oxidative stress. To date all of the commercial strains tested are sensitive to these juices and reduce fermentation capacity. We have also confirmed the inhibitory role of previously identified lactic acid bacteria in yeast fermentation but have 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 progeny cells 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 the need for continued induction. We have identified several other genera of lactic acid bacteria as well as acetic acid bacteria from arrested wines that are also capable of inducing the [GAR+] prion in wine yeast. We have received samples from over 53 wineries that have suspected bacterial inhibition of fermentation and have been able to isolate [GAR+] pion-inducing bacteria from many of these wines. Further we have shown that bacteria isolated from stuck wines, when grown in growth-permissive media then removed from the medium via filtration the inducer is still present in the filtrate and capable of  inducing the prion in wine strains of S. cerevisiae. Thus the inducer may be present in the absence of viable bacteria if the fermentation had bacteria present at some point. Controlling the presence of these bacteria is therefore important under production conditions.