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60 P R A C T I C A L W I N E R Y & V I N E YA R D June 2015 W I N E M A K I N G mentation. Or, alternately, these lower levels of acetic acid and ethyl acetate in these fermentations infected with atypi- cal acetic acid bacteria may not be as attractive to fruit flies, and thus the bacteria may not be vectored to other fer- mentations within the winery. Therefore, identifying the inhibitor and mitigating its impacts may be more advantageous than monitoring levels of the bacterium. Identifying the inhibitor is important to enable rapid screening of different yeast strains for tolerance to this com- pound, rather than needing to do co- fermentations. Four independent isolates of Gl. cer- inus were identified from arrested fer- mentations, and all four strains were able to induce [GAR + ] prion state. Like Gluconobacter oxidans, Gl. cerinus does not appear to directly inhibit yeast growth but rather induces formation of the [GAR + ] prion, which, in turn, produces the inhibition. The analysis revealed bacterial strain differences within a species with respect to level of induction of the [GAR + ] prion on both yeast strains UCD932 and EC1118. The two yeast species are used because EC1118 is more readily induced (shows denser spots of growth at higher dilutions) than UCD932. In a related study, we are directly assessing the sulfur dioxide tolerances of these bacterial strains. Many of the prob- lematic wines had negligible or low sul- fur dioxide levels. However a few of the affected commercial fermentations did have 50 ppm total SO 2 added, so these strains may potentially be more resistant to SO 2 than previously thought. This will require further analyses with accurate determination of molecular SO 2 levels. Assessment of the impact of juice oxidative stress and vitamin content on yeast fermentation progression A second objective of our research involves assessment of juice oxidative stress, vitamin content and proline levels in relation to low and normal amino acid levels on fermentation rates. A compo- nent of this objective was the screen- ing of a set of commercial, winery and vineyard yeast isolates to identify those that, in addition to UCD522, might show greater tolerance of these inhibitory con- ditions. Preliminary screening experi- ments showed that high proline by itself in a nutrient-sufficient environment had little to no impact on fermentation. Prior analysis conducted on juices that were difficult to ferment have shown accumulation of mannitol and changes ORGANIZED BY WORLD LEA DER IN WINE TECHNOLOGY 3 rd — 6 th NOVEMBER 2015 Fiera Milano (Rho) Italy info@simei.it / simei.it 26 TH EDITION INTERNATIONAL ENOLOGICAL AND BOTTLING EQUIPMENT EXHIBITION