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62 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 13/04/15 11:47 proline in the berry minimizes an effect of these phenolic compounds. Yeast cells are similarly sensitive to these conditions but accumulate mannitol to minimize the effects. We confirmed the presence of mannitol in sluggish fermentations from fruit with these chemical signatures, and propose that high proline might lead to changes in the cell membrane that may impact nutrient acquisition or inhibit the protective role of mannitol. Solutions for winemakers Detailed yeast strain sensitivities with respect to bacterial interactions or must nutritional conditions have yet to be fully delineated. However, a few takeaways are in order. Winemakers may want to alter the lev- els of sulfur dioxide used when pressing or crushing grapes in order to knock out bacteria that can trigger the processes that we now know can lead to a sluggish or stuck fermentation. They also should be careful about blending grapes from vineyards known to have certain bacte- rial strains or have historically produced sluggish or stuck fermentations. As we come to know more about sus- ceptibility of different yeast strains to conditions leading to stuck fermenta- tions, winemakers could add specific strains that have the ability to overpower inhibitory vineyard bacteria or can sur- vive nutritionally difficult conditions. PWV Bibliography 1. Agenbach, W.A. 1977 "A study of must nitrogen content in relation to incomplete fer- mentation, yeast production and fermentation activity." Proc. S. Afr. Soc. Enol. Vitic. 66–87. 2. Bell, S.-J. and P.A. Henschke. 2005 "Implications of nitrogen nutrition for grapes, fermentation and wine." Aust. J. Grape Wine Res. 11: 242–295. 3. Bely, M., J.M. Sablayrolles and P. Barre. 1990 "Automatic detection of assimilable nitrogen deficiencies during alcoholic fermentation in oenological conditions." J. Ferm. Bioeng. 20: 246–252. 4. Bisson, L.F. 1999 "Stuck and sluggish fermenta - tions." Am. J. Enol. Vitic. 50: 107–119. 5. Brown, J.C. and S. Lindquist. 2009 "A heri- table switch in carbon source utilization driven by an unusual yeast prion." Genes Dev. 23: 2320–32. 6. Dhar, P. 2012 "Metabolomic profiles and etha - nol yields of commercial Saccharomyces strains during Chardonnay fermentation." MS Thesis, UC Davis. 7. Daniel, F., D.F. Jarosz, J, C.S. Brown, G.A. Walker, M.S. Datta, W.L. Ung, A.K. Lancaster, A. Rotem, A. Chang, G.A. Newby, D.A. Weitz, L.F. Bisson and S. Lindquist. 2014 "Cross-kingdom chemical communication drives a heritable mutually beneficial prion based transformation of metabolism." Cell 158: 1083–1093. 8. Maisonnave, P. I. Sanchez, V. Moine, S. Dequin and V. Galeote. 2013 "Stuck fermenta- tion: Development of a synthetic stuck wine and study of a restart procedure." Int. J. Food Microbiol. 163: 239–247. 9. Quain, D.E. and C.A. Boulton. 1987 "Growth and metabolism of mannitol by strains of Saccharomyces cerevisiae." Microbiol. 133: 1675–1684. 10. Shen, B., S. Hohmann, R.G. Jensen and H.J. Bohnert. 1999 "Roles of sugar alcohols in osmotic stress adaptation. Replacement of glyc - erol by mannitol and sorbitol in yeast." Plant Physiol. 121: 45–52. 11. Solomon, P.S., O.D.C. Waters and R.P. Oliver. 2007 "Decoding the mannitol enigma in filamentous fungi." Trends Microbiol. 15: 257–262. 12. Stockert, C.M. L.F. Bisson, D.O. Adams and D.R. Smart. 2013 " Nitrogen status and fermentation dynamics for Merlot on two rootstocks." Am. J. Enol. Vitic. 64: 195–202.