Issue link: http://winesandvines.uberflip.com/i/513570
June 2015 P R A C T I C A L W I N E R Y & V I N E YA R D 57 W I N E M A K I N G played low YAN values and sluggish fermentations. During the 2014 harvest, amino acid levels were monitored in this block. This analysis showed that the proline to argi- nine ratio is high from the onset of vérai- son and continues to increase throughout ripening. This increase is due both to loss of arginine and net synthesis of proline. For reasons that are unclear, this process seems to be retarded in vines infected with red blotch as they show a reduced pro:arg ratio along the spectrum of ripen- ing with an initial ratio of 56 rising to 70 at harvest as compared to an initial ratio of 144 for the uninfected vines rising to 233 at harvest. We determined that increased proline appears to have an inhibitory effect on yeast fermentation capability under con- ditions of low nitrogen and low vitamins in the must. The effect appears to be vari- able with respect to the commercial yeast strains tested, with strain D254 being more susceptible to low vitamin condi- tions regardless of must proline content as compared with UCD2895, EC1118 and UCD522, while EC1118 was only suscep- tible to these conditions if must proline levels were elevated. Under conditions of low amino nitro- gen and vitamin supplementation, D254 was inhibited by higher proline levels that did not appear to affect the other yeasts tested. Note that all strains in the experiment were inhibited by low amino nitrogen musts without vitamin supple- mentation regardless of proline content. Emerging inhibitory microbial competitors The second class of chronic problem- atic fermentations is characterized by an atypical microbial profile of the juice as determined by ETS Scorpion analyses conducted by the wineries. The micro- bial profiles of these juices show very low levels of Lactobacillus kunkeei (less than 50 cells/mL), variable but still low levels of other lactobacilli and pediococci (also less than 50 cells/mL), with very high populations of un-typed acetic acid bac- teria (more than 2,000 cells/mL). These analyses were conducted post- arrest of the fermentation, and it is chal- lenging under such circumstances to know if the organism caused the arrest or merely opportunistically started growing after the yeast were negatively impacted. S tuck and sluggish fermentations not only incur a financial cost to wineries but also negatively impact perceived wine quality. The causes of arrest of yeast fermentation during wine production are relatively well understood. 4 Judicious management of fermenta- tion nutrients, microbial competition and temperature dramatically reduces the risk of arrest of fermentation. 1,2,3,4 It is also important to select the correct strain for the juice/must conditions, as strains vary in nutrient requirements and tolerance of stress during growth and fermentation. 1,2,3,4,6,8 Key factors in strain selection are eth- anol tolerance, nitrogen requirements and sensitivity to temperature shocks and presence of other microbes. Such information is often available from yeast suppliers and directly applicable com- mercially in most cases. However there remain instances of sluggish and arrested fermentations for juices that meet all the existing criteria for successful completion of fermentation. We have provided fermentation guide- lines that have significantly reduced the incidence of problem fermentations in the wine industry (wineserver.ucdavis. edu/industry/enolog y/fermentation_ management/problem_key/index.html). In spite of this success, there remain incidences of arrest of fermentation in California that are not due to any well- documented causes. The goal of our American Vineyard Foundation-funded project on assess- ment of difficult-to-ferment juices is to uncover the causes of these chronically challenging wine fermentations that are non-responsive to current best practices of fermentation management. These juices are often sourced from the same vineyard or block of a vineyard along with other similarly managed vineyards and blocks displaying normal fermenta- tion kinetics. In recent years there has been an increase in the incidence of difficult- to-ferment juices, defined as those that show problematic fermentation issues regardless of nutrient supplementation or yeast strain. 6 Some of this is due to the practice of harvesting at higher Brix values, which places more stress on the yeast for completion of fermentation including increasing nutrient needs of the population. High proline content/low YAN juices However, there are many cases in which juice derived from fruit from the same vineyard with similar sugar and nutri- tional profiles display very different fer- mentation profiles when inoculated with the same strain. 12 One class of these juices displays an atypically high proline to arginine (pro:arg) ratio and is, overall, low in total nitrogen (YAN). The pro:arg ratio in proline-accumulating fruit is generally in the range of 6 to 20, and these difficult juices often display pro:arg ratios greater than 50, and in some cases higher than 200. YAN values for these grapes are in the range of 50 to 80, well below that needed to support a fermentation to dryness. Our research has shown that simple addition of nitrogen as diammonium phosphate to acceptable YAN levels does not significantly improve fermentation performance, nor does increasing the arginine content to a near-normal ratio or adding a complex nitrogen supple- ment. This suggests that while the low nitrogen is a problem, it is not the only problem associated with these juices, and we suspect that whatever factors are driving the increase in proline within the berry are also negatively impacting the yeast or that other elements of the grapevine stress response are leading to the presence of inhibitory conditions within the juice. In working with one commercial Chardonnay vineyard with high pro- line to arginine ratios, we determined that increased nitrogen in the vineyard improved YAN, reduced the pro:arg ratio and restored fermentation rate. But YAN was improved with two years of nitrogen fertilization, and fermentation gradu- ally improved over four years, thus the inhibitory factor does not immediately respond to vine nutritional improvement but fermentation does eventually become more normal. A block of Cabernet Sauvignon in the Oakville Research Station vineyard dis- Linda F. Bisson, professor, Vidhya Ramakrishnan, Lucy Joseph, Koichi Sakamoto, Peter Luong, Qinqwen Fan, Yan Luo and Gordon Walker, Department of Viticulture & Enology, University of California, Davis BY When winemakers do everything correctly: The Challenges of difficult-to-ferment juices