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28 WINES&VINES March 2016 H undreds of articles about stuck fermentations cover how to avoid them as well as restart them. It is an art that relies on experience and intuition about what went wrong with a particular fermentation. The traditional methods that help to avoid stuck fermentations in- clude assessment of spoilage organism potential, nutritional status of the initial must and viability of the yeast used relative to the style of wine desired. Many laboratories servicing the wine indus- try offer assessment panels to test a winery's grapes, but in most cases winemakers don't use that service until it is too late. The sooner the winemaker understands there is a problem, the easier it will be to begin a triage of the situation and the better the resulting wine will be. Even when a winemaker thinks all bases are covered, occasionally fermentation for a certain wine won't perform the same way it did in previ- ous seasons. As a wine consultant and wine- maker, this has always been a bothersome situation to resolve: Why would this wine, this season, not complete fermentation? This was the case with wines from a recent client of mine. The winery had a series of wines with fermenta- tions that had started normally. Then, sometime after the lag phase had passed and the yeast was on the con- cluding side of the fermentation curve, the rate slowed to a crawl and apparently stopped at about 2° Brix. After trying all the traditional paths to restart the fermentation, they asked me to figure out what might be causing this situation. When I reviewed the wines organoleptically, I found no off odors. A n a l y s i s s h o w e d volatile acidity was below any thresh- old and, there- f o r e , n o t a problem. VA is one of the more frequent causes o f s t u c k f e r - mentations at this level, and remedial efforts that would exacerbate that situation should be avoided. However, one of the techniques the client had tried was to introduce lysozyme, which is frequently added to reduce the bacterial load in a fermentation. Lysozyme is most effective against gram-positive bacteria, which meant that after the lysozyme addition, the wine still could have had Acetobacter or other acetic acid-producing bacteria present. Macro oxygenation is a technique frequently used to help fermentations. Yeast are facultative organisms, meaning they can survive both in aerobic and anaerobic environments. In a nor- mal fermentation during lag phase, yeast grows rapidly and lives off the dissolved oxygen that is in the must. Eventually the organism population exceeds the capacity of the must to bring more oxygen in from pumping over, and so the yeast switch from aerobic metabolism to anaero- bic metabolism. That is when aug- mentation of yeast growth is helped by injection of up to 60 mg/L per day of oxygen into a must, which can encourage the yeast to continue to grow at maximal rates. This technique is not without some peril, as Acetobacter are not facultative organisms. With- drawing oxygen slows and then stops growth of the acetic acid bacteria during fermentation. If you know that there are Acetobacter organisms present, you need to mon- itor any additions of oxy- g e n t o m a i n t a i n t h e health of the wine. It was with much caution that I broached the possibility of adding oxy- gen to these fermentations. At this time I re- membered a news brief published by Linda Bisson from the University of California, Davis, about a new concept of yeast metabolic regula- tion by bacteria that involved prions. Use of the environment The magazine Cell published an article by Daniel F. Jarosz et al. titled "An Evolutionarily Conserved Prion-like Element Converts Wild Fungi from Metabolic Specialists to Generalists." In the ar- ticle, the authors described a novel means of controlling yeast use of glucose through secretion of a prion that switches the yeast from exclusively metabolizing glucose as their food source to becoming a generalist in use of available carbon sources for metabolic activity (see "Switching Rate Tuned to Ecological Niche"). As described in the article, use of the prion acts as if it is a genetic mutation but instead functions as an epigenetic mechanism, causing a fundamental change in the attacked organ- ism's use of its environment. The prion control is niche-based and reversible (see "Bet-Hedg- ing Properties of the [GAR + ] Prion"). The action of the yeast prions in restricted nutritional environments stresses the cellular protein-folding network. So, evolutionarily, the yeast has retained the ability to hedge its sur- vival by being able to switch carbon sources based on its local environment, except that in the wine-fermentation environment, bacteria have caused this to happen at precisely the wrong time. It happens when resources are at their most critical level for the yeast. This ability to switch carbon sources is an evolutionarily conserved trait of Saccharomyces cerevisiae. Several types of typical wine bacteria such as strains of Lactobacillis and a couple of Pediococcus have the ability to induce this switch in carbon sources. Once this switch occurs, it can remain present in a given culture for many generations of yeast growth. Over time and under presentation of a new environmental set of conditions, S. cerevisiae will be able to be- come a glucophial again. The process takes the [gar - ] state (glucose repressed) where the yeast n RICHARD CAREY The Role of Bacteria in Stuck Fermentations Winemaking