Absent an inoculation, all yeasts found in grape must and wine will originate from one or more of the following sources: vineyard, grapes, or winery processing equipment (E.J. Bartowsky, Bacterial spoilage of wine and approaches to minimize it, Letters in Applied Microbiology). According to Loureiro and Malfeito-Ferreira (Spoilage yeasts in the wine industry, International Journal of Food Microbiology 86, 2003), mature, healthy grapes harbor microbial populations (yeasts, lactic and acetic acid bacteria, filamentous molds) at levels of 103 - 105 CFU/g (colony forming unit -- a measure used in microbiology that indicates the number of micro-organisms present in a water sample (www.legionella.com/cfu)), levels that vary based on environmental conditions (rainfall. temperature, grape variety, the application of chemicals in the vineyard). Yeasts resident on grape berries tend to congregate in areas where juice might escape (Loureiro and Malfeito-Ferreira). Wine-associated yeasts are identified in the table below.
As seen from the foregoing, yeasts exist in the vineyard environment and on healthy grapes but, for our purposes, the intersection of yeasts and damaged grapes is of significance. Grapes can be damaged in any number of ways (hail, birds, etc.) but it is the damage caused by phytopathogenic molds that is of greatest interest. The effects of these molds on grapes and wine are spelt out in the table below.
According to Woolford et al., (Genome Survey Sequencing of the Wine Spoilage Yeast Dekkera (Brettanomyces) bruxellensis, Eukaryotic Cell 6(4), April 2007), Brettanomyces bruxellensis is a major microbial cause of wine spoilage worldwide and results in significant economic loss. The yeast makes "the winery itself a primary habitat surviving in the walls ... interior surfaces of presses and fermentation tanks, or on the wood of barrels." From these positions the microbe is well situated to "colonize the fermenting must or maturing wine." Wines infected with Brettanomyces will exhibit aromas of mousiness, wet wool, burnt plastic, horse sweat, or barnyard. According to the same authors, the severity and frequency of Brettanomyces has been on the uptick as winemaking has trended towads wines with higher levels of residual sugar and that are unsulfited, unfiltered, and aged on lees.
Brettanomyces is exceptionally dangerous because it has all of the characteristics of Saccharomyces cerevisiae (ethanol-tolerant, facultatively anaeorobic, can exist without mitochondrial DNA, ferments preferentially in the presence of high glucose under aerobic conditions) but extend beyond it in that, while slower growing, "it can assimilate a wider variety of carbon choices." These conditions lead to the following progression in a Brettanomyces-contaminated alcohol fermentation (Woolford et al.):
- Saccharomyces cerevisiae dominates throughout primary fermentation then is replaced by D. bruxellensis during the maturation phase when ethanol concentration is high and minimal amounts of sugar remain.
It was long thought that Brettanomyces contamination was a result of poor hygiene in wineries but contamination persists even in the face of intensive hygiene efforts on their parts (Renouf et al., Interactions between Brettanomyces and other yeast species during the initial stages of winemaking, Journal of Applied Microbiology 100 (6), June 2006). The research cited in this post seems to indicate that Brettanomyces can enter the winery through sour rot and can then take up residence within the facility and contaminate batches of wine essentially at will. Intensive hygiene efforts can clean up an infected location but re-contamination is potentially just another sour-rot affected batch away. Vigilance in screening for sour rot would seem to be in the best interest of the winery.
Grape must is nutrient-rich and ethanol poor, a candy store for most micro-organisms. The longer those conditions exist, the more non-Saccharomyces yeasts will thrive. With the exception of Brettanomyces, most wine-spoilage yeasts are ethanol-intolerant so the quicker that high ethanol levels are obtained, the less opportuntiy for these micro-organisms to proliferate. The issue with natural fermentations is that they start slowly and take longer to get up to levels in which S. cerevisiae thrives. And it is in that period that the fermented wine becomes susceptible to the travails of wine-spoilage yeasts.
The remaining disadvantages of natural wine fermentation will be coverd in a number of upcoming posts.
©Wine -- Mise en abyme