What is a spoilage microbe?
When it comes to wine microbes, a lot of times we think about splitting them into two categories: “good” and “bad.” In the simplest terms, we think of Saccharomyces cerevisiae as the “good” yeast and we think of Oenococcus oeni as the “good” bacteria, and pretty much everything else is considered “bad.”
What makes a microbe bad?
If we think about microbes in society, we’re used to thinking about germs that have a negative impact on human health. Similarly, a spoilage organism in wine can be defined by the undesired characteristics it causes. Uncontrolled acetic acid bacteria lead to unpleasant vinegar aroma, Brettanomyces yeast contamination leads to smoky/leather/manure/medicinal aromas, and so on. In contrast, “good” yeast and bacteria are the ones that do what we want – namely, they efficiently convert sugar to alcohol or convert L-malic acid to L-lactic acid – with minimal unwanted effects.
Sensory impacts of wine microbes
We’ve all learned that a good wine yeast, typically S. cerevisiae, performs the following conversion through a series of reactions:
C6H12O6 (glucose) à 2 C2H5OH (ethanol) + 2 CO2 (carbon dioxide)
But it turns out it’s a lot more complicated than that, because this main pathway involves many intermediate compounds that can branch in lots of possible directions and form other products.
In fact, every strain of yeast and every strain of bacteria does a whole bunch of different things as part of their metabolism. As winemakers and wine consumers, we just happen to like what some do better than what others do.
Can “spoilage” yeast be a good thing?
Non-Saccharomyces wine yeasts are typically considered spoilage organisms, specifically because they do not perform the same way as the more-desired S. cerevisiae. They tend to have lower fermentation rates and lower ethanol tolerance, and they may produce elevated levels of compounds like volatile acids or hydrogen sulfide.
However, the perception of non-Saccharomyces yeasts may be changing. Some winemakers and consumers are recognizing the value of a more complex microbial community during fermentation, particularly if they believe it reflects the naturally-occurring yeasts in the vineyard. Because the metabolism of these yeasts differs from that of S. cerevisiae, they can do things that commercial S. cerevisiae can’t, and some of those things have a positive impact on the wine. For example, S. cerevisiae lacks the β-glucosidase enzyme that releases some classes of desirable aroma compounds, while many non-Saccharomyces grape and wine yeasts express the needed enzyme.
When good microbes go bad
If we define a spoilage microbe by its unwanted effects, then even our “good” strains of yeast and bacteria are capable of causing spoilage characteristics. For example, a stressed S. cerevisiae fermentation can produce high levels of unpleasant sulfur compounds, resulting in aromas of rotten eggs, sewer gas, or garlic. Despite being “good” bacteria, the O. oeni used for malolactic conversion can cause problems, too, including overpowering diacetyl (buttery) aromas or geranium taint through the metabolism of sorbic acid.
Context is key
With all this in mind, it makes sense to characterize a spoilage organism not by what it is, but by whether or not we like what it does. Like so many things in winemaking, what a given microbe does is dependent on chemical and environmental factors. And like so many things in wine tasting, whether we like it is dependent on context, expectations, and preference. For example, aged red wines from the Rhône or southern Italy often exhibit Brett-derived aromas, and many consumers appreciate that as typical of the region and the style. Then again, individual consumers may have a strong aversion to a certain characteristic that leads them to always consider it a fault – as Jennie likes to say, manure belongs in fertilizer, not in wine.
For more information
Register for the free Research and Winemaking webinar on Microbial Spoilage on August 3rd
Download the free Wine Faults fact sheet series or reach out to us at firstname.lastname@example.org