Scram, Spoilage! Understanding Wine Chemistry to Maintain Stability. No one wants spoiled wine and your winery’s microbial landscape shouldn’t have you shaking in your Blundstones. Although it is impossible to maintain a truly sterile cellar, monitoring specific analyses can effectively diagnose and prevent problems at all stages of winemaking.
A good quality control program incorporates reactive lab analysis in the event of a problem and routine analysis of key parameters to track trends over time. In this article, we’re going to discuss 4 key parameters and why they should be measured throughout the winemaking process.
Volatile Acidity (VA)
What is Volatile Acidity?
Volatile acidity refers to the steam-distillable acids present in wine, primarily acetic acid but also lactic, formic, butyric, and propionic acids (Buick and Holdstock 2003). VA is often measured as acetic acid (the primary component) because acetic acid is more easily and accurately measured than VA. In low concentrations, volatile acidity can enhance fruitiness, but in higher concentrations, it causes an unpleasant vinegar off-aroma.
Where does it come from?
VA is not naturally occurring in grapes. Instead, it is a byproduct of microbial activity. It can be produced by unwanted microbes like spoilage organisms or by wanted microbes like yeast and malolactic bacteria when they are stressed by fermentation conditions like temperature, alcohol, or pH.
Why monitor it?
There are many reasons why monitoring VA over time is useful:
- There are legal limitations to acetic acid in wine (1.2 g/L in whites and 1.4 g/L in reds). Checking VA frequently allows you to intervene before approaching these concentrations.
- Acetic acid, a major contributor to volatile acidity, is inhibitory to yeast. It can cause problems at concentrations > 0.6 g/L and cause stuck fermentations at concentrations > 0.8 g/L. Checking VA upon receiving fruit and prior to fermentation can help avoid this issue.
- Rising VA can indicate microbial problems that may result in additional off-aromas, off-flavors, hazes, and other wine quality or stability concerns. It is often more feasible to consistently check VA than to check other indicator compounds or to run full microbial analysis. Such microbial issues may include:
- Vineyard-originating non-Saccharomyces yeast species can produce large amounts of VA pre-fermentation. The resulting acetic acid can be inhibitory to Saccharomyces yeast and active non-Saccharomyces yeast can delay the onset of fermentation.
- Saccharomyces yeast can produce VA when stressed by fermentation conditions. Rising VA during fermentation can indicate that yeast are stressed and the fermentation may become sluggish or stuck.
- Lactic acid bacteria (including malolactic bacteria) are generally problematic after fermentation and can produce VA and a variety of other off-odors.
- Acetic acid bacteria can cause problems pre- and post-fermentation producing VA, other off-aromas, and compounds that can inhibit clarification and filtration.
- Brettanomyces yeast can produce VA in addition to “barnyard” off-aromas.
What is malic acid?
Malic acid is a naturally occurring acid in grapes. It is commonly involved in malolactic fermentation.
What is malolactic fermentation?
Malolactic fermentation (MLF) occurs when malic acid is converted into lactic acid by lactic acid bacteria. Malolactic fermentation can be intentionally induced with commercial bacteria strains or it can also occur spontaneously if not prevented (typically prevented with SO2).
Some effects of MLF include:
- Increased stability: spoilage organisms cannot utilize lactic acid
- Reduced acidity: malic acid is more tart than lactic acid
- Sensory impact: MLF can produce buttery aromas and flavors, and can lead to a smoother, rounder mouthfeel.
Most red wines undergo malolactic fermentation to minimize microbial risk during barrel aging. However, many white and rose wines do not undergo MLF as a way to preserve acidity and freshness.
Why monitor malic acid?
If malic acid is decreasing prior to or during alcoholic fermentation, this may indicate unwanted/unexpected microbial activity that should be addressed to avoid off-aromas and flavors and to avoid competition with the chosen yeast strain (in extreme cases).
If malolactic fermentation was induced, malic acid should be monitored to make sure it is completely depleted. If the MLF gets sluggish or stuck, the malic acid remains an energy source for spoilage organisms.
If malolactic fermentation was not induced, malic acid should still be monitored to ensure that it has not spontaneously begun and the prevention measures are still effective.
SO2 (and pH)
What is SO2 and where does it come from?
SO2 is naturally produced by yeast during fermentation and is also added as a preservative for its antioxidant and antimicrobial properties. It exists in multiple forms in wine and is often measured and reported as free and total SO2.
What’s the difference between free and total SO2?
To understand free and total SO2 measurements, it is important to first understand how SO2 behaves in juice and wine.
In wine, SO2 exists in two main forms: molecular (SO2) which has antimicrobial and antioxidant properties, and bisulfite (HSO3–) which does not. Bisulfite can exist as an ion, or can be found bound to sugar, polyphenols, and other juice/wine components.
Free SO2 is a measure of molecular SO2 and un-bound bisulfite. Total SO2 is the sum of free SO2 and bound bisulfite.
Molecular SO2 & pH
Disclaimer! This section gets a little technical so feel free to skip to the next, but it helps give context to why monitoring Free SO2 and pH are so important.
pH determines the portion of free SO2 present in the molecular form. More molecular SO2 is present at lower pHs.
It is generally accepted that:
Molecular SO2 = free SO2 / (1+10pH-1.8)
Molecular SO2 concentrations should be >0.8 g/L to achieve the antimicrobial effects (the antioxidant properties can be achieved at lower molecular concentrations). This means, as pH increases, more SO2 must be added to achieve a 0.8 g/L molecular concentration.
Why monitor SO2?
SO2 is volatile and free/molecular levels will decrease over time in tank or barrel. SO2 will need to be periodically added to ensure antimicrobial and antioxidant protection.
Ideally, free SO2 should be measured both before and after additions. As discussed above, there are many factors that can affect how much SO2 ends up in the molecular form, and this should be verified after an addition.
Why track alcohol over time?
Unlike the other parameters discussed in this article, changing alcohol is generally not related to potential spoilage or stability concerns. However there are still good reasons to monitor it:
- Alcohol can increase (or decrease) over time if cellar temperature and humidity control are off. This may allow the wine to creep into a new tax bracket.
- Tracking alcohol over time is a good way to catch cellar mistakes. While not common, things can go wrong with making additions to wine, messing up tank transfers, etc…
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Buick, D., Holdstock, M. 2003. The relationship between acetic acid and volatile acidity. AWRI Tech. Rev.(143): 39-43.
Margalit, Y. 1997. Concepts in wine chemistry. The Wine Appreciation Guild Ltd: South San Francisco, CA, USA: 255–257.