Dr Victoria Hughes – R&D Manager/VIC State manager, Dr Carly Gamble – Technical Manager/WA State manager, and Dr Alana Seabrook – Operations manager
Volatile acidity (VA) is found in all wine to varying degrees. Whilst it can be derived from chemical oxidation, it is primarily derived in winemaking by microbial action. In a year where the sanitary state of the fruit may be compromised due to heavy rainfall, split berries and rot, grapes can even arrive at the winery with high starting VA. At high levels, VA can inhibit alcoholic and malolactic fermentation as well as impacting resulting wine quality. This article seeks to outline the definitions of VA, the sources and strategies to avoid high levels.
Total VA is made up of a number of low molecular weight compounds, with the most prominent being acetic acid (over 93%)(Buick and Holdstock, 2003). Ethyl acetate which has a nail polish aroma may also be present along with others in lesser amounts. As a consequence, the standard practice in Australian wineries is the measurement of acetic acid via enzymatic testing. Acetic acid can be easily and affordably determined. The test for analysing total VA involves steam distillation which can be extremely time consuming.
Comparison data shows that there is an excellent correlation (r2 >0.94) between volatile acidity as determined by steam distillation and acetic acid measured enzymatically using an autoanalyser (Buick and Holdstock, 2003).
Analysis of 353 wines showed that average volatile acidity results obtained by steam distillation are only slightly higher than those obtained using an enzymatic based method. This is to be expected given that steam distillation measures acetic acid, as well as additional volatile acids.
| Wine Type | Number of Samples Analysed |
Average Volatile Acidity Result (g/L) |
Average Acetic Acid Result (g/L) |
|---|---|---|---|
| Red | 194 | 0.64 | 0.55 |
| White | 139 | 0.47 | 0.40 |
| Fortified / Sweet | 20 | 0.60 | 0.52 |
| Total | 353 | 0.57 | 0.49 |
Table 1. Comparison of average results obtained by steam distillation and autoanalyser (Buick & Holdstock (2003).
Measurement of acetic acid can be performed using a benchtop spectrophotometer and commercially available test kits, with excellent correlation to results obtained by steam distillation (r2 = 0.9961). Vintessential acetic acid test kits may be used on a portable spectrometer like the Absorbance one where results are calculated automatically, or a standard spectrophotometer with the use of a calculation worksheet. Larger laboratories or wineries may have access to an autoanalyser enabling them to use the autoanalyszer equivalent.
Enzymatic analysis of acetic acid is therefore an excellent way to determine volatile acidity, requires less specialised equipment, less training and allows higher throughput of samples. It can also be performed quickly and cost efficiently by your local Winechek laboratory.
White wines that don’t go through malolactic fermentation often present VA levels of less than 0.2 g/L acetic acid. Red wines that go through alcoholic and malolactic fermentation present levels ranging from 0.4 -0.8 g/L acetic acid, but may go as high as 1-1.2 g/L. A well-integrated premium red wine may have a VA of 1 g/L and not have an obvious acetic, or volatile character, while another wine with the same level will have distinct characters pointing to a fault. The maximum legal limit for wine produced in Australia is 1.5 g/L of volatile acidity excluding sulphur dioxide, expressed as acetic acid (Food standards 4.5.1.5).
The sanitary state of the fruit harvested can have an impact on VA. Bird peck and rot are examples of physical effects on the berry that causes the grape sugars to be exposed to the environment. This can cause Acetobacter and Gluconobacter species to proliferate and produce high levels of VA before alcoholic fermentation has even commenced.
VA is best measured routinely throughout the winemaking process. VA levels above 0.8 g/L acetic acid are inhibitory towards yeast and will inhibit further alcoholic fermentation after this point. If alcoholic fermentation has not been completed and the levels of VA are above 0.8 g/L, it is recommended to remove a proportion of the VA by using reverse osmosis or blending. Once the levels are below 0.8, alcoholic fermentation may be re-started.
Acetobacter spp are an oxygen loving bacterium, commonly present in all unfiltered wines at levels off 1000 cells/mL. Ullaged tanks and barrels provide an opportunity for the Acetobacter species to proliferate and produce significant levels of acetic acid in several days. Heat, oxygen and residual sugar all contribute to ideal conditions for Acetobacter growth. By routinely topping barrels and maintaining an ideal level of molecular SO2 this can be avoided.
Non-Saccharomyces yeast such as H. uvarrum can produce limited levels of VA if allowed to proliferate in the early stages of fermentation (Albertin et al., 2014). Saccharomyces cerevisiae yeast produce some acetic acid during the exponential growth phase of the yeast in the first third of fermentation. This typically ranges from 0.2 – 0.5 g/L of acetic acid. In general, levels above this are not produced by the yeast but other species including Lactic acid bacteria (LAB) and Acetic acid bacteria (AAB). Lactic acid bacteria are also able to produce some VA in the range of 0.15-0.3 g/L. By the end of alcoholic and malolactic fermentation, a VA of 0.6-0.8 g/L or below is common. Elevated levels of VA are typically attributed to the presence of spoilage bacteria, namely Acetobacter species but sometimes Lactobacillus and Pediococcus species. Lactobacillus and Pediococcus species are more common in wines with elevated pH and they can utilise glucose, tartaric acid and glycerol to produce acetic acid.
Delays in alcoholic or malolactic fermentation whereby the wine is not protected by SO2, can lead to proliferations in acetic acid bacteria and cause high levels of VA. Oenococcus oeni, the species commonly inoculated for malolactic fermentation (and routinely the species that naturally proliferates in the absence of inoculation) can consume sugar and produce low levels of volatile acidity. Commercial strains are selected to ensure minimal production of VA and typically have a preference for metabolising malic acid over glucose (Ref). Excess VA produced in a struggling alcoholic fermentation is more likely due to a high population of AAB rather than sugar metabolism by O.oeni. Co-inoculation of yeast and bacteria is a common practice which does not typically present with high levels of VA.
Brettanomyces bruxellensis can produce volatile acidity however only in aerobic fermentation. In wine B. bruxellensis typically grows anaerobically (without the presence of oxygen).
Understanding the sanitary state of the harvest is the first indicator of possible VA production. Compromised fruit with sugars exposed are likely to have a higher population of AAB and pose a higher risk of downstream VA production. The use of SO2, bioprotection, or simply inoculation of a commercial strain of S. cerevisiae to a fermentation which would otherwise have been left to spontaneously ferment may assist with preventing the proliferation of further levels of AAB.
Inoculating with commercial strains of yeast and bacteria has a two fold effect of reducing VA. Firstly by using strains that have been identified as not having excessing VA production and secondly but inoculating high levels of known yeast and bacteria which prevents the proliferations of less desirable species that may produce VA.
Any gaps between the start of alcoholic fermentation and the start and finish of MLF pose a risk of unknown species of LAB and AAB proliferating. Minimising the time whereby the wine is not protected with SO2 will assist in reducing possible spoilage. Understanding that warmer temperatures and exposure to oxygen promote the growth of AAB is also important when considering fermentation parameters.
References
Albertin, W., Bely, M., Marullo, P., Masneuf-Pomarede, I., Miot-Sertier, C., Coulon, J., . . . Colonna-Ceccaldi, B. (2014). Oenological prefermentation practices strongly impact yeast population dynamics and alcoholic fermentation kinetics in Chardonnay grape must. International Journal of Food Microbiology, 178(C), 87-97.
AWRI (2018), Volatile Acidity, Grapegrower & Winemaker, Issue 648: 16.
Buick, D., and Holdstock, M. (2003). The relationship between acetic acid and volatile acidity. AWRI Technical Review (143): 39-43.
Illand, P., Bruer, N., Edwards, G., Caloghiris, S., and Wilkes, E. (2013). Chemical Analysis of grapes and wine: techniques and concepts 2nd edition. Patrick Illand Wine Promotions Pty Ltd.
Ribéreau-Gayon, P.; Glories, Y.; Maujean, A; Dubourdieu. 2006. Handbook of Enology Second Edition Volume 1: The Microbiology of Wine and Vinifications. Chichester: John Wiley & Sons Ltd: 59, 191, 238.
Sponholz, W.R. 1993. Wine Spoilage by Microorganisms. Fleet, G.H. ed. Wine microbiology and biotechnology. Singapore: Harwood Academic Publishers: 395–420.
Boulton et al. 1998
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