Volatile Acidity in Wine Making
Drew Horton and Matthew Clark
Dept. of Horticulture, University of Minnesota
Volatile Acidity, or "VA", is caused by a type of bacterial spoilage which produces large amounts Acetic acid (vinegar) which is a serious wine fault, the metabolization of acetic acid and alcohol (ethanol) can produce ethyl acetate which smells like nail-polish remover and is also a serious wine fault.
The legal limit of VA in finished wines is: 1.2 gram/liter in whites, 1.4 g/l in reds, 1.2 g/l in dessert wines and for late-harvest wines picked at 28 °Brix or above it's 1.5 g/l for whites and 1.7 g/l in reds.
High VA wines can only be brought within legal limit by "blending away" with low VA wine, or by the use of specialized reverse osmosis filtration.
Most or all of the following can be considered "best practices" in all wine making, but are especially important when working with poor condition or otherwise "compromised" or degraded fruit.
How to minimize VA production in the winery
Clean, clean, clean and SANITIZE your winery and all surfaces and equipment that come into contact with wine as effectively, thoroughly, and as often possible. Pay particular attention to keeping floors and drains as clean as possible.
Harvest fruit as quickly, cleanly and as cool as possible, and proceed to crushing/de-stemming and/or pressing as soon as possible. Do not attempt to use obviously damaged, degraded or rotted fruit. Sorting of fruit prior to processing is advised if it can be done cool, quickly and cleanly.
Use a 50 ppm "shock" dose of sulfur (SO2, aka potassium meta bisulfate (KMBS)) on all juices or musts as soon as possible just prior to or during initial grape-reception and processing to reduce the bacterial "load'. This dose should be sufficient to eliminate native yeast fermentation or other microbes being transported into the winery. Do NOT use "cold-soak” methods or encourage 'wild" or "native" yeast fermentations when VA risk is high.
Encourage healthy and fast-starting fermentations through proper yeast hydration strategies that include the use of a yeast hydration nutrient. A yeast hydration nutrient is DIFFERENT from a yeast fermentation nutrient, and the difference is important. The ultimate goal is to reduce any fermentation stresses and encourage a quick and complete fermentation by the use of adequate yeast nutrition and managed temperature control. Each yeast has a preferred temperature range that you should be aware of and will be indicated on the original packaging or in the supplier catalog.
Choose and use pure strains of yeast that are proven to be low producers of volatile acidity, including: "BC" (a Bayanus yeast), Lalvin "C" and "DV10", and ICV "OKAY”. These yeast do not actually lower VA, rather they are proven strong fermenters with a fast start and completion. All commercial "pure" strains of yeast are chosen based, among other things, on the fact that they are naturally low producers of VA.
Eliminate or control the population of fruit flies by keeping all fermentations covered with a physical barrier (even thin plastic sheeting held in place by string and a bungee cord) and the use of fruit fly traps in the winery (a glass or jar with an ounce or two of sweet or red wine in it with a drop of dish soap makes a cheap and effective trap, or commercial "fly strips" or "tape" can also help).
Keep all winery waste (stems, skins, lees, and pressed grape pomace) as far away from winery as possible, at least 100 yards away from winery, or further if possible, do not keep grape-waste near the winery for extended periods.
Avoid cross-contamination of different lots of wine, sanitize all sample devices, valves, and wine thieves with a sulfur/citric acid solution or 40 per cent ethanol solution (i.e. cheap vodka, which is an inexpensive, available and effective surface sanitizer).
The recipe for effective SO2/citric acid solution is 3 grams of citric acid and 0.5 gram of KMBS per liter of water. This should produce a solution with a pH of 2.2, and active SO2 level of 250 ppm.
Other acid-based, iodine-based, or quaternary ammonia based sanitizers are highly-recommended.
Minimize oxygen exposure during all phases of production and storage by eliminating or reducing head space in vessels.
Minimize oxygen exposure during processing, settling, racking or transfers and during storage by "sparging" (i.e., displacing the air) all tanks and hoses with a food- grade, inert gas (CO2, nitrogen or argon) prior to use and/or filling. Keep any headspaces sparged with inert gases at least twice a week.
Dry ice can help in minimizing oxidation by its evaporation into carbon dioxide gas which displaces air/oxygen during processing, pressing, and prior to filling tanks. One should be careful with the handling of dry ice and be aware of the safety hazards of using this product, including its storage.
Minimize or eliminate any head space in storage vessels, and if necessary, spray a mixture of sulfur (SO2 aka KMBS) and water on the exposed surface of stored wine by mixing 0.5 gram of KMBS with one liter of clean water, this 250ppm SO2 solution can be sprayed directly on to the surface of wine in storage, especially if the presence of a "film yeast" is noticed. If large amounts of film yeast form, attempt to siphon or rack the clean wine under the film yeast to another container, leaving the contaminated "film" behind. “Early” sterile filtration may be used to reduce or eliminate bacteria load prior to storage or maturation.
At ALL times, keep the stored or maturing wines at an appropriate Free SO2 level, which should be checked and adjusted at least once per month.Keep all wines in barrels topped monthly at least and at an appropriate free-SO2 level, based on the wines pH (Molecular SO2 chart: http://srjcstaff.santarosa.edu/~jhenderson/SO2.pdf)
All white wines, fruit wines, and sweet wines should be maintained at 0.8 molecular SO2. Only dry red wines can be maintained at the lower 0.5 molecular SO2 level.
A "cool" or cold cellar is better than a "warm" cellar, acetic acid bacteria produce faster in a warm environment. A very clean and very cool (50-56°F / 10-13 °C) cellar/work area is a great help in slowing and reducing bacterial spoilage.
The website "vinoenology.com" is an excellent resource for various wine making "calculators" including: SO2 additions, acid additions, pH adjustments, blending, copper sulfate additions and fortifications, etc.
IN SHORT: KEEP IT CLEAN, KEEP IT COOL, PROPERLY AND TIMELY MANAGE THE USE OF SO2, AND MINIMIZE OXIDATION/OXYGEN EXPOSURE AT ALL TIMES.
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