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.
Articles of interest:
Alert: Spotted Wing Drosophila (SWD) in Grapes
Matthew Clark1, Eric Burkness2, Drew Horton1, and Bill Hutchison2
1Grape Breeding & Enology Program, Dept. of Horticulture, University of Minnesota
2MN Extension IPM Program, Dept. of Entomology, University of Minnesota
Typically, when grape berries have intact skin, female SWD are not able to penetrate the berry skin to lay eggs. However, despite the relatively low susceptibility of grapes to being infested by SWD, as grapes age, skins become thinner and there is a greater risk for splitting and other fruit injury to occur (hail, birds, disease, general deterioration). With the very wet growing season in 2016, we have observed increased berry splitting in tight cluster cultivars. In varieties like Marquette and the Frontenac series, we have observed fruit breakdown. This degradation in berries (skin and pulp) could likely be attributed to the increased berry water content. Additionally, we have been observing necrosis on Marquette rachis leading to some berry decay but have not been able to determine if this is pathogenic or a stress response. In most years, harvest of Frontenac can be delayed as a benefit to juice chemistry as it often “hangs well” allowing acids to decline. This is not recommended in 2016.
Once fruit skin is compromised, or the fruit skin has deteriorated sufficiently, female SWD will begin laying eggs in the fruit. Despite typically low populations of adult flies, relative to other fruit crops, if fruit becomes susceptible to egg lay, populations can grow exponentially in a few short weeks reaching thousands per trap per week (http://www.fruitedge.umn.edu/swdtrap#grapes). With the limited success and/or options for insecticide use, the best options for managing SWD populations in grapes is to use production practices that minimize fruit splitting, disease, bird damage, and fruit deterioration. If fruit starts showing high levels fruit damage or deterioration, growers should consider harvesting fruit as soon as possible.
Vineyard monitoring can help inform the grower if and when the SWD are present. Although grape is not a preferred host, SWD may be present in other fruits adjacent to vineyards, including in gardens, greenhouses, composting fruit, and native plant populations. These SWD are opportunistic and will exploit damaged or deteriorating grape berries. Through monitoring, the grower can determine what steps are needed to use insecticides, although there is inconclusive evidence that insecticides are effective. Weekly monitoring will help determine what insect pests are a problem, and if their is increased presence of SWD or other pests.Traps can be purchased from catalogs such as Great Lakes IPM (http://www.greatlakesipm.com/2016%20Catalog%20Web.pdf) or traps can be constructed using the following instruction sheet (http://www.ipm.msu.edu/invasive_species/spotted_wing_drosophila/monitoring)
Vineyard sanitation is also critical to minimize fallen fruit or missed clusters during harvests. Most wine makers are familiar with fruit flies in the winery and in discarded grape must. One should consider strategies to restrict the fly movement. Consider covering waste must with fine mesh or tarps until all fruit has been harvested from the vineyard as this compost could serve as a source of continued fly infestation. Small amounts of damaged fruit/must can be placed in clear plastic bags where heat from the sun will kill larvae and flies, although this may have limited success in the fall as temperatures decline. Do not compost or bury the material as neither of these methods have been shown to eliminate SWD. Winter conditions in Minnesota are not favorable for SWD as the insects are not able to survive.
In the Winery
Fruit flies, including SWD, are adept at transferring microbes including acetobactor from berry to berry in the vineyard and from juice to wine and to equipment. These microbes are responsible for the volatile acidity (VA) that taints the wine with aromas of acetic acid and ethyl aldehyde. You will smell and taste this in the vineyard, and these berries should be discarded (although not to the vineyard floor). The use of traps in the winery fermentation area can help eliminate flies that either enter the winery on their own or those that come in with fruit at harvest. The traps used for monitoring are suitable. Simple vessels with a small volume of vinegar, sweet wine, or red wine, and a drop of unscented dish detergent are cheap, effective traps that can be set out daily. Removing must and waste regularly and sanitizing winery surfaces will help to reduce fruit flies and microbial problems.
Winery practices to control VA with sulfur dioxide (SO2) are really no different than controlling wild yeast and other bacteria that come in on the berries and are controlled through managed fermentations. However, tainted juice prior to fermentation will be difficult to manage unless it falls below detection thresholds after winemaking through blending. During primary fermentation, racking and pressing may eliminate some of the VA in addition to more SO2. Other advanced techniques include reverse osmosis and filtration to reduce the microbe population. During and after fermentation, some of the bacteria may live in a film on the juice/wine surface and not be effected by the dissolved SO2. Again, best practices to reduce oxygen at the surface in the vessel will reduce the chances for VA. Select vessels with minimal head space and consider sparging with inert gases.
Some Follow up from the event can be found here with links to important documents.
The MGGA has put together a one day MDA class with local MDA officials to help alleviate your concerns.
What: MDA Mock Inspection Class with Inspectors and Q&A sessions.
When: April 29th from 9:30am-2:30pm
Cost: $10 MGGA Members/ $40 non MGGA members
Where: Two Rivers Vineyards and Winery
6111 Us Hwy 10 Ramsey, MN 55303
Two Rivers Vineyard & Winery
Schedule: 9:30-10 Registration
10-12 Mock Production area and Tasting Room Inspection with Open Q&A with Inspectors
12-12:30 Lunch (provided)
12:30-2:30 Winery Processing with MDA Inspectors and more Q&A
Register at the following link:
On 2/19/2016, buds were chopped for winter survival measurements on 7 sentinel varieties at the University of Minnesota research vineyards at the Horticultural Research Center. The data represented here are just an example using a small sample number of canes, buds, and cultivars. Only 4 canes, 8 buds each were evaluated (32 total buds). We urge you to begin sampling your own vineyards as there is some evidence of damage. At this point, no changes would be needed to pruning based on these results at this location. More complete data will be presented in coming weeks.
Cultivar Primary Bud Survival
Frontenac gris 97%
Frontenac blanc 84%
La Crescent 91%
St. Croix 91%
Some Early images from a grafting ‘experiment’ with Ampelopsis as the scion wood (top part as a cultivar), and a hybrid rootstock from the UMN breeding program.
Starting to see callus formation at the graft union. We use the Ryset grafting tool which just like cutting puzzle pieces.
On the bottom we see callus where roots will soon start to form. The next step for us is to dip the top of the plants in melted wax (covering the bud and graft union) to prevent excessive evaporation, and to stick them in a potting media for rooting in the greenhouse. We will also likely remove the wire ties to prevent girdling as the stems begin to grow radially.
We are feeling pretty optimistic, but only time will tell if these grafts are compatible.
Savor Minnesota is a rare opportunity to taste everything Minnesota wineries have to offer, as well as samples from dozens of craft breweries and food producers, all in a single afternoon. The sixth-annual Savor Minnesota is Saturday, April 16, 2016, at Canterbury Park in Shakopee. Explore the wines produced in Minnesota in this event sponsored by the Minnesota Farm Winery Association.
Developed at the University of Minnesota, ‘Itasca’ is the newest wine grape that will be available to growers in 2017. Offering disease resistance, winter hardiness, and improved wine chemistry such as lower acid than other hybrids, ‘Itasca’ will create new opportunities for grape growers and wine makers in Minnesota and across cold-climate wine growing regions. Read more about it here.
Registration is now open for the 9th Annual International Cold Climate Wine competition. Click on the logo to learn more.
Winter is Minnesota can be one of the most challenging times for the grape plants. It's the main reason V. vinifera varieties aren't grown here. Learn a little bit about whats going on in the vineyard in winter.
Wine making is a rewarding career, but is not free from headaches. A wine maker's nightmare is the re-fermentation of sweet wines and the instability of some wines. This blog entry addresses the topic and offers some strategies to avoid and mitigate a potential devastating re-ferment.
Are you curious if your wine is finished with malolactic fermenation? Here is a quick reminder on how to test with paper chromatography.
ALERT: September 27, 2016. Spotted Wing Drosophila (SWD) in Grapes: A short memorandom on SWD in Minnesota and associated volatile acidity. Read more here.
Fall vineyard managment should focus on managing insects, vertebrate pests, rots, and diseases that will impact the vines in the next growing season. Making quality wines requires disease intervention and sorting, as infected fruit will impact wine quality. Read more here.
“From Vine to Glass: Understanding the Flavors and Aromas of Cold-Hardy Grapes and Wine”
Tuesday, May 17th*, 2016
12:00 Noon Eastern (11:00 am Central)
7:00 pm Eastern (6:00 pm Central)
*Please note this is a date change from the original date of May 10th.
Join Anne Fennell of South Dakota State University, Adrian Hegeman of the University of Minnesota and Somchai Rice of Iowa State University as they discuss their research conducted on Marquette and Frontenac as part of the Northern Grapes Project.
Friday April 29, 2016
This Saturday April 16, 2016
The University of Minnesota releases its news wine varieity 'Itasca' on April 4, 2016
Experimenting with different grafting techniques including grafting Ampelopsis with a hybrid rootstock.
Early bud chop counts on cold-hardy cultivars at the HRC