Despite our best efforts in the vineyard and good decision making in the winery, many winemakers still face a few lots from 2018 with higher VA than desired. As we learn the lessons of this vintage, and consider our options going forward, here are a few notes on volatile acidity, its causes, limits and treatment options.
By definition, volatile acidity measures the “acids that can readily be removed by steam distillation” (1). The principle volatile acid in wine is acetic acid which presents as a vinegary aroma/flavor. Other acids included in the volatile fraction include propionic (fatty), butyric (rancid butter) and C6-C12 acids (goaty). VA is reported in grams/liter, and, in the US, standardized to acetic acid equivalents, as this is the primary volatile acid. (In Europe, sulfuric acid equivalents are used, so if you are sending samples to a lab overseas, be aware of the difference in scale.) A wine’s total acidity includes fixed acids (such as tartaric and malic) that are not removed by steam distillation, and the volatile acids. (1)
Acetic Acid Butryic Acid
The Federal limit of VA is 1.2 g/L for white wines, 1.4 g/L for red wines and 1.2 g/L for dessert wines. For late harvest wines produced from grapes picked at higher than 28 Brix, the limits are 1.5 g/L for white wines and 1.7 g/L for red wines. (2, 3). In the production winery, volatile acidity is most often measured by a Cash Still or enzymatic analysis. If you are using a Cash Still, be aware the SO2 can interfere with the measurement, so you need to do the appropriate corrections. If you are measuring with an enzymatic kit, realize you are really measuring acetic acid specifically. Other volatile acids are likely to be present as well, so the VA is likely higher than what your enzymatic is reading. (3)
Propionic Acid
The sensory threshold for detection of VA is often cited as 0.7 g/L while at 1.4 g/L, wine takes on the odor and flavor of vinegar. (1) This threshold is very subject to matrix effects, meaning that some wines will show their VA at lower levels and some will hide it even at higher levels. As a general rule of thumb, VA will seem more prevalent in lighter bodied, delicate wines and less apparent in wines with higher sugar or alcohol. In addition, high levels of ethyl acetate may enhance the perception of volatile acidity. Volatile acidity interacts with other flavors in the wine to affect its sensory properties. VA magnifies the perception of acids and tannins. (4)
References
There are several reasons why a wine might have high VA. Although VA can be formed by the chemical hydrolysis of hemicellulose when wine is aged in oak, most volatile acidity comes from the activity of microbes such as oxidative acetic acid bacteria, malic acid bacteria, oxidative yeasts, and mold. (1) Not all VA is considered a fault. It is normal to have small amounts produced by yeast during fermentation (0.3 g/L). At this level, VA can even add complexity. Acetic acid is also an important intermediate in the production of acetate esters that generate much of the wine’s young fruity character. (1) VA accumulation can happen at many different stages in the winemaking process. Following is an outline of the most common causes.
Fruit that is compromised on the vine or during harvesting can lead to high VA wines. One culprit is the oxidative bacteria Gluconobacter. It is frequently present on at a rate of 10^2 cells/g on healthy fruit. However, on damaged fruit, it can be found in concentrations up to 10^6 cells/g. This microbe is good at metabolizing sugar, but it produces a copious amount of volatile acidity as a result. (1)
Another source of volatile acidity comes from the fungi that infect fruit with bunch rot. Bunch rots include oxidizing enzymes that lead to degradation of phenolics and anthocyanins. These fungi also reduce nitrogen and sugar levels. Reduced nitrogen can lead to further accumulation of VA by stressed yeast during fermentation. (2)
Compromised fruit also contains elevated levels of oxidative yeasts such as Klockera and Hanseniaspora. These produce both acetic acid and ethyl acetate and can be especially active at low temperatures such as those found during cold soak or during a long lag phase of fermentation. (1)
When picking compromised fruit, it is important to limit the time grapes spend in bins, as damaged fruit provides a sugar source for any bacteria, yeast, or mold that are present on the grapes. If grapes have been damaged by mold, they likely have higher levels of acetic acid bacteria already resident as a result. A fast processing and start to fermentation is often the best option to limit accumulation of volatile acidity. (2)
The majority of VA accumulation during fermentation is a function of yeast metabolism. If yeast are stressed by a lack of nitrogen or adverse fermentation temperatures, they can produce acetic acid as a result. Zoecklein (3) provides an excellent summary of these conditions. However, excessive VA is not producing during normal fermentations.
Fermenting juice is generally a low oxygen environment, which helps limit VA accumulation even with compromised grapes. Gluconobacter diminish during this time and are replaced by Acetobacter (also present on the grapes). Though Acetobacter is not producing acetic due to lack of oxygen, they are still present and will come back to metabolic activity later in aging.
VA can start to accumulate in the later stages of fermentation if fermentation lags and CO2 is no longer produced at high enough rate to protect the wine. Acetobacter can start to convert ethanol to acetic acid when oxygen is present. Lactic acid bacteria can metabolize sugars and produce a high level of VA as a result. LAB other than Oenococcus can cause high levels of acetic acid from the metabolism of citric, malic, tartaric, glucionic acids. Each of these needs oxygen to work and is more common in warm environments with low SO2 and pH higher than 3.5. Many wineries inoculate for malolactic fermentation in order to control the strain of bacteria and limit production of acetic acid as well as diacetyl and other byproducts. (1)
VA accumulation during aging can be chemical or biological. Even in properly stored wines, an increase of 0.06 – 0.12 g/L is expected per year in new wood due to the breakdown of wood hemicellulose by acid in wine. (3) However, spoilage during aging is usually due to microbes.
Acetic acid bacteria convert ethanol to acetic acid in the presence of oxygen. They can remain viable in wine for years under anaerobic conditions and can substitute quinones for oxygen in metabolism allowing them to continue to metabolize (slowly) when no O2 is present. (1) Storage in oak barrels allows for small amounts of oxygen (30 mg/L/yr) (3) while cellar practices such as racking and topping provide a brief bolus of oxygen to the wine. Population levels of Acetobacter have been reported to increase during racking. (1) If headspace is allowed to develop by lack of topping, increased surface area as well as available oxygen can lead to rapid VA accumulation.
Another potential source of VA during aging is Brettanomyces. Though Brett infection is characterized by accumulation of 4-ethyl phenol (4-EP) and 4-ethyl guaiacol (4-EG) that lead to barnyard and band-aid flavors and odors, Brettanomyces can also lead to rapid accumulation of VA. Zoecklein (4) cites two studies, one that reported an increase from 0.31 g/L to 0.75 g/L acetic acid in a white wine after 20 days of incubation with Brett, and another that reported 0.62 g/L under the same conditions. Like Acetobacter, Brettanomyces will utilize any oxygen that is present to metabolize sugar and produce acetic acid.
To limit VA accumulation by Acetobacter and Brettanomyces, keep the pH of the wine low, minimize O2, and keep SO2 topped up. Storage in low temperature cellars also slows down microbial metabolism. Also, keep bungs sealed tightly, and limit the number of times you pull the bung. Due to evaporation of water and ethanol through barrel staves, a slight vacuum develops in tightly bunged barrels that leads to a low oxygen environment in the headspace.(3) In Bordeaux, it is common practice to top the barrel, seal the bung tightly, turn it at an angle, and not touch it again for 6 months. The tannins in the wine coupled with the vacuum in the headspace are sufficient to protect wine from oxygen. (M. Finot, personal communication, Dec 2018)
References
Due to the difficulties of the 2018 growing season, many winemakers had at least one or two lots in the winery with higher VA than desired. Unfortunately, once VA is present, there are limited options for remediation: masking with oak or sweetness, blending, and removal with reverse osmosis or nanofiltration technology. Following is a review of the principles behind VA removal. We begin with a general discussion of semipermeable membranes and why VA removal uses reverse osmosis. If you are familiar with these concepts already, you can skip to the second section outlining the specifics of reverse osmosis for VA removal.
Regardless of the type of filter being used, filtration itself depends on the principles of permeability and concentration. Filters act as semipermeable membranes, which provide a barrier that allows some particles to cross while impeding other particles. This is true for the filter pads used before bottling as much as the nanofilters found in the Sweetspotter machine. Filters can be constructed to sort particles based on a number of factors including size, charge, hydrophobicity, and others, however the filters we are concerned with sort primarily based on size.(1)
The size of molecules in solution are measured by units of molecular weight called the Dalton. If a membrane has a pore size of 100 Daltons, this means 50% of the molecules that are 100 Daltons are expected to pass through that membrane.(2) You can think of this in terms of making baskets with different sized balls. If you use a tennis ball in a regulation basketball hoop, rim shots are likely to go in, since the hoop is so much larger than the ball. If you try this with a basketball, the probability of the ball bouncing off the rim and out of the hoop is greater. If you try with a beachball, it will not go through. Many reverse osmosis filters are around 100 D. Given the molecular weights of common molecules found in wine (Table 1) we would expect water and ethanol to pass through this membrane easily, most of the acetic acid to pass, and some of the the ethyl acetate to pass through. When measured, it turns out that 75% of the ethanol, 60% of the acetic acid, and 40% of the ethyl acetate cross the membrane.(3)
Table 1Molecular Weights of common wine chemicals. Adapted from Smith (2014)
The overall “size” of a particle really refers to its three dimensional structure. Proteins, for example, are long chains that fold in on themselves to form three dimensional shapes. This is analogous to a flat sheet of paper vs. a crumpled up sheet of paper. The membrane will sort based on whatever shape the molecule takes in the solution. Also, sometimes molecules interact with other molecules in ways that make them seem bigger to the membrane. Molecules in water based environments can even interact with the water itself. This is especially true for charged particles (ions), so any molecules in their ionized form will not pass as easily through the membrane (or may not pass at all). This many lead a membrane to retain particles that, based on weight alone, should be able to pass through (4).
VA removal is a membrane driven process that takes advantage of the permeability of membranes to separate out these small molecules of the wine while leaving the others intact. Most of the constituents of wine are much larger than the pore size of the membranes used for this process. By running the wine through the filter, water, ethanol, acetic acid and ethyl acetate are allowed through while tannins, anthocyanins, proteins, acids and most flavor molecules are retained.(2) The liquid that crosses the membrane is called permeate while the liquid that does not cross is called retentate. The retentate makes up the filtered wine while the permeate is either discarded or treated and returned to the wine, depending on the application.
Figure 2 This membrane is permeable to water but not sugar. The water moves by osmosis from the side with less sugar (more water) to the side with more sugar (less water) to dilute the sugar solution.
The other principle that governs reverse osmosis is concentration. In solution, molecules naturally move by diffusion from areas of higher concentration to areas of lower concentration. The difference in concentration across a distance is called a concentration gradient. Molecules will only move in response to their own gradient, not the gradients of other molecules. For example if you have a chamber separated by a membrane that is permeable to both salt and sugar, and you add salt to one side of the membrane and sugar to the other, each will diffuse to the other side to even out the concentration. The presence of salt will not deter sugar from diffusing. (4)
Water also moves according to this principle. The diffusion of water is called osmosis. When thinking about osmotic movement it is important to remember that in any solution, the osmotic pressure represents the sum total of all of the chemicals dissolved in that water. If a solution is 5% sugar, it is 95% water. If it is 10% sugar, it is 90% water. So, if you have two chambers separated by a membrane that is permeable to water but not sugar, and you fill one chamber with 5% sugar solution and the other with 10% sugar solution, the water will diffuse from the 5% solution to the 10% solution by osmosis.
With that in mind, picture this: you have a membrane separating two chambers. The membrane pore size is 100 Daltons. If you start with wine on one side of that membrane, what will happen?
(1) Initially, water, ethanol, and acetic acid will cross the membrane while the remainder of the wine remains on the original side. There is no water, ethanol or acetic acid on the other side, so they are diffusing down their concentration gradients.
(2) Very shortly, there will be a solution of water, acetic acid and ethanol of equal concentration on both sides of the membrane.
(3) All net diffusion will quickly stop when the concentration of these three components is equal on both sides of the membrane. In reality, water will diffuse back to the other side, as there are more chemicals dissolved in the liquid on the original side, as all those phenolics, acids and flavor molecules are still there.
That is not a very efficient membrane filtration. To counter this movement of water (osmosis) back through the membrane, and to force even more ethanol and acetic acid through, a physical pressure is applied on the original (retentate) side of the membrane. This provides the force to push more solutes and water through the membrane. However, due to the pore size, only those solutes small enough to pass continue across.(4) This is the basis of reverse osmosis: a process by which a solvent passes through a porous membrane in the direction opposite to that for natural osmosis when subjected to a hydrostatic pressure greater than the osmotic pressure.
The use of filtration for VA removal relies on the principles of permeability and concentration. Most are crossflow systems, using long parallel tubes to provide surface area and tangential flow to keep the membranes from getting clogged. As Jamie Goode explains it,
a portion of the wine is put through a tube under pressure, with walls made of a filtration membrane. Crossflow keeps the membrane from clogging, but requires a lot of surface area of membrane, so these systems use a column consisting of lots of small tubes. Water, acetic acid and alcohol pass through the filter to form permeate, after which it can be treated and returned to the wine. (5)
There are several different kinds of membranes used in cross flow filter systems with different separation applications depending on the type of filter. Table 2 gives an overview of pore sizes for membranes and what they are each used for. Reverse osmosis is the primary type used for VA removal.(1)
Table 2 Types of crossflow filters used in winemaking
The process of reverse osmosis can be applied at several steps in winemaking for different purposes. (5)
Despite relying on the same principles, machines outfitted for one purpose are not also optimized for other applications (must concentration and VA removal, for example) due to differences in pressure needed to overcome osmosis in juice vs. wine. Also, different membrane sizes may be needed.(6)
Figure 3 Steps to VA removal
To remove of VA specifically, three steps are needed. The Chateau Hetsakais website (7) outlines these fully:
(1) Run the wine though the RO filter to create a permeate of alcohol, acetic acid, and water while the leftover (retenate) is essentially the same wine with lower alcohol, less water and less acetic acid. For must concentration, one would stop here and discard the permeate.
(2) Reduce the pH of the permeate to less than 3.0 using a pH correction column. This shifts more of the acetic acid into its molecular form rather than its ionic form. The molecular form is the form that binds to the resin in the VA removal column in the next step, so the larger proportion of the acetic acid in this form, the better the binding efficiency will be.
(3) Once the pH has been shifted, the permeate is run through the acetic acid binding resin. The permeate increases in pH because as it binds to the resin, it displaces KOH, a base. After the permeate has traversed the column holding the acetic acid removal resin, the remaining permeate is closer to the starting pH and has much less acetic acid. This solution is then returned to the wine.
The process is similar for removal of ethyl acetate, however the pH shift is different due to the ionization of this molecule, and a different resin is used. Due to the importance of pH and availability of binding sites, preparation and regeneration of the columns is necessary, and pH must be monitored throughout the process. The Chateau Hatsakais website (insert hyperlink: http://chateauhetsakais.com/filtering-reverse-osmosis/) has a complete step by step guide of the practical steps involved in running one of these machines.
Despite this great technology, RO is often accompanied by flavor stripping and a shift in pH.(2, 5, 6) This is due in part to the type of membrane being used. Reverse osmosis membranes can be described as tight or loose. Tight membranes have smaller pore size and only allow water, ethanol, acetic acid and ethyl acetate to pass. However, these membranes also require higher pressure and energy requirements and have slower flow rates. Loose filters allow water, alcohol, acetic acid and ethyl acetate as well as 4-EP and 4-EG to pass (so they can be filtered out), but malic and tartaric acids can pass through the membrane as well, leading to large changes in pH that must be corrected later. These membranes have lower energy and pressure requirements and faster flow rates, so they are in common use. (6)
Smith (2017) advocates for only using tight membranes with MW cutoff of 80 Daltons. His argument is that, though these machines often come equipped with pH adjustment, if acids are making it through the membrane, most likely flavor molecules are too. Tight filters would retain acids and small sized flavor molecules. However, he points out that machines that use looser filters run at decreased pressure and increased flow rate. Smith acknowledges that machines such as the Sweetspotter by VA Filtration, though it uses loose RO membranes, is the “only alternative for micro-winery outside California (unless you have one of your own).” (This may have changed since Smith’s book.) These machines are outfitted for low energy operation that allows plug-in to 20 amp/110V wall outlet and can be used without refrigeration. He also compliments its “terrific instruction manual” and acknowledges that to substitute tighter membranes would lead to “impractically slow” permeate flow.
Each machine will be outfitted slightly differently, so if you are considering using this technology on your wine, make sure to contact the leasing company or manufacturer for specific instructions, and consider the pros and cons of this intervention. In his chapter on reverse osmosis, Jamie Goode (5) quotes Roger Boulton, renowned enologist from UC Davis, as cautioning there are “no independent published reports of the sensory effects of this treatment compared to a control, only proprietary claims and selective testimonials”. Different wines will respond differently to this procedure, but we will try to have examples of wines before and after treatment at several of the WRE tastings this year, in case you are considering this intervention yourself.
References
(1) Carey, R. (2017). Membrane Filtration - Wines and Vines (June 2017)
(2) Smith, C. (2014). Postmodern Winemaking: Rethinking the Modern Science of an Ancient Craft by Clark Smith. University of California Press
(3) Vinovation (n.d.) Uses of RO. Retrieved from http://www.vinovation.com/custequip.htm. Jan 1, 2019.
(4) Jackson, R. S. (2014). Wine Science: Principles and Applications(4 edition). Amsterdam: Academic Press.
(5) Goode, Jamie (2014). The Science of Wine: From Vine to Glass Second Edition. University of California Press. Berkely and Los Angeles.
(6) Smith, C. (2017). Selecting a Machine for Reverse Osmosis - Wines & Vines (Jan 2017)
(7) Filtering (reverse osmosis). http://chateauhetsakais.com/filtering-reverse-osmosis/. Retrieved 1/1/18.