The More I Study, the Less I Know (Apparently)

Every time I try to get behind a topic for the Stage 1 exam, I’m surprised by how little I actually know. And I’m quite sure this is a me problem, not a general MW Stage 1 condition—because most of the information is in the Diploma programme. Some of it might have made a fleeting stop in my short-term memory, but it never quite settled into real knowledge. Other parts? Probably just never registered at all.

Take something like volatile acidity (VA). You kinda know it. You recognise it in the glass, you know it’s a by-product of fermentation and also a sign of bacterial spoilage—but when, how, and why it occurs, or how to prevent and correct it? You don’t really think about that… until now.

The MW question I came across reads:
“Describe the steps a wine technician would take to confirm raised volatile acidity in a bottled red wine sample and identify the source of the problem.” (2024)

So first: what is volatile acidity?

Volatile acidity refers to the total of volatile (i.e., easily evaporated) acids in wine, including acetic, malic, carbonic, and sulfurous acids. But in reality, acetic acid accounts for over 96%¹ of it—so that’s the one measured. Naturally, it’s a by-product of yeast fermentation, but it can also result from bacterial spoilage—specifically, acetobacter reacting with oxygen. That reaction produces acetic acid, which can then combine with ethanol to form ethyl acetate, the compound responsible for vinegary and nail polish remover aromas (the use of acetone as the Korean word for nail varnish helps a lot here).

Note to self: Spontaneous or sluggish fermentations often lead to elevated VA, sometimes even stuck fermentations. Need to look into the why here.

Once VA gets too high—usually above 0.7 g/L², where the off-smells become obvious—you’re in trouble. EU legal limits cap VA at 1.2 g/L for reds and 1.07 g/L for whites and rosés. Techniques like membrane filtration or ion exchange can help reduce VA.

Traditionally, blending was the only fix. But in the late 1990s, reverse osmosis (RO) started gaining traction. The early results weren’t great—flavours were stripped along with the faults. Eric Stern once wrote in Wine Business Monthly: “The operation was a success, but the patient died.” That said, things have come a long way since. Even though wine is too niche a market to warrant membranes made specifically for faults like VA or Brett, some winemakers now believe acetic acid (due to its low molecular weight) can be selectively removed with the right combination of membrane pore size and pressure.

In these systems, the permeate (the part passing through the membrane) goes through an ion-exchange column with charged resin beads that neutralise the acetate group. It’s then recombined with the retentate (what was held back), effectively “cleaning” the wine.

But still—this raises the eternal question of cost-effectiveness. Equipment, labour, time, and sensory consequences all play a role. Not to mention: how much does the process change the character of the wine?

Which leads to the better question: how do we prevent VA in the first place? Unsurprisingly, oxygen is the main culprit.

One thing that stuck with me in this study session: VA levels above ~0.5 g/L usually result from ideal conditions for acetobacter³ (and gluconobacter, but let’s leave that aside for now). So if those conditions aren’t allowed to develop, you can avoid the issue.

‘Normal amounts’ of VA occur during different steps of the winemaking process, starting at grape reception (damaged grapes have higher level of acetobactor), during yeast fermentation, bacterial fermentation (MLF), storage in oak barrels and any other moment when the wine is in contact with oxygen, e.g. racking, blending, topping up etc. The key is to control these environments to reduce the growth of acetobactor and therefore avoiding the uncontrolled development of acetic acid and ethyl acetate. Temperature-controlled fermentation, keeping ullage level to a minimum, reasonable use of SO₂ as both antioxidant and microbial agent and usage of commercial yeast for fast results are often mentioned as prevention.

In essence, managing VA is about finding the right balance between controlling the environment and allowing the wine to express itself naturally. It’s often said that red wines will always contain some acetobacter unless membrane filtered, and many believe that a small amount of VA—around 0.5 g/L—can actually add aromatic lift and complexity. As with so much in winemaking, there is no black-and-white answer. I once read that no winemaker sets out to make bad wine; rather, each one works toward quality within the limits of their grapes, their cellar, brand positioning and their philosophy. VA, like many things in wine, could be seen in that grey area between fault and feature—and understanding it is a step toward the better quality we all aim to achieve.

That’s it for now on VA, nerding out.

---

Process	Approximate pore size	Removes	Pressure
Reverse osmosis (RO)	<0.001 μm	Salts, acids, small molecules	Very high
Nanofiltration (NF)	~0.001–0.01 μm	Small organics, divalent ions	High
Ultrafiltration (UF)	~0.01–0.1 μm	Proteins, polysaccharides	Moderate
Microfiltration (MF)	~0.1–1 μm	Yeasts, bacteria	Low

1. Robinson, J., Harding, J. and Vouillamoz, J. (2015) The Oxford Companion to Wine. 4th edn. Oxford: Oxford University Press ↩︎
2. Hoyle, P.G. (1982) ‘Volatile Acidity – An Update’, The Australian & New Zealand Grapegrower & Winemaker, (282), pp. 25–31. Available at: https://www.awri.com.au/wp-content/uploads/2018/03/s1982.pdf (Accessed: 13 May 2025). ↩︎
3. Ideal conditions for acetobacter are:
   – Temperatures between 30 and 40 degrees
   – High pH of 3.5-4.0
   – Low alcohol concentrations (which could explain the link to spontaneous fermentations)
   – Lack of SO₂ and presence of O₂ ↩︎