How Is Vinegar Made? A Second Fermentation Nobody Wanted
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The French name means sour wine, which is exactly what it is: alcohol that a particular family of bacteria has oxidised into acetic acid. For most of history this was a failure, since a barrel of wine left exposed to air becomes vinegar whether the owner wants it or not, and the technology of winemaking is substantially the technology of preventing it. What makes vinegar interesting is that the same accident turned into a preservative, a medicine, a cleaning product and one of the five basic tastes.
The chemistry
Vinegar is the product of two fermentations in sequence. In the first, yeast converts sugars into ethanol and carbon dioxide, which is how wine, beer, cider and any other alcoholic base is made. In the second, acetic acid bacteria, chiefly of the genera Acetobacter and Gluconacetobacter, oxidise the ethanol into acetic acid and water. The crucial difference between the two is oxygen: yeast fermentation works without it and the bacterial step requires it, which is why a sealed vessel makes wine and an open one makes vinegar. The bacteria are aerobic, form a film on the surface where the air is, and cannot work in a full closed container. Finished vinegar is typically four to eight percent acetic acid, and the pH around two to three is what does the preserving, since most spoilage organisms and pathogens cannot grow in it, which is the whole basis of pickling.
The methods
Industrial and traditional production differ in how fast oxygen is delivered to the bacteria:
- •The Orleans process, the traditional slow method, in which barrels are partly filled and left with air access for weeks or months while a surface film converts the alcohol, producing a complex result and tying up a great deal of capital
- •The generator or trickling process, developed in the nineteenth century, which trickles the liquid over a packed bed of wood shavings colonised by bacteria, exposing a large surface to air and cutting the time to days
- •Submerged fermentation, the modern industrial method, which blows air through the liquid in a stirred tank and completes the conversion in roughly twenty-four hours, which is how most cheap vinegar is made
- •The mother, a cellulose film produced by the bacteria and visible as a cloudy disc or strands, which is harmless, is a sign of live culture and is filtered out of most commercial products for appearance
- •Pasteurisation and filtration, which stop further change and remove the mother, and whose absence is what raw or unfiltered on a label means
The types and what they come from
Every vinegar takes its character from its alcoholic base and from what happens afterwards. Wine vinegar, red or white, reflects the grape. Cider vinegar comes from apples and is the one with a large body of health claims attached. Malt vinegar is made from unhopped beer and is a British and northern European staple. Rice vinegar, milder and less acidic, is central to Chinese, Japanese and Korean cooking, and sweetened rice vinegar is what makes sushi rice. Chinese black vinegar is made from glutinous rice or sorghum and aged, giving a smoky depth. Distilled white vinegar is made from fermented industrial alcohol, contains essentially nothing but acetic acid and water, and is the cleaning and pickling product. Balsamic is a category of its own and the label is doing a great deal of work: traditional balsamic from Modena is made from cooked grape must, aged for a minimum of twelve years and frequently twenty-five in a series of shrinking barrels of different woods, and is sold in small bottles at high prices, while the inexpensive product is wine vinegar with concentrated grape must and frequently caramel colouring added.
What it is used for
The applications follow from the acidity. In preservation, the low pH inhibits bacteria, which is the basis of pickling and of the shelf stability of sauces and condiments. In cooking, acid balances fat and sweetness, brightens flavours and is one of the five basic tastes, and it performs specific chemical jobs: denaturing protein in ceviche and in marinades, reacting with bicarbonate of soda to generate carbon dioxide for leavening, preventing the oxidation that browns cut fruit, and helping to set poached eggs. In cleaning, dilute acetic acid dissolves the calcium carbonate in limescale and soap scum, which is a genuine chemical action rather than folklore, and it is ineffective against grease, which needs a surfactant, and is not a reliable disinfectant despite its reputation. Historically it had a medical and industrial life as a wound wash, as the basis of the drink given to Roman soldiers, and as a solvent, and the practice of drinking diluted vinegar for health, currently marketed around apple cider vinegar, has a body of small studies suggesting modest effects on blood glucose after meals and much weaker support for everything else claimed.
The takeaway
Vinegar is alcohol oxidised into acetic acid by aerobic bacteria, so the same liquid that yeast made in a sealed vessel becomes vinegar in an open one, which is why winemaking is largely the art of excluding air. Production methods differ only in how fast oxygen reaches the bacteria, from months in slow barrels to a day in aerated tanks. The type reflects the alcoholic base, traditional balsamic is cooked grape must aged twelve years or more, and the acidity does the preserving, the flavour balancing and the limescale removal.