Infographic on the chemistry of different types of vinegars. First, an overview of vinegar production is shown, explaining the fermentation of sugars and starches into ethanol, then the subsequent oxidation of ethanol to acetic acid. Six different varieties of vinegar are then explored: Distilled vinegar, balsamic vinegar, wine vinegar, apple cider vinegar, malt vinegar, and rice vinegar. Chemical compounds of interest are highlighted for each, and also detailed in the post accompanying this image.
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The first vinegars were accidents of oxidation, wine left out too long which turned sour. The word vinegar even derives from the Latin for ‘sour wine’. Today, varieties of vinegar range from balsamic to rice vinegar, produced on a commercial basis. This graphic takes a look at some of the chemical quirks of the different types.

Making vinegar

Vinegar production has come a way from leaving out wine and waiting. Regardless of the type of vinegar, they are all made using the same basic chemical premise. The fermentation of fruits or cereals transforms sugars and starches into alcohol (ethanol). A second fermentation, using acetic acid bacteria, converts ethanol into acetic acid, the main acid in all vinegars.

Traditional methods let acetic acid bacteria grow on the surface of the fermenting alcohol. This is slower because the bacteria need oxygen to carry out their acetifying work. Simpler methods involve filling a barrel with alcohol, adding the bacteria, and leaving them for months. The more common method for producing distilled and industrial vinegars sees wooden vats filled with vinegar-soaked wood shavings. The alcohol slowly drips through, converting to vinegar itself as it does so.

The obvious disadvantage of these processes is their speed or lack of. So modern industrial methods of vinegar production often use submerged fermentation. In giant tanks, pumps bubble air through the alcohol, reducing the time taken to produce vinegar from months to a matter of hours.

Vinegar varieties

Producers use a range of raw materials for vinegars but for simplicity, we can split these into grain vinegars and fruit vinegars. Grain vinegars include white and distilled vinegar, as well as malt vinegar and rice vinegars. Fruit vinegars include wine vinegars, balsamic vinegar and apple cider vinegars.

Distilled vinegar is the colourless one, and as simple as vinegar gets. The ‘distilled’ in the name isn’t a reference to the vinegar, but to the alcohol it is made from, distilled after production from barley malt or corn. Beyond acetic acid, there aren’t a lot of other compounds in distilled vinegars. though acetaldehyde, acetone, and ethyl acetate are present in small amounts. Ethyl acetate is often the only ester present in distilled vinegars, partly explaining the differences in the aroma.

Fermenting wines produces wine vinegars, giving red wine, white wine, or sherry vinegar depending on the type of wine used. The variety of compounds present is much wider than in distilled vinegars. Organic acids from the grapes, including tartaric acid, are present. Polyphenols and other phenolic compounds originate from both the grapes and the barrel-ageing some varieties undergo.

Balsamic vinegar is often grouped with wine vinegars, but is subtly different. Instead of wine, it’s produced from reduced grape must (crushed grape juice containing the seeds, skins and stems). Traditional balsamic vinegars age for at least 12 years in wooden barrels, concentrating flavours and spawning a range of compounds. These include 5-acetoxymethyl-2-furaldehyde, a key component contributing to balsamic vinegar’s long-lasting sweet taste. ‘Balsamic vinegar’ alone is not a protected term, and cheaper products which imitate the traditional version are made from a blend of grape must and wine vinegar, with colourings and thickeners added.

Grapes aren’t the only fruit from which vinegars are made: Apple cider vinegars are also popular. Like wine vinegars, they contain organic acids, including malic and citric acids from the apples. In both types of vinegars, other alcohols, such as propanol, react to form varying acids and esters during fermentation, accounting for the more complex flavours.

Malt vinegar is the most popular example of grain vinegar in Western countries. It’s made from fermented malted barley, essentially unhopped beer. Chemically, malt vinegars are less complex than fruit vinegars, and contain limited different organic acids — though researchers have detected lactic acid in some samples. Malt vinegar often contains more branch-chained organic compounds than fruit vinegars, contributing to its flavour and aroma.

Rice vinegars, made by fermenting rice wine, are another example of grain vinegars, and come in a range of varieties. The majority are less acidic, sweeter, and milder in flavour than other types of vinegar. A notable exception is Chinese black vinegar, such as Zhenjiang vinegar, which has a distinct, smokey flavour. Acetoin, found in many rice vinegars, gives a buttery flavour. In some varieties, furfural and pyrazines can contribute toasty flavours.

Vinegar processing

After production, commercial vinegars are usually filtered to remove sediment and pasteurised. Its acidity means vinegar isn’t often contaminated, but unpasteurised vinegar can still contain the acetic acid bacteria that turned it into vinegar in the first place. These don’t pose a health problem, though can form a biofilm known as the ‘mother of vinegar’. Made of acetic acid bacteria and a type of cellulose, it looks a bit like a weird alien jellyfish lurking in the bottle.

Despite its unsettling appearance, the mother of vinegar is harmless. Historically, it was used as a kind of sourdough starter for vinegar, added to alcohol to produce vinegar at home. Less palatably, it’s also the favoured food of vinegar eels, nematodes which survive at the low pH values in vinegar. Before that completely puts you off vinegar, be reassured that filtering removes any vinegar eels present, so you’re not likely to find any swimming around in a bottle in your cupboard.

Acetic acid’s other uses

Acetic acid may be the key component of vinegar, but vinegar accounts for a small percentage of acetic acid produced worldwide every year. Most acetic acid is produced industrially from petrochemicals rather than through fermentation. There’s more detail on some of acetic acid’s other uses in this previous post on the site.

References and further reading