How fruit becomes vinegar
I've been making fruit vinegars at home for a while now. It's slow, it's cheap, and it's the closest thing I've found to having a small lab in the kitchen. You set it up, then microbes do the work for weeks. Here's what's actually happening in the jar.
Two fermentations, two sets of microbes
The first thing to understand is that vinegar doesn't come straight from fruit. It's made in two stages, by two completely different groups of organisms.1
Stage one: sugar to alcohol
Yeast eats the sugar in the fruit juice and turns it into alcohol and carbon dioxide. That's the same process as making wine or cider. In fact, at the end of this stage, that's what you've got. Yeast does this best without much air, so this stage is usually done in a closed jar or demijohn with an airlock to let the gas out.
Stage two: alcohol to acid
Then acetic acid bacteria take over. They turn the alcohol into acetic acid, which is the sharp, sour taste of vinegar. Unlike the yeast, these bacteria need oxygen. So for this stage you want the opposite set-up: a wide jar with a cloth over the top, so air can get in but fruit flies can't.
This switch catches a lot of people out. If you leave stage two sealed up, very little happens. If you leave stage one open, you can end up with bacteria starting too early and a weak, unbalanced result.
You can skip stage one by starting from a ready-made cider or wine. But making your own from fruit means you control the flavour from the start, and that's half the fun.
The mother
After a week or two, a pale, rubbery layer starts to form on the surface. This is the "mother of vinegar". It looks unpleasant the first time you see it, but it's a good sign.
The mother is a mat of cellulose made by the bacteria themselves, with bacteria and some yeast living in it. The cellulose is mostly produced by bacteria in groups called Komagataeibacter and Gluconacetobacter, while Acetobacter is often the most common acetic acid bacterium in it.1 It floats on top because that's where the oxygen is.
A piece of mother from a previous batch, or a good splash of raw, unpasteurised vinegar, gets a new batch going far faster. It also makes the mix more acidic straight away, which helps keep other microbes out while the bacteria get established.
Things I've learned the hard way
- Warmth matters. Batches in a warm spot are noticeably faster than ones in a cold kitchen in January.
- Wide beats tall. More surface area means more air for the bacteria. A wide jar works better than a tall narrow bottle.
- Leave it alone. Moving the jar can sink the mother. A new one will form, but it slows things down.
- Taste rather than time it. It's ready when it tastes like vinegar, not like cider or wine with a sour edge. That can take anything from a few weeks to a few months.
- Know mould from mother. A mother is smooth, pale and jelly-like. Fuzzy spots that are blue, green, black or white and dry-looking are mould. That batch goes in the bin.
A note on safety
Homemade vinegar is great for dressings, drinks and cooking. But don't use it for pickling or preserving. Its acidity varies from batch to batch, and food safety guidance says pickling needs a vinegar of known strength, at least 5% acidity, to stop harmful bacteria growing.2 For pickles, use shop-bought vinegar with the acidity on the label.
Why fruit vinegars
Most shop vinegar is there purely for sourness. A good fruit vinegar actually tastes of the fruit. Raspberry, blackberry and plum all make vinegars you'd happily use on their own. A spoonful in soda water makes a decent soft drink too.
The bit I like most
I studied zoology, so I can't help seeing it as ecology in a jar. One group of organisms changes the conditions so that a second group can thrive. Then the acid the second group makes keeps most competitors out. It's a tiny, managed ecosystem sitting on the kitchen counter.
References
- Mother of vinegar, Wikipedia, with links to the underlying studies.
- Penn State Extension, Advice about vinegar.