Last autumn, after watching a television series on French bakeries, I created my own sourdough starter.

Having read various websites about feeding and maintaining starter at home, I was confused about the acetone — nail varnish remover — smell. Most sites advise dumping half the starter then adding fresh flour and water to what remains of the original and stirring well.

As my starter was still rather young at the time, the acetone smell was frequent.

A good friend of mine with an excellent knowledge of chemistry asked one day how the starter was going. I explained my concern about the acetone. He said it was nothing to worry about. In fact, it is part of the fermentation process.

He generously offered to write a guest post for me and my readers. Without further ado, here it is:

Yeasts

Yeasts are naturally-occurring micro-organisms. The strains used in brewing and baking consume carbohydrates and sugars as sources of energy. They produce alcohols and carbon dioxide as part of their digestive processes.

In the presence of air, alcohols oxidise naturally in a two-stage process. In the first stage, aldehydes and ketones are produced. These typically have strong aromas. Acetaldehyde smells of pears, whilst acetone smells of nail varnish remover. Many of them are actually used as artificial flavourings for foods. For example, acetaldehyde is used to flavour boiled sweets.

In the second stage, and with air still present, the aldehydes and ketones oxidise further to produce acids. It is not possible to produce acids without first producing aldehydes and ketones.

Brewers aim to stop this oxidation process, since they wish to retain as much alcohol as possible. A glass of wine, if left standing in air, gradually turns to vinegar, which is another form of acid.

However, bakers wish to allow both stages of the oxidation process to occur in order to produce acids, since these are an integral part of a sourdough starter. Acids such as lemon juice taste sour; hence the term sourdough for a bread starter that contains acids.

Yeasts also grow best in slightly acidic environments, so they are able to live in sourdough starters. However, they also need a continuous supply of carbohydrates and sugars, which is why these starters must be fed regularly.

Both stages of the oxidation process can also occur in the human body. After drinking alcohol, the aldehydes and ketones can cause a strong smell on the breath. The acids that are then formed can cause stomach upsets. Most indigestion and hangover cures work by trying to neutralise these acids.

With an adequate food supply, yeasts will continue to consume the carbohydrates and sugars. They will therefore produce alcohols and carbon dioxide all of the time. The natural oxidation process is also continuous. So, with healthy yeasts, a sourdough starter will always contain some alcohols at the start of the oxidation process, some aldehydes and ketones that have completed the first stage and some acids that have completed the second stage. It should always have a slight smell.

If the smell is very strong, there may not be enough air in the starter for the oxidation process to complete successfully. Under these circumstances, the starter should be left open to the air for a short time, simply to let some carbon dioxide out and some more air in. However, it should not be left open for too long, since the aldehydes and ketones can evaporate, particularly in a warm room. If that happens, there will be nothing left to turn into acids. So, after a few minutes, the starter should be covered again.

Overall, the smell of pears or nail varnish remover in a sourdough starter is a good sign. It shows that the yeasts are producing alcohols and that the natural oxidation process to make acids is under way. Do not be put off.

I hope that you found this as helpful as I did. Normally, the advice in the penultimate paragraph works.

When it does not — and this is the case only if the starter is left unused or unfed for several days — I simply add a rounded teaspoon of flour along with water to the starter and stir well.

More about my sourdough experience tomorrow.

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