Mother of vinegar

Mother of vinegar from a wine vinegar pot

I'm currently visiting my parents in Switzerland, in a lovely region located between the lake Léman (lake Geneva) and the mountains of Jura (see Nyon région tourisme). On that occasion my father gave me a tour of his vinegar pots, and even fished out the mother of vinegar from inside the pot so that I could take a picture. Have a look at this biofilm of acetic acid bacteria: it's a large and thick disc with the color of a liver!

As their name implies, these bacteria transform alcohol, for instance from wine, into acetic acid. From what I read during a quick overview of the literature, common acetic acid bacteria include species of the genera Acetobacter and Gluconobacter. 



Both Acetobacter and Gluconobacter operate under aerobic conditions, but interestingly only Acetobacter can fully process the organic acid into water and CO₂; Gluconobacter cannot proceed further than acetic acid. Nevertheless, Acetobacter strains are often preferred for vinegar production because they are better acid producers. But obviously, I do not know for sure what species is (are) present in my father's vinegar pots!

Vinegar pots

For the acidification to occur properly, the alcohol concentration should not be too high, which would inhibit the action of the acetic acid bacteria. For the same reason, the wine used to make vinegar should not contain SO₂ (often added to the wine to prevent microbial growth).


The acetic acid bacteria are great to make vinegar, but they can cause oenological problems in wine making. The acetic acid bacteria are naturally present on grapes and in the freshly pressed juice, but their number tend to decrease during the alcoholic fermentation (Joyeux, 1984). During the wine making conditions are kept anaerobic so acetic acid bacteria usually cannot grow; however, acetic acid bacteria can survive and proliferate if temperature and oxygenation are not tightly controlled. Usually wine is not filter-sterilized prior to be put into bottles, so a small number of acetic acid bacteria can still be in there. If the wine is stored for a long time in improper conditions (for instance a corked bottle standing upright), acetic acid bacteria can form a biofilm in the oxic zone and spoil the wine…

Wine vinegar is only one type among many types of vinegar that are produced in the world: cider, fruit, malt, rice and many other starting materials can be used to make vinegar. Just pick up your favorite one! 

Vineyards in Signy, Switzerland

References:

• Joyeux A.,  Lafon-Lafourcade S. and P. Ribéreau-Gayon (1984). Evolution of acetic acid bacteria during fermention and storage of wine. Applied and Environmental Microbiology. Vol.48, pp. 153-156.