While some people know vinegar is a product of fermentation, even fewer know that bacteria, not yeast, is the essential agent. Acetic acid bacteria—the general term for vinegar fermenting bacteria—are present throughout our environment especially in fruits.
They long ago evolved to survive not only by metabolizing simple sugars (i.e. glucose) but the alcohol ethanol. Acetic acid is a waste byproduct of the metabolism of the ethanol. In addition, like many other bacteria, acetic bacteria can form a community inside of a biofilm made of cellulose. This is the origin of the famous “mother of vinegar”.
What are acetic acid bacteria? Acetic acid bacteria belong to the class Alphaproteobacteria which is a pretty wide group of bacteria. They have several key characteristics:
- They are rod shaped
- They are gram-negative (this is due to the composition of their cell wall)
- They may or may not be motile (have flagella to move around)
- They metabolize sugars and ethanol among other compounds for energy
- They are extremophiles, or bacteria that can survive in extraordinary environments. In the case of acetic acid bacteria they can survive in harsh acidic environments of low pH that kill most other bacteria. They do this by allowing acid in the cell and normalizing themselves to their environment, not keeping the acid out
The first identification of acetic acid bacteria was made by Louis Pasteur. He correctly identified the microbial origin of vinegar fermentation. He was not the first to guess at a biological origin though. The German amateur scientist Friedrich Kützing correctly inferred in the early 19th century that the mother of vinegar was likely biological in origin. He called it the “vinegar plant” and believed it to be a sort of algae he called Ulvina aceti. Later, Dutch biologist Christian H. Persoon believed the mother of vinegar to be a fungus which he termed Mycoderma or ‘fungal skin’.
Louis Pasteur, through careful experimentation, correctly deduced its bacterial origin and understood that the bacteria were obligate aerobes (required oxygen) which they used to ferment ethanol to acetic acid. He named the bacteria Mycoderma aceti, in a nod to Persoon. He published his findings in 1864 and in November of 1867, the city leaders and vinegar makers of Orléans invited Pasteur to the city to speak to the vinaigriers about his discoveries and how to improve vinegar production.
Later researchers, however, in studying the bacteria dispensed with the ‘Mycoderma‘ term entirely. Martinus Beijerinck, a Dutch microbiologist, is credited with first using the term Acetobacter in 1898. Other researchers followed this nomenclature. Amongst the first and most well known strains was Acetobacter aceti. There was also Acetobacter pasteurianus, Acetobacter orleanensis, Acetobacter schützenbachii, and Acetobacter xylinum. Almost all of these were associated with the traditional ‘slow’ or Orléans methods of vinegar production in barrels using mothers to absorb air and make vinegar. The strain Acetobacter schützenbachii was an exception, first isolated in the newer packed generators.
As the 20th century went on and more research was done, more physiological differences as well as differences in the ability to metabolize certain salts or sugar alcohols led to a debate on changing the names for acetic acid bacteria. The legendary Toshinobu Asai, a Japanese researcher who focused on acetic acid bacteria, was one of the key enablers in this trend.
Asai split acetic acid bacteria into Acetobacter and Gluconobacter. There were several differences but the main difference ended up being that Acetobacter metabolized ethanol well and glucose less well and was also able to ‘overoxidize’ acetic acid to water and carbon dioxide if it ran out of ethanol. Gluconobacter did not oxidize ethanol well but did metabolize glucose well. Acetobacter was split again in the 1980s into Acetobacter and Gluconacetobacter. The latter took all the bacteria that could overoxidize acetic acid (including some like Acetobacter xylinum). Finally several years ago, Glucanoacetobacter was itself split into Glucanoacetobacter and Komagataeibacter. Thus we have three genera of acetic acid bacteria involved in vinegar production. What are their differences? Generally:
- Acetobacter – primarily the bacteria involved with traditional ‘slow’ or Orléans methods of vinegar production in barrels or drums
- Glucanoacetobacter – mostly acetic acid bacteria found in fruit and other glucose rich sources. Cannot form cellulose (mother of vinegar). There are a few species that are present in high acid vinegar fermentations (G. Entanii).
- Komagataeibacter – this genus contains most species that are involved in the modern industrial production of vinegar. Many varieties, but not all, form mother of vinegar but many can survive high acid levels (6%+ acetic acid)
What does this all mean? First, there are many more species of acetic acid bacteria than brewing yeast. Second, because acetic acid bacteria are hard to cultivate and mass produce, vinegar makers do not use specific species or strains intentionally like wine or beer makers use specific yeast strains. It is too hard to obtain a pure bacterial culture and even harder to keep it pure in an industrial environment.
At Supreme Vinegar we sell mother of vinegar and though we have an idea of what they are (16s rRNA DNA sequencing is the default method to approximate what the bacteria are) many bacteria can be present in each culture. For now, understanding acetic acid bacteria is much more art than specifically picking out a brand of yeast.