Many of you have surely accidentally left a bottle of wine or beer open. After a while, the contents of the bottle no longer taste like beer or wine, but like acid. Vinegar has formed. But what exactly has happened?
What is Vinegar?
Vinegar is a liquid made up of water and acetic acid. The acetic acid in the final product is derived by fermenting an alcoholic liquid or by diluting purified acetic acid. Depending on the type of vinegar, other ingredients, such as flavoring agents or artificial coloring, may also be present.
The U.S. Food and Drug Administration mandates at least 4 percent acetic acid in table vinegar. The leftover alcohol content may not exceed 0.5 percent. Vinegar essence has an acetic acid content higher than 15 percent and is typically used for cleaning and disinfecting. Glacial acetic acid refers to highly concentrated (99 to 100 percent) acetic acid.
There are a huge variety of vinegars produced around the world. See our chart, “A Vinegar Lineup,” for an overview of several of them. A good vinegar should taste fruity (depending on the base used), not bland or dull, and shouldn’t taste “off.” Faking the pungent taste with other acids or pungent-tasting substances is forbidden in commercial vinegars. Pungent spicing is not allowed, though hot spices in spiced vinegars are. Many vinegars are preserved by sulfurization or the addition of up to 2 percent table salt by volume to prevent cloudiness, vinegar mother formation, vinegar eels, etc., in the final product.
Vinegar at a Glance
Name: Acetic acid (systematic IUPAC name: ethanoic acid)
Chemical formula: CH3COOH
Melting point: 63 degrees Fahrenheit
Boiling point: 244 degrees Fahrenheit
Density: 1.05 grams per milliliter
Molar mass: 60.05 grams per mole
Appearance: Generally clear with no more than a few vinegar eels; occasionally slightly cloudy. Cloudiness is desirable in naturally cloudy vinegars.
Coloring: The coloring of the vinegar depends on the fruit mash. White wine vinegars are clear, whereas red wine vinegars are very dark. The color can also be changed by adding caramel coloring or fruit.
Vinegar Production Methods
Vinegar can be produced naturally, via alcoholic fermentation followed by acetic fermentation, or artificially in a laboratory. Carbon-14 radioactivity testing can reveal whether a particular vinegar is natural or synthetic. Worldwide, about 20 percent of vinegar is produced synthetically and about 80 percent microbiologically.
The most commercially important production method is the Monsanto process, which catalyzes acetic acid production from methanol and carbon monoxide under pressure.
This produces an 80-percent acetic acid solution, which is then diluted to produce vinegar essence or synthetic table vinegar for industrial food preservation.
Vinegar is produced naturally by fermenting an alcoholic liquid. Therefore, two fermentations are required to produce vinegar from a fruit juice or mash: an alcoholic fermentation, which is performed anaerobically by yeast, and an acetic fermentation, performed aerobically by fungus.
Acetic fermentation happens in two stages. First, acetaldehyde is formed from alcohol and oxygen:
Then, the fungus Micrococcus aceti (already present in beer, wine, and cider) continues fermenting the solution until acetic acid is produced:
The ideal temperature for this fermentation is 77 to 86 degrees Fahrenheit, at which 1 ounce of alcohol can produce about 1 ounce of acetic acid. In theory, the initial percent alcohol should match the final percent acetic acid, though in practice 1 percent alcohol often produces about 0.8 percent acetic acid.
If the fermentation consumes all or almost all the alcohol, the acetic acid will be oxidized into carbon dioxide and water, reducing the vinegar concentration and creating a strong musty smell (overoxidation).
Acetic fermentation produces ethyl acetate as a byproduct, which is responsible for the glue-like smell during fermentation. This smell serves as a signal that the fermentation is proceeding correctly. The fermentation is nearly finished when the smell has vanished.
The Mother of All Vinegar
While vinegar is being made, a “vinegar mother” usually forms. This term refers to a collection of vinegar bacteria and the cellulose it secretes, and looks different depending on the type of bacteria involved.
A dry mother floats on the liquid and looks dry from above. It’s smooth when young, and wrinkles as it ages, but doesn’t thicken. A wet, shiny mother looks like a thin, separate liquid layer on the surface of the liquid. It may climb up the container sides, and often causes cloudiness. A mucosal mother grows like a layer of jelly under the surface of the liquid, growing thicker as it ages.
Vinegar bacteria (Acetobacter spp.) are found in all organic, sugar-containing substances (such as fruits, roots, and sap) that can undergo alcoholic fermentation with yeast. They’re transferred by wind or insects from one food source to another, and grow as soon as alcohol, heat, and air are available.
Vinegar bacteria can be divided into groups based on their origin, their reaction to changes in temperature, their skin formation, their reaction to changes in acidity, and their alcohol tolerance. “Fast” bacteria are a specially cultivated group not found in nature. Good vinegar bacteria should have a high alcohol-oxidizing capacity over a set period of time (without overoxidizing), high temperature stability, and resistance to high acetic acid concentrations.
Staying Within Tolerance
Alcohol: Some vinegar bacteria can only tolerate an alcohol content of 6 percent, while others can tolerate as much as 13 percent. The growth medium and how well-nourished the bacteria are play a crucial role in their tolerance.
Temperature: The minimum temperature for bacterial growth is between 43 and 50 degrees; the optimal temperature, depending on bacteria type, is between 66 and 93 degrees; growth ceases between 95 and 113 degrees. Bacteria that can grow at relatively high temperatures are best for vinegar production, because they can generate high local temperatures in the fermentation vessel.
Acetic acid: Acetobacter growth generally ceases or becomes very slow at an acetic acid concentration of 2 to 3 percent, but they can still produce vinegar up to a concentration of 11 percent. However, their behavior varies drastically and depends on the growth vessel and conditions.
Nutrients: To grow and thrive, the vinegar bacteria also need minerals, such as potassium, magnesium, sulfate, and phosphates, as well as sources of nitrogen. Ideal nutrient sources therefore include malt extract, beer, extract-rich wines, fruit wines, fruit juices, and nitrogen-rich yeast water and yeast decomposition products.
Heating Up the Competition
Vinegar bacteria can generate the heat they need to grow by oxidizing alcohol. It’s important that the bacteria not get too cold, but also that enough heat dissipates to avoid overheating. The optimal temperature is 77 to 99 degrees Fahrenheit, but if the bacteria reach this optimal temperature, their productivity decreases. For maximum efficiency in vinegar production, the temperature in the vessel should be slightly lower than optimum; remember that the temperature at the edge and bottom of the vessel will be lower than in the central and upper areas.
Reprinted from The Artisanal Vinegar Maker’s Handbook by Bettina Malle and Helge Schmickl (ISBN 978-1-943015-02-3) by permission of Spikehorn Press.
Both Bettina Malle and Helge Schmickl earned masters of science in chemical engineering and doctorates in technical sciences from the Vienna University of Technology. They married in 2002, and together have authored books and conducted workshops on vinegar, distilled spirits, and essential oils. They live in Klagenfurt, Austria.