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The word vinegar, not surprisingly, comes from the French for “sour wine,” and one of the ways to make vinegar is to ferment fruit past the point where it is simply alcoholic, and turns very tart instead.
Lactic fermentation is a process that involves anaerobic bacteria consuming sugar and converting it to lactic acid, ethanol, and more bacteria. When the fermentation involves only the lactic bacteria (as in yogurt or vegetable fermentation), the culturing stops at the point where there are no more sugars to be consumed, and the bacteria die off.
In fermented drinks such as water kefir or kombucha, or in fermented fruits, there is an additional presence of yeast, either added to the mixture for fermentation, present in the material being fermented, or present in the culture. The yeast promotes the additional production of alcohols in the fermentation. If the fermentation is aerobic (exposed to oxygen), an additional type of bacteria is present — acetobacter – which converts the available alcohol into acetic acid. Acetic acid is what makes a vinegar acidic.
Another process for making vinegar involves the fermentation of distilled alcohol. The resulting colorless liquid is then diluted to a standard strength and filtered of all impurities.
There are two ways of describing the strength of vinegar: its acidity, which is measured as a pH value, and its concentration (sometimes referred to as grain strength), which is the amount of acidity in a given volume of liquid and which is measured in percent.
To measure the pH of vinegar, simply use a pH strip. The resulting color will tell you how strong the vinegar is. Distilled white vinegar usually measures around pH 2.4, with a strength of 5%. Apple cider vinegar will have a pH of about 4.25 to 5.0. The lower the pH, the more acid the vinegar is.
If you use vinegar for starter liquid for making kombucha, it should have a pH no higher than 4.5, and a strength of at least 5%. The vinegar should be pure of any additional flavors, sugars, or bacteria. Distilled white vinegar is ideal for starting kombucha.
To determine the amount of acetic acid in the vinegar liquid, you need special equipment. You can buy acid test kits at a beer- and wine-making supply shop for only a few dollars, but these are designed to test the acidity of wine, and generally will not measure an acidity of more than 1%. To be described as “vinegar,” a liquid must have at least 4% volume of acetic acid.
Test kits for measuring this quantity of acid usually run around $50 to $100. Essentially, you would add the vinegar being tested to a flask, and dilute it with water. You then add phenolphthaline, a chemical that will change color at different levels of acidity.
In a special tube flask marked with measurements in milliliters, you put a solution of sodium hydroxide, a caustic chemical that must be handled with care. This is called the titrating solution, and is slowly dripped into the vinegar-phenolphthaline mixture until the vinegar solution turns pink. At this point you can measure how much of the sodium hydroxide you have used.
Ideally, the procedure is repeated three times for accuracy. Once you have a reliable value for the volume of titrating solution, you must do some math to translate that amount into a standard measurement of grams of acetic acid in a liter of vinegar. That would be the percent strength of vinegar.
This degree of accuracy is generally not something you would do as a home hobbyist making vinegar in the kitchen. If you have made vinegar using wine or fruit scraps, consider it a delightful addition to salad dressings or recipes.
Of course, people have been using vinegar for preservation for centuries without such testing. If you choose to use homemade vinegars for pickling projects, you should at least measure the pH value, and ensure that it is no higher than 4.0. This pH will protect against most harmful bacteria, and against the formation of mold.
For safety, however, if you need vinegar of a certain strength for pickling, we advise that you are best off using a commercial vinegar that has been tested and certified to have a certain strength.