Acetic acid is the active ingredient in vinegar, but the two aren’t identical. Vinegar is a dilute solution of acetic acid in water, typically containing 4% to 12% acetic acid along with small amounts of other compounds. Pure acetic acid, by contrast, is a concentrated chemical that can burn skin and corrode surfaces. The distinction matters for cooking, cleaning, and safety.
What’s Actually in Vinegar
Standard household vinegar is roughly 5% acetic acid and 95% water. That ratio holds true whether you’re looking at white distilled vinegar, apple cider vinegar, or wine vinegar. The FDA requires any product labeled “vinegar” to contain at least 4% acetic acid, while specialty varieties like balsamic or sherry vinegar may reach 6% to 7%.
What separates vinegar from a simple acetic acid solution is everything else in the bottle. Fermented vinegars contain amino acids, sugars, vitamins, minerals, polyphenols, and other organic acids that develop during the fermentation process. These trace compounds give each type of vinegar its distinct flavor and color. A bottle of apple cider vinegar tastes nothing like a lab-grade acetic acid solution diluted to the same concentration, because the fermentation creates hundreds of flavor compounds that pure acetic acid lacks.
How Vinegar Gets Its Acetic Acid
Vinegar production is a two-stage fermentation. First, yeast converts sugars from fruit, grain, or another source into alcohol. Then a group of bacteria called acetic acid bacteria take over. These bacteria sit on specialized membranes in their cells and convert the alcohol first into acetaldehyde, then into acetic acid. The process is efficient because the bacteria release acetic acid directly outside their cells rather than having to transport it from the inside out.
The starting material determines the type of vinegar. Apples yield apple cider vinegar, grapes produce wine vinegar, and grain alcohol becomes white distilled vinegar. But the core chemistry is always the same: bacteria oxidize alcohol into acetic acid.
There is also synthetic acetic acid, manufactured from petroleum-derived chemicals rather than fermentation. It’s chemically identical at the molecular level, but scientists can distinguish the two by measuring carbon isotope ratios. Fermented vinegar has a slightly different carbon fingerprint than synthetic acetic acid, and labs can detect adulteration when more than 15% to 20% of a “vinegar” product has been replaced with the synthetic version.
Concentration Changes Everything
At 5%, acetic acid is the harmless tangy liquid you splash on salads. As concentrations rise, the safety profile changes dramatically. Cleaning vinegar sold in stores typically contains 6% to 10% acetic acid. Horticultural vinegar, used as a weed killer on gardens and lawns, runs 10% to 20%. At concentrations between 10% and 25%, acetic acid acts as an irritant to skin and eyes. Above 25%, it becomes corrosive and requires handling in a ventilated fume hood.
The most concentrated form, called glacial acetic acid, is nearly 100% pure. It’s a hazardous industrial chemical that can cause severe burns on contact. This is the same molecule sitting in your pantry at one-twentieth the concentration, which illustrates why the percentage matters far more than the name on the label.
Why Acetic Acid Preserves Food
Vinegar’s long history as a food preservative comes down to what acetic acid does to bacteria. When bacterial cells absorb acetic acid, the molecule breaks apart inside them, dropping their internal pH and disrupting their metabolism. Concentrations as low as 0.1% can slow the growth of dangerous strains like E. coli O157:H7, and most bacteria are killed at just 0.5%. Acetic acid also interferes with bacterial DNA replication and drains the energy reserves of pathogens like Salmonella.
For home pickling, vinegar needs to be at least 5% acidity to safely prevent the growth of harmful microorganisms. This is sometimes expressed as “50 grain” on labels. Using vinegar below that threshold for canning or preserving creates a real food safety risk.
When Vinegar Can Cause Problems
Even at food-safe concentrations, vinegar isn’t completely benign. Drinking it undiluted or in large amounts can worsen heartburn and other inflammatory conditions in the upper digestive tract. Over time, frequent exposure to vinegar can erode tooth enamel. If you’re using vinegar as a health tonic, diluting it further and drinking it through a straw helps reduce contact with your teeth.
The real danger comes from confusing product types. Cleaning vinegar and horticultural vinegar look similar to cooking vinegar on a shelf but contain two to four times the acetic acid. They should never be consumed. Always check the label for the acidity percentage before using any vinegar product in food.