Acetic Acid
What it does
Acidity, preservative, flavour.
How it's made
Made by fermentation (vinegar) or synthesis.
Acetic acid is the acid that gives vinegar its sharp taste and smell, and in food it is used to add sourness and to preserve. It occurs naturally wherever sugars or alcohol are fermented, and the traditional food-grade route still relies on this natural process rather than laboratory chemistry.
The classic method is a two-stage fermentation. First, yeasts convert sugars (from fruit, grain or other crops) into alcohol; then acetic-acid bacteria of the genus Acetobacter oxidise that alcohol into acetic acid using oxygen from the air. Industrially this is speeded up in large vessels where the alcohol-containing liquid is trickled over a bed of bacteria or aerated vigorously to give the microbes plenty of oxygen.
The resulting vinegar or dilute acetic acid is then filtered and standardised to the required strength. A large volume of acetic acid for industry is also made synthetically from petrochemical feedstocks, but the material used to acidify and flavour food is generally the fermented, food-grade product, labelled accordingly.
Possible reactions
Generally safe; it is the acid in vinegar.