Acetic Acid (HAc)
Definition - What does Acetic Acid (HAc) mean?
Acetic acid is one of the three major acids created during fermentation. It is a volatile acid, which means that it is unstable and prone to reacting with oxygen or yeast to create faults in a wine. Since it is a naturally occurring acid, winemakers cannot prevent acetic acid from appearing in their wines; however, most tasters cannot detect acetic acid under 600mg/L. Winemakers often try to keep the level of acetic acid under that mark, using the acid’s harsher characteristics to support the aroma and flavor of the wine (instead of overwhelming it). Acetic acid smells and tastes exactly like vinegar, which is why, when it is present in high quantities, it is considered the vinegar taint.
WineFrog explains Acetic Acid (HAc)
Acetic acid is a by-product of fermentation. It is created during microbial metabolism - when wine yeasts eat up the sugars to create alcohol, they also produce a small amount of acetic acid. Acetic acid becomes a fault when it is created through other, unintended chemical reactions. For example, lactic acid, which forms during malolactic fermentation, converts glucose into acetic acid; this means that, if there is still sugar in a wine after malolactic fermentation, the wine could become very vinegary. Another way that acetic acid gets into a wine is through botrytis (noble rot). Dessert wine products using Botrytis have high levels of acetic acid due to the fact that naturally occurring grape yeasts get beneath the skin of the grape and into the pulp/must of a wine. Grape yeasts produce a lot more acetic acid than wine yeasts.
Acetic acid is the most odorous and bitter of all the naturally occurring wine acids. If left unchecked, it becomes a wine fault, but there are many winemakers who use it to support the flavors and aromas of their wines. The ideal level of acetic acid in a wine is 300mg/L; at this level, it is undetectable to the taster, but it provides a strong acid base for the other components of a wine, like the fruit, tannins and other tastes. In fact, when acetic acid reacts with esters, it produces acetate esters that contribute to a wine’s fruit character.
Finally, acetic acid is the acid that changes a wine into vinegar (with the help of the acetobacter bacteria). When a wine is left open and exposed to oxygen, the acetobacter bacteria converts ethyl (alcohol) into acetic acid (vinegar).