Fixed acidity refers to the combination of natural acids found in grapes (tartaric, malic and citric) or formed during malolactic fermentation (lactic). Volatile acidity refers to the combination of volatile acids created during fermentation, or as a consequence of microbial changes. Its strength is an indication of a wine's degeneration. The most important volatile acid is acetic acid, the existence of which indicates the transformation of wine into vinegar. Total acidity is the combination of all the acids contained in a wine or must. This concept is usually measured in grams of tartaric acid per litre.