Lactic acid is the type of acid that is not naturally present in wine grapes but can occur in wine during fermentation, specifically through a process known as malolactic fermentation. This process involves the conversion of malic acid, which is found in grapes, into lactic acid by lactic acid bacteria. This fermentation is commonly employed in the production of certain wines, such as many red wines and some full-bodied white wines, to soften the acidity and add complexity to the flavor profile.
In contrast, tartaric acid is naturally found in grape juice and is one of the primary acids in wine, playing a critical role in stabilizing the wine's overall structure. Citric acid is also naturally occurring in grapes, albeit in small quantities, and can be added to enhance flavor or acidity. Acetic acid, while typically an unwanted spoilage product resulting from bacterial activity, can also be present in very small amounts as a byproduct of fermentation processes. Therefore, lactic acid's formation through fermentation processes makes it distinct from the other acids listed, as it requires specific microbial action and modification of pre-existing components from the grapes.