Malolactic fermentation while the wine is still macerating can be risky in some specific cases. If the grapes have not been crushed, there are still many whole berries in the juice and therefore sugar.
Among the lactic bacteria, there are homofermentative and heterofermentative strains. Heterofermentative strains use glucose via the pentose phosphate metabolic pathway to form a significant amount of acetic acid as well as lactic acid in D- and L+ form (normally only L+).
A vinegar or yogurt smell may then develop. This is lactic spoilage.
If inoculation or co-inoculation is used, homofermentative strains should be chosen to avoid this problem. In the case of fermentation with indigenous yeasts, MLF on the skins can sometimes be risky but this remains an exception. It should be noted that the risks are accentuated when the grapes are not sulphited.