Controlling bacteria early in the fermentation is important because some types of lactic bacteria prefer sugar to malic acid. Most experts recommend treating grapes with 30 milligrams per liter of SO2 at the crusher, even if ML fermentation will be encouraged later in the winemaking process. Even at this low level, the sulfur dioxide is effective in limiting early bacterial growth. After a few days of fermentation, the sulfur dioxide level in the wine will be very low, and it will no longer inhibit the desired ML fermentation. First, a neglected, red fermentation overheats, and the high temperature kills the yeast.
Fermentation abruptly stops, and considerable sugar remains in the wine.
The wine is warm, and the wine contains sugar. In this condition, wine is extremely vulnerable. Some lactic bacteria are always in wine, and when the wine is in this condition, these bacteria often attack the remaining sugar. The wine is warm, and large amounts of acetic acid can be produced quickly. A wine can spoil in a short time under these condition.