Acetobacter is also known as acetic acid or vinegar bacteria. As the name implies, it can grow in wine and produce vinegar (acetic acid). Acetobacter can only develop in the presence of oxygen and often becomes a problem in wines that are being aged in untopped containers. It can be controlled with a combination of sulfur dioxide and keeping barrels and tanks topped and full after fermentation

Lactobacillus and Pediococcus are both forms of undesirable malolactic bacteria. Lactobacillus typically grows in high pH red wines that have a stuck fermentation. It grows on the sugar present from the incomplete alcoholic fermentation and produces large quantities of acetic acid. Because it usually grows in high pH musts (greater than 3.75), sulfur dioxide is less effective. The best treatment is prevention, adding acid to a high pH must, and maintaining a healthy fermentation that completes to dryness.

If Lactobacillus has already become established, lysozyme (an antimicrobial enzyme that is effective at high pH), can be added to control growth. Pediococcus produces an off-aroma that is described as “vegetal” or “dirty socks” and often comes from cooperage that has not been kept clean. Like all spoilage microbes, it can be prevented by a combination of keeping winery equipment and cooperage clean, adding pure inoculums of yeast and malolactic bacteria for fermentation and addition of sulfur dioxide.

Brettanomyces is yeast that can grow in wine without the presence of oxygen or sugar. It usually shows up in high pH red wines while they are being aged and it produces an aroma that is described as “barnyard” or “horse sweat.” Brettanomyces grows very slowly and some winemakers feel that a small amount of its aroma can add complexity to a wine. However, too much of this character can be seen as a flaw and most winemakers avoid it.

Sulfur dioxide usually does not kill Brettanomyces but prevents it from growing. As a wine ages, the level of free SO2 diminishes, and a wine that has been adjusted with sulfur dioxide prior to being placed in barrels may require supplementary additions as it is being aged to prevent Brettanomyces from developing.

The presence of sulfur dioxide will also inhibit malolactic bacteria and help to prevent malolactic fermentation (MLF) from occurring, leaving more of the natural acidity in the final wine. This method is preferred for fruit-forward wines with crisp acidity that will not receive a great deal of barrel ageing.

If sulfur dioxide is not added prior to alcoholic fermentation, more oxidative browning will occur and, within a few days of pressing, the juice will take on a brownish hue. While this dark color may cause alarm, you should not be overly concerned because after fermentation the dark, oxidized phenols will settle out, leaving a wine that is much brighter and more appropriate in color for a white wine.

Since the phenols that can oxidize have been removed from settling and racking, there is less of a potential for the wine to oxidize after fermentation and the wine will be more color-stable and better able to age. This method of pre-fermentation oxidation does diminish some of the fruity and floral aromas in the final wine.

The absence of sulfur dioxide allows MLF to commence after primary fermentation is complete, lowering the acidity of the wine, and making it more microbiologically stable. After MLF is finished, sulfur dioxide should be added to protect the wine. If MLF is not desired, the wine should be adjusted with sulfur dioxide after primary fermentation is complete.

For red wines, it is a good idea to add a small amount (about 30 to 40 ppm), of sulfur dioxide immediately after the grapes have been crushed. While not absolutely necessary, this will discourage the growth of spoilage organisms such as lactobacillus and allow the yeast to get a good start on the fermentation without competition from other microbes.

By the end of primary fermentation, the majority of the free SO2 will be bound up by compounds present in the grape skins and there will not be so much residual SO2 that the growth of malolactic bacteria is inhibited. After MLF is complete, sulfur dioxide should be added to protect the wine during ageing.

For both red and white wines, as they age, free sulfites are bound by other compounds that are present in the wine resulting in a gradual lowering of the effective amount of sulfur dioxide in wine. For this reason, it is always a good idea to monitor the SO2 level during ageing and to addmore as needed.