The Yin and Yang of Bacterial Fermentation

In December, 2017, I wrote about the lactic acid bacteria (LAB), Oenococcus oeni that we used to conduct Malolactic Fermentation. Oenococcus oeni is one of four genera of the lactic acid family that includes Lactobacillus, Leuconostoc and Pediococcus able to survive the unfavourable conditions (low pH, high ethanol concentration and low nutrients) present in wine. Oenococcus oeni is the most well adapted wine-associated species and is used almost exclusively for the induction of malolactic fermentation (MLF) in red, white and sparkling base wines
The only other family of bacteria commonly found in grape juice and wine is Acetobacter. While LAB is generally thought of as providing positive attributes, the members of Acetobacter are generally thought of as being spoilage organisms when found in wine, due to the formation of major oxidized products, such as acetaldehyde and acetic acid.

The modulation of wine flavour by bacteria

Organism	Compound	Aroma/Flavor Attribute
Acetobacter Gluconobacter oxydans, Acetobacter aceti, Acetobacter pasteurianus, Gluconacetobacter liquefaciens and Gluconacetobacter hansenii	Acetic Acid	Vinegar
Heterolactic bacteria	Mannitol spoilage	Sliminess and production of a vinegary-estery, slightly sweet taste
Oenococcus oeni and Lactobacillus	Acetaldehyde metabolism → acetic acid and ethanol
Oenococcus oeni	2,3-Butanedione (Diacetyl)	Buttery, butterscotch
Lactic acid bacteria and acetic acid bacteria	p-coumaric and ferulic acids → vinyl and ethyl phenols

In secondary fermentation with beneficial bacteria, oxygen is the enemy that encourages the growth of Acetobacter and vinegar flavors. Keeping the wine environment anaerobic encourages Lactic acid bacteria that will convert the harsher malic acid to lactic acid perhaps with an accompanying formation of diacetyl (aka buttery flavors). Take home message: top off all wines to keep the influence of oxygen and by extension, the growth of undesirable bacteria to a minimum.
References:
1. Scott Laboratories Malolactic Bacteria.
2. Swiegers, Jan, Bartowsky, Eveline, Henschke, P.A. & Pretorius, I.S., Microbial modulation of wine aroma and flavour, Australian Journal of Grape and Wine Research, 2005, 11. 139 - 173. 10.1111/j.1755-0238.2005.tb00285.x.