Turning juice into wine involves a process called fermentation in which yeast, an anaerobic bacteria, is introduced into and allowed to interact with the juice from the grapes. If certain other bacteria become trapped in the juice with the yeast, however, the outcome may be very different than expected.
Fermentation is a fairly simple process. When sugar is added to yeast, alcohol and carbon dioxide are produced. In wine making, the yeast feeds off of the two types of sugars found in grapes; glucose and fructose. This reaction produces ethanol and fermentation occurs. The type of grapes used to make a particular type of wine determines the type of sugars present and thus influences the flavor of the ethanol produced. The type of container the wine is stored in, such as an oak barrel, will influence the fermentation process as will the temperature of the area in which the containers are stored. Red wines prefer slightly warmer temperatures of 70-80 degrees while a white wine will ferment better at a cooler 60 degrees.
Acetic Acid Bacteria
Acetic acid bacteria are one type of bacteria that disrupt the wine making process by interfering with the fermentation process. Their presence is often responsible for turning wine into vinegar. According to the website WineBusiness.com, acetic acid bacteria--AAB--are dependent on oxygen to thrive and therefore reproduce most at times when the grapes or resulting wine are exposed to air. Another factor important to the survival of AAB in wine is pH. Even a slight drop, for example a decrease in pH from 3.7 to 3.5, can affect their ability to maintain population count. The presence of acetic acid bacteria can lead to spoilage of wine as a result of the production of several compounds such as excessive acetic acid (essentially vinegar), ethyl acetate, acetaldehyde, dihydroxyacetone and acetoin. Each of these byproducts has the potential at high enough concentrations to greatly alter the taste of the wine in unappetizing ways.
Lactic Acid Bacteria
A study performed at the Iowa State University on the affect of lactic acid bacteria--LAB--on wine spoilage describes LAB as the microorganisms responsible for fermented foods such as sauerkraut and yogurt. These bacteria are commonly found on the leaves of the grape plant as well as on the grapes themselves. Once introduced into the wine making process they can cause undesirable taste changes and odors in the wine produced. Several factors encourage the growth of LAB and thus lead to wine spoilage. These include the fermentation of sugars and citric acid, presence of oxygen or carbon dioxide--depending on type of bacteria and the presence of inorganic salts and carbohydrates. The extent of the damage in each case is due in part to the type of bacteria as well as the type of wine being created.