Alcoholic Fermentation

Alcoholic fermentation is the best known of the fermentation processes, and is involved in several important transformation, stabilization, and conservation processes for sugar-rich substrates, such as fruit, and fruit and vegetable juices. Alcoholic fermentation is carried out by yeasts and some other fungi and bacteria. The first step of the alcoholic fermentation pathway involves pyruvate, which is formed by yeast via the EMP pathway, while it is obtained through the ED pathway in the case of Zymomonas (bacteria). In the following step, the pyruvate is decarboxylated to acetaldehyde in a reaction that is catalyzed by the enzyme pyruvate decarboxylase (Figure 2).

The redox balance of alcoholic fermentation is achieved by the regeneration of NAD+ during the reduction of acetaldehyde to ethanol, which is catalyzed by alcohol deydrogenase. The ATP yield of alcoholic fermentation is 1 or 2 mol of ATP per mole of glucose oxidized via the ED and EMP pathways, respectively. Zymomonas mobilis is the most important bacterial species that is able to perform alcoholic fermentation. The habitat of this species is the lymph of tropical trees, such as the palma tree from where it was originally isolated. Z. mobilis was proposed and used as a starter for ethanol production at the industrial level, although at present alcoholic fermentation carried out by yeast is better known and has been more thoroughly investigated. Natural alcoholic

Table 2 Examples of industrial fermentation and their producer microorganisms

Fermented food and animal feed

Microorganisms involved

Fermentation

Undesired fermentation

Fermented meat (salami)

Lactic acid bacteria

Lactic

Fermented milk

Lactic acid bacteria

Lactic

Alcoholic

Fermented milk

Lactic acid bacteria and yeast

Lactic and alcoholic

Cheese

Lactic acid bacteria; propionic

Lactic and sometimes

Mixed-acid and butyric

bacteria

propionic

acid

Bread and bakery products

Yeast and lactic acid bacteria

Alcoholic and heterolactic

Beer

Yeast

Alcoholic

Lactic and mixed acid

Wine

Yeast

Alcoholic

Lactic

Olive brines

Lactic acid bacteria

Lactic

Butyric

Silage

Lactic acid bacteria

Lactic

Amino acids

Butyric

Mixed-acid

Coffee

Lactic acid bacteria and yeast

Alcoholic and lactic

Cocoa

Yeast and lactic acid bacteria

Alcoholic and lactic

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