Fermentation and its Uses

Fig. 1 - Glycolysis

Fermentation, according to Louis Pasteur, is essentially respiration in the absence of oxygen. This, however, is an oversimplification of the process.

More technically, fermentation involves the breakdown of carbohydrate molecules to form an intermediate product, pyruvate and a small amount of energy. This stage of the process is known as glycolysis, and it occurs inside all living cells (see figure 1).

The pyruvate molecule may then be changed into products such as ethanol, lactic acid, carbon dioxide, hydrogen, citric or acetic acid, depending on the type of organism and the conditions. Further metabolism of these products may occur if oxygen becomes available – one example of this is the oxidation of ethanol to acetic acid when wine is spoiled.

The Production of Alcohol

Beers, wines and spirits are all produced by fermenting various carbohydrates. Yeasts do this naturally to sugars; a property that has been utilised by humans for thousands of years. Wine was probably first produced in the Middle East as long ago as 5000 BC, and evidence of a process involving crushed grapes dating from 6500 years ago has been found in Macedonia.

Fig, 2 - Fermentation Vats

Modern wine makers combine additional yeast varieties to those present naturally in the harvested grapes, and the process of fermentation from sugar to ethanol and carbon dioxide occurs in large steel or plastic vats or oak barrels (see figure 2). The carbon dioxide produced is either allowed to escape, or trapped to form sparkling wines.

Yeasts are also instrumental in the production of beer, manufactured since around 6000 BC. Unlike wine making, the source of carbohydrates is partly germinated malt barley rather than grape sugar (see figure 3). The reason malt grain needs to be germinated is that this procedure produces enzymes that begin the conversion of starches into sugars. Hops are added to produce a bitter flavour during the brewing process, and also to act as a preservative.

Spirits such as whiskey, brandy and Cognac are produced using distillation to further concentrate the ethanol to around 37-43% (as opposed to around 12% in wines). Additional sugar may be added to produce liqueurs.

Ethanol is also produced industrially on a large scale for use as a biofuel. This has traditionally involved a two step fermentation procedure using aerated tanks containing the yeast Saccharomyces cerevisciae and substrate carbohydrates. Recent studies have shown that the bacterium Zymomonas mobilis produces a larger quantity of ethanol and, unlike yeast, does not become inhibited by high ethanol concentrations. Interestingly, Z. mobilis can also be genetically modified to convert a larger range of sugars into ethanol.

The Production of Citric Acid

Fig. 3 - Germinated Malt Barley

Citric acid is a useful product in both the food and pharmaceutical industries; it is used in food as a preservative and to produce an acidic, sour taste in soft drinks and other beverages. In the pharmaceutical industry it can be used as a buffering agent and to clean equipment.

Citric acid is formed by the fermentation of a molasses substrate by the fungus Aspergillus niger. The biochemical pathway involved includes the production of pyruvate in glycolysis, followed by its conversion to citric acid via the condensation of acetyly co-enzyme A and oxaloacetate. The presence of specific amounts of trace elements such as iron have proved to be important in this process.

Acetic Acid Production

In the presence of the Acetobacter bacterium and oxygen, fermented carbohydrates, ciders or wines can be converted to vinegar (acetic acid). The result is usually a 5% solution of acetic acid.

Acetic acid is used in diluted form in the food industry as a condiment and pickling agent. It is also employed in industry as a solvent and an important reagent in many organic synthesis reactions.

Lactic Acid Production

Again, this product is formed after pyruvate has been produced in the glycolysis pathway. The presence of lactic acid bacteria (Lactobacillus bulgaricus, Streptococcus thermophilus or similar species) usually results in the conversion of two pyruvate molecules to lactic acid or lactic acid, ethanol and carbon dioxide.

In the food industry, lactic acid fermentation is used in the production of yoghurt, sauerkraut, pickles and cheeses. Other uses for lactic acid include tanning leather, manufacturing lacquers and inks, as a moisturising agent in cosmetics and in the production of polylactic acid. Polylactic acid, which can be used to make biodegradable plastics, is finding many uses in the biomedical industry, as well as in areas such as clothing and food packaging.

A Versatile Reaction

Fermentation certainly produces a diverse range of chemicals and is obviously a key reaction in many industries. The one thing all these processes have in common is an initial culture containing carbohydrates and a particular species of microorganism. Although in all cases pyruvate is produced as an intermediate step, the microorganisms - whether bacteria, yeasts or fungi - will each determine the specific end fermentation products.

References

Gunasekaran, P., and Chandra Raj.,K., 'Ethanol Fermentation Technology', Madurai Kamaraj University, ias.ac.in, accessed 14/4/2010

Mudgeee Grape Grower's Association, 2010, 'How is Wine Made?', mudgeewine.com, accessed 13/4/2010

Temph.info, 2010, 'Acetic Acid Fermentation', tempeh.info, accessed 12/4/2010