Fermentation of Alcoholic Beverages, Essay Example

Role of Yeast in Fermentation of Alcoholic Beverages

Introduction

Process of fermentation has a long history, as even ancient Egyptians knew the secret of brewing the beer. Though fermentation itself had been used for thousands of years, it was considered to be a magical process, but not a logically explicable one. By the 17th century, role of yeast in fermentation of alcoholic beverages was unknown, though yeast was known to be necessary. There were two points of view then – first one claimed that yeast was essential for the fermentation process, while other disagreed that the fermentation process was purely chemical (Wong). In the middle of 19^th century this argument was resolved by Louis Pasteur’s work. He recognized yellow cells as yeast but failed to recognize grey rod- and sphere-shaped as bacteria. He also showed that the different fermentation products produced were invariably accompanied by specific microorganisms. It was only later when he came across the fact that grey mass was actually bacteria.

What drinks are usually produced by means of yeast fermentation? Manufacture of such alcoholic beverages as wine, cider, sake, beer and distilled spirits (e.g. whisky), beer, liquor involves fermentation of yeasts. The most widely used yeasts are Saccharomyces cerevisiae or in the case of beers, usually S. carlsburgiensis. Chemical reaction including yeasts is used not only in fermentation of alcoholic beverages, but also in baking. While for beverages the key product of the reaction is ethanol, for bread key product is carbon dioxide. ‘The production of alcohol occurs best in the absence of oxygen’ (Wong).

Yeasts and Fermentation

‘Fermentation is an energy-releasing form of metabolism in which both the substrate (initial electron donor) and by-product {final electron acceptor) are organic compounds’ (Jackson, 2008, p.355). Alcoholic fermentation is the most common type of fermentation, and it is charachterisic for S. cerevisiae. For such kind of fermentation ethanol acts as the final electron acceptor (though for yeast it is a by-product), whereas glucose is the preferred electron donor (substrate) (Jackson, 2008, p.355).

Ronald Jackson in his book Wine Science (2008) explained the term yeast as a ‘collection of fungi that possess a particular unicellular growth habit – cell division that involves budding (extrusion of a daughter cell from the mother cell) or fission (division of the mother cell into one or more cells by localized ingrowths)’ (p. 363). Unlike most filamentous fungi yeasts possess only one nucleus.

Yeast is an indivisible part of fermentation process. But why this very species had been chosen for the fermentation? A number of other microorganisms can produce glucose without oxygen and provide by-products which usually include alcohols (isopropanol and butanol) and short-chained organic acids (formic, acetic, lactic, etc.) (Marx, 1989, p.30) Also, most of the microorganisms that are capable of anaerobic metabolism can produce ethanol, but they are unable to produce it in necessary quantities because their cell membranes cannot tolerate the alcohol impact. In the course of history yeast had been found to be one of microorganisms that are capable of tolerating concentrations of ethanol up to 18 per cent of the fermentation broth (Marx, 1989, p.30, Jackson, 2008, p.). That is why percentage of alcohol in wine and beer cannot exceed approximately 16%. In order to produce beverages with higher concentrations of alcohol, the fermented products must be distilled (Wong). S. cerevisiae can grow on simple sugars or even on common table sugar. Saccharomyces yeast is also well known as an addition to human food and thus it ideal for producing alcoholic beverages, too.

Fermentation Process

As it becomes clear from information above, yeast is a producer of ethanol in glucose medium. When yeast grows without oxygen all the ATP that is needed for the growth is gen­erated by the process of glycolysis (Lea, Piggott, 2003, p.30). Equation for brewery fermentation can be described as (Lea. Piggott, 2003, p.31):

It becomes clear that yeast converts one molecule of glucose to two molecules of the alcohol plus two molecules of carbon dioxide (Marx, 1989, p.29). In the process of fermentation the microorganism gains two molecules of the energy ATP, which is used to maintain other cellular activities. ‘If no cell mass were produced, 51 per cent of the glucose would be converted to alcohol. However, the actual yield is closer to 45 per cent because some of the glucose is convened to cell mass and to by-products, such as glycerol’ (Marx, 1989, p.30).

As in the equations there are major products of reaction, there are some other by-products of fermentation, which quantities are negligible in comparison to those of main products. Yeasts produce wide variety of other substances and compounds such as esters, carbonyl products, sulfur compounds, glycerol, organic acids, and the flavor compounds (Lea, Piggott, 2003, Jackson, 2008). ‘Glycerol is produced in the fermentation by the reduction of dihydroxyacetone phosphate, and this reaction regenerates NAD⁺ when the supply of acetaldehyde is inadequate’ (Lea, Piggott, 2003, p.31).

Wine Production

Production of wine is a bit different from that of beer, as fermentation of wine can occur without addition of yeasts. Jackson described the peculiarities of wine fermentation and use of yeasts; he pinpointed that for wine fermentation it is common practice to use yeasts from previous fermentation. He called this consecutively used material an inoculum. In wine production, skins of the grape contain endogenous yeasts and they are main source of the yeast inoculum. As a result of introducing previous yeasts to the new ones one can get mixed yeasts and ensure that next fermentation will be like previous (2008). Also, inoculation of fermentation is an essential part of wine and beer making.

Vinification is a process quite different from brewery and it includes different processes. But as in beer production ethanol is produced by yeasts. There are two main organisms that are involved in wine making: Saccharomyces cerevisiae and Oenococcus oeni, and they are in some way different in selectively employing fermentative metabolism (Jackson, 2008). ‘S. cerevisiae is so adapted to fermentative metabolism that it can generate as many ATP per second as is normally generated by respiration’ (Jackson, 2008, p.355). Oenococcus oeni influences fermentation of lactic acid and it is less adapted to grape juice medium than S. cerevisiae. Also, both S. cerevisiae and O. oeni can tolerate reasonably high ethanol concentrations.

Fermentation agents are essential for production of ethanol, and yeast is a good choice. Though all yeasts of chosen species are suitable, there are different demands for them to produce different drinks. This variety of demands requires genetic modification and selection. For example, brewing yeasts must posses next properties:

• ‘rapid fermentation rate without excessive yeast growth; efficient utilization of maltose and maltotriose with good conversion to ethanol;
• ability to withstand the stresses imposed by the alcohol concentrations and osmotic pressures encountered in brewing;
• reproducible production of correct levels of flavor and aroma compounds;
• ideal flocculation character for the process employed; good handling characteristics’ (Walker, 1998, p.288).

In reality brewing yeasts are far from ideal characteristics as some their aforementioned properties are limited and there is a room for future investigation.

Conclusion

Production of alcoholic beverages has been part and parcel of human life. Though the recipes differed, fermentation was discovered many thousands years ago by different nations. It was the way of producing ethanol from liquids that contained glucose. Yeast fermentation has gone a long way from the magic ritual to casual manufacturing process. In the long run we know perfectly well what happens on the every stage of this complicated process and know how to rule it or inhibit it.

Anaerobic fermentation of yeast produces carbon dioxide, ethanol and energy. For yeast’s point of view all these products are by-products. Though carbon dioxide can be used in bakery, it is ethanol that is valued in alcoholic beverages fermentation.

To summarize the research it is essential to note that in fact, fermentation is the vital part of alcohol beverage production. And it cannot be denied that without yeast it would not happen. Yeast may be an essential component of must, like in production of beer. In case of wine S. cerevisiae is added in order to catalyze the reaction and enhance action of grape’s skin yeast. Yeasts can be specially grown or endogenous, but their presence is essential to generate things we appreciate alcohol for.

Works Cited

“Role of Yeast in Production of Alcoholic Beverages” Yeast and Alcoholic Beverages: Beer, Wine and Liquor. June 18, 2009 < http://www.botany.hawaii.edu/faculty/wong/BOT135/Lect14.htm>

Jackson, Ronald S. Wine Science: Principles and Applications Academic Press, 2008 Lea, A. G. H., Piggott, J. Raymond Fermented beverage production Birkhäuser, 2003 < http://books.google.com/books?id=mLntKU3H-14C&printsec=frontcover&hl=en&source=gbs_navlinks_s>

Marx, Jean L. A Revolution in biotechnology Cambridge University Press, 1989

Walker, Graeme M. Yeast physiology and biotechnology John Wiley and Sons, 1998