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, 2009). 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).

Ronald Jackson in his book Wine Science 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)’ (2008). 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) 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, Jackson, 2008). 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). Equation for brewery fermentation can be described as (Lea. Piggott, 2003):