2.3 Ethanol fermentation
Fermentation is a metabolic process (one that is required to sustain life) in which living cells break down larger molecules into smaller ones. In fermentation, the living yeast uses sugars, rather than oxygen, to break down the complex energy-rich molecules into simpler ones, releasing energy in the process. You will see this illustrated in the brewing process in Week 2 via the addition of barley grains. This energy is harvested by the cells, to generate a molecule called adenosine triphosphate or ATP, which is the principal energy transfer molecule used in many biochemical processes that occur in living organisms. A series of enzymes within the yeast catalyse the sugar breakdown (thereby increasing the rate of the reactions which breakdown the sugar) . This occurs over several steps starting with sucrose, common table sugar, which has a formula of C12H22O11. In the presence of a specific enzyme and water, sucrose splits, forming two molecules of glucose (C6H12O6).
From this point a series of enzyme-catalysed reactions occur, turning glucose into ethanol and carbon dioxide. You will learn more about enzymes and the role they play in this process in Week 5. The overall reaction – that is, the equation for fermentation – is shown in Equation 2.
Humans have hijacked this process to great effect, notably in brewing and baking. In brewing, as you will learn in Week 2, the ethanol is the desired product. On the other hand, the generation of carbon dioxide is essential for the baker, as this gas allows their bread (or other product) to rise (or leaven). While the rising process also results in the production of ethanol, this evaporates due to the high temperatures used in the baking process.
What conditions may affect the fermentation reaction?
Fermentation is a biological reaction so temperature will affect the reaction as enzymes tend to have an ‘active window’ between certain temperatures, with a particular temperature being optimal for effective activity. Outside this window the enzymes do not work efficiently and the reactions will not happen.
The presence of air (oxygen) will have an effect on this reaction. The fermentation reaction occurs in anaerobic conditions (without air). The presence of oxygen can affect the rate of reaction. Different yeasts are affected in different ways – some work fine in aerobic conditions, as you will see later in this course.
The types of grain used will mean different starches and therefore sugars present. As these molecules are the basis of the reaction their chemical make-up will have a significant effect on reaction performance. Starch is a complex polysaccharide, i.e. it consists of numerous smaller sugar units joined together that need to be broken down by enzymes before the yeast is able to use them as raw materials for fermentation.
All ethanol contained in alcoholic beverages is produced in this manner, i.e. fermentation carried out by yeast. Different yeasts are key to brewing, with many strains used. You will learn more about these in Week 2 of the course.