Saccharomyces cerevisiae (yeast) is asugar fungus used in different types of foods and beverages because of its uniquephysiology and the key roles it plays in food fermentations.
Yeast is naturallyfound growing on the skins of fruits, plant leaves, surface of the skin, in theintestinal tracts of warm-blooded animals, soil and on flowers. However, thisyeast is found in the skins of grapes and other fruits. Most yeasts will thrivebest in a neutral or slightly acidic pH environment and each type of yeast hasslightly different temperature range. Saccharomyces cerevisiae’s growthtemperature is between 32.3°C-45.4°C and can survive in the presences orabsence of oxygen.
In the presence of oxygen, the yeast will undergo aerobicrespiration, converting carbohydrates into carbon dioxide and water. LuisPasteur was the person who first identified and used saccharomyces cerevisiae asthe key microbe for wine and bread making in 1856. It was around a year laterwhen it became popular for most industries to use it.
This type of yeast has beengenetically manipulated and engineered to suit the specific needs for the fermentationprocess. Saccharomyces cerevisiae has been labeled as GRAS (generally regardedas safe) and is the most common yeast out there for food and beverage making.It has been studied as a eukaryotic model organism and was the first tocompletely sequence.Saccharomyces cerevisiae chemicalformula is C:H (1.612): O (0.557): N (0.158).
This means for every 1 mole ofcarbon, there is 1.613 moles of hydrogen, 0.557 moles of oxygen, 0.158 moles ofnitrogen. On a much smaller scale, there is also 0.012moles of phosphorus,0.003moles of sulfur, 0.003 moles of magnesium, 0.
022 moles of potassium, and0.001 of calcium. Fermentation systems ranged fromopened and closed squares to horizontal and vertical cylindrical vessels. Themost common style for batch is the cylindroconical fermenter, an enclosedvertical cylindrical vessel with a cone shaped base because they are cheaper topurchase and operate, the efficiency of production, and it doesn’t take up alot of room. Before any beer can be made, all the equipment must be sanitized,and the operator must maintain good hygiene to ensure low unwanted microorganismsgetting into the beer. The chosen grain and barely are placed into a cheesecloth and into the water. They undergo mashing; the steeping ofgrains, releasing the flavours and sugar that are needed for the fermentationprocess.
The mixture is heated and held at different temperature for periods oftime to activate the enzymes in the grain to convert the starches tofermentable sugars (glucose, sucrose, fructose). This process can take up to1-2 hours and as the result, the mixture is now mashed. The mixture nowconsists of the freed starches and sugars. The cheese cloth with the grains areremoved, leaving the mixture of water and sugars, known as wort. The wort goesthrough the boiling process for 45-90 minutes.
Hops are added as a bitteringagent and to add aroma of the beer for 15 minutes or until “flame out”. Flame outis when the heat source is either turned off or the boiling has stopped. The longerthe hops are in, the more of the piney or fruity smells are present in thebeer. The aroma and flavor of the beer depends on the kind of hop used. Themixture must be cooled before the saccharomyces cerevisiae can be added becauseit is temperature sensitive. This type of yeast can undergo both aerobic andanaerobic respiration. For Fermentation, the yeast starts breaking down thedifferent forms of sugar present in the wort.
The sugars present are typicallymonosaccharides (glucose, fructose), disaccharides (galactose, sucrose, and maltose),trisaccharide (maltotriose). The yeast first breaks down the monosaccharides byusing different enzymes both inside and outside the cell. Sucrose is brokendown into glucose and fructose by the invertase enzyme. Once the complex sugars are broken down into monosaccharides,the yeast can use them. In the presents of oxygen, aerobic respiration occursin the mitochondria of the yeast.
Using other enzymes, two pyruvates producedfrom glycolysis, are decarboxylase to form acetaldehydes and carbon dioxide. Laststep is to had hydrogen ions to the aldehyde to form ethanol. The hydrogen atomis from the NADH made during glycolysis and converts back to NAD+. This forms adefense that will make sure no bacteria growth on the mixture. However, somebacteria are purposely added to the mixture for different tastes. Some breweriesused open fermentation vessels, which can expose the beer to infection butimprove the process of removing the yeast.
On the industrial scale, the yeastis removed by filtration and bottled. Conditioning and carbonation occur in asecondary container, so the beer is not exposed to the dead yeast.