Production of yeasts
Ripening yeasts, like lactic acid bacteria, are also produced in specialised fermentors under strict hygiene conditions but with different nutrient sources and growth parameters. Notably, yeast fermentation needs to be conducted under aerobic (with oxygen or air) conditions. They are concentrated and mainly available as freeze-dried powders.
The baker’s yeast is commercially produced on a nutrient source which is rich in sugar (usually molasses: by product of the sugar refining). The fermentation is conducted in large tanks. Once the yeast fills the tank, it is harvested by centrifugation, giving an off-white liquid known as cream yeast. This is further processed into any of several different forms:
- Compressed yeast: still widely used commercially, it is a soft beige solid block with limited storage properties.
- Active dry yeast: dried yeast presented in granules or beads that needs to be rehydrated before it can be used.
- Instant yeast: vacuum packed fine powder that has become popular in home breadmaking, as it is easy to use
Yeasts in food production
Yeasts have two main uses in food production: baking and making alcoholic beverages. They have been used in this way since ancient times – there is evidence that ancient Egyptians used yeast in breadmaking, and we have been making fermented drinks like beer and wine for millennia.
Baked goods like bread rise because of the presence of yeast as a raising, or leavening, agent. The most common yeast used in breadmaking is Saccharomyces cerevisiae. It feeds on the sugars present in the bread dough, producing the gas carbon dioxide. This forms bubbles within the dough, causing it to expand. Other ingredients in the mixture have an effect on the speed of the fermentation – sugar and eggs speed it up; fats and salt slow it down.
Several different yeasts are used in brewing beer, where they ferment the sugars present in malted barley to produce alcohol. One of the most common is Saccharomyces cerevisiae, the same strain used in breadmaking; this is used to make ale-type beers and is known as a top-fermenting yeast as it forms a foam on the top of the brew. Bottom-fermenting yeasts, such as Saccharomyces pastorianus, are more commonly used to make lagers. They ferment more of the sugars in the mixture than top-fermenting yeasts, giving a cleaner taste.
The alcohol in wine is formed by the fermentation of the sugars in grape juice, with carbon dioxide as a byproduct. Yeast is naturally present on grapeskins, and this alone can be sufficient for the fermentation of sugars to alcohol to occur. A pure yeast culture, most often Saccharomyces cerevisiae, is usually added to ensure the fermentation is reliable. Sparkling wine is made by adding further yeast to the wine when it is bottled. The carbon dioxide formed in this second fermentation is trapped as bubbles.