Yeast Production

Yeast ProductionBaker’s Yeast is a commercial preparation consisting of dried cells of one or more strains of Saccharomyces cerevisiae. Bakers use yeast as a leavening agent in the rising of dough for baking. A secondary contribution of yeast to bread is flavoring and aroma. Baker’s yeast is a high volume, low value product, with 3 million ton being produced per annum on a global scale.

Yeast production is fairly simple operation. It consists in developing a selected strain in a growth medium adjusted to the requirements of the yeast for a fast multiplication. Cane or beet molasses is the primary raw material and S. cerevisiae is the main yeast strain for baker’s yeast production. Molasses contains 45 to55 weight percent fermentable sugars, in the forms of sucrose, glucose, and fructose and it supplies all the sugar that yeast needs for growth and energy along with part of the needed nitrogen. The amount and type of cane and beet molasses used depend on the availability of the molasses types, costs, and the presence of inhibitors and toxins. Usually, a blend consisting of both cane and beet molasses is used in the fermentations. Before it is fed to the yeast, concentrated molasses is diluted with water, clarified, and heat sterilized. A variety of essential nutrients and vitamins is also required in yeast production. The nutrient and mineral requirements include nitrogen, potassium, phosphate, magnesium, and calcium, with traces of iron, zinc, copper, manganese, and molybdenum. Normally, nitrogen is supplied by adding ammonium salts, aqueous ammonia, or anhydrous ammonia to the feedstock.

Yeast strains that are suitable for bread production are selected under very strictly controlled circumstances and allow them to reproduce until several hundred grams of yeast are available. This small quantity of yeast is then transmitted to the industrial production facilities where it is grown until adequate quantities are obtained of what is called “mother yeast”. This mother yeast is the basis for the production of the actual baker’s yeast. The mother yeast is introduced into fermenters. These are large fermentation tanks in which nutrients and sugar, in the form of molasses are added on the one hand, and huge amounts of sterile air on the other. Air compressors blow this air through a ventilation system into the tanks and through the mother yeast. Temperature, pH value, airflow and molasses supply are parameters that are permanently controlled. During fermentation, the yeast is constantly cooled in order to prevent the temperature developed by the yeast during its growth from rising too high.

After fermentation, the yeast is fully-grown and the suspension is subsequently centrifuged in order to separate the yeast from the remainders of molasses. The yeast cream is quickly cooled down to 4°C and stored in refrigerated tanks. The yeast cream in those refrigerated tanks is then processed into 3 kinds of yeast products. Part of the yeast cream is sold as liquid yeast. The remaining yeast cream is further processed into fresh or dry yeast.

The yeast cream is transferred to a rotating vacuum filter and is spread out on the filter cloth of this rotating drum, thus removing the remaining water until the desired dry substance and consistency are obtained. Next, the yeast is scraped off the drum. Some of the yeast flakes obtained is pressed and block yeast is formed. The blocks are stored under refrigerated conditions. The remaining yeast that is rubbed off the rotating vacuum filter is taken to drying units where the yeast is dehydrated according to a very specific procedure in order to create a granular structure. Dry yeast has a moisture content of maximum 5%, but it becomes viable again after contact with water. This yeast is carefully vacuum-packed in aluminum bags.