Yeasts are unicellular fungus very common in the environment, but are usually isolated from sugar-rich material. There are more than 50,000 fungal species thus far identified, 500 of which are classified as yeasts. Perhaps the best known of these are the strains of Saccharomyces cerevisiae used in a variety of industrial operations. Once the specific strains are isolated and selected for their desired fermentation qualities and traits, whether it is in baking, brewing, wine making or as probiotics, they must be maintained under rigorous parameters to insure their activity, purity and genetic integrity.
Saccharomyces reproduce by budding, where by a small bud, or daughter cell, is formed on the parent cell. The nucleus of the parent cell splits into a daughter nucleus and migrates into the daughter cell. The bud continues to grow until it separates from the parent cell, forming a new cell. Some yeasts reproduce by binary fission instead of budding, whereby they divide the genetic information and split into two new organisms. To grow yeast needs a fermentable substrate composed of sugar, nitrogen, phosphorous, vitamins and minerals. Traditionally molasses has been the substrate of choice but as world commodity markets change other substrates are being used along with molasses. In an industrial setting, yeasts are produced by a controlled aerobic fermentation (growth with air) where yeast biomass, and not alcohol, is the result. Pure strains of the “mother” culture are maintained in the laboratory, and then transferred to the plant process, where they are grown in a nitrogen-enriched sugar medium, fortified with vitamins and minerals for optimum cell activity. After the appropriate number of generations, the mature cell mass is separated from the spent liquid, then delivered as liquid cream yeast, filtered and sold as compressed yeast or filtered and dried into instant dry yeast. Another process step is autolysis (self-digestion) of the yeast cells to liberate the cell nutrients. The autosylate can be simply dried or it can be separated to produce a fully soluble yeast extract, and cell walls
Torula yeast is generally known as nutritional yeast and classified into Candida utilis (Pichia jadinii). Torula is a budding fungi and cosmopolitan in nature. It was first described by Persoon in 1794. Torula yeast (in dried form) is permitted by US FDA for direct addition to food for human consumption (21 CFR 172.896). The first commercial production of torula yeast was started in Germany during WWI from the by-product of cellulose and lignin products industry which has significant amounts of pentose that are not easily metabolized by S.cerevisiae. In the midst of ongoing demand of torula yeast for food applications, food manufacturers in US and other countries are producing primary grown torula yeast utilizing sugar feed stocks ensuring pure yeast and consistency batch after batch. It is pasteurized and dried to produce a fine, light grayish-brown powder. The quality of torula yeast is very much dependent on the substrates and production methods employed.
The strains of S. cerevisiae are employed in three main industrial processes:
- For the production of alcoholic beverages such as wine, beer, sake, potable spirits, and to a large extent, industrial alcohol and fuel alcohol, where under anaerobic or low-oxygen conditions the yeast ferments carbohydrates and converts it into alcohol and carbon dioxide.
- As a leavening agent for the production of bread and baked goods, where it converts the fermentable sugars present in dough into the gas carbon dioxide. This causes the dough to expand or rise as gas forms pockets or bubbles.
- The production of biomass, extracts, autolysates, and flavor compounds. The yeast used in such processes can be either primary grown or spent yeast from the brewing and distilling industries.
Deactivated yeast, usually Saccharomyces cerevisiae, is also used in nutritional supplements popular with vegans and the health conscious, where it is often referred to as “nutritional yeast”. It is an excellent source of protein and vitamins, especially the B-complex vitamins. Some brands of nutritional yeast are fortified with vitamins or minerals.
Some probiotic supplements use the yeast Saccharomyces boulardii to maintain and restore the natural flora in the large and small gastrointestinal tract.
There are yeasts capable of utilizing unusual substrates that are of importance in waste disposal. Some can degrade palm oil mill effluent, TNT (an explosive material), and other hydrocarbons such as alkanes, fatty acids, fats and oils. They can also tolerate high concentrations of salt and heavy metals, and are being investigated for their potential as a heavy metal biosorbent.
Yeasts have recently been used to generate electricity in microbial fuel cells, and to produce ethanol for the biofuel industry. The fuel ethanol process utilizes sugar cane, field corn, or other cereal grains as a source of starch that will be broken down into simple sugars that yeast will be able to ethanol. Saccharomyces yeasts have been genetically engineered to ferment xylose, one of the major fermentable sugars present in cellulosic biomasses, such as agriculture residues, paper wastes, and wood chips.
The yeast species S. cerevisiae is also extremely important as a model organism in modern cell biology research, and is one of the most thoroughly researched eukaryotic microorganisms. Researchers have used it to gather information about the biology of the eukaryotic cell and ultimately human biology.
The inactivated torula yeast is widely used as flavorings and flavor enhancer in processed foods, pet foods, and animal feeds. It has a very clean flavor profile and does not come with bitter/strange taste like other yeast. Torula yeast contains relatively higher amounts of nucleotides that help to enhance flavors. It has a slightly yeasty odor and gentle, slightly meaty taste. Torula yeast is a good source of nutrients for pet foods and is also used for honey production, aquaculture, as a media additive in industrial fermentation and insect cell culture, or as an attractant for the management of the olive fruit fly.