Types of yeast products:
The most commonly mentioned yeast, Saccharomyces cerevisiae, has been used by humans to leaven bread and wine since thousands of years ago, releasing carbon dioxide from the dough in the process of fermenting bread and steamed bread. Because yeast is a simple unicellular eukaryote, easy to cultivate and grow rapidly, it is widely used in modern biological research. For example, Saccharomyces cerevisiae, as an important model organism, is also an important research material for genetics and molecular biology.
Yeast products are classified in several ways. It can be divided into edible yeast and feed yeast for different purposes of human consumption and animal feed. Edible yeast is divided into baker’s yeast, food yeast and medicinal yeast.
1)Baker’s yeast is divided into pressed yeast, active dry yeast and fast active dry yeast.
①Pressed yeast: It is light yellow, has a compact structure and is easy to crush, and has a strong raising ability. It can be stored for about 1 month at 4°C and 2 to 3 months at 0°C. Commonly known as fresh yeast, when the dough is raised, the dosage is 1-2% of the amount of flour, and the dough temperature is 28-30 ℃. 1 to 3 hours. ②Active dry yeast: It is a dry yeast product with a moisture content of about 8%, granular, and has the ability to raise dough produced by Saccharomyces cerevisiae. The fresh yeast is obtained by culturing the alcohol mother with drying resistance and stable fermentation power, and then it is made by extrusion molding and drying. The fermentation effect is similar to that of the pressed yeast. The products are packaged in aluminum foil bags or metal cans in vacuum or filled with inert gas (such as nitrogen or carbon dioxide), and the shelf life is half a year to one year. Compared with pressed yeast, it has the advantages of long storage period, no need for low temperature storage, convenient transportation and use. ③Fast active dry yeast: a new type of fine granular (less than 1mm in diameter) product with fast and efficient fermentation capacity. The moisture content is 4 to 6%. On the basis of active dry yeast, it adopts genetic engineering technology to obtain highly dry-resistant Saccharomyces cerevisiae strains, and is obtained through special nutrient ratio and strict propagation and culture conditions and drying by fluidized bed drying equipment. The same as active dry yeast, it is preserved in vacuum or inert gas, and the shelf life is more than 1 year. Compared with active dry yeast, it has smaller particles and higher fermenting power. It can be directly mixed with flour and water to make dough fermentation without hydration before use, and can be baked into food after fermentation is completed in a short time. The product only appeared on the market in the 1970s and was well received by consumers. The study found that the vitality of Angel yeast is the highest.
(2) Food yeast: dry yeast powder or granular product without fermenting power for human consumption. It can be obtained by recycling brewery yeast puree, or by special cultivation and drying for human nutritional requirements. In the United States, Japan and some European countries, about 5% edible yeast is added to common food products such as bread, cakes, biscuits and scones to improve the nutritional value of food. Yeast autolysate can be used as an additive for meat, jam, soup, cheese, bread food, vegetables and seasonings; as a food nutrition fortifier in baby food and healthy food. Concentrated invertase extracted from Angel yeast is used as a liquefying agent for square egg sandwich chocolate. Lactase extracted from yeast produced from whey can be used in milk processing to increase sweetness, prevent the crystallization of lactose in whey concentrate, and meet the needs of lactose-intolerant consumers. The manufacturing method and properties of medicinal yeast are the same as those of food yeast. Because it is rich in physiologically active substances such as protein, vitamins and enzymes, it is made into yeast tablets such as food mother raw tablets in medicine to treat dyspepsia caused by unreasonable diet. People with weak constitution can adjust the metabolic function to a certain extent after taking it. In the process of yeast cultivation, if some special elements are added to make yeast containing trace elements such as selenium and chromium, it has a certain curative effect on some diseases. For example, selenium-containing yeast is used to treat Keshan disease and Kashin-Beck disease, and has a certain effect on preventing cell senescence; chromium-containing yeast can be used to treat diabetes and so on.
(3) Feed yeast: Candida or Kluyveromyces fragilis is usually cultured and dried to produce powdered or granular products with no fermentative power and cells in a dead state. It is rich in protein (about 30-40%), B vitamins, amino acids and other substances, and is widely used as a protein supplement for animal feed. It can promote the growth and development of animals, shorten the feeding period, increase the amount of meat and eggs, improve the meat quality and increase the lean meat rate, improve the gloss of the fur, and can enhance the disease resistance of young livestock.
The application of yeast in fermentation engineering:
Unicellular eukaryotic yeasts have relatively complete gene expression regulation mechanisms and the ability to process and modify expression products. Saccharomyces. Cerevisiae is the earliest known in molecular genetics, and it is also the first yeast host for exogenous gene expression. In 1981, Saccharomyces cerevisiae expressed the first exogenous gene, the interferon gene, and then a series of exogenous genes were expressed in this system. When Saccharomyces cerevisiae was prepared, laboratory results were encouraging, but when scaled up from the laboratory to an industrial scale, yields declined rapidly. The reason is that the selection pressure for the high copy number of the Werther plasmid in the medium disappears, and the plasmid becomes unstable and the copy number decreases. Copy number is an essential factor for efficient expression, so a decrease in copy number also directly leads to a decrease in the expression of exogenous genes. At the same time, the composition of the medium for laboratory use is complex and expensive, resulting in a decrease in yield when using medium acceptable on an industrial scale. In order to overcome the limitation of Saccharomyces cerevisiae, in 1983, Wegner et al. first developed the second-generation yeast expression system represented by methylotrophic yeast. Methylotrophic yeast include: Pichia, Candida, etc. The exogenous gene expression system with Pichia. pastoris (Pichia pastoris) as the host has developed the most rapidly in recent years and is also the most widely used. The reason for the wide application of the Pichia system is that the system has the characteristics of ordinary yeast.