Yeast - is a single-celled fungus that belongs to the classes Ascomycetes, Basidiomycetes and Deuteromycetes. There are 372 known species of yeast, belonging to 39 genera. Yeast (YeAst) is a type of microorganism that was first used by humans in making bread, brewing, etc. In recent years, its application has become more and more widespread. Yeast cells can be eaten and used as feed, and valuable drugs such as nucleic acids, coenzyme A, cytochrome C, and coagulants can be extracted; their metabolites are used to produce vitamins, organic acids, and enzymes. Preparations, etc.; at the same time, it can also be used for petroleum fermentation and petroleum dewaxing. Yeast also often causes harm to humans. Saprophytic yeasts can spoil food, textiles and other raw materials; a few hypertonic yeasts such as SAcchArcmyces rouxii and honey yeast can spoil honey and jam; some are polluting bacteria in the fermentation industry. They consume alcohol, reduce yields or produce bad odors, affecting product quality. The shape and size of yeast: Most yeasts are single cells, generally round, oval or lemon-shaped, with a size of about 1 to 5 × 5 to 30A. Various yeasts have certain sizes and shapes, and they vary with each other. The age of bacteria and environmental conditions vary. Cell structure of yeast: The cell structure of yeast is very similar to the basic structure of bacterial cells, including cell wall, cell membrane, cytoplasm, nucleus and its contents (model diagram of yeast cell structure) ) An important difference between yeast and bacteria is that yeast has an obvious nucleus. The nucleus is round, usually in the center of the cell, and has a nuclear membrane, nucleolus and chromosomes. The main component is deoxyribonucleic acid (DNA), which is the "information center" of the cell, that is, the control center of the metabolic process. Plays an important role in reproduction and genetics. The reproduction methods of yeast are divided into two types: asexual reproduction and sexual reproduction, with asexual reproduction being the main one. Asexual reproduction is divided into budding and fission.

Mold is the collective name for "filamentous fungi". Any fungus that grows into velvety, cotton-like or spider web-like mycelium on the substrate is called mold. In terms of classification, molds are divided into algal fungi, ascomycetes and deuteromycetes. In addition to being used in traditional brewing, sauce making and other fermented foods, molds have been widely used in the fermentation industry in recent years to produce alcohol, citric acid, penicillin, griseofulvin, gibberellin, amylase, fermented feed, etc. The shape and structure of mold: Mold cells are composed of branched or unbranched hyphae, and many hyphae are intertwined together and are called mycelium. The average diameter of the hyphae is 2 to 10A, which is several to dozens of times larger than that of ordinary bacteria and actinomycetes, and is similar to yeast. (Diagram of nutritional hyphae of mold) Characteristics of mold colonies: Mold colonies are composed of branched hyphae. Because the hyphae are thicker and longer, the colonies formed are looser, fluffy, cotton-like or spider web-like, and are generally several to dozens of times larger than bacterial colonies. Colonies are often light-colored or white at first. When spores of various colors grow on the colony, due to the different shapes, structures and pigments of the spores, different structures and colors often appear on the surface of the colony, such as yellow, green, and cyan. , black, orange and other colors. The pigments produced by some molds can diffuse into the culture medium, causing the front and back sides of the culture medium to show different colors. Therefore, colony characteristics are also one of the main basis for identification of molds. Molds reproduce in various ways, mainly by forming asexual and sexual spores. Generally, when mold mycelium grows to a certain stage, it first reproduces asexually, and at a later stage, sexual reproduction structures are produced on the same mycelium to form sexual spores. Molds that are closely related to human life and food production include Rhizopus, Mucor, and Phoebe of the class Phycetes; Monascus of the class Ascomycetes; Aspergillus and Penicillium of the class Deuteromycetes, etc. Mucor is widely used in the food industry. For example, Mucor tallow can produce hydroxybutanone and lipase. Mucor reuteri is not only used in the brewing industry, but can also be used to make fermented bean curd, and Mucor racemosa is used to make tempeh. Some Mucor species can also be used for the conversion of steroid compounds to produce oxalic acid, lactic acid, succinic acid and glycerol. Rhizopus often appears on starchy foods, causing food to become moldy and spoilage. It can convert starch into sugar, so it is the main saccharifying bacterium for distiller's yeast or koji medicine used in my country, North Korea, and Southeast Asia. Rhizopus is the main strain in small medicinal koji and folk liqueur koji in various provinces in my country. Aspergillus is an important strain in the fermentation industry, pharmaceutical industry, and food industry. In my country, Aspergillus is used to make sauce, and it is the main strain used by the people to make wine and vinegar koji.

Modern industry utilizes Aspergillus to produce various enzyme preparations (amylase, protease, pectinase, etc.) and organic acids (citric acid, gluconic acid, etc.), which can be used as saccharified feed in agriculture. There are also some Aspergillus species that can produce substances harmful to the human body, such as aflatoxins. Commonly used Aspergillus species in industry include Aspergillus niger, Aspergillus oryzae and Aspergillus terreus. Monascus is used to produce red yeast and is still an excellent natural food colorant, such as red fermented bean curd. There is also Monascus purpureus, which can produce α-amylase, maltase, etc., which can be used to hydrolyze starch and produce glucose. In recent years, it has been used industrially to produce glucoamylase preparations.