yeast
English name: yeast
Yeast is some unicellular fungi, not a unit of phylogenetic classification. Yeast is the earliest microorganism used in the history of human civilization. At present, there are more than 1 kinds of yeasts. According to their ability to produce spores (ascospores and basidiomycetes), yeasts can be divided into three categories: the strains that form spores belong to ascomycetes and basidiomycetes. Those that do not form spores but propagate mainly through budding are called incomplete fungi, or "false yeast". At present, it is known that most yeasts are classified into Ascomycetes. Yeast is widely distributed in nature, mainly growing in acidic and humid sugary environment, for example, it is most common in the interior and surface of fruits, vegetables, preserves and orchard soil.
[ Edit this paragraph] Physiology
Yeast lives in obligate or facultative aerobic life, and obligate anaerobic yeast is unknown at present. In the absence of oxygen, fermented yeast obtains energy by converting sugar into carbon dioxide and ethanol.
C6H12O6 (glucose) →2C2H5OH (alcohol) +2CO2↑
in the process of brewing, ethanol was preserved; In the process of baking bread or steaming steamed bread, carbon dioxide initiates dough, while alcohol volatilizes.
In the presence of oxygen, yeast converts glucose into water and carbon dioxide. For example, the steamed bread and bread we eat are all expanded by yeast in the presence of oxygen.
[ edit this paragraph] chemical element composition
the chemical composition of yeast is related to the culture medium, culture conditions and the physiological state of yeast itself.
In general:
Average elemental composition of yeast cells (%) As follows:
carbon -47 hydrogen -6.5 oxygen -31 nitrogen -7.5~1 phosphorus -1.6~3.5
the content of other elements is very small (%)
calcium -.3~.8 potassium -1.5-2.5 magnesium-.1 ~ .4 sodium-.
iron-9-35 copper: 2-135 zinc: 1-16 cobalt: 15-65
[ Edit this paragraph] Features
Most yeasts can be isolated in an environment rich in sugar, such as some fruits (grapes, apples, peaches, etc.) or plant secretions (such as cactus juice). Some yeasts live in insects. Yeast is a unicellular eukaryotic microorganism. The morphology of yeast cells is usually spherical, oval, sausage-shaped, oval, lemon-shaped or lotus root-shaped. It is much larger than the single-celled individual of bacteria, generally 1~5 microns '5~2 microns. Yeast can't swim without flagella. Yeast has a typical eukaryotic cell structure, including cell wall, cell membrane, nucleus, cytoplasm, vacuole, mitochondria, etc., and some also have microsomes. Cell morphology of yeast Cell morphology of yeast Micrograph of cell structure of yeast Colony of yeast.
The colony characteristics of most yeasts are similar to those of bacteria, but larger and thicker than bacterial colonies. The colony surface is smooth, moist and sticky, which is easy to stir up. The colony texture is uniform, and the colors on the front and back, the edges and the center are uniform. Most colonies are milky white, a few are red and some are black. Colony of beer yeast colony of red yeast colony of various yeasts.
[ Edit this paragraph] Reproduction
Yeast can reproduce asexually by budding or sexually by forming ascospores. Asexual reproduction means that when the environmental conditions are suitable, a bud grows from the mother cell and gradually grows to a mature size and then separates from the mother. When the nutritional status is not good, some yeasts that can have sexual reproduction will form spores (usually four), and then germinate when conditions are suitable. Some yeasts, such as Candida, cannot reproduce asexually.
Growth conditions of yeast
Nutrition:
Yeast needs similar nutrients like other living organisms. Like bacteria, it has a set of intracellular and extracellular enzyme systems to break down macromolecules into small molecules that can be easily utilized by cell metabolism.
water content:
Like bacteria, yeast must have water to survive, but yeast needs less water than bacteria. Some yeasts can grow in an environment with very little water, such as honey and jam, which shows that they are quite tolerant to osmotic pressure.
acidity:
yeast can grow in the range of pH 3-7.5, and the optimum pH is pH4.5-5..
Temperature:
Generally, yeast cells cannot grow at a temperature lower than the freezing point of water or higher than 47℃, and the optimum growth temperature is generally between 2℃ and 3℃.
Aerobic gas:
Yeast can grow in both aerobic and anaerobic environments, that is, yeast is facultative anaerobic bacteria. In the absence of oxygen, yeast breaks down sugar into alcohol and water. In the presence of oxygen, it decomposes sugar into carbon dioxide and water, and in the presence of oxygen, yeast grows faster.
[ Edit this paragraph] Use
The most frequently mentioned yeast, Saccharomyces cerevisiae, has been used by human beings to ferment bread and wine for thousands of years, and carbon dioxide will be released from the dough during the fermentation of bread and steamed bread.
As a simple single-celled eukaryote, yeast is easy to cultivate and grows rapidly, so it is widely used in modern biological research. As an important model organism, Saccharomyces cerevisiae is also an important research material in genetics and molecular biology.
[ Edit this paragraph] Product category
There are several classification methods for yeast products. The different purposes of human consumption and animal feed can be divided into edible yeast and feed yeast. Edible yeast is divided into bread yeast, food yeast and medicinal yeast.
(1) baker's yeast is divided into pressed yeast, active dry yeast and fast active dry yeast.
① pressed yeast: a block product with water content of 7 ~ 73% produced by Saccharomyces cerevisiae. It is light yellow, has a compact structure and is easy to crush, and has strong hair-raising ability. The product which can be stored at 4℃ for about 1 month and at ℃ for 2 ~ 3 months was originally obtained by squeezing and dehydrating the centrifuged yeast milk with a plate-and-frame filter press, so it was called pressed yeast, commonly known as fresh yeast. When leavening, its dosage is 1 ~ 2% of the flour, the leavening temperature is 28 ~ 3℃, and the leavening time varies with factors such as yeast dosage, leavening temperature and dough sugar content, generally 1 ~ 3 hours.
② active dry yeast: a granular dry yeast product with about 8% water content produced by Saccharomyces cerevisiae. Fresh yeast is obtained by using alcohol mother with dry resistance and stable fermentation power, and then it is made by extrusion molding and drying. The fermentation effect is similar to that of pressed yeast. Products are packed in vacuum or aluminum foil bags or metal cans filled with inert gas (such as nitrogen or carbon dioxide), and the shelf life is six months to one year. Compared with pressed yeast, it has the advantages of long preservation period, no need for low temperature preservation, convenient transportation and use.
③ Fast-active dry yeast: a new type of fine granular (less than 1mm in diameter) product with fast and efficient fermentation. The moisture content is 4-6%. On the basis of active dry yeast, it adopts genetic engineering technology to obtain a highly dry-resistant Saccharomyces cerevisiae strain, which is obtained by special nutrition ratio, strict proliferation culture conditions and drying with fluidized bed drying equipment. Like active dry yeast, it is preserved in vacuum or inert gas, and its shelf life is more than 1 year. Compared with active dry yeast, it has smaller particles and higher fermentation power. When it is used, it can be directly mixed with flour and added with water to make dough for fermentation, and can be baked into food after fermentation in a short time. This product only appeared in the market in the 197s, and was well received by consumers. It was found that Angel Yeast had the highest activity.
(2) food yeast: a dry yeast powder or granular product that has no fermentative ability and is for human consumption. It can be obtained by recycling yeast mud from breweries, or by specially cultivating and drying it for the requirements of human nutrition. The United States, Japan and some European countries add about 5% edible yeast powder 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, vegetables and seasonings. It is used as a food nutrition enhancer in baby food and health food. The 5'- nucleotide prepared from yeast autolysis extract can be used as an additive to strengthen food flavor in combination with monosodium glutamate (see). The concentrated invertase extracted from Angel Yeast was used as liquefier for chocolate with square eggs. Lactase extracted from yeast produced from whey can be used in milk processing to increase sweetness, prevent the crystallization of lactose in whey concentrated solution, 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 protein, vitamins, enzymes and other physiologically active substances, it is made into yeast tablets, such as raw mother tablets, to treat dyspepsia caused by unreasonable diet. People with weak constitution can adjust their metabolic function to a certain extent after taking it. In the process of yeast culture, if some special elements are added to make yeast containing trace elements such as selenium and chromium, it has certain curative effect on some diseases. For example, selenium-containing yeast is used to treat Keshan disease and Kaschin-Beck disease, and has a certain effect of preventing cell aging; Chromium-containing yeast can be used to treat diabetes.
(3) feed yeast: usually, candida or Kluyveromyces fragilis is cultivated and dried to produce a powdery or granular product with no fermentability and dead cells. It is rich in protein (about 3 ~ 4%), 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 lean meat rate, improve the gloss of fur, and enhance the disease resistance of young livestock.
[ Edit this paragraph] Harm
Some yeasts are harmful to living things or appliances, for example, Rhodotorula will grow on wet furniture such as shower curtains; Candida albicans will grow in moist human epithelial tissues such as vaginal lining.
[ edit this paragraph] the role of yeast
I. composition of yeast genome
before the sequencing plan of Saccharomyces cerevisiae began, about 2,6 genes encoding RNA or protein in yeast were identified by traditional genetic methods. By sequencing the whole genome of Saccharomyces cerevisiae, it was found that there were 5885 open reading frames coding for specific protein in the whole genome sequence of 1268kb. This means that there is a gene encoding protein in the yeast genome every 2kb on average, that is, 72% of the nucleotide sequence of the whole genome consists of open reading frames. This shows that yeast genes are arranged more closely than other higher eukaryotes. For example, in the genome of nematodes, there is a gene encoding protein every 6kb on average; In the human genome, a gene encoding protein can only be found every 3kb or more on average. The compactness of yeast genome is due to the short interval between genes and the scarcity of introns in genes. The average length of the open reading frame of yeast genome is 145bp, that is, 483 codons, the longest is an open reading frame with unknown function (491 codons) on chromosome XII, and a few open reading frames are more than 15 codons. In the yeast genome, there are also genes encoding short proteins, for example, PMP1 gene encoding plasma membrane protein lipids composed of 4 amino acids. In addition, the yeast genome also contains: about 14 genes encoding RNA, arranged at the long end of chromosome XII; 4 genes encoding SnRNA, scattered on 16 chromosomes; 275 tRNA genes belonging to 43 families are also widely distributed in the genome. Table 1 provides an overview of the distribution of yeast genes on each chromosome.
table 1 yeast chromosome profile
chromosome number
length (bp) Gene number tRNA gene number
I 23× 13 89 4
II 87 188 41 13
III 315× 13 182 1
IV 1531974 796 27
V 569 22 271 13
VI 27× 13 129 13. II 19936 572 33
VIII 561×13 269 11
IX 439886 221 1
X 745442 379 24
XI 666448 331 16
XII 178171 534 22
XIII 92443 45 9 21
XIV 784328 419 15
XV 192283 56 2
XVI 94861 487 17
sequencing revealed a wide range of base composition changes in the yeast genome. Most yeast chromosomes are composed of GC-rich DNA sequences and GC-deficient DNA sequences in different degrees and in a wide range. This change of GC content is related to chromosome structure, gene density and recombination frequency. The regions with high GC content are generally located in the middle of chromosome arm, and the gene density in these regions is high; The regions with low GC content are generally close to telomeres and centromeres, and the number of genes in these regions is relatively poor. Simchen et al. confirmed that the relative incidence of yeast genetic recombination, that is, double-strand breaks, is coupled with the GC-rich regions of chromosomes, and the recombination frequencies of different chromosomes are different. The recombination frequencies of smaller chromosomes ⅰ, ⅲ, ⅳ and ⅸ are higher than the average recombination frequency of the whole genome. ?
another obvious feature of yeast genome is that it contains many DNA repeats, some of which are identical DNA sequences, such as rDNA and CUP1 genes, Ty factor and its derived single LTR sequence. There are a large number of trinucleotide repeats in the open reading frame or gene spacer, which has attracted great attention. Because some human genetic diseases are caused by changes in the number of trinucleotide repeats. There are more DNA sequences with high homology with each other, which are called genetic redundancy. The ends of many chromosomes in yeast have highly homologous regions with a length of more than tens of kb, which are the main areas of genetic abundance, and these regions are still undergoing frequent DNA recombination processes. Another kind of genetic abundance