Microbial products produced by fermentation culture with sugar, starch and other industrial and agricultural by-products as raw materials. It is short for yeast. Yeast is the largest microorganism directly consumed by human beings. 1986, the annual output of baker's yeast in the world was10.8 million tons (calculated by 30% solids). Yeast is rich in protein, fat, sugar and B vitamins, as well as enzymes, coenzymes, RNA, sterols and some metabolic intermediates. Some yeasts, such as Saccharomyces cerevisiae, can convert sugar into ethanol and carbon dioxide under anaerobic conditions.
A brief history of development In 2300 BC, people began to make bread with "old yeast" containing yeast. From the residual models of bakeries and breweries unearthed in Thebes, Egypt, it can be seen that as early as 2000 BC, human beings have made good use of yeast to make fermented food and wine. In BC13rd century, the technology of baking bread spread from Egypt to the Mediterranean and other regions. 1680 A. Van Leeuwenhoek discovered yeast cells from a drop of beer with a microscope. Soon, humans began to consciously use yeast (beer yeast paste) to make dough. The importance of yeast has gradually attracted the attention of industry.
/kloc-In the mid-9th century, the industrial revolution in Europe produced a large number of densely populated areas, which required large-scale industrial production of baker's yeast to meet the needs of bread production. 1846, Austrian M. Marckhoff established the world's first yeast factory in Vienna. The factory uses grain as raw material, adopts mild ventilation culture method, and obtains yeast and alcohol at the same time, which is called "Vienna method". Because yeast is separated from the culture solution by a press, the product is called "pressed yeast". 1876, L. Pasteur of France published the theory that oxygen in the air can promote yeast reproduction, which laid the foundation for large-scale ventilation culture to produce yeast. At the beginning of the 20th century, due to the appearance of yeast centrifuges, Denmark and Germany began to use Zuluf method to produce yeast, that is, the sugar solution slowly flowed into the aerated fermentation broth, commonly known as "fed-batch culture method" and "batch culture method". Chulaf's method has high product yield, low raw material consumption and easy process control, which has been used up to now and has been continuously improved and perfected. Since the 1920s, the raw materials for yeast production have been expanded to use molasses, wood hydrolysate, sulfite pulp waste liquor and molasses alcohol waste liquor. In the 1960s, factories with hydrocarbons such as oil, coal and natural gas and their secondary processed products (such as acetic acid, ethanol and methanol) were established one after another, which changed the long-standing tradition of using carbohydrates as raw materials.
Shortly after the outbreak of World War I, Germany began to study the production of yeast by modern methods to solve the problems of food shortage and high production cost. So far, production practice and scientific development have provided conditions for the production of active dry yeast. The outbreak of World War II objectively promoted the development of yeast production. Because the pressed yeast has high water content and is perishable, it needs to be transported by refrigerated trucks, which can not meet the requirements of special wartime environment, resulting in the large-scale production of active dry yeast. From 65438 to 0945, some military institutions and factories in the United States and Europe produced more than 4 million pounds of active dry yeast to meet the urgent needs of war preparation. Active dry yeast mainly supplies baking industries such as bread and cakes, and has been extended to the application of brewing wine, mainly wine and other fruit wines. Due to the development of genetic engineering and drying technology, a new type of fast-acting dry yeast with high fermentability, which can be directly mixed with flour to make dough, appeared in the late 1960s, and was first developed and produced by Gusteau Company in the Netherlands.
Yeast production in China started at 1922. Before 1949, only Shanghai Li Dahua Hygiene Food Factory and Shanghai Xinya Enzyme Factory produced baker's yeast, and the annual output was only 12t (calculated by dry yeast). In 1950s, yeast production in China developed greatly, and dozens of factories were established, forming an independent industrial system. In the early 1980s, the number of yeast factories rapidly increased to more than 40. The output of yeast powder in Guangdong province ranks first in the country. By 1988, two factories with an annual output of 2kt quick-acting dry yeast have been built. In addition, Jiangsu, Henan and other places use monosodium glutamate waste liquid and alcohol waste liquid to build factories to produce feed yeast, with an annual output of 100 ~ 500t. The types of bread yeast increased from single pressed yeast to active dry yeast and fast active dry yeast. The production of edible yeast, medicinal yeast and feed yeast has also developed to varying degrees. 1985, the total output of yeast in China reached 1 1kt, of which baker's yeast was about 5kt.
The world yeast production is developing towards large-scale and automation, and the production process has been controlled by computer, with high labor productivity. For example, the yeast factory of Danish Alcohol Company produces an average of 200 tons of pressed yeast per person per year. The production of baker's yeast in Gist company in the Netherlands is large, with an annual output of 200,000 tons, half of which is processed into quick-acting dry yeast for export; Le Schaaf in France is150 kt; Universal Foods USA is 120kt.
Product classification There are several classification methods for yeast products. According to the different purposes of human consumption and animal feed, it can be divided into edible yeast and feed yeast. Edible yeast is divided into bread yeast, food yeast and medicinal yeast.
Bread yeast can be divided into pressed yeast, active dry yeast and fast active dry yeast.
① Squeezed yeast: a block product with water content of 70 ~ 73% produced by Saccharomyces cerevisiae. Pale yellow, compact structure, easy to crush and strong fuzzing ability. It can be stored at 4℃ for about 65438 0 months and at 0℃ for 2 ~ 3 months. The product was originally obtained by squeezing and dehydrating the centrifuged yeast milk with a plate-and-frame filter press, so it is called squeezed yeast, commonly known as fresh yeast. When fermenting, its dosage is 1 ~ 2% of flour, the fermentation temperature is 28 ~ 30℃, and the fermentation time varies with factors such as yeast dosage, fermentation temperature and sugar content of dough, and is generally 1 ~ 3 hours.
② Active dry yeast: a dry yeast product containing about 8% moisture and having fermentation ability. Fresh yeast was obtained by using alcohol mother with dry resistance and stable fermentation power, and then it was made by extrusion and drying. The fermentation effect is similar to that of pressed yeast. Products are packed in aluminum foil bags or metal cans filled with vacuum or inert gas (such as nitrogen or carbon dioxide), and the shelf life is from half a year to 1 year. Compared with pressed yeast, it has the advantages of long shelf life, no need for low temperature storage, convenient transportation and use.
③ Fast-active dry yeast: a new fine-grained product (diameter less than 65438±0mm) with fast and efficient fermentation ability. The moisture content is 4-6%. On the basis of active dry yeast, it uses genetic engineering technology to obtain a highly dry-resistant Saccharomyces cerevisiae strain, which is dried by special nutrition ratio, strict proliferation culture conditions and fluidized bed drying equipment. Like active dry yeast, the shelf life is longer than 1 year when it is stored in vacuum or inert gas. Compared with active dry yeast, its particles are smaller and its fermentation ability is higher. When in use, 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 1970s, and was well received by consumers.
Food yeast is a dry yeast powder or granular product for human consumption, which has no fermentation and reproduction ability. It can be obtained by recycling yeast mud from brewery or by special cultivation and drying according to the requirements of human nutrition. The United States, Japan and some European countries add about 5% edible yeast powder to common foods 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 condiments. 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 combine with monosodium glutamate to enhance food flavor (see nucleotide seasoning). The concentrated invertase extracted from yeast is used as liquefier for chocolate with square eggs. Extracting lactase from yeast produced in whey can be used to increase sweetness in milk processing, prevent lactose crystallization in whey concentrated solution, and meet the needs of lactose intolerant consumers.
The preparation method and properties of medicinal yeast are the same as those of edible yeast. Because it is rich in protein, vitamins, enzymes and other physiological active substances, it is made into yeast tablets, such as raw mother tablets, to treat indigestion caused by unreasonable diet. People with weak constitution can regulate metabolic function to some extent after taking it. In the process of yeast culture, if some special elements are added to make the yeast contain trace elements such as selenium and chromium, it will have certain curative effect on some diseases. If selenium-containing yeast is used to treat Keshan disease and Kaschin-Beck disease, it can prevent cell aging to some extent; Chromium-containing yeast can be used to treat diabetes.
Feed yeast is usually made from Candida or Kluyveromyces fragilis by culture and drying. It is a powdery or granular product, which has no fermenting power and dead cells. 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 cycle, increase the amount of meat and eggs, improve the meat quality and lean meat rate, improve the luster of fur and enhance the disease resistance of young animals.
Product quality The main quality index of baker's yeast is fermentation ability, that is, the amount of carbon dioxide (expressed in milliliters) discharged by fermentation of a certain dough at a certain time and temperature. At present, the commonly used method in the world is Haydak Dough Method. The United States, western European countries and China all adopted this method. The Soviet Union used the increased volume of the dough after fermentation to calculate the fermentation power of yeast. Romania calculates the fermentation power of yeast by sinking the dough into water and calculating the time it takes for the dough to float to the water surface. Because the measuring conditions used in yeast factories are different, such as temperature, time, yeast dosage and dough type, there is no unified international standard. The general range of fermentation capacity is 500 ~ 1200. The higher the numerical value, the higher the fermentation power of yeast and the better the product quality. Protein and B vitamins are the main contents of food yeast and medicinal yeast. Protein content is the main grading standard of feed yeast.
physiology
Yeast is specialized or facultative aerobic, and there is no specialized anaerobic yeast at present. In the absence of oxygen, fermented yeast obtains energy by converting sugar into carbon dioxide and ethanol.
C6H 12O6 (glucose) →2C2H5OH+2CO2
In the brewing process, ethanol is retained; In the process of baking bread or steaming steamed bread, carbon dioxide triggers dough, while alcohol volatilizes.
breed
Yeast can reproduce asexually by budding or sexually by forming ascospores. Asexual reproduction refers to growing a bud from the mother cell under suitable environmental conditions, gradually growing to a mature size, and then separating from the mother cell. When the nutritional status is not good, some yeast that can reproduce sexually will form spores and germinate again when the conditions are suitable. Some yeasts, such as Candida, cannot reproduce asexually.
manufacturing method
The pure yeast was expanded and cultured step by step in agar slant test tube for many times, then proliferated and cultured in fermentor, and then the yeast product was obtained by centrifugal separation, squeezing and drying. The following figure shows the process of producing bread yeast from molasses.
different
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.
use
Saccharomyces cerevisiae is the most frequently mentioned yeast. It has been used to ferment bread and wine for thousands of years, and carbon dioxide will be released from dough during the fermentation of bread and steamed bread.
In the pharmaceutical industry, yeast and its products are used to treat some dyspepsia, and can improve and regulate the metabolic function of human body. Therefore, the production of medicinal yeast plays an important role in yeast industry.
Yeast is a simple single-celled eukaryote, which 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.
harm
Some yeasts are harmful to organisms or electrical appliances. For example, Rhodotorula will grow on wet furniture such as shower curtains. Candida albicans will grow in moist human epithelial tissues such as the inner wall of vagina.
yeast
In animal husbandry, yeast is widely used as concentrated feed to increase the content of protein in feed, which plays an obvious role in improving the meat production, egg production and milk production of livestock, improving meat quality and fur quality.
① Strain: The strain used to produce baker's yeast is Saccharomyces cerevisiae. The strains used to produce food yeast and medicinal yeast are Saccharomyces cerevisiae and Grape Juice Yeast. The strains used to produce feed yeast include Candida utilis and Kluyveromyces fragilis, and the latter can also be used to produce edible yeast and yeast autolysate.
② Raw materials: sugar beet molasses (see sugar beet), sugar cane molasses (see sugar cane) and grain raw materials. Beet molasses has a high sugar content (about 50% reducing sugar), and the yeast produced is lighter in color. The biological oxygen demand (BOD) in yeast wastewater is high because of betaine which cannot be used by yeast. The sugar content of sugarcane molasses is slightly lower than that of beet molasses, and the biotin content and ash content necessary for yeast growth are higher. When producing yeast, if 80% beet molasses and 20% sugarcane molasses can be used, the yeast obtained is better in quality and quantity. However, no matter what kind of molasses contains impurities that hinder the growth and reproduction of yeast and affect the quality of the final product, it must be treated before it can be used in production. The commonly used treatment methods are sulfuric acid or phosphoric acid heating treatment clarification method. After the molasses is diluted, a small amount of acid is added, the temperature is raised to 90 ~ 65438 000℃, and the temperature is kept for 0.5 ~ 65438 0.0 hour, then lime milk is added to neutralize the sugar solution until the pH value is about 5.0, and a clear sugar solution is obtained through natural sedimentation or mechanical separation for yeast growth and reproduction.
Corn, wheat and potatoes can also be used as raw materials for yeast production. However, because yeast can't directly use starch, it must be hydrolyzed into sugar with acid or enzyme. Due to starch hydrolysis and other factors, yeast culture conditions are different from molasses. In addition, sulfite pulp waste liquid, wood hydrolysate, whey, alcohol waste liquid and monosodium glutamate waste liquid can all be used as raw materials for producing feed yeast. The Soviet Union and other countries produce feed yeast with petroleum processed products such as n-alkanes and methanol as raw materials.
In order to ensure the growth and reproduction of yeast, in addition to supplying the above carbon sources, a certain amount of nutrient salts such as ammonium phosphate, ammonium sulfate, magnesium sulfate, ammonia water and urea must be added as nitrogen and phosphorus sources. In order to make baker's yeast have high fermentation ability, nitrogen source and phosphorus source should be added in a certain proportion. In addition, biotin, pantothenic acid, inositol and thiamine are the basic elements of yeast growth and reproduction, which can be supplemented appropriately according to the types of products and raw materials used.
(3) Proliferation culture: under strict aseptic conditions, yeast cells in flasks (500-5000 ml), Karl Fischer bottles (10l) and seed tanks (500- 10000L) are gradually expanded to increase the number of yeast cells, and then the seed tanks are inoculated. Continue to use this yeast as seed mother for culture to obtain the second generation yeast. The third generation yeast obtained by the same method is commercial yeast. From triangular flask culture to mother tank culture, 12 be' wort is generally used as culture medium, and micro-aeration culture is carried out at 30℃ 12 ~ 24 hours. If the seed tank is large, part of wort and part of molasses can be used as culture medium and cultured at 30℃ for 12 ~ 14 hours. After the culture, the growth of yeast should be examined by microscope, and the yeast cells should be uniform in size, strong and free of miscellaneous bacteria. In general, about 5kg of fresh yeast (calculated by pressed yeast) can be obtained by 500l mother tank culture.
The propagation of commercial yeast is carried out by aeration and culture in a 50 ~ 200m3 fermentor. The yeast in the mother tank is added to the fermentor and mixed with a certain amount of water to a certain concentration. Under the condition of ventilation, sugar solution and nutrient salt were added in proportion and cultured at 30℃ for 12 ~ 18 hours. In the process of culture, the residual sugar is controlled at 0. 10 ~ 0.50g/ 100ml, and the pH of the culture solution is adjusted to 4.5 0.5 with sodium hydroxide or sodium carbonate. The ventilation rate varies with the type of fermentor, culture conditions and yeast species, and is generally about 1: 1, that is, the same volume of air as the fermentation broth is introduced every minute. Foam generated in the culture process is defoamed with edible oil or synthetic defoamer. At the end of culture, the yeast concentration is generally 4 ~ 6% (calculated on a dry basis). Under ideal conditions, yeast cells can grow exponentially within 2.5 hours, and 100g sugar can be transformed into 56.7g stem cell material.
(4) separation and pressing: after the stage of yeast proliferation and culture is finished, the yeast is separated from the fermentation broth by a centrifuge, and the yeast milk is washed with water for 2-3 times to remove the by-products of yeast metabolism, impurities and miscellaneous bacteria in the fermentation broth. Generally, the final yeast milk contains 18 ~ 20% yeast (calculated as pressed yeast). If the color of yeast milk is dark, the number and quantity of washing can be increased, or a small amount of acid can be added to the washing water to increase the washing effect. Normal yeast milk is milky white or slightly beige. Yeast milk is dehydrated by plate-and-frame filter press or vacuum drum filter. Dehydrated yeast is cake-like with a water content of 70-73%. Add a little cooking oil, adjust the water content, and squeeze it into block products (such as 50g and 500g) with a certain shape and weight by an extruder, pack them with wax paper, and store them at 0-4℃ for sale.
⑤ Drying: Drying fresh yeast into active dry yeast is a process with high technical requirements, and it is necessary to avoid yeast losing its fermentation power due to heating during the drying process. The traditional drying method is to squeeze fresh yeast into a cylinder (2 ~ 3 mm long), and then dry it continuously or intermittently in a box dryer, with the hot air temperature not exceeding 40℃. The equipment used in this method is simple, but the drying time is long and the fermentation power is greatly lost. At present, few manufacturers produce active dry yeast by this method.
Fluidized bed drying equipment is widely used in the world to produce commercial active dry yeast and fast active dry yeast, which are divided into continuous and intermittent types. During the drying process, yeast particles are in a boiling state. At the initial stage of drying, the water content of yeast is high, and the air temperature can reach 100 ~ 150℃, so the dehydration speed of yeast is fast. In the later stage of drying, the air temperature should be properly lowered to keep the yeast temperature between 30℃ and 40℃, and the total drying time is about 65438 0 hours, which varies with the shape, charging amount and air condition of the dryer. Before drying, adding some emulsifier to fresh yeast can improve the rehydration performance of dry yeast, enhance the resistance of yeast to thermal drying and reduce the loss of fermentation power. Commonly used emulsifiers are sorbitan monostearate, sucrose ester and citric acid ester, and the addition amount is 0.5 ~ 2.0% of yeast dry matter. In order to prevent the oxidation of dry yeast, a small amount of antioxidant such as butyryl hydroxyanisole can be added, and the addition amount is 0.65438 0%. Due to the addition of these compounds, the storage stability of active dry yeast is greatly improved.
The drying of food yeast, medicinal yeast and feed yeast is relatively simple. Because these products do not need to retain the fermentation power of yeast, centrifugal spray drying and drum drying can be used.