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The Production Process of Bread Yeast
- Nov 28, 2018 -

The yeast production process as we know it today took about 50 years between the end of the 19th century and the 1920s to formalize. During this period, the new production process of bread yeast mainly includes:

(1) wide application of ventilation culture: in the 1970s, the discovery of Pasteur effect laid a theoretical foundation for the ventilation culture method of yeast, that is, under aerobic conditions, the reproduction speed of yeast was accelerated, while the yield of alcohol was decreased. In 1877, BusebiusBrun of Copenhagen proposed a method of producing yeast by ventilation culture. 1886 a yeast factory using ventilation culture was born in Berlin, Germany. The effect of ventilation greatly increased the mass concentration of yeast and decreased the mass concentration of alcohol, which separated the yeast industry from the alcohol industry.

(2) fluid-adding process: from 1910 to 1920, scientists in Denmark and Germany first invented and improved fluid-adding process. Today, the process remains the only viable way to maximize biomass while minimizing ethanol production. In this method, the mass concentration of sugar in the culture medium is low at the beginning of culture, and the concentrated sugar solution is continuously added in the culture process, so that the mass concentration of sugar in the culture medium remains low throughout the culture process. By using this method, high yeast yield and high yeast concentration can be obtained.

(3) use molasses instead of grain as raw material: in the 1920s and 1930s, cheap waste molasses gradually replaced corn, malt and other grains as the main raw material for yeast production, further reducing the production cost of yeast. Molasses as raw material has three advantages: first, it simplifies the yeast production process and eliminates the complex raw material processing process in the production process of grain as raw material; Molasses is a waste product in the sugar industry, and its price is much lower than that of grain. The nutrition of molasses is richer than that of acid hydrolyzed sugar. Molasses contains a variety of vitamins are necessary for the growth of yeast, such as biotin, pantothenate, inositol, thiamine and so on, which is conducive to the growth of yeast and product quality.

The development stage of active dry yeast is from the 1920s to the present. In the early 1920s, due to the great progress in drying technology, it was possible to dry pressed yeast (CY) into active dry yeast (ADY). With the improvement of drying technology and the wide use of additives or protectants, the product quality of active dry yeast has been improved and the variety has been diversified. At present, the development of the yeast industry has enabled the active dry yeast to control the sales market and be widely used in the production of bread and pastry.

At present, the main raw material of commercial bread yeast production is sugar beet molasses or sugarcane molasses. The fermentation process is carried out in the fermentation tank with cooling coil and strong ventilation device. Under these conditions, the amount of alcohol produced by anaerobic or aerobic fermentation can be reduced to a minimum. The fermentation process requires the addition of supplemental media, i.e., fluidized media, and proper ventilation.

The process of yeast culture can be divided into three stages, namely, adaptation, accumulation and maturation. After yeast is inoculated, the period of time that the cell needs from quiescent state to begin a large number of reproduction is yeast adapt to period, time is about 0.5h to 2h differ, depend on culture condition and seed state and decide. For the frozen seeds separated by centrifugation, yeast cells are completely dormant, and the direct entry into flow culture will prolong the adaptation period. Thus seeds are activated before formal flow culture. In this way, the contamination of impurities in the early stage of fermentation can be inhibited and the growth can be synchronized in the early stage of fermentation. After the activated seeds enter the fermenter, they can immediately enter the flow culture.

After the end of the adaptation period, yeast cells enter the logarithmic growth period of rapid growth and reproduction, with continuous flow of sugar solution and nutrient salt, and a large amount of air. At the beginning of the logarithmic growth stage, due to the low cell concentration in the culture medium, the flow acceleration of sugar solution and nutrient salt is also low and the ventilation volume is small. With the process of flow culture, the flow acceleration of sugar solution and nutrient salt also increases. At the middle stage of fermentation, the ventilation volume reached the maximum, the cell growth rate and the flow acceleration rate of the sugar solution also reached the maximum. At the later stage of yeast accumulation, nutrients are generally no longer added, and the rate of yeast increase may gradually decrease, so the rate of flow acceleration of sugar solution is also appropriately gradually reduced.

At the end of the accumulation phase, an appropriate maturation phase is required. During this period, nutrients are not added and ventilation volume is reduced. If nutrients such as nitrogen sources are not used up in the culture medium, and the sugar content is close to zero, a certain amount of sugar solution should be supplemented at the same time of a small amount of ventilation, so that nitrogen is exhausted and the spores fall off. The maturation period is generally 0.5~2h, depending on the cell status and the status of residual nutrients in the medium.

The post-treatment of fermentation liquid includes centrifugation, concentration, washing, pressing or filtration. The filter cake can be extruded into semisolid yeast blocks (30% of the solid material) and used as fresh yeast after packaging. If active dry yeast is produced, the pressed yeast can be extruded into thin strips and dried at a rapid low temperature. Additives are used in the drying process to produce highly stable dry yeast with low moisture content and to reduce the loss of activity. Active 1000 yeast contains about 95% solids. The results of yeast cultivation should be light in color and fresh in smell. The cells of the bacteria should be separate and of uniform size.

 

Nov.28, 2018

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