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IV. INDUSTRIAL PRODUCTION OF YEAST (13-17)

In 2003, an estimated 2.8 million metric ton of baker's yeast, expressed as compressed yeast, was produced throughout the world. Half was produced on the continent of Europe, with the other half equally divided between the Americas and the rest of the world. Regardless of their physical forms (compressed, dried, etc.), the yeasts produced are not the same in the various regions of the globe, because manufacturers have to adapt them to the local bread-making conditions. However, they have one problem in common; ensuring that the fermenting capacity of the product they supply is consistent and stable. This is achieved by controlling the quality of raw materials, production technology and logistics.

A. Raw Materials

The raw materials used must satisfy the nutritional requirements necessary for the growth and multiplication of yeast cells.

Molasses is the substrate of choice, both economically and technically. Viscous, highly colored liquids, their composition varies according to the sugar-making process from which they derive, and the quality of the sugar-beet or cane harvest; 77-82% of dry matter essentially provides sucrose as a source of carbon (45 to 55%), minerals, trace elements, and vitamins. However, they supply very little nitrogen for protein synthesis (the betaine they contain cannot be assimilated) and not enough minerals such as phosphorus, magnesium, and often zinc. The problems they cause are rarely related to any deficiencies but are more likely to be due to the presence of elements that are toxic to yeast. These may be substances that derive from farming methods or sugar-producing processes: fungicides, quaternary ammonium, sulfites, but also an excess of minerals (Na2+), short-chain fatty acids, and a great many trace elements.

In order to homogenize the composition of the nutrient medium and reduce the risks of toxicity, all batches are analyzed and tested before they are mixed (up to 10 or 12 different sources).

After an initial filtration to remove coarse foreign elements, the molasses is diluted and heated to reduce the viscosity. Clarifiers continuously remove the fibers and colloids by centrifugation. The next is flash pasteurization: the molasses is heated to 130°C under pressure for a very short time. This treatment removes vegetative forms of microbial contaminants as well as Clostridium and Bacillus spores, without caramelizing or degrading the sugars. After it has cooled down on plate exchangers, the molasses is stored in a buffer tank, ready for use.

Cane molasses, which contains a lot of fibers and colloids, is difficult to process but has a very high nutrient content. When it is mixed at a level of 20% with sugar beet molasses, biotin does not need to be added.

The minerals that are added as nutrients (phosphoric acid, ammonium salts, ammoniac) or to control pH (sulfuric acid, soda) comply with food standards, in particular for heavy metal content. The water for diluting the medium is potable and chlorinated to avoid any contamination.

B. Storage and Protection of Strains

Thousands of strains have resulted from research studies but less than about 10 are used in large-scale industrial production. Manufacturers store them in their own laboratories but also, as a precautionary measure, in public collection centers. With the development of identification methods, the claims of patent applications dealing with the creation and improvement of strains can be described more accurately. The methods of storing strains vary; for example, — 80°C on a glycerol medium with subcultures made after 1 to 5 years, or 4°C on a gelose medium and subcultures after 1 to 3 months.

C. Propagation

How yeast cells multiply (wrongly called fermentation) is of major importance in carrying out the yeast manufacturer's two objectives.

The first relates to quality. A specific strain must satisfy many criteria, which are as follows

The best fermentative power in a specific application (''normal,'' sweet, or acid doughs) or fermentation kinetics that are most suited to a given bread-making method (sponge and dough, Chorleywood bread process, etc.)

The nature of the finished product: compressed yeast, rehydratable dry yeast or instant yeast

The stability of fermenting power

The second objective has to do with economics. The manufacturer must achieve the quality he seeks for the best price. The result is a compromise between:

Yields (% of yeast obtained in proportion to the substrate used)

Productivity (rate of multiplication and optimum use of tank equipment)

It is evident, therefore, that there are many complex and sometimes contradictory obstacles. To untangle the knot, engineers and researchers rely on data on the biochemistry and physiology of cells. Although attempts at modeling have been made in order to optimize production, experience and a certain degree of pragmatism are still called for.

Yeast is propagated in sequences during which cells multiply under different conditions. Multiplication has two aims, which follow on from each other:

The first is to massively increase the cell population so that a final so-called commercial generation can be seeded.

The second is to multiply the yeast so that its biochemical composition and physiological condition meet the requirements of producer and user.

The number and method of these stages can differ noticeably from manufacturer to manufacturer, but the principle is the same. In each stage, enough yeast is produced to inoculate the next stage (Fig. 12). This is carried out using a series of containers, from lab to plant, whose contents must be managed extremely well when the industrial stage is reached.

The third industrial generation (commercial generation) must finish with a maturing phase, the aim of which is to ensure the yeast is stable. It means making the yeast, at the end of the manufacturing process, synthesize as many stock sugars as possible (trehalose and glycogen) and reducing the rate of budding to very low levels. In order to do this, the yeast is deprived of nitrogen, supplying it with molasses only and the temperature is slightly increased above the 32 °C ± 2, which is the average temperature during production.

Next comes the separation of yeast cells from the wort. This operation takes place continuously using centrifugal equipment where the yeast is washed and a cream is obtained (i.e., a suspension of cells in water, at a dry matter concentration of between 15 and 20%). This figure is equivalent to about 50% of cells, in volume. Above 23% dry matter, the suspension becomes too viscous and cannot be pumped. This cream is cooled down to 4°Cin

Figure 12 Yeast propagation stages. D = day.
Figure 13 Yeast manufacture flow chart.

a plate exchanger and kept at this temperature in storage tanks. About 400 metric tons of yeast (compressed yeast equivalent) will be finally collected 10 days after the first test tube was cultured in the laboratory. The principle of yeast manufacture is summarized in Fig. 13).

French yeast producers, who are concerned about environmental regulations, have concentrated on investing in methods of pollution control. After the yeast cream has been separated, there is a great deal of effluent that cannot be discharged, as it is, into the environment. The most diluted effluent is treated by aerobic or anaerobic biological methods. The must from which the yeast has been removed, and which still contains nonassimilated organic nitrogen (betaine) and mineral salts rich in potassium sulfate, is very highly concentrated by evaporation and inverse osmosis. The vinasse obtained is used as a fertilizer, spread on directly as manure or in the form of potassium extracts after crystallization, decantation, and drying. It can also be used in animal feed after treatment to reduce the potassium content. Pollution control treatments have a considerable effect on production costs.

V. COMMERCIAL YEASTS A. Strains

The strains used in France and in Europe as a whole are of the ''rapid adaptation to maltose'' type. They were developed in the early 1960s in response to straight bread-making methods that had appeared as a result of high-speed mixing used in bread-making with no added sugar. Because of their poor performance in very sweet doughs (over 15% of sucrose / flour), osmotolerant strains already used for the large export markets were introduced to the French market at the end of the 1980s. The difficulty bakeries had in using several qualities of yeast resulted in research work into strains with a wide range of applications (Table 5).

B. Biochemical Compositions Adapted for Applications

The need to increase the rate of fermentation, particularly in an industrial environment, is met by yeasts with a high nitrogen, and therefore protein, content. The major drawback of these yeasts is their instability, which is due to higher enzyme activity and a storage sugar

Table 5 Comparison of Commercial Strains and Wide-Range Laboratory Strains

Type of yeast and strain

Fermenting power lean dough (base 100)

Fermenting power 25% sweet dough (base 100)

Invertase activity (Ui)a

Commercial—rapid adaptation to maltose Commercial—osmotolerant Lab—hybrid, wide range Lab—genetic engineering wide range

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