In this article we will discuss about the production of baker’s yeast industrially. Learn about:- 1. History of Baker’s Yeast 2. Outline of the Production of Bakers’ Yeast 3. Yeast Strains 4. Raw Materials and Process Required for Production of Baker’s Yeast 5. Principles of Aerobic Growth 6. Practice of the Aerobic Growth of Bakers’ Yeast.
- History of Baker’s Yeast
- Outline of the Production of Bakers’ Yeast
- Yeast Strains
- Raw Materials and Process Required for Production of Baker’s Yeast
- Principles of Aerobic Growth
- Practice of the Aerobic Growth of Bakers’ Yeast
1. History of Baker’s Yeast:
The art of fermenting doughs from cereals was practiced before recorded history. This and the production of liquid, fermented mashes from cereals are closely related processes. It is likely that the liquid from a fermented mash was drunk as a slightly alcoholic beverage, while the semisolid mash was formed into dough and baked.
Even today yeast strains used in the production of ale and bread are those of a single species, Saccharomyces cerevisiae. Until well into the middle of the 19th century bakers obtained their yeast from breweries. At that time lager beer strains of Saccharomyces uvarum (synonym- S. carlsbergensis) were introduced into Central Europe and later the United States.
These strains do not tolerate the high osmotic pressure in dough, and bakers were forced to look for another source of yeast. Distillers’ yeasts which are also strains of S. cerevisiae perform reasonably well in bread baking, but they are difficult to separate from the distillers’ mash. This led to the establishment of a separate industry which produced bakers’ yeast on a large commercial scale for sale to bakers and for home baking.
It is entirely possible to produce baked goods by keeping a portion of an actively fermenting bread sponge and by using it to inoculate a new sponge. This is how active fermentations were perpetuated on farms or in bakeries before bakers’ yeast became available as a commodity. This method of inoculation from preceding fermentations is still used to a considerable extent in the wine industry, and it is common in the brewing industry.
There are two important reasons why this method of propagation is not practical for the commercial production of baked goods. Both reasons have to do with the plastic consistency of doughs. Yeast grows very slowly in doughs, and very long fermentation and proof times are required unless the doughs are inoculated with large numbers of yeast cells.
Secondly, the semisolid, plastic nature of doughs makes it difficult to store and transfer them for subsequent use and to mix them with more flour and water. Table 14.1 shows that the number of yeast cells per gram of dough exceeds that of a starting beer or wine fermentation by a factor of about 50.
During fermentation, there is a 5- to 10-fold growth in the number of yeast cells in beer and wine, while there is scarcely any growth in dough during the short period of a bakers’ fermentation. These differences explain the requirement of the baking industry for a large-scale commercial source of yeast.
The earlier bakers’ yeast plants, well into the 20th century, produced a mixture of alcohol and bakers’ yeast from grain mash fermentations. Both alcohol and yeast were sold. Beginning with the early decades of the century, yeast producers used higher aeration and, from about 1920, incremental feeding of their fermentations.
Such highly aerobic, incrementally fed (fed batch) fermentations produce more yeast and very little alcohol so that ethanol is not recovered any longer. During the past 100 years production has shifted from a typical distillers’ fermentation to a highly aerobic fermentation of molasses worts which is now characteristic of the production of bakers’ yeast.
2. Outline of the Production of Bakers’ Yeast:
The principal carbon and energy source for the production of bakers’ yeast is cane or beet molasses. The nitrogen sources are ammonia, ammonium salts, and urea, and the phosphorus source is ortho-phosphates or phosphoric acid. The fermentation medium is also supplemented with minerals (magnesium and trace minerals) and vitamins (biotin and thiamin).
The final trade fermentations are carried out under highly aerobic conditions, and with incremental feeding of the molasses wort. This fermentation is carried out at a pH between 4 and 6, at a temperature of 30°C, and, for periods of 8 to 20 hr. The multiplication of yeast cells is 5- to 10-fold, and the concentration of yeast solids may reach 4 to 6% at the end of the fermentation period.
After the fermentation the yeast cells are concentrated by centrifuging to a yeast cream of 15 to 20% solids. This cream is cooled and it is either pressed in a filter press or filtered in a rotary, vacuum filter. The resulting yeast press cake is extruded in the form of semi-plastic blocks and packaged in wax paper, or it is crumbled and sold in bulk. The slightly moist yeast cakes or the crumbled yeast is called “bakers’ compressed yeast.”
For the production of active, dry yeast the press cake is extruded in the form of fine strands. These are dried in tunnel driers on endless steel mash belts, in rotary louver driers, or in air lift driers. Compressed yeast has solids content of about 30%; active dry yeast contains 90 to 95% yeast solids.
3. Yeast Strains:
Bakers’ yeasts are strains of S. cerevisiae. They are propagated by pure culture methods in the laboratories of yeast producers. Suitable strains are also available from known public culture collections. In contrast to the brewing, distilling, and wine industry, the yeast strains used by bakers are not proprietary strains.
That is, they are freely available to anyone wishing to make single cell isolates for his own culture collection. The only protection which a producer could obtain for a proprietary strain would be through appropriate patents. For almost all of the commercially available yeasts such patent protection has not been sought.
Within the last 10 years there have been some exceptions to this general rule. In 1969 Lodder et al. described the hybridization of bakers’ yeast strains, and several patents have since been issued which specifically aim to protect such hybrids.
One of the major problems in improving yeast strains by hybridization is the difficulty of evaluating many hundreds of hybrids with regard to all of the qualities required by a bakers’ yeast. Some of these qualities are related to the production of such yeasts- yield, growth rate, stability on storage of the compressed yeast, ability to withstand drying.
Others are related to performance of such hybrids in the bakery- gas production rates in lean doughs, in doughs for normal white pan bread, in sweet doughs or in frozen doughs.
Attempts have also been made to hybridize S. cerevisiae with Saccharomyces rosei in order to confer better osmotolerance to the yeast. This is particularly important if such yeasts are to be used in cookie doughs which are now made with chemical leavening. At this time such hybrids are not used commercially.
4. Raw Materials and Process Required for Production of Baker’s Yeast:
The most widely used carbon sources are cane and beet molasses with a fermentable sugar concentration between 50 and 55%, and a Brix of about 80°. Table 14.2 shows the constituents of molasses as a percentage of total solids. Since molasses is a by-product of the sugar industry there may be considerable variations in composition. Yeast manufacturers have no control over the composition of molasses and are rarely able to choose the raw material on the basis of known performance.
The pH of molasses is somewhere in the range of 6.5 to 8.5. Yeast ferments and assimilates sucrose as rapidly as invert sugar (glucose plus fructose) because of its high invertase activity. The fructose moiety of the trisaccharide, raffinose, is fermented by bakers’ yeast. The residual melibiose moiety is fermented by some strains and not by others.
Other compounds in molasses, such as acetic acid, lactic acid, succinic acid, tartaric acid, and glycerol, can be assimilated in the presence of mono- and diglycerides. Some amino acids can serve as both carbon and nitrogen sources.
2. Nitrogen Sources:
The nitrogenous compounds in cane or beet molasses cannot be relied upon to serve as an adequate source since only some of them are assimilated. This fraction varies with the different types of molasses. Ammonia nitrogen and that of many of the amino acids is assimilated. Glutamic and α-aminobutyric acid can serve as sources of assimilable nitrogen; aspartic acid, alanine, glycine, and lysine can serve as partial sources.
In practice most of the required nitrogen is supplied by added ammonium salts, liquid ammonia, or urea. The nitrogen of nitrates is not assimilated.
Bakers’ yeast requires biotin for growth, and compressed yeast contains about 0.75 to 2.5 ppm of this vitamin (dry weight basis). Cane molasses supplies ample amounts of biotin (0.5 to 0.8 ppm); beet molasses does not (0.01 to 0.02 ppm). Therefore, at least 20% of cane molasses has to be blended with beet molasses in the preparation of the feed wort, or the feed has to be supplemented with synthetic biotin.
At current prices for biotin such additions are economically feasible. If urea is used as a source of nitrogen, higher amounts of biotin are required. L( + )-Aspartic acid can partially replace biotin, and L( + )-aspartic acid plus oleic acid can completely replace biotin in growth media for bakers’ yeast.
Bakers’ yeast will adapt to the absence or a deficiency of pantothenate and inositol, but these vitamins are required for optimum growth. They are generally present in sufficient quantities in molasses.
For optimum growth it is also advisable to supplement the thiamin content of molasses with this vitamin. Thiamin is almost quantitatively taken up by bakers’ yeast during growth. Sufficient thiamin is usually added to the medium to obtain a content of 50 to 10 μg per g of final yeast solids because it improves the activity of compressed yeast in dough systems.
Other vitamins are present in molasses in sufficient quantities or they are not needed for yeast growth. Literature references on the requirements of bakers’ yeast for various vitamins are usually based on the concentration of the vitamins in the growth media but fail to give the amount of yeast grown. Therefore, they are not particularly helpful for the practice of commercial fermentations.
For growth and good performance in fermentations, bakers’ yeast requires the addition of phosphates. The amounts added should give a final composition of the yeast of 2.5 to 3.5% P2O5 for yeasts containing 7 to 9.5% nitrogen (all based on dry weights). Phosphates are almost quantitatively taken up by the yeast during growth. The common sources of phosphorus are phosphoric acid, alkali phosphate salts, or ammonium phosphate. The latter can also serve as a source of nitrogen.
Molasses contains sufficient potassium to supply the requirements of yeast for this element. The same is generally true of calcium, but molasses has to be supplemented with a magnesium salt, generally magnesium sulfate. Molasses contains sufficient sources of sodium and sulfur to supply these elements. Yeast ash contains 0.4 to 0.5% sodium as NaO2 and 0.2 to 0.25% sulfates as SO3. If sodium chloride is added to yeast cream to aid in its filtration, the sodium concentrations may be somewhat higher.
Bakers’ yeast also requires the presence of some elements in trace amounts. These are Fe, Zn, Cu, Mn, and Mo although the information in the scientific literature leaves some doubt as to whether these are the only trace elements required. As with vitamins, requirements are generally expressed in terms of nutrient concentration in the medium without reference to the amount of yeast grown.
Therefore, quantitative interpretation is difficult. In general such trace elements are supplied in sufficient quantity by molasses with the possible exception of zinc. This metal may be added in the form of zinc sulfate.
5. Fermentation Activators and Inhibitors:
Many products have been reported to be activators of yeast growth, such as flour milling waste, sludge from aerobic digesters, etc. It is probable, that such reports are based on the stimulatory effects of these materials in growth media which have been deficient in one or another vitamin or, trace element. Occasionally, well defined plant growth factors, such as indolyl acetic acid, have been reported as yeast growth stimulants but as far as is known these are not used on a commercial scale.
SO2 inhibits yeast growth but concentrations up to 800 ppm in molasses can be well tolerated. S. cerevisiae adapts well to the presence of even higher concentrations of SO2 as is known from the use of this species in the wine industry where fermentations are often carried out in the presence of 80 to 100 ppm of SO2.
Molasses contains variable amounts of nitrate which can be reduced to nitrite by bacterial action during the production of yeast. A considerable loss in yield has been reported for concentrations of 0.004 to 0.001% nitrite.
6. Other Carbon and Energy Sources:
Any sugar-containing raw material or any starchy material that can be hydrolyzed to fermentable sugars may serve as a carbon and energy source for the production of bakers’ yeast. These sugars are sucrose, maltose, glucose, fructose, and mannose. Lactose is not fermented by bakers’ yeast, and galactose is fermented only very slowly.
Such sugar-containing raw materials may be sugar cane juice or molasses, grape juice concentrates, date juice, wood hydrolysates, starch hydrolysates, or waste sulfite liquor. Up to the present time economics have dictated the use of molasses. Waste sulfite liquor is used to some extent in Finland.
This liquor from paper pulp mills contains a mixture of hexoses and pentoses at very low concentrations. S. cerevisiae assimilates only the hexoses, and consequently very large volumes of liquor have to be passed through the fermentors.
During the past 5 years the economic picture has changed so that the use of molasses is not as attractive as it has been in the past. There are two basic reasons for this change, both of a technical nature. The first is the improved recovery of sucrose from beet or cane juice which leaves a molasses with a lower concentration of fermentable sugars and with a higher concentration of compounds which are of no value to the yeast producer.
In Europe molasses is now available with a fermentable sugar concentration of 40% as compared with a traditional concentration of 50-55%. A newly developed process is capable of removing a major portion of sucrose from molasses. This results in a substrate with very low concentrations of fermentable sugars and high concentrations of inorganic and organic (non-sugar) compounds.
The second problem is the high BOD of fermentor effluents if molasses is used as fermentation substrate. Obviously, this problem will be further aggravated if the fermentable sugar content of molasses is reduced and that of organic non-sugar compounds is increased.
Together these problems have rekindled interest in the use of alternate carbohydrate sources for the production of bakers’ yeast. At present the only available sources which are technically acceptable and economically attractive are starchy materials from cereal grains, principally from corn (maize).
Grain mashes were used traditionally for the production of bakers’ yeast until the 1930s, and high quality bakers’ yeast can be produced with them. Of course, starchy grains require conversion to fermentable sugars, a task which can be carried out with bacterial and fungal amylases more efficiently than in the early decades of the century. Alternately, corn syrups of high dextrose equivalent can be used as a suitable carbon source.
Ethanol is aerobically assimilated by bakers’ yeast after a period of adaptation. However, it is not a satisfactory raw material for the production of bakers’ yeast.
Losses of ethanol through aeration of the fermentor are negligible for bakers’ yeast fermentations which produce less than 0.1% ethanol in the fermentation medium.
For most anaerobic batch fermentations the increase in yeast cell mass follows a predictable pattern. At the beginning of the fermentation there is slow growth and fermentation may be barely perceptible. This early lag phase is followed by a period of exponential growth, that is, growth where cell divisions take place at identical intervals.
This phase is often called “stormy” in the beer and wine industry. Finally, the exponential growth phase is followed by the latent phase in which growth declines or stops completely. Fermentation continues during the latent phase, although at a reduced rate, until the amount of substrate is exhausted. This pattern is well known from the fermentation of alcoholic beverages.
In contrast, aerobic, continuous fermentations show exponential growth throughout the length of the fermentation. For such fermentations the increase in yeast cell mass can be expressed as the generation time or the specific growth rate constant for exponential growth. Generation time is simply the time required for each doubling of the yeast population.
The specific growth rate constant (μ) is defined by the following equation:
Upon integration, one obtains In (Pt/PO) = μ x t; and for t = 1 hr, one obtains μ = In (Pt/P0) or 2.31 x log10 (Pt/PO). This is the specific growth rate for exponential growth, and for continuous fermentations its value is identical with that of the dilution rate.
The relation of μ to generation time is expressed by the following equation:
Table 14.3 Shows Some of the Actual Values Spanning a Practical Range:
Commercial yeast fermentations are so-called “fed batch” fermentations. They are carried out with incremental feeding of the substrate for growth. There is no simultaneous removal of the fermentor content, and the fermentation must be finished when the fermentor is full. The total time for such fermentation is generally between 8 and 20 hr.
In the course of the fermentation, the growth rate drops and the generation time increases from 3 to 5 hr and finally to 7 hr. This leads to an 8-fold multiplication of yeast cells in a 15 hr fermentation period.
The maximum theoretical yield coefficient under anaerobic conditions is Ys = 0.075, which means a yield of 7.5 kg of yeast solids per 100 kg of fermentable sugar. Under strictly aerobic conditions the higher possible yield is Ys = 0.54. The purpose of a bakers’ yeast plant is the production of cell mass, and, therefore, fermentations are carried out under aerobic conditions which maximize yield.
However, two additional conditions have to be met to obtain maximal yields. The growth rate, μ, must not exceed values of about 0.2, and the amount of substrate present at any one time must not exceed a given limiting value. Figure 14.1 shows the effect of dilution rate (or specific growth rate) on yeast yield.
Strikingly similar results have been obtained by Meyenburg (1969) except that his yields did not drop until a value of μ = 0.23 had been reached. At growth rates below 0.2 the respiratory quotient (RQ) = Qco2/Qo2 is about 1. At higher growth rates carbon dioxide development is greatly accelerated, the RQ increases rapid
ly, and ethanol is formed from the fermentable sugar. This state is called “aerobic fermentation.” For practical purposes it is, therefore, necessary to keep growth rates below 0.20.
The amount of oxygen required for yeast growth is in the neighborhood of 1 g of O2 per g of yeast solids. The elemental composition of yeast has been determined by Harrison (1967) to correspond to the formula C6H10NO3. The elemental composition C = 45%, H = 6.8%, N = 9.0%, O = 30.6% as determined by Wang et al. (1977) approximates the above formula when translated into percentages by weight.
The required oxygen may be supplied by pure oxygen gas, or in the form of hydrogen peroxide, but in practice it is always supplied by blowing air through the liquid fermentor contents.
The capacity of an aeration system to transfer oxygen from air into the liquid phase is expressed in terms of its volumetric oxygen transfer coefficient, KLa (hr-1), where KL is the oxygen transfer coefficient (m/hr) and a is the interfacial area between air bubbles and liquid per unit volume of liquid (m2/m3 or 1/m).
In actual practice, the rate of oxygen transfer in a fermentation system is often expressed in terms of millimoles O2/liter-hr, which is equal to KLa x C*, where C* is the equilibrium concentration of dissolved oxygen in the liquid phase (millimoles O2/liter). One can readily calculate that an oxygen transfer rate of 140 mM/liter-hr is required to produce 4.5 g of yeast solids per liter per hr.
The efficiency of aeration systems is quite variable. For fermentors without agitators the efficiency of oxygen utilization may not be higher than 20%; that is, for incoming air with an oxygen concentration of 22% the exit air would have an oxygen concentration of 17%. That means that the amount of air blown through the fermentor would have to be 5 times the amount that can be calculated from the required oxygen transfer rate.
Better dispersion of the incoming air, and hence smaller bubbles and a larger bubble surface area, can be obtained by mechanical agitation at the point at which the air enters the fermentor. With such systems 40 to 50% of the available O2 can be transferred to the liquid phase and utilized by the yeast. Such systems have been described by Aiba et al. (1965).
Oxygen transfer rates are often determined by the so-called sulfite oxidation method. This measures the rate of oxidation of a solution of sulfite to sulfate in the presence of a metal catalyst. The method is very useful for determining the effect of variables (agitator speed, addition of surfactants, etc.) oxygen transfer in a given fermentor.
Its usefulness for predicting the performance of aerating systems for producing a given amount of yeast is somewhat limited because the rate of oxygen transfer is affected by the catalyst used (Co or Cu) as well as by other extraneous factors.
The addition of surface active agents affects the oxygen transfer rate and the effect on the oxygen transfer coefficient (KL) may differ from that on the bubble surface area (a). This is another reason why actual aeration requirements cannot be accurately predicted from sulfite oxidation measurements. They have to be determined by varying the aeration rate during actual fermentations and determining the effect of these variations on yeast yield.
The level of dissolved oxygen during fermentation can be determined in the fermentor by oxygen electrodes which are commonly available. They can be calibrated by aerating the fermentor medium in the absence of yeast and assuming an oxygen concentration of 7.7 ppm at 30°C under these conditions.
During an actual fermentation these values may be considerably lower and often below 0.4 ppm (or about 5% that of the saturation level). Figure 14.2 shows some of these values as they vary throughout fermentation for an air sparged, nonagitated fermentor with an oxygen transfer capability of 150 mM per liter per hr.
If the fermentor is aerated with gas mixtures containing a very high percentage of oxygen, there is a disturbance of yeast metabolism. Glucose consumption per g of yeast produced increases, ethanol is formed, and the ability of the yeast to ferment and to respire glucose is impaired. Figure 14.3 shows such conditions for various percentages of oxygen in the air supply.
The most favorable conditions for yeast growth are obtained at oxygen levels between 21 and 30% in the gas. At such levels the RQ approaches 1, ethanol production is negligible, and yield is maximized. Dellweg et al. (1977) have suggested several reasons for the effect of hyperbaric oxygen pressures on yeast fermentations.
Up to this point it has been assumed that oxygen transfer from the gas to the fermentor liquid is the limiting step in oxygen transfer rates and in the rate of oxygen uptake by yeast cells. The effect of bulk mixing on microbial systems has rarely been studied. Einsele et al. (1978) worked with continuous, aerobic fermentations of bakers’ yeast.
Mixing times were determined as the response of intracellular NADH to a glucose step change during the continuous fermentation. The response time of this particular system was 4.4 sec. This indicates that bulk mixing times are only important if they are long in relation to the response time of the yeast cells.
Concentration of Fermentable Sugars:
It is well known that the rate of glucose fermentation, that is, the production of ethanol and CO2, is faster under anaerobic conditions than under aerobic conditions. This is called the Pasteur Effect. The presence of glucose at higher concentrations inhibits respiration even in the presence of excess oxygen.
This is also expressed in a decrease of the levels of enzymes of the electron transport system and of the citric acid cycle. The respiratory coefficient is a sensitive tool for determining the degree of glucose repression of respiration. Wang et al. (1977) have calculated the percentages of glucose which are fermented or respired for different values of RQ.
Dellweg et al. (1977) report that “a yeast culture growing at 1.1 mM glucose (0.2 g/liter) shows the highest respiration rate. It is, of course, lower at lower glucose concentrations, but above this concentration oxygen uptake is again diminished as the result of the so-called glucose effect”.
The carbon source, glucose or fructose, must therefore be provided in very small concentrations. This can be achieved by incremental feeding as a sugar or molasses solution. In practice maximal yields are obtained by limiting the growth rate to μ= 0.2 or less, and by incremental feeding which gives an RQ of approximately 1 and which reduces ethanol formation to negligible levels.
Bakers’ yeast can tolerate a fairly wide range of hydrogen ion concentrations. It can be grown at pH levels between 3.6 and 6 with optimum levels between 4.5 and 5. At lower pH levels contamination by bacteria is minimized. But the adsorption of coloring material from the molasses substrate at low pH values must be considered. Therefore, most commercial fermentations start at low pH levels.
At the end of the fermentation the pH is raised by the addition of ammonia or alkali. Kautzmann (1969C) suggests a starting pH of 4.2 to 4.5 and a final pH of 4.8 to 5.0. The pH values of a commercial fermentation are also shown in Fig. 14.2. Eroshin et al. (1976) determined optimum yields with S. cerevisiae (not necessarily a bakers’ strain). In continuous fermentations with a growth rate of μ = 0.1, he found optimum yields at a pH of 4.1.
White (1954) determined the generation times for bakers’ yeast growth at various temperatures and found the following values- 5 hr at 20°C, 3 hr at 24.5°C, 2.2 hr at 30°C, 2.1 hr at 36°C, 4 hr at 40°C, and approximately 8 hr at 43°C. Similar results have been obtained by Keszler (1967) with a top fermenting brewers’ yeast strain of S. cerevisiae. But temperature optima for maximum yield are lower. Eroshin et al. (1976) obtained maximum yields at 28.5°C. In practice yields are optimized by constant fermentation temperatures between 28° and 30°C.
Yield of bakers’ yeast is best expressed as the yield coefficient, Ys, that is, grams of yeast solids per gram of substrate used. For practical purposes this value is about 0.5. Several investigators have reported somewhat higher values. Chen (1959) and Chen and Gutmanis (1976) obtained a yield coefficient of 0.5; Oura (1974) reported a value of 0.52 and Dellweg etal. (1977) a value of 0.54.
A certain amount of substrate is used for the maintenance of the metabolism of the cells. For fermentations with incremental feeding Wang et al. (1977) found that 0.08 g of sugar was required by 1 g of yeast cell solids per hr for maintenance. Since the maintenance requirement is probably independent of growth rate, one can expect higher yields at higher growth rates, at least up to a growth rate of μ = 0.18. At higher growth rates yields are reduced because aerobic fermentation interferes with yeast growth.
In the trade, yields of bakers’ yeast are often expressed in terms of compressed yeast as a percentage of the molasses used. This makes sense for practical purposes of production costs since molasses is bought and compressed yeast is sold on a per kg (lb) basis. But it makes it difficult to judge the efficiency of the operation since the sugar content of molasses and the solids content of compressed yeast vary. In the United States compressed yeast generally has a solids content of 30%, while in Europe it is often assumed to be 27 or 28%.
The effect of high osmotic pressure in inhibiting yeast growth and fermentation is well known. Most investigations have dealt with the ability of various yeast species to ferment foods with high sugar concentrations such as jams and jellies. With regard to baked goods Windisch et al. (1976) have investigated the effect of very high osmotic pressures in cookie doughs on yeast fermentation. S. cerevisiae is not osmophilic yeast, yet there are appreciable differences among strains of bakers’ yeast. It is also known that the conditions of growth affect the osmotolerance of the yeast.
White (1954) found that bakers’ yeast grown with incremental feeding is more osmotolerant than yeast grown in set fermentations. He also described the effect of growth rate on osmotolerance. The addition of salt decreases the specific growth coefficient of bakers’ yeast.
The conditions affecting osmotolerance in bakers’ yeast have not been studied sufficiently. This is unfortunate because the osmotolerance of bakers’ yeast has assumed greater importance with higher sugar concentrations in modern bread formulations and with the production of high sugar sweet goods.
In commercial practice yeast concentrations of 4 to 6% (yeast solids) are obtained. But growth rate decreases rapidly at concentrations exceeding 3 to 4%. Fries (1962) found a generation time of 4 hr at the beginning of a commercial fermentation and of 8 hr at the end.
Many reasons have been advanced for this drastic reduction in growth rate, for instance, the exhaustion of growth factors, the inability of nutrients (sugar, oxygen) to reach the surface of the yeast cells due to increased viscosity or inadequate mixing, increasing concentrations of inhibitors which may be present in molasses, increased osmotic pressure, and others.
In particular it has been suggested that the increase in CO2 concentration in the fermentor accounts for the decrease in growth rate. However, Chen and Gutmanis (1976) tested this hypothesis by growing bakers’ yeast under conditions approximating those of a commercial fermentation. Aeration was carried out with gas streams containing 21% oxygen but varying concentrations of CO2. Inhibition of yeast growth was negligible below 20% CO2 in the gas mixture, and slight inhibition was found at the 40% level.
Actually, one can grow bakers’ yeast to concentrations exceeding 10% yeast solids in experimental fermentations provided a sufficient transfer of oxygen can be achieved. Other factors may, of course, play a role at higher yeast concentrations. At a concentration of 10% yeast solids the cell volume may occupy 25% of the liquid fermentor volume, and at concentrations of 22 to 23% solids the mass cannot be pumped any more. That means that there is certainly a practical limit to the concentration of yeast solids that can be achieved in the fermentor.
During the propagation of bakers’ yeast the supply of molasses is reduced toward the end of the fermentation period in order to “mature” the yeast. The maturing period results in compressed yeast with somewhat lower fermentation activity but better storage stability, and in yeast with reduced numbers of budding cells. A strict comparison of the glycolytic activity of bakers’ yeast cells at various stages of the cell cycle has not been undertaken except by Meyenburg (1969).
Extensive work with Schizosaccharomyces pombe has shown that this yeast can be grown in synchronous culture, and that the rate of glycolysis increases in a linear fashion between the synchronous divisions and with a doubling of the rate of increase at each division. It is unlikely that synchronous growth can be induced in S. cerevisiae as readily as in S. pombe, at least in a manner that would permit industrial application.
Figure 14.4 shows the results that can be achieved if one starts with an inoculum of yeast cells which are at an identical stage of the cell cycle. After several cycles of growth, periodicity is less pronounced and finally abolished. The figure does indicate a pronounced change in the respiratory quotient at various stages of the cell cycle.
Molasses is received at about 80° Brix and with a fermentable sugar concentration of 50 to 55%. For use in fed batch fermentations it is diluted and clarified. Clarification is required to remove insoluble solids which would otherwise be carried into the compressed yeast, and in order to lighten the color of the final yeast. Molasses is generally diluted to a Brix of 40° and the pH is adjusted to about 5 with acids such as sulfuric acid.
Insoluble solids are removed in a desludger centrifuge, that is, a solid bowl centrifuge with intermittent discharge of the sludge. Beet molasses may also be clarified by filtration but cane molasses is difficult to filter. The clarified molasses is then sterilized in a high-temperature short-time heat exchanger and cooled.
It is important that the molasses not be heated too long to prevent darkening and the destruction of sugars. This is also true of the diluted molasses wort which should not be stored at elevated temperatures before use in the fermentation. Other substrates do not require particular preparation and can be fed from individual feed tanks to the fermentor.
The construction material is generally stainless steel and in modern factories wort storage tanks, tanks for storage of other liquid nutrients, and all piping are also of stainless steel. The size of commercial fermentors may vary greatly but fermentors for the final trade fermentation with a volume of 200 m3 or more are common. One of the critical considerations in the design of a fermentor is the ratio of height to diameter.
Oxygen transfer is greatly improved with greater height of the liquid column, but for very tall fermentors the capital investment for a given volume is higher. Above a liquid height of 3 to 5 m the air must be supplied by compressors to overcome the hydrostatic pressure of the liquid column. For squat fermentors air blowers or self-priming aerators may be used.
Bakers’ yeast fermentations need not be carried out under conditions of absolute sterility because of the low pH. Therefore, the fermentation vessel need not be pressurized which lowers the cost of production in comparison with fermentors of equal size used in the production of enzymes or antibiotics.
It must be re-emphasized that the supply of oxygen is the most critical factor for the construction of the fermentor and for the conduct of the fermentation. It is relatively easy to add molasses, acids, ammonia, minerals, or vitamins to fed batch fermentation, but it is difficult and costly to add oxygen, and this nutrient is usually the limiting factor in achieving high productivity. High oxygen transfer rates can be achieved with specially designed aeration equipment.
In the United States, simpler systems of supplying oxygen are generally used. Air is fed into the fermentor through perforated tubes at the bottom of the tank. Rosen (1977) describes such a stationary system which consists of a horizontal tube with 24 side tubes provided with 30,000 holes of a diameter of 1.5 mm.
It is generally believed that holes of very small diameter are required to create air bubbles of small diameter and hence to improve oxygen transfer. This view has been questioned since the diameter of air bubbles in a turbulent liquid does not depend on the diameter of the air outlet orifice once the air bubble has reached a distance of about 10 cm from that orifice.
Distribution of air bubbles may be assisted by internal agitators and most United States fermentors now have such aeration systems. The use of agitators lowers the requirement for air volume but adds to the cost of the equipment and may have a slightly higher total energy requirement. Air requirements are often stated in terms of volume of air per fermentor volume per min (VVM).
Such figures are not meaningful in comparing oxygen transfer capabilities of various fermentors since this depends also on bubble size and dwell time of the air bubbles. The critical factors which determine the oxygen transfer capability and hence productivity of the fermentor are the oxygen transfer rate and the density of the mixture of air and fermentor liquid.
The latter is, of course, a measure of that portion of the fermentor volume occupied by liquid. Fermentors with simple air spargers have lower oxygen transfer rates but a higher usable fermentor volume than systems in which the air bubbles are finely dispersed and have a long dwell time.
Cooling can be carried out with internal cooling coils or by means of external heat exchangers. Bakers’ yeast fermentations are carried out at 28° to 30°C. The cooling requirements are great since about 3.5 kcal are generated per g of yeast solids produced (aerobically).
There is considerable foaming in highly aerated bakers’ yeast fermentations. This can be reduced by addition of suitable anti- foaming agents which may be silicones, fatty acid derivatives, or other edible surface active materials. Antifoam agents affect oxygen transfer in a complex manner. They tend to depress oxygen transfer coefficients but increase the total interfacial area between air bubbles and fermentor liquid.
Bakers’ yeast fermentations are not carried out under conditions of exponential growth. In fed-batch fermentation a constant feed rate does not permit exponential growth but can provide only for a constantly diminishing growth rate. Figure 14.6 shows some experimental and some commercial molasses feed rates as a function of fermentation time. For comparison a truly exponential curve has been included.
A diminished growth rate does not mean diminished productivity since a large yeast population at lower growth rates may have greater productivity than a small population with a high growth rate. Productivity is here meant to be “grams of yeast solids produced per liter fermentor volume per hour.”
This is the decisive factor which determines the capacity of a fermentor for the production of bakers’ yeast. Productivity varies from a low value at the beginning of the fermentation to higher productivity in the latter stages. On the average one can expect a productivity of 3 g/liter-hr for a 15 hr fermentation in which yeast solids concentration increases from 0.6 to 5.1%.
The time of a bakers’ yeast fermentation may vary from 10 to 13 hr for a 4-fold multiplication and from 16 to 20 hr for an 8-fold multiplication. Shorter fermentations can be started with higher initial yeast concentrations and hence improved productivity during the fermentation. However, the turnaround time of the fermentor (charging, discharging, and cleaning) becomes a higher percentage of the total available time.
Toward the end of the fermentation the feed rate is sharply reduced in order to permit “maturing” of the yeast cells. This maturity is expressed in a low percentage of budding yeast cells and greater stability of the compressed yeast on storage. Panek (1975) has provided a more convincing rationale for the physiological changes during this maturation period.
In the commercial production of bakers’ yeast ethanol formation is minimized in order to maximize cell yield. Ethanol concentrations in the fermentor liquid should be kept below 0.1% and preferably below 0.05%. In that case the yield of bakers’ yeast with 30% solids will be 80 to 100% of the weight of the molasses used. The yield of yeast solids based on fermentable sugar in molasses is 45 to 50%.
In some European countries it is customary to produce both yeast solids and ethanol in the course of production. This is not economical since low concentrations of yeast solids and low concentrations of ethanol raise the cost of recovery of both materials disproportionally. It may be justified on the basis of legal and economic considerations regarding the production of potable alcohol.
Sequence of Fermentations:
The bakers’ yeast fermentations always been the final or trade fermentation stages. This final stage is, of course, decisive for yield and productivity of a plant and it is most important for product quality. In actual operation the trade fermentation is preceded by a sequence of smaller fermentations in which the pitching yeast is grown.
Table 14.4 shows a typical sequence of such fermentation stages. The first two stages are called R1 and R2. They are carried out in small scale equipment and under conditions of complete sterility. The R1 ferrrientor is inoculated from a laboratory grown pure yeast culture (for methods of handling pure cultures). There is, of course, no incremental feeding during the earlier stages of the fermentation and yields are low. Fermentation times for the early stages may be from 8 to 16 hr.
Beginning with the third stage, R3, the fermentors are not pressurized and a low level of contaminants begins to appear. At the end of the F3 stage enough pitching yeast has been grown to divide the contents of the fermentor and to pitch 3 additional fermentations. The same is true at the end of the F4 stage. One may also separate the yeast from the F3 stage by centrifuging and storage of the yeast cream for later pitching of the F4 stage.
This has two advantages. It increases the flexibility of the plant operation by permitting staggered operation of the final fermentation stages, and it reduces the number of bacterial contaminants. The entire sequence shown in Table 14.4 involves 24 generations of yeast.
Harvesting of Yeast Cells:
Yeast cells are separated from the fermentor liquor of the final fermentation by centrifuging in a vertical, nozzle type, continuous centrifuge which develops a G of 4000 to 5000. This is sufficient to affect complete separation of the cells which have a water content of 62 g per 100 g of cells and a density of 1.133 g/cm3.
During the first pass through such centrifuges the yeast concentration can be tripled and on additional passes (with or without washing with water) a concentration of 18 to 20% yeast solids can be obtained. This pumpable liquid which has a whitish appearance is called yeast “cream.” It may be stored for several days at temperatures between 1° and 4°C without detriment to yeast quality.
Yeast from the cream is further concentrated by filtration (or by pressing which accounts for the word “compressed yeast”). Filtration can be carried out with plate and frame filter presses without use of a filter aid. More commonly it is done with rotary, continuous vacuum filters. The filter surface of the rotary filter must be coated with an edible material since very small concentrations of this coating may be found in the press cake.
The best material is potato starch or sago palm starch which has a sufficiently large granule size to permit efficient filtration. Such filters produce a press cake of about 27 to 28% solids. Higher solids levels can be achieved on this equipment if the cream is salted just before filtration.
The osmotic effect of the salt treatment reduces the moisture content of the cells. The salt is then removed directly on the filter by water sprays. The resulting crumbly mass of yeast cells is called a press cake. A scientific investigation of the filtration and extrusion of bakers’ yeast has been reported by Sambuichi et al. (1974).
The yeast press cake is now mixed in a blender with small amounts of emulsifiers and/or cutting oils. These additives, which may be of the order of 0.1 to 0.2%, facilitate extrusion and provide a better, lighter appearance of the yeast cake. The emulsifiers used are mono- or diglycerides, sorbitan esters, or lecithin.
The yeast press cake is now extruded through nozzles in the form of thick strands with a rectangular cross section. These are cut into appropriate lengths to form the well-known shape of packaged bakers’ yeast with weights of 1 lb or 500 g. The 500 g (1 lb) cakes are wrapped in wax paper and packaged 22.5 kg (50 lb) to a case.
Today most of the compressed yeast sold in the United States to wholesale bakers is packaged in the form of a crumbled cake with irregularly shaped particles in 22.5 kg (50 lb) plastic lined bags. The free flow characteristics of such crumbled yeast can be improved by admixture of hydrophobic and hydrophilic additives.
Starting with a yeast cream with a temperature of 3° to 4°C, the mass warms up through the filtration, mixing, extrusion, and packaging operations. Therefore, immediate and rapid cooling is required after packaging. This is done in refrigerators with vigorous circulation of cold air and with stacking of the cases or bags in such a manner that the air can circulate freely around them. Generally a 24 to 48 hr cooling period is required. Compressed yeast is shipped to bakers in refrigerated trucks and stored in the bakery in the refrigerator until used.
It is entirely feasible to ship bakers’ yeast in the form of a pumpable yeast cream. Some attempts to introduce this method of distribution have been made but have not been successful, probably because of the requirement for refrigerated storage tanks at the bakery. The shipment of refrigerated yeast creams in Russia has been described by Volkova and Roiter (1973) and Pasivkin (1973).
Stability of Compressed Yeast:
The storage stability of compressed yeast at refrigerator temperatures of 5° to 8°C is quite good. To some extent stability depends on processing conditions, for instance, low nitrogen concentrations in the yeast and a low percentage of budding cells (less than 5 to 10%) favor storage stability.
In general there is a loss of 3 to 5% of gassing activity over the period of 1 week. During storage there may be some loss of moisture. A small rate of respiration of endogenous carbohydrate leads to a slight decrease in this fraction and to a relative increase in the nitrogen content of stored yeast.
A liquid yeast cream could be kept satisfactorily at 4° to 6°C for 10 days and at 20°C for 1 day. For storage at 23°C the activity of Finnish compressed yeast showed no drop in activity but after a 2 week storage period the drop was precipitous. For storage at 35°C about one-third of the gassing activity was lost on 2 days storage. The loss of activity is generally greater for maltose fermentations than for the fermentation of glucose, fructose, or sucrose.
Efforts have been made to correlate the degree of instability with changes in cell constituents. The best correlations could be obtained with the trehalose content of yeast. High trehalose content indicates full “maturation” of the cells which occurs at reduced growth rates toward the end of the fermentation. The higher the trehalose content the better the storage stability of the yeast.
Compressed yeasts with nitrogen concentrations above 8% (based on solids) have a shorter shelf-life than yeast with nitrogen levels of 7% or below. Therefore, yeasts with high nitrogen levels and high rates of gas production as they are used in the United States are always shipped under refrigeration. Such yeasts have now been introduced into Canada.
In Europe where such yeasts are known as “Schnellhefen” (fast yeasts), refrigerated shipment and storage are also recommended. Yeasts which have to be distributed at ambient temperatures or which require an exceptionally long shelf-life (for instance, consumer yeasts) are produced at nitrogen levels between 6 and 7%.
The stability of compressed yeast can also be checked by determining the effect of storage on viability. The most reliable method is a yeast plate count which reveals the number of cells capable of reproduction. Staining techniques with methylene blue are commonly used but their accuracy is questionable.
Parkhinen et al. (1976) found good correlation between live cell counts and staining methods if he used mixtures of live cells and cells which had been killed by heating. But if death occurred as the result of storage at 35°C the correlation was poor as shown in Fig. 14.7. The authors obtained better results with fluorochromes, such as primuline, than with methylene blue. Yeasts stored at 5°C lost only 1 to 2% in viability during a 16 day storage period. Storage at 20°C led to the death of 10% of the cells in 16 days.
Since bakers’ yeast is not grown under pure culture conditions, it contains various microbial contaminants. The most numerous are lactic acid bacteria belonging generally to the genera Lactobacillus or Leuconostoc. The total bacterial count which generally reflects the presence of these bacteria is usually between 104 and 109 cells per g. Carlin (1958) has reported somewhat higher values.
These particular contaminants are without influence on the production of bread in normal commercial operation. They are a good source of “sourdough” organisms. In liquid pre-ferments they are reduced to one-tenth their original numbers. Some coliform organisms and sometimes E. coli can be found.
The same bacteria also occur in active dry yeast. Most bacteria are more sensitive than yeast to drying and storage in dried form. Therefore, total bacterial counts in active dry yeast are reduced, and a further reduction takes place on prolonged storage.
Yeast is an excellent growth medium for molds, and mold growth often occurs on yeast cakes which have been stored for 3 to 4 weeks or longer in the refrigerator. On rare occasions massive infections with Oidium lactis (machine mold) or with other wild yeasts occurs. These yeasts generally belong to species with a faster growth rate than S. cerevisiae.
The following species have been identified- Saccharomyces paradoxus, Candida utilis, Torulopsis minor, Candida krusei, and Candida mycoderma; and C. krusei. C. mycoderma, C. tropicalis. Trichosporon cutaneum, Torulopsis Candida, and Rhodotorula mucilaginosa. The most common genera are Candida and Torulopsis. Microbiological techniques and media for the detection and estimation of bacteria, yeasts, and molds in bakers’ compressed yeast have been described by Fowell (1967).
Some type of automatic control has been practiced in the industry for some time. For instance, automatic control of the pH at a set value or at a pH which rises slowly during the fermentation is entirely practical. The principles of such control devices are similar whether one wishes to control the pH or some other variable.
It requires a sensor, a recorder, and a device which activates a solenoid valve whenever a preset value has been reached or t exceeded. The principles are basically the same as those used for thermoregulation of laboratory water baths. The problems with such systems are engineering and maintenance problems.
Apart from pH control, which is relatively simple and reliable, there is an incentive to control the addition of molasses feed. This is important because underfeeding leads to a loss of productivity and overfeeding leads to the production of ethanol and a loss in yield.
At the present time molasses wort additions are preset so that given amounts of the wort are fed to the fermentor in given time intervals. Such feed curves have been developed pragmatically. In practice the ability of yeast to grow at a given rate depends among other things on the amount of pitching yeast originally present, on the amount of fermentable sugars in molasses, on the availability of essential nutrients in adequate amounts, on the availability of sufficient oxygen, and on the physiological state of the yeast.
All of these factors show some variation in practice, and as a consequence yeast growth rates vary from batch to batch. It would indeed be highly desirable to feed molasses wort “on demand,” that is, based on the concentration of yeast at any given moment and on the concentration of glucose in the fermentor liquid.
But neither of these two variables is currently suitable for on-line monitoring. Automatic ceil counting and optical density measurements are not precise enough for the measurement of cell solids, and the determination of glucose requires the removal of yeast (catalase) prior to the enzymatic assay.
Some other parameters can be used to obtain indirect measurements of the cell concentration at any given time. Some of these show good promise. The oldest method depends on the determination of ethanol in the effluent gas and throttling of the wort feed as soon as this concentration exceeds a pre-set value.
New sensors for continuous monitoring of ethanol in the effluent gas are now available and permit full control of feed additions. These so-called hydrocarbon analyzers react with all oxidizable organic compounds, but it is assumed that ethanol is the major oxidizable constituent of the exit gas.
Feed control may also be based on the level of dissolved oxygen. This requires a reliable and sufficiently-sensitive oxygen electrode. Such a device has been used by Miskiewicz et al. (1975) in a bakers’ yeast fermentation. The control device was set at a point corresponding to a 12% saturation of the fermentor liquid with oxygen. Molasses additions are automatically reduced if the level of oxygen falls below this pre-set level.
Finally, the RQ, that is the rate of carbon dioxide evolution divided by the rate of oxygen uptake, can be used to regulate molasses feed. For respiration of a carbohydrate the QCo2 should equal QO2, that is, the RQ should be 1. Figure 14.8 shows the extent of ethanol production at RQ values higher than 1.
For a cellular yield of 0.5 g per g of sugar used, the RQ should be regulated at 1.04. For a well conducted bakers’ yeast fermentation the uptake of oxygen in the fermentor and the evolution of carbon dioxide show a constant RQ between 0.95 and 1.
This figure also gives an indication of the precision which can be achieved with automatic measurement of the fermentor exit gas. The principle of RQ measurement for automatic feed control to the fermentor has also been used by Aiba et al. (1976) and Whaite et al. (1978). All authors have used computers for on-line analysis of the output of sensors.
There are still technical questions to be overcome before this type of sophisticated control can be applied to commercial fermentations. Such difficulties reside in oscillations of the system and in the requirement for accurate delivery of the feed by pumps. But the principle of computer control has been adequately demonstrated to be applicable to bakers’ yeast fermentations. “Speed of computation, often a difficulty with rapidly responding systems, is not a real problem with microbiological cultures, since any metabolic delays are substantially greater than computational ones”.
Bakers’ yeast may be produced in continuous fermentation which would permit better utilization of the total available fermentor volume. Since a maturing step is required to obtain stable compressed yeast, at least 2 vessels in series are required.
One such continuous fermentation was carried out commercially in England during the 1960s and has been adequately described by Olson (1961), Sher (1961), and Burrows (1970). The system operated as a 5-stage, open, homogeneous, and continuous fermentation. The total operating time of a continuous run lasted from 5 to 7 days.
One can foresee several problems with continuous bakers’ yeast fermentations, but it is not quite clear why this particular operation was discontinued. The growth of contaminants certainly presents greater problems in a continuous fermentation than in a fed batch culture involving several stages.
Morphological changes have been observed after several days’ operation of continuous fermentations. These consist of cell elongations similar to those which have been observed in continuous brewing systems.
Effects of Ultrasound:
The use of low frequency ultrasound on bakers’ yeast cream, compressed bakers’ yeast, and liquid pre-ferments has been investigated by a group of Russian workers. A noticeable increase in fermentation rate of bread doughs has been claimed resulting in a 25 to 30% decrease in fermentation time. So far confirmatory reports from other groups of investigators are lacking.
The production of Active Dry Yeast (ADY) is a technical triumph. Bakers’ yeast is the only vegetative microorganism which is dried commercially without significant loss of viability of the cells. While there is rarely significant loss 6f viability there is always some loss in bake activity, and this loss may be larger or smaller depending on processing conditions.
Therefore, active dry yeast has not replaced compressed yeast in many wholesale bakery operations, at least not in areas with good means of refrigerated distribution. ADY has largely replaced compressed yeast in consumer and institutional markets where storage stability is decisive.
It is entirely possible to dry compressed yeast by crumbling it or by extruding it in the form of small strands, and by spreading it on paper in a dry room. The resulting dry product shows a considerable loss in baking activity hut could be used to raise doughs. One can also mix, for instance, 1 kg of compressed yeast with 5 kg of low moisture starch or flour and obtain an ADY with relatively low moisture content. Apart from these simple procedures the literature reveals numerous more sophisticated methods of dehydration beginning in the 1920s. The history of the development of ADY has been reviewed by Frey (1957).
None of the earlier methods calling for the addition of substantial amounts of dry, edible materials (flour, starch, calcium salts) has been commercialized. At present the only methods used on a commercial scale start with yeast press cake, which is subdivided before drying into thin strands or smaller particles, Drying is carried out with currents of air at temperatures which keep the temperature of the yeast itself below 40°C.
Continuous belt tunnel driers are widely used for the commercial production of ADY. In this method the strands of extruded press cake are deposited on the wire mesh screen of an endless belt. This belt carries the yeast through several drying chambers in which the airflow is directed through the yeast bed, alternately in an upward or downward direction.
Drying times vary from 2 to 4 hr and air inlet temperatures from 28° to 42°C. Tunnel drying may also be carried out as a batch process for smaller installations. In this case the yeast strands are layered onto the wire mesh of rectangular screens. These screens are mounted on racks which are rolled into the drying tunnel. The results of this type of drying are comparable to the continuous tunnel driers.
Recently air lift (fluidized bed) driers have been used for the commercial production of ADY. This can also be done on a continuous or batch basis.
Generally air lift driers can be operated with shorter drying periods because they permit the use of more finely granulated yeast press cake. Drying times of 1 to 2 hr are satisfactory but much shorter drying times (as short as 10 min) have been suggested in the patent literature.
Using an airstream at 100° to 150°C at the beginning of the drying period and keeping the temperature of the yeast particles within the 25° to 40°C range, drying times of 10 to 30 min have been used. Temperatures of the yeast particle up to 50°C are not detrimental at the end of the drying period.
Air lift driers are also available for continuous operation. The use of a vibrating screen may provide some advantages in obtaining a uniform suspension of yeast particles in the airstream. The drying of yeast press cake with a “vibrating fluidized bed method” has been described by Russian workers.
Finally, roto-louver type driers are used for the production of ADY in the form of small, round pellets. Drying times are generally between 10 and 20 hr in a drum which carries the yeast press cake on the inside and which rotates at about 4 rpm. Hot air is blown through louvers in the surface of the drum and passes through the yeast bed.
Other drum drying methods, including vacuum drum drying, have been described by Russian workers. Vacuum drying can also be carried out with tunnel driers in which liquid yeast cream is spread on endless, wide steel belts. Spray drying has not been particularly successful on a commercial scale. Freeze drying results in considerable losses in viability.
Traditionally ADY has been made without the use of any additives, but within the last 20 years some additives have been tried and used on a commercial scale. These additives are usually emulsifiers which improve the rehydration characteristics of ADY, particularly if the yeast is dried to low moisture levels.
Sucrose esters and sorbitan esters have been used. In commercial practice sorbitan monostearate is used in concentrations between 0.5 and 2% based on the dry weight of yeast solids. Protection of low moisture ADY against oxidative deterioration is obtained with the addition of butylated hydroxyanisole (BHA) at levels of about 0.1% based on yeast solids. Such yeasts are available in the trade under the name “protected ADY.” Storage stability of such yeasts is greatly improved and approaches but does not match the complete removal of oxygen from the package.
During the drying of bakers’ yeast the rate of water removal is rapid until a moisture content of 15 to 20% is reached. It is assumed that the water removed is “free” water. At a moisture content of about 20%, changes in viability and cell wall permeability are minor.
Harrison and Trevelyan (1963) find an optimum rehydration temperature of ADY of 21°C for a 23% moisture level but 42.5°C for a 5.8% moisture level. There is some loss of cell solids during the early stages of drying due to increased respiration at temperatures between 30° and 40°C.
As drying is continued below 15 to 20% moisture, the rate of drying decreases sharply. Respiration stops and the yeast cell leaches constituent compounds into the surrounding water if it is rehydrated. When losses of gassing activity or viability occur, this will be apparent at moisture levels below 15%. There is undoubtedly injury to the yeast cell which is not restricted to the cell wall.
Beker (1977), who has done the most extensive work in this area, documents such changes in morphology. For instance, invaginations in the cytoplasmic membrane of compressed yeast have a depth not exceeding 50 nm, but in ADY they extend to a depth of 200 nm. There is a positive correlation between the concentration of trehalose in bakers’ compressed yeast and its ability to withstand drying. Krasnikov et al. (1975), who accelerated the rate of drying by application of a high frequency field, specified a level of at least 11 to 12% of trehalose (based on solids).
It is not clear whether the presence of trehalose is the cause of the stability of yeast during drying or whether it merely reflects conditions of aerobic growth or a low protein concentration of the yeast.
Generally ADY is dried to a moisture content of 7.5 to 8.5%. This level represents a compromise between the demand for good bake activity which is higher at higher moisture levels and good stability which is better at lower moisture levels. For use in baking ADY is rehydrated in water.
If the yeast is rehydrated in cold water, about 20 to 25% of the cell solids leach out of the yeast cell. This results in a considerable loss in baking activity. Rehydration in warm water minimizes this leaching. A temperature of 40°C is optimal. The time required for rehydration is generally less than 5 min.
Leaching of solids from the yeast can be entirely prevented by vapor rehydration, but this is not practical in bakeries. Leaching is also minimized if the ADY is added directly to the dry flour before mixing. Presumably this reduces leaching because both the flour and the yeast compete for the small amount of available water.
For direct addition to flour the ADY has to be finely ground or the particles of ADY must have a small diameter such as those obtained by air lift drying. For such air lift dried ADY the bake activity is often better if it is added directly to the flour than if it is separately rehydrated in water.
The leaching phenomenon has been investigated by Herrera et al. (1956) and Peppier and Rudert (1953). The phenomenon is not well understood. Beker (1977) has provided an excellent review and possible explanations based on the behavior of bio-membranes.
Stability of ADY, Form of Particles, and Packaging:
ADY from the roto-louver process consists of small pellets with a diameter of about 1 to 2 mm. The surface of the pellets is smooth which tends to improve their stability in air. ADY from tunnel drying methods takes the form of irregularly shaped, highly fissured strands of a diameter of 1 to 2 mm and of varying length. This yeast can be ground to a powder which passes a No. 20 mesh sieve.
This type of yeast is routinely sold to bakers without protective packaging in polyethylene lined bags or drums. It loses about 7% of its bake activity per month at ambient temperatures and has a useful storage life of 1 to 2 months. If it is packaged in an inert atmosphere or under vacuum it loses only about 1% of its activity per month with an annual loss not exceeding 10%.
Air lift dried ADY usually has very small cylindrical particles with rounded ends and a diameter of less than 1 mm. It generally has a moisture content of 4 to 6% and is less stable in air, which makes protective packaging mandatory.
Protective packaging for ADY means packaging in an atmosphere with less than 2% oxygen and preferably less than 0.5% oxygen, or vacuum packaging. Yeast for the consumer trade is usually packaged in foil pouches in which the aluminum foil is laminated to a heat sealable plastic film such as Pliofilm or Saran.
Such films are also suitable for larger packages in 500 g and 10 kg sizes. The inert gas for the smaller consumer packages (7 g) is commonly nitrogen. Larger foil bags are usually sealed under vacuum and so are 2 lb tins. Tin cans containing 10 to 15 kg ADY must be flushed with nitrogen before sealing; ADY adsorbs carbon dioxide readily.
If this gas is used with larger packages, a sufficient vacuum is created to give the appearance and performance of vacuum packaging. While the size and shape of the yeast particles affect their stability in air, they do not affect their stability in protective packages.
Chemical Composition of ADY:
The regular ADY of commerce has a moisture level of 7.5 to 8.5%. Air lift dried yeasts and protected ADYs usually have lower moisture levels (4 to 6%). The nitrogen level is normally about 7% (based on solids) except for some highly active yeasts with nitrogen levels up to 9.5%. The concentration of phosphorus expressed as P2O5 is generally one-third that of the nitrogen.