Prof. S. V. Lebedev.

Yeast as a Feed, Food, and Therapeutic Agent.

Popular movements, prolonged wars, and similar large-scale and long-lasting events of state importance, by disrupting the normal course of a country’s life, usually lead to the breakdown of its food-supply apparatus, to temporary impoverishment in food and feed resources, and not infrequently to famine. In connection with this, there invariably arise efforts to replace the customary essential food products with new ones, previously unused but more accessible under the temporarily established conditions.

The great war of 1914 just experienced, and the events of the subsequent years, likewise caused an acute nutritional crisis in the belligerent countries, compelling a persistent search for new ways of obtaining the necessary food resources. Among the products proposed for nutrition—first in Germany and later in Russia—yeast was also put forward.

The grounds for this were: a) the richness of yeast in proteins, b) its accumulation in large quantities as a waste product at breweries, c) the centuries-old use of yeast as food in the form of a constant, though quantitatively small, constituent part of bread, d) the ease and rapidity of industrial production of yeast, and, finally, e) the presumed possibility of producing yeast cheaply by synthesizing its protein mass at the expense of nitrogen obtained from the air.

In connection with this, during the war Germany advanced and practically carried out: on the one hand, the use of yeast wastes from brewing for human nutrition and, on the other, the production by an industrial-factory method of so-called “food yeast” (Nährhefe). With regard to the latter, apart from brief reports on the success of the industrial and technical solution of this task, the author was unable to find in the literature any fuller elucidation of the question. It is possible that this is a consequence of the local scarcity of the literature for the corresponding years. This circumstance necessitates, further on, in individual cases, citing certain investigators and works not from the original sources, but from other articles in the literature available to the author that concern one or another essential aspect of the question under discussion.

The situation is presented much more fully with respect to the conditions and results of using waste brewer’s yeast for feed and food purposes. Raw or dried beer yeast, previously washed free of hop impurities, at one time during the war became in Germany a customary component of the population’s food ration. In connection with this, all yeast stocks in those years were collected. This possibility of using beer yeast for nutrition had to a considerable extent been prepared in Germany long before the war by the “Berlin Fermentation Institute,” which for many years had been developing the question of feeding brewer’s yeast to livestock. This question was first raised by Prof. Lindner¹) in 1904. Then, over the following years, it was discussed from various sides by Völz²), Haysuck³), Kellner⁴), Paeschner⁵),

1) Jahrbuch der Versuchs- und Lehranst. für Brauerei, 1904, p. 455.
2) Pflüger’s Archiv, 1905, p. 380.
3) Wochenschr. für Brauerei, 1910, p. 241; Jahrb. d. Vers. u. Lehranst. für Brauer, 1911, p. 282.
4) Deutsch. Landwirtsch Presse, 1910, p. 59; Landw Versst., vol. 63, p. 209.
5) Wochenschr. für Braur., 1910, p. 373.

Pott¹, Delbrück², Baudrexel³, and other investigators. The work was carried out both with moist pressed yeast, containing about 70% water, and with dried yeast, whose moisture content averaged 10%, with a content of 50–58% crude protein, 2–4% fat, 25–30% nitrogen-free extractive substances, and 6–8% ash. At the same time, the study of the conditions of feeding yeast and the results obtained was conducted on various animals, such as: horses, cows, sheep, pigs, dogs, rabbits, rats, mice, pigeons, geese, and chickens. Yeast was fed in mixtures with dried potatoes⁴, with potato stillage⁵, with barley⁶, with brewer’s grains, chaff, and similar additions. Attention was also paid to how fattening with yeast⁷ affected the quality and taste aspects of the meat and milk of the fattened animals, and no negative indications were found. With regard to the physiological effect of the utilization of the yeast substance by the animal organism, the works of the above-mentioned investigators established that this effect, though not the same in different animals, is in general nevertheless considerable. According to Völz, for example, in sheep it reached as much as 84%, whereas in chickens and in domestic birds generally it did not rise above 66%. Not all constituent parts of yeast are assimilated equally. Völz’s experiments⁸ on dogs established the digestibility of the organic substances of yeast at up to 70%, and of its proteins at up to 84.2%, whereas, according to Kellner⁹, in rams 84.6% of the nitrogenous substances and up to 89.5% of the dense organic substances of yeast were digested.

The good digestibility of yeast, in connection with its high protein content, is the basis for interpreting this material (especially in the form of dried yeast) as a valuable, highly concentrated feedstuff, suitable for convenient feeding to domestic animals in mixture with the other cheap and readily available feeds already named and with other feeds.

Thus, the question of the applicability of yeast for the nourishment of the animal organism had already, before the war, been sufficiently elucidated, which, in connection with individual experiments conducted on man, gave grounds for assuming the applicability of yeast also for human nutrition. Obviously, the taste aspects of yeast as such have their significance; however, the latter cannot be decisive in the question of the applicability of yeast for human nutrition, since tastes are very diverse, and, moreover, over time they change. For this reason, not infrequently that which at one time was sharply rejected by the masses, being regarded as completely unacceptable for food, later became a widely distributed necessary food product, of which the potato may serve as an example. As is known, its introduction into use in Europe, and in particular in Russia, was associated with stubborn resistance, protest, and even popular disturbances. In considering yeast as a material for human nutrition, one must likewise not forget that it is only a food raw material, and therefore yeast, like grain bread, raw potatoes, fresh meat, and so forth, may be entirely different in taste from the culinary and other products made from them for food after the appropriate preliminary kitchen processing.

Of considerably greater, and indeed essential, significance is the question of the chemical composition of yeast, its digestibility, and also the physiological effect on the human organism of the yeast consumed by it. The possibility of using yeast for human nutrition, which in prewar times had not represented any practical interest, was nevertheless touched upon in certain works, for example by Völz and Baudrexel¹. It was established that assimilation of the total dry matter of yeast may reach up to 86%, with a coefficient of utilization of yeast nitrogen of 60–70%. In regard to the degree of assimilation of yeast nitrogen, the experiments of Prof. Slovtsov², carried out later, are also cited. This latter investigator sees the relatively low digestibility of yeast proteins as being due to the unfavorable technical conditions under which food is prepared from yeast and to the preparation of the yeast raw material itself, which for food purposes is usually supplied in the form of yeast dried in one way or another at a high temperature. For this reason, the resulting food product proves to be poorly assimilated by the human organism. In connection with this, Prof. Slovtsov believes that the future of the practical use of yeast lies, on the one hand, in improving the technique of producing yeast raw material and, on the other, in developing methods for preparing good dishes from yeast. In Germany during the war, an extensive set of recipes was created in this area; however, most such recipes do not yield good dishes. According to Ressmon and Mayer³, yeast may be used for nutrition by increasing its addition to the dough being mixed in breadmaking up to 2.5%. The possibility of very substantial additions of yeast to baked bread was shown by S. V. Lebedev⁴ through the use of special drying methods that preserve the enzymes of the added yeast and a distinctive method of preparing dough fermented with this yeast. Under such conditions it proved possible to obtain “bread” with a protein content of up to 25–30% by weight of its dry matter. In appearance the product was a well-baked bread, but with the indicated high protein content it possessed a peculiar spicy taste, which some liked, but which in the majority caused an unpleasant taste sensation. Reducing the amount of yeast added brings the taste closer to that usual for bread.

In considering the question of the suitability of yeast for human nutrition, one must first of all dwell on the composition of yeast. In this respect it must be noted that the latter is not the same for different species and races; and even in the case of quite definite yeasts, it is highly unstable and variable depending on the totality of many conditions, such as: the composition of the nutrient medium, the duration and temperature of fermentation, the degree of access of air, the duration and conditions of storage of the yeast obtained, and so forth. In yeast, as a food product, besides the chemical composition one cannot fail to take into account the presence of various enzymes and vitamins. In the literature there are sufficient data on the chemical composition of yeast, both in the form of elementary analysis of yeast and with respect to the definite chemical compounds contained in it that are essential for vital processes.

Below are given some of these data concerning yeast, directly in the form in which they are obtained: i.e., with a moisture content of up to 70%.

Nägeli and Löw¹) found, per 100 parts by weight of yeast:

The ash content in the dry matter of yeast ranges from 2.5% according to Echlosberg and Wagner, to 7.5–8.9% according to Mitscherlich and Bill²), but it also reaches considerably higher values; for example, Hirt³) gives an analysis of yeast with an ash content of 9.94%.

Regarding the composition of yeast ash, the following data are available⁴):

¹) Chemie und Physiologie d. Malzes u. d. Bieres, Prior 1896, p. 355.
²) Die Gärungschemie, A. Mayer 1906, p. 112.
³) Wochenschr. f. Brauer, 1911, p. 177.
⁴) Continuous Alcoholic Fermentation, S. V. Lebedev, 1915, p. 30.

From the foregoing it is evident that the content of all the constituent substances of yeast is subject to considerable fluctuations, proteins not excepted. The quantity of the latter, calculated on the dry matter of yeast, varies within very wide limits, from 31 to 75%, which corresponds to a variation in nitrogen content from 5 to 12%.¹

In view of the fact that moist yeast is very unstable and quickly spoils, it is usually dried for preservation. In most cases, yeast is dried by brief heating to 120°–140° through metal walls—for example, rollers heated by steam—from which the layer of dried yeast is immediately scraped off. The resulting product, intended as feed for livestock or for human nutrition, is a yellowish, very hygroscopic powder, rather pleasant to the taste. In the air it attracts moisture and changes somewhat; when kept in closed vessels, however, it remains a durable product for a long time. It is obvious that fresh moist yeast and dried yeast, as just indicated, differ from one another in chemical composition, owing to the effect of high temperature on the plasma and, in general, on the contents of the cell. In addition, yeast dried in this way no longer possesses the enzymes destroyed by such high heating. Feed and food yeasts obtained before and during the war were usually dried by the method indicated above. According to data from individual investigators, the composition of dried brewer’s yeasts of various origins is given below.

¹ Die Gärungsführung, Delbrück und Hayück, 1911, p. 2.

Thus, dried brewer’s yeast is characterized by an abundance of proteins, a considerable ash content, and the presence of certain quantities of carbohydrates, including glycogen, pentosans, cellulose, and several other representatives of this group of chemical compounds. A characteristic feature of the ash is the abundance of phosphoric acid and potassium salts. The chemical composition of yeast gives grounds for regarding yeast as a valuable feed and nutritive agent¹, which is also consistent with the results of experiments in feeding yeast².

Considerably earlier than the establishment of the suitability of brewer’s yeast as feed, its medicinal properties had been noted against impurity of the skin, emaciation, general weakness of the organism, and so on. In connection with this, brewer’s yeast became known and long ago became a folk remedy. The reason for this should apparently be sought in the varied enzymes and vitamins of yeast. Obviously, in the dry brewer’s yeast usually obtained for feed yeast by drying (up to 140°), the enzymes and vitamins, and consequently the medicinal properties as well, are destroyed or are weakened to a considerable degree. The use of brewer’s yeast in medicine for treatment³ begins in 1886, when in Germany physicians proposed yeast as a remedy against various diseases, such as diphtheria, infantile paralyses, measles, scarlet fever, cancer, and erysipelas. In France, yeast was recommended for boils, carbuncles, and smallpox. Later yeast was proposed for gastro-intestinal catarrh in children. In most cases, the effect of yeast on the organism was established from the results of separate, so to speak, episodic cases of its use in one or another patient. Therefore the causes of the action of yeast were understood very differently. There existed, for example, the opinion that yeast, taken internally, suppresses abnormal fermentation in the intestines, thereby eliminating the organism’s predisposition to furunculosis. On the other hand, in connection with the discovery in yeast of a strong endotryptase, the possibility was admitted of a bactericidal action of yeast taken by the organism. Some noted that after yeast was given to dogs infected with staphylococci and streptococci, a strong

an increase in the opsonic index. Buchner ascribes the therapeutic action of yeast to the zymase contained in it. All the above-mentioned data—partly empirical, partly theoretical assumptions—led to the appearance on the market and to the spread of various medicinal preparations obtained from yeast. Most such preparations consist of brewer’s, less often distiller’s or baker’s, yeast dried in one way or another; in most cases they are pure, or else contain additions of starch, asparagine, and other substances that have no significance for the actual action of the preparations.

Such yeast preparations are marketed under various names. Among those that have become widespread may be noted: levuretin (Feigel’s), Germany; furuferulin (Weirich’s), tripase (Ridel’s), dry wine yeast, dry beer yeast, Levure de bière (Belgium), cerevisin (France), mycodermin (Austria), Roos tablets, zymin, Roos’s cerolin, and others. The usual prescription of beer or baker’s yeast in fresh form is 10–15 grams 2–3 times a day. Dry yeast is usually given to an adult in doses of 1–3 teaspoonfuls per day.

As to the suitability of yeast for treating animals, it may be noted that this remedy is applicable in cases of plague in dogs. In this connection yeast is an excellent preventive agent when given to healthy puppies, and also a reliable therapeutic agent for animals already ill. This is confirmed by many years of experiments by Shrop, as well as by Kramarenko¹ and Kravtsov². Brewer’s yeast is given 2–3 times a day by the teaspoonful; ordinary baker’s yeast is also used, recommended for an adult animal at 10–20 grams per day mixed with milk.

Everything set forth above served as the basis for the question of using yeast for nutrition being placed, during the war, already in the sphere of broad industrial tasks. In Russia it was raised in 1916–1917 in the Central Military-Industrial Committee by its department for the utilization of distilleries and breweries that had been shut down since the beginning of the war. It arose in connection with the shortage of meat products that had become apparent in the country and the existence of more than 3,000 idle factories of the kinds named. The commission created to carry out this task found the existing literature on the question insufficient and deemed it necessary to develop the question further, both with respect to the utilization of yeast by the human organism and its effect upon it, and in the sense of obtaining yeast food raw material. With regard to the latter, two tasks were set: first, to devise a method for converting raw brewer’s yeast into so-called “food yeast,” i.e., into a dry product that is stable in storage and convenient for transport; and second, to develop practically convenient methods for obtaining one or another kind of yeast most suitable for nutritional purposes.

The work that unfolded in the part concerning the study of the digestibility and nutritive value of yeast was carried out by a number of investigators under the direction and with the direct participation of Prof. B. I. Slovtsov, while the botanical-technical side of the question was entrusted to Prof. G. A. Nadson.

Most of the experiments considered below on the study of the digestibility by the human organism of yeast were carried out on so-called “nutritional yeast,” obtained by V. V. Vvedensky’s method at the Petrograd Kalinkinsky brewery from waste brewer’s yeast. The product was obtained by briefly drying liquid yeast, poured in a thin layer onto a rotating metal surface heated to 140° C. The dried yeast was immediately removed from the heated surface by a special

with shavings. It is obvious that the yeast obtained in this way, when dried, loses its enzymes. This circumstance must be taken into account when considering the results of experiments cited below, according to the data of Prof. Slovtsov,¹ on the assimilability and nutritive value of such “nutritive yeasts.”

The composition of the latter, in the form obtained at the Kalinkin plant, is as follows: 5–10% water, 42.5–53.05% protein, 4.4–6.9% ash, 1.28–14.8% fat, and 11.9–19.5% carbohydrates. The caloric value of this food product, yielding chiefly proteins and ash constituents, is sufficiently high. To clarify the relative nutritive value of this product, a comparison² of food products analogous in their character is given below.

The comparison presented shows that dry yeast, in its chemical composition and caloric value, gives grounds for assigning it to valuable nitrogen-containing food products, while at the same time it is rich in such a necessary ash constituent as phosphoric acid. However, for a final judgment on the suitability of yeast for nutrition, chemical composition alone—giving the content of “protein” and “non-protein” nitrogen and other ordinarily determined constituent parts—even when caloric value is present, is insufficient. For this it is necessary to establish the nutritional suitability of yeast protein in relation to its chemical composition.

As is known, at the present time proteins are commonly divided into complete and incomplete, and the question of which of these categories the given proteins belong to is essential in evaluating them with respect to their suitability for nutrition.

The essence of this is that complete proteins contain all those various amino acids which are necessary to meet all the requirements, in this respect, of the organism nourished by such protein. Therefore, if the amount of so-called “complete protein” supplied to the organism corresponds to its

¹ “Nutritive Yeasts,” B. I. Slovtsov. News of the Commissariat of Public Health, 1919, No. 7–12, p. 28.
² Ibid., p. 31.

of the daily requirement, the latter, also qualitatively, proves to be fully satisfied by this protein, which is a source of all the amino acids necessary for the nutrition of the organism.

The second category of proteins, called “incomplete,” is characterized by the absence among the amino acids forming these proteins of a greater or lesser number of those required for the nutrition of the organism. Such an example may be gluten, which contains no cystine or phenylalanine, and also protamines, consisting chiefly of diamino acids, etc. Under these conditions, proteins of this kind, even when present in quantities sufficient in the daily ration to provide the organism with the required minimum of nitrogenous substances, nevertheless do not supply the full sum of amino acids needed by the organism; and if their deficiency is not made up by administering additional proteins containing the missing amino acids, then, when nourished with such incomplete proteins, the organism experiences partial starvation, which leads to nutritional disturbance and serious diseases.

Thus, the question of the nutritional suitability of yeast depends on the composition of yeast proteins. According to the analyses of Zalesskii and Shatelov²), the nitrogen in yeast protein is distributed as follows:

N—amino-acid          . . .  7.38%— 8.75%
N—diamino-acid        . . . 13.07%—11.85%
N—monoamino-acid      . . . 79.06%—79.40%

According to Stutzer, in yeast proteins there are: 10.1% nitrogen in the form of peptides and amino acids, 63.8% in the form of non-phosphorous proteins, and 26.1% in the form of phosphorous proteins. The character of the proteins, according to Wroblewski³), is very varied: there are proteins of the albumin, globulin, mucin, nucleoproteid types, as well as albumoses and peptones. The sulfur and phosphorus content corresponds to:

SO₃; in % of dry matter . . . 0.693—0.696
P₂O₅;  "   "  "      "   . . . 2.749—2.945

The sulfur content in the protein gives grounds for assuming the presence in it of thio-amino acids.

Upon complete hydrolysis of yeast, and also upon autolysis, a series of final decomposition products is obtained, chiefly of the amino-acid type, namely: leucine, isoleucine, valine, phenylalanine, tyrosine, glutamic acid, aspartic acid, lysine, arginine, histidine, tryptophan, cystine. Glycocoll is absent. In addition, yeast contains various organic compounds, among which are purine bases, especially hypoxanthine, xanthine, and guanine. Besides these, there are organic acids, especially nucleic acid, etc.

All the investigations noted, as well as others, indicate that yeast contains a protein varied in the composition of the amino acids forming it, which gives grounds for regarding yeast protein as a complete protein and, consequently, suitable for nutrition.

In considering yeast from the point of view of its applicability for human nutrition, one must take into account not only its suitability for this purpose with respect to its chemical composition, but it is also necessary to consider certain morphological features of this microorganism and, first of all, the properties of the yeast envelope.

As is known, yeast is a unicellular, chlorophyll-free organism belonging to the sac fungi and to the family Saccharomyces. There exist an extremely large number of species and races of yeast. Yeast cells

¹) “Nutritional Yeasts,” B. I. Slovtsov. Izv. Kom. Zdravookhr., 1919, No. 7—12, p. 31.
²) Proceedings of Kharkov University, 1915 (according to Slovtsov).
³) Journ. prakt. Chem. 64 (1901)—33.

They have a round, oval, or elongated, and in general quite varied, shape, depending not only on the species and race, but also on the physiological condition, the age of the given cell, and the external conditions acting upon it in various ways. The diameter of a yeast cell ranges from 2 to 10 microns. Each cell consists of protoplasm with a cell nucleus. It contains vacuoles, glycogen, droplets of fat, crystals of calcium oxalate, and other inclusions. The cell is covered by an elastic membrane, very thin in young cells and thicker in old cells, on average about 0.5 micron thick. Yeast spores have a thicker membrane, reaching 0.7–0.9 and even more—up to 1 micron. The yeast membrane, in contrast to its contents, possesses great strength¹. During the process of autolysis of yeast, its membrane remains intact, and in dead cells it dissolves only in the presence of bacteria or tissues that dissolve cellulose. The membrane of the yeast cell is an anatomically and chemically complex formation. According to Lindner², its composition includes: cellulose, pectin-like, plasmatic and, consequently, protein substances, and mineral elements. The cellulose of the yeast membrane differs from ordinary cellulose found in plant cells. By the accepted methods of obtaining cellulose, i.e., by treatment with alkalis and acids, it is not possible to obtain fiber from yeast. Nor is it dissolved by an ammoniacal solution of copper oxide. Many reactions characteristic of ordinary fiber do not apply to the cellulose of the yeast membrane. Therefore some investigators regard the fiber of the yeast membrane as differing³ from ordinary fiber, calling it “yeast gum” and by other names.

According to Ducloux, the yeast membrane contains, depending on age and other conditions, 5.9–15.1% of its weight of this substance. Czapek⁴ showed that yeast cellulose is heterogeneous and consists of insoluble achrocellulose and erythrocellulose, which passes into solution when heated under pressure. Erythrocellulose dissolved in water is slowly hydrolyzed by saliva, yielding d-glucose. Hydrolysis of the yeast membrane gives grape sugar, mannose, galactose, and pentoses. Complete dissolution of the yeast membrane is brought about by strong sulfuric or chromic acids.

What has been considered above regarding the yeast membrane and its ability successfully to withstand various influences does not, however, prevent yeast from causing fermentation of the medium surrounding it. This indicates that the yeast membrane must be regarded as semipermeable and therefore incapable of serving as an insurmountable barrier to the passage, under certain conditions, of the contents of the yeast cell beyond its limits into the surrounding medium, and consequently also into the digestive tract of man and other animals when they are fed with yeast. However, the degree of this possibility is of essential practical importance and requires special clarification.

In dwelling on this question, it should be noted that the conditions of assimilation of yeast by the animal organism will differ depending on the form in which the yeast is used for feeding, i.e., whether it will be killed and deprived of its enzymes by the action of the high temperatures of drying, or whether it will consist of enzymatically active cells in the form of fresh active yeast or in a form which, though dried, has nevertheless, thanks to slight heating, retained its enzymes. It is obvious that the enzymatic activity of yeast intended for nutrition promotes the development of internal, varied processes of autolysis of yeast during the period of its presence

them in the alimentary tract of the organism. This circumstance, by causing the transformation of the colloidal, non-diffusible mass of plasma and other substances contained in the yeast cell into new, simpler products, readily diffusible through the yeast membrane, thereby facilitates for the organism nourished with yeast the possibility of assimilating them.

Thus, the dissolution and extraction of the contents of the yeast being digested, owing to their enzymatic nature, is partly carried out by the yeasts themselves, which reduces the expenditure of effort by the organism digesting them.

As regards the possibility of collision or mutual destruction between the enzymes developed by the yeasts and the enzymes secreted in the digestive tract of the organism receiving the yeasts, such crossing is to a considerable extent excluded, since the action of the yeast enzymes is concentrated chiefly within the yeast membrane, whereas the enzymes of the alimentary tract of the organism exert their action mainly outside the yeast cell. Moreover, this delimitation of the sphere of action of the one and the other active enzymes, owing to the already noted, in its place, extremely great resistance of the yeast membrane, is invariably maintained from the beginning to the end of the stay of the yeasts in the alimentary tract of the organism digesting them.

In connection with the clarification of the significance of the enzymatic nature of yeasts in the process of their assimilation by the human organism, it is obvious that, in order to elucidate the suitability of yeasts for nutrition, it is expedient to illuminate this question by direct experiment, and moreover on such yeast material as would be subjected to the action of digestive processes only from the side of the organism receiving them, i.e. excluding the internal enzymatic work of the yeasts themselves; and therefore, in the present case, enzyme-free yeast material should be taken.

This also has a certain practical significance in that the majority of so-called “fodder” or “nutritive” yeasts, owing to the conditions of the commonly accepted methods of producing them, inevitably prove already to have been deprived of their enzymes, destroyed by high heat during drying.

An experimental elucidation of this question is given by Prof. Slovtsov¹ in his interesting experiments, conducted with respect to the already mentioned dry yeasts: on the one hand, directly with their assimilation by man; on the other, with the digestion of the same yeasts by natural gastric juice.

The conditions of the experiments of the latter category are given below:

“Air-dry substance (powder from yeast), previously extracted with ether, was digested for two weeks at \(37^\circ\) C with natural gastric juice; the juice was drained off three times and replaced with fresh juice. The residue was digested in a thermostat for six weeks with artificial pancreatic juice of considerable strength. Two similar phenomena gave the following results:

From the above it is evident that prolonged digestion by pepsin and trypsin converts the main mass of the nitrogenous substances of yeast into the dissolved state, although the coefficients of digestion are lower than in the digestion of meat.

In experiments on the assimilation of dry yeast in humans, the daily ration given to them, 30–50 g of yeast, was administered together with other food, such as bread, butter, porridge, sugar, etc. It was found that the coefficient of assimilation of nitrogen of the whole mixed food was 84.05%, with a coefficient of assimilation of yeast nitrogen of 53.89%.

Microscopic studies of yeast cells that had passed through the digestive tract of man (Slovtsov) and animals (Nadson) showed that the yeast membranes of the majority of such cells were destroyed and already deprived of their contents.

The effect of dry, enzyme-free yeast on the secretion of digestive juices by the human organism is not great. According to the investigations of L. A. Orbelí¹), yeast constitutes a protein food that ensures for itself the influx of the necessary digestive juices and does not impose greater demands on the human digestive apparatus than ordinary protein food familiar to people in the form of meat. With regard to the effect of dry yeast and its decoctions on the activity of digestive enzymes, Prof. Slovtsov established their favorable action on trypsin and invertase, their indifferent action on pepsin, and a somewhat retarding action on ptyalin of saliva.

The assessment of the actual suitability of yeast for nutrition is best revealed by feeding experiments and by determining the coefficient of assimilation of the principal constituents of yeast, which is given below in the form of summarized results of the investigations of Völz and Deutschland.

All that has been set forth above gives grounds for recognizing dry, enzyme-free yeast as a valuable food raw material for the organism of a healthy person.

The significance for the sick, as well as for weakened organisms and children, of dry yeast deprived by high heating during drying of enzymes and living cells was elucidated in 1917–18 by numerous experiments organized by Prof. B. I. Slovtsov and carried out in Leningrad clinics and hospitals under the direction of specialists and professors: G. A. Smirnov, A. K. Palenko, S. Ya. Kulnev, M. G. Nikitin, and others. These experiments, confirming the previously obtained observations already noted above, yielded very favorable results. The general conclusion, in Prof. Nadson’s brief formulation, is as follows: “It may be regarded as actually proven that dry brewer’s yeast is a dietary food product and at the same time possesses therapeutic properties. Containing neither living cells nor enzymes, dry yeast is nevertheless an excellent therapeutic agent... In Vinkel’s opinion, the therapeutic action of yeast belongs to its nucleoproteids, rich in phosphorus and iron, and partly to vitamins; in Borsikow’s opinion³)—to nucleins and salts.”

¹) Cited according to Slovtsov.
²) Nadson, “Brewer’s Yeast as a Food Product.” Yearbook 1923–24. Section of Brewers V. A. I., p. 125.
³) Biochem. Ztschr. Bd. 43 (1913), 418.

Prof. B. I. Slovtsov¹ believes that “it is necessary to distinguish between yeast preparations that contain enzymes and those that do not contain enzymes, since their action will obviously be different. For example, bread with yeast, prepared by Prof. S. V. Lebedev in Tomsk and preserving the yeast enzymes, proved useful in secretory disorders of the stomach, reducing hypersecretion and increasing the digestive power of the gastric juice—an action not possessed by the bread of our (enzyme-free) nutritional yeasts.”

In view of the novelty and distinctive features of the above-mentioned bread, burdened with a considerable quantity of yeast, it seems necessary to dwell also on this medicinal bread tested in Tomsk. The latter appeared in 1913 as the result of the present author’s effort to develop a means for the mass use of yeast for food purposes in the form of a product convenient both for use under everyday conditions and acceptable as food with respect to taste. Naturally, in this direction, among other things, the idea also arose of bread burdened with yeast. In view of the excessive effect of dough fermentation when an excessively increased quantity of yeast was used, as well as other phenomena connected with this, it turned out that the capacity of dough for yeast, in terms of the possibility of obtaining bread, is very limited. In connection with this, in order to weaken the yeast added to the dough, it first had to be subjected to slow, prolonged drying at such relatively low temperatures as would not greatly affect the enzymes of the yeast. Drying was carried out in a specially constructed apparatus with good regulation of temperature conditions, using an electric heating device. Drying was conducted with a continuous current of air.

Three kinds of dry yeast were obtained, differing in the temperature conditions of drying within the limits: 26–41°C, 30–54°C, and 30–70°C. Correspondingly, drying of the yeast lasted 6, 5, and 4 hours. In this process, from starch-free yeasts pressed on filter presses, 28.5–32.6% of dried yeast was obtained by the indicated method.

All experiments were conducted exclusively on pure cultures of XII race top-fermentation (upper) yeasts of the Berlin Fermentation Institute. The yeasts were prepared by the usual aerated method on wort made from barley and rye malt. The fermentative power of the dried yeasts, determined by the usual method², corresponded, for yeasts dried at: (26–41°C) 1.99–2.2 g; (30–54°C) 2.41–2.68 g; (30–70°C) 3.17–3.37 g.

Thus, the dried yeasts were enzymatic. The short-term stay of the dough in the oven for 20–30 minutes during baking produces within it a heating that is not capable of destroying the enzymes of the yeast, owing to which the baked bread also proves enzymatic. This latter property is revealed in the fact that bread rich in yeast, placed after its final cooling under a glass bell, after several hours shows quite clear signs of alcoholic fermentation taking place in it. This indicates the preservation in yeast bread, even after baking, of the relatively low-resistant zymase, and consequently also of other enzymes more resistant with respect to temperature, including proteolytic—protein—enzymes, which are of great importance in the process of digesting food. Thus, yeast bread contains not only nutritive substances, but also agents for digesting them, which may have substantial significance in certain diseases, and also for weak organisms.

Yeast bread was prepared from good white wheat flour, tap water, dry yeast, and table salt, taken in a definite proportion, which varied depending on the desired content of nitrogenous substances in the finished product. By gradually increasing the amount of yeast introduced, it proved possible to obtain bread of good appearance with a content of nitrogenous substances, calculated as proteins, of up to 32.4%, reckoned on the dry matter of the product. The bread was baked in an ordinary bread-baking oven at 230°C, in the form of rolls of 160–200 grams each. With a higher yeast content, after baking the product outwardly appeared as a well-baked, finely porous, rather dense mass of well-kneaded bread, of the same color and appearance, but with a distinctly pronounced spicy, specific aroma and taste, partly reminiscent of cheese.

As the amount of added yeast was reduced, these properties weakened and disappeared. The specific smell and taste appeared more strongly in yeast dried at a lower temperature, namely 26–41°C.

Most of the experiments were carried out with bread of the average composition presented below, recalculated to the dry matter of the product. In the same form the composition of the flour used for baking the yeast bread is also presented.

The considerable addition of dried yeast created quite unusual conditions for preparing the dough. For bread of the indicated composition these conditions amounted to the following.

For the preparation of the dough there were taken:

125 parts by weight of tap water.
100 ” ” wheat flour
50 ” ” yeast dried at 26–41°C.
1.5 ” ” pure table salt.

Procedure for preparing the dough and baking the bread:

The dough is kneaded with water at a temperature of 7–9°.
Rising time 60–55 min., kneading down of the dough—6 min., second rising 13–18 min.; dividing the dough into rolls of 160–200 g each; proofing of the rolls on the sheet 8–12 min.; baking in an ordinary bread-baking oven 25–30 min.

The composition of the yeast bread obtained and the presence in it of enzymes, as well as vitamins, on the one hand argues for its considerable value as a nutritive product, and on the other indicates the possibility of applying it for therapeutic purposes. Numerous cases in 1913–14 of the use of this bread by various persons in the city of Tomsk in quantities of 100–250 grams per day showed its applicability for the mild and rapid elimination of skin impurities and certain disorders of the stomach and intestines. Some noted a noticeable improvement in general well-being. There was a case, conducted under the observation of Prof. Valedinsky, of successful elimination of the consequences of poisoning by fish poison.

Systematic experiments to study the action of yeast bread in various diseases were conducted by Prof. I. M. Levashov at the Therapeutic Hospital Clinic of Tomsk University in 1915–16. Owing to conditions unfavorable for this work, connected with the general experiences of the country at that time, these experiments were interrupted; they did, however, yield some interesting results, a brief summary of which Prof. I. M. Levashov gives in the formulation cited below.

“From the experiments carried out at the Therapeutic Hospital Clinic of Tomsk University with Prof. S. V. Lebedev’s yeast bread, one may note 5 cases of a severe form of Diabetes mellitus—diabetes—where, by means of a carbohydrate-free diet, it was possible to reduce the excretion of sugar from 5 to 1.5%; the administration of 50 g of ordinary bread led to an increase to 2.5–3%, whereas yeast bread caused a decrease in sugar excretion to 1%, with a marked improvement in the patient’s general condition. The prescription of this bread, in a daily amount of 100–200 g, gave positive results in gastric and cutaneous diseases. In gastric diseases a particularly appreciable effect was obtained in subacid catarrhs. It should also be noted that a very severe form of Pemphigus with a poor prognosis was successfully treated¹). The positive results of the experiments with yeast enzymatic bread, prematurely interrupted by the conditions of that time, speak in favor of the desirability of further study of this question.”

The possibility of using yeast for feed, food, and therapeutic purposes is of profound interest not only in itself, but, in connection with achievements in the field of utilization of atmospheric nitrogen, acquires exceedingly great significance in technical, industrial, and economic respects. At the same time a new branch of the technology of alcoholic fermentation is taking shape, in which the aim is no longer alcohol in the form of one or another alcoholic beverage, but the production of proteinaceous nutritive and feed products; along with this, as has already been shown by Prof. G. A. Nadson²) and others, the production of fatty nutritive yeasts is also possible. Under such conditions the phenomenon of alcoholic fermentation acquires for humanity a new meaning and the extraordinary value of a mighty means of subsistence, leading to an inexhaustible source of protein substances of microbiological origin, obtained at the expense of the nitrogen of the air. This indicates that broad horizons are now opening before the technology of alcoholic fermentation, and that new theoretical, technical, and industrial tasks are being put forward, the identification and clarification of which is a current task of the present time.

¹) The case concerns the illness of Prof. V. P. Elyashev; since it was impossible at that time to obtain the indicated bread, it was necessary to switch to simple factory pressed yeast, used at 100 g in three doses per day. The yeast intake lasted about 4–5 weeks, and improvement occurred after 1.5–2 weeks.

²) “Fetthefe,” Nadson and Konokotina, “Wochenschr. f. ‘Brauerei’,” 1924, Bd. 41, No. 46. “Fatty Yeasts,” Nadson and Konokotina. Publications of the Main Botanical Garden—Petrograd, vol. 22, I—1923.

1. Izvestiia Komiss. Zdravookhraneniia. Petrograd. Proceedings of the Apothecaries, 1919, No. 7–12, p. 45. ↩↩↩↩↩↩↩

2. (See p. 17). Loss of weight in 24 hours during fermentation of 5 g of yeast in a 10% solution of 100 cubic cm of sugar at 30°C. ↩↩↩↩↩↩

3. Czapek—Biochemie der Pflanz., I vol., p. 508. ↩↩↩↩

4. Ibid. ↩↩↩

5. Völz—Jahrb. d. Vereins d. Spirit. Fabr. in Deutschl., 1912, p. 301. ↩

6. Also Landw. Jahrb., 1912. ↩

7. Völz and Baudrex—Wochenschr. f. Breur, 1913, No. 11. ↩

8. Pflüger’s Archiv, 1905, p. 380. ↩

9. Landw. Versst., vol. 63, p. 209. ↩