A Treatise on Baking

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Yeast

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Chapter IV

YEAST

YEAST

I. YEAST DEFINED AND EXPLAINED

A. BOTANICAL DEFINITION

Yeast is a microscopic one celled plant belonging to the group known botanically as “fungi” and ordinarily multiplies by a process known as budding, and which causes fermentation when placed under suitable conditions.

1. Derivation of Word Yeast

Our word yeast comes from the old Sanscrit “yas” meaning to seethe or boil,—descriptive of its action in suitable sugary solutions, from which it produces alcohol and the gas, carbon dioxide.

B.     BAKERS’ YEAST

1. Different Types of Yeast

There are many different species of yeast just as there are different types of apples or corn. The species of yeast used for baking purposes is termed: “Saccharomyces Cerevisiae” and commercially, the word yeast usually refers to this species. However, there exist different strains of yeast within this one classification alone,—some stronger and more valuable for baking purposes than others.

2. Bakers’ Yeast and Brewers’ Yeast

From a commercial standpoint, two groups of yeast are often mentioned :

(a) bakers’ compressed yeast. This type of yeast is ideally adapted for bread making.

(b) brewers’ yeast. For baking purposes, this type of yeast is vastly inferior both in baking and keeping qualities to bakers’ compressed yeast manufactured today,—although years ago, before the organization of the yeast industry, brewers’ yeast was employed to some extent in bread making.

3. Description of a Yeast Plant

Each individual yeast plant is round or oval in shape and measures about 1/3000 inch in diameter.  Each yeast cell is surrounded by a thin membrane of cellulose, and the interior of each cell is made up of finely granular protoplasm.

4. Compressed Yeast Sold to Bakers

A pound of compressed yeast consists of billions of yeast cells, pressed together in a solid mass and thus freed from excess water. The compressed yeast sold in this country for bakers’ use is a special variety selected and cultivated to develop extreme hardiness, high vitamin content,

uniformity and strong enzymatic power. The aim of the yeast manufacturer is to produce a yeast which is properly balanced in its fermenting and conditioning powers and which will create the best character of fermentation in the dough batch so as to make possible the production of the highest quality bread.

II. THE DISCOVERY OF YEAST

A. ANCIENT KNOWLEDGE REGARDING YEAST

Yeast or leaven was used in a crude way, thousands of years ago, by the ancient Egyptians. However, individual yeast cells were not recognized until a Dutchman by the name of Leeuwenhoek in 1680 constructed a microscope by means of which yeast cells were first made visible to the human eye.

While Leeuwenhoek reported a description of these cells, he failed to recognize them as living organisms associated with fermentation and thereby just missed a very important discovery.

For nearly a century the exact nature of yeast remained a mystery. In 1803, Thenard claimed that yeast caused fermentation. Several other investigators of that time supported this contention but in 1840 Von Liebig vigorously disputed this theory.

B. PASTEUR’S DISCOVERY

For several years a controversy waged on this subject,—but no definite information was established until in 1859, Pasteur after considerable experimentation first demonstrated positively that yeast was a living organism, capable of growth and reproduction, and that yeast was actually the cause of fermentation.

Because of this, Pasteur, one of the most famous scientists the world has ever known, has rightly earned his title: “The Father of Fermentology.”

The experimental work of Hansen and Biichner,—in addition to that of Pasteur,—formed the basis from which the present commercial manufacture of yeast has evolved.

III. THE COMMERCIAL PRODUCTION OF BAKERS’ YEAST

A study of yeast and how it is grown commercially is fascinating. While each yeast plant is extremely tiny and only visible by means of a microscope,—the daily output of a yeast factory is measured in tonnage.

In this country, the manufacture of yeast on a commercial scale began about sixty years ago.

A. SEED YEAST

One of the greatest treasures possessed by the yeast manufacturer of today is his choice strain of seed yeast particularly selected and cultivated in order to develop hardiness and uniformity.

B. FOOD FOR COMMERCIAL GROWTH OF YEAST

This seed yeast is placed in a carefully prepared food which is in the form a rich extract, made from raw materials of the highest quality— just as seed corn is planted in fertile soil. The quality and strength of yeast depends primarily on its culture. The raw materials used may be in the form of molasses, grain or any other such substance provided they furnish the nutrition which the yeast needs for optimum growth. The food media or “wort” on which the yeast is grown must be sterilized and furnish exactly the nutrients which these tiny plants need for ideal development and multiplication. These food substances consist of natural sugars plus necessary diffusible nitrogen substances which if deficient in the raw material used are supplied by the addition of suitable nitrogen salts. The necessary mineral matter or inorganic salts are also supplied and all of the food for the commercial growth of yeast is dissolved in water. All these nutrients must be present in definite form and in definite amounts.

Air which has been washed in a special way is blown into this food media or extract in order to supply the yeast plants with the oxygen which they need for optimum growth and multiplication and to remove carbon dioxide which is inhibitive to yeast growth.

C. TEMPERATURE CONTROL AND SANITATION

An accurate temperature regulation must be maintained at all times during the process and absolute cleanliness must prevail throughout in order to prevent the invasion of “wild” and undesirable yeasts and bacteria.

D. HOW YEAST GROWS AND MULTIPLIES

The seed yeast originally placed in the food extract multiplies by a process known as budding, which takes place very rapidly under proper conditions.

Each tiny plant assimilates nutrition from the food media and in a very short time a bulge occurs on one side of the yeast cell. This bulge or bud enlarges until it is nearly as large as the parent cell, and then it too puts out a bud and the first plant also produces another bud. In this manner these buds develop rapidly into matured cells which finally break away from the parent cell and the process of multiplication is repeated.

Thus the nutrient substance of the food extract is assimilated by the living yeast imparting to it the power of initiating and sustaining the fermentation process in the dough batch.

In each yeast factory, countless millions of yeast plants are grown every hour. Each crop of yeast, after being grown to its proper maturity is separated from the spent food extract by means of centrifugal machines somewhat similar to the cream separators of the modern dairy. It is then cooled, filtered, pressed, cut, wrapped, refrigerated and marketed.

In one sense the yeast manufacturer may be considered a “scientific farmer.” The commercial cultivation of high quality yeast is not simple. It represents a complicated process requiring chemical skill and the continued application of knowledge which has resulted from untiring research work.

E. HIGH QUALITY OF BAKERS’ YEAST

Yeast when delivered to the baker is quite alive and vigorous,—ready to perform its vital function in the dough batch.

Modern scientific investigation and the continuous search for new and more efficient methods of manufacture has resulted in the exceptionally strong and uniform compressed yeast which is now being produced and sold throughout this country and Canada.

IV. THE PRINCIPAL QUALITIES OF GOOD YEAST FROM THE BAKERS’ STANDPOINT

A. STRENGTH OF YEAST

1. Definition

Strength of yeast means the power of yeast to raise and condition the dough batch throughout the entire dough stage.

The word strength when applied to yeast means longevity or the power to initiate and sustain the entire process of fermentation of the dough until this action is terminated by the heat of the oven.

2. Thermal Death Point of Yeast

One very important characteristic of yeast strength is its thermal death point or the temperature at which its fermentative activity in the dough is killed by the heat of the oven.

Through constantly improved methods, the yeast manufacturer of today produces for the baker uniformly strong yeast,—one which will function vigorously in the dough from start to finish,—not merely exhibiting- a flashy start with subsequent weakening.

3. The Advantage of Strong Yeast

Strong yeast raises and conditions a dough faster and to better advantage than weak yeast. Strong yeast is absolutely necessary for the production of the highest quality yeast raised baked goods.

B. UNIFORMITY AND DEPENDABILITY

1. General Definition

Uniform yeast is yeast which can be depended upon to raise and condition the dough batch in the same way and at the same rate day in and day out,—all other conditions remaining the same. This means absolutely dependable yeast.

2. Two-Fold Meaning of Yeast Uniformity

There are two meanings which can be applied to the uniformity of yeast and both are very important from the baker’s point of view.

(a) GENERAL UNIFORMITY. This means that the yeast delivered and used on different days will bring about the same results in the dough batch in the same period of time provided of course, that equal amounts are employed, all other conditions remaining the same.

(b) BALANCED UNIFORMITY. Yeast does not merely raise the dough. It performs many other useful and independent functions which are explained in detail later on. Each of these separate actions of yeast is of utmost importance in the production of quality baked goods. It is essential that the yeast used should be of such a character that these various different independent functions will proceed at a proper relative rate or in other words—balance. For instance: It is important that the carbon dioxide gas which raises the dough shall be produced by the yeast at a rate which will best conform to the speed at which the gluten is softened or mellowed by the action of the yeast. Such a condition is largely dependent on the relative strength and action of the different enzymes in the yeast. By careful selection and cultivation the modern yeast manufacturer is able to furnish the baker with yeast which is properly balanced in this respect. This essential characteristic of bakers’ yeast may well be termed “balanced” uniformity.

3. Practical Importance of Yeast Uniformity

To the baker, uniformity of yeast is of fundamental importance for it is the uniform strength of his yeast which permits him to adhere to a daily standard schedule of bakeshop operation with the assurance of the production of uniform quality yeast raised products each and every day. Without uniform yeast, a uniform loaf is practically impossible.

C. PURITY OF YEAST

By this is meant the absence of foreign wild yeasts or undesirable bacteria from bakers’ yeast. As explained previously, bakers’ yeast rep-

resents one strain of yeast selected and grown for commercial use because of its particular properties which make it ideally adapted for bakers’ use. There are of course, many other kinds of yeasts and bacteria floating in the air, which may not be ordinarily harmful, but nevertheless, are highly undesirable in bakers’ yeast. Their action in the dough causes what is termed wild fermentation resulting in the production of undesirable flavors and, in general, inhibits and interferes with the normal healthy fermentation of the dough, thereby impairing the quality of the baked goods produced.

The modern yeast manufacturer exercises extreme care to avoid the invasion of foreign organisms in bakers’ yeast for he realizes that purity of yeast is a factor of utmost importance to the baker.

D. HARDINESS

Hardiness means the ability of yeast to retain its baking strength throughout shipping and handling until used. Like other living plants yeast is naturally perishable and in order to maintain its maximum leavening power, it is necessary that care be exercised so as to insure proper shipping and delivery conditions. The compressed yeast supplied to bakers in this country and Canada represents only yeast cells which have been grown to proper maturity so as to develop extreme hardiness.

E. CONSISTENCY, FEEL AND FRACTURE

Bakers’ compressed yeast should be firm and “springy” to the touch, fairly brittle and should break with a clean, sharp fracture without undue crumbling and should not have the consistency of butter. Yeast should exhibit a very slight moistness.

F. TASTE AND ODOR

It is difficult to describe the taste and odor of yeast in any other way than “yeasty.” This flavor is clean and pleasant, somewhat resembling that of apples.

G. APPEARANCE

When received by the baker, yeast may range from a light cream color, with a slight yellowish or grayish hue to a darker yellow or almost pale brown shade. The variations in color are usually caused by unavoidable changes in the water or slight variations in the color of the raw material used in the preparation of the food extract on which the yeast was grown.

Sometimes, though rarely, the outside of a cake of yeast may be whitish, while the inside possesses a more normal yellowish tinge. This thin white coating is usually due to a slight drying out on the surface which is of no material importance.

Within a considerable range, these differences in yeast color are not significant of the quality of the yeast.

V. THE COMPOSITION OF YEAST

The primary importance of yeast in bread-making is due to the life and activity of the yeast by which the panary fermentation of a dough is made possible. This of course, is dependent on the fact that the yeast is alive and that its enzymes are active.

A. AVERAGE ANALYSIS OF YEAST

The actual chemical composition of yeast, while of secondary importance is of interest and an average analysis of fresh yeast is stated below:

The above constituents are present in yeast in the form of hundreds of complex compounds, formed in the yeast plants during their development and growth, by the mysterious workings of nature. The moisture content of yeast is not present as free water outside of the yeast cell, but represents a definite part of the live yeast plant in the same way as the natural moisture contained in apples, potatoes or other vegetables.

B. VITAMIN AND ENZYMES IN YEAST

The vitamin content of yeast consists mainly of Vitamin B, an essential food factor without which the human body could not live. Many ordinary foods are lacking in Vitamin B, and while to date, it has been practically impossible to isolate the vitamin substance of yeast from its other constituents,—feeding tests have shown that yeast is one of the richest sources of this important food factor.

The actual life activity of yeast in a dough whereby panary fermentation is made possible depends on the presence and function of minute quantities of certain enzymes in the yeast. Much is yet to be learned about the exact nature of these enzymes and they have never been isolated in a pure state. In view of the importance of the yeast enzymes, a brief explanation in this connection is given in the following paragraphs.

VI. ENZYMES AND THE FUNCTION OF THE YEAST ENZYMES IN THE DOUGH

A. ENZYMES DEFINED

An enzyme may be defined as a very minute substance produced by living organisms which, by its mere presence under suitable conditions, brings about or speeds up certain chemical changes. For instance, our food is digested through the agency of certain enzymes,—and starch is converted into sugar by enzjTne action. The particular enzymes in yeast, often called “ferments” are responsible for panary fermentation. These enzymes may be compared to traffic policemen, inasmuch as they start and control the various conversions in the dough without actually being a part of these changes. It is also remarkable to note that a very small quantity of enzymes are capable of converting a very large quantity of substances from one form to another without actually being destroyed or changed.

B. ACTION OF ENZYMES SPECIFIC

The action of enzymes is specific—that is to say—each class of enzymes has only one particular chemical conversion which it will bring about. For instance, one kind of enzyme will break up sugar into carbon dioxide and alcohol, and this is the only direct function it can perform. Another class of enzyme will change one form of protein in another class of compounds but can do nothing else. Still another class of enzyme will act on malt sugar but not on cane sugar.

C. THE MAIN ENZYMES OF YEAST AND FUNCTION OF EACH

There are several different enzymes in yeast,—each performing its particular function in the dough batch. It is impossible to isolate these enzymes in a pure state, and there is no organism known other than yeast which contains the same combination of enzymes in the same relative proportion. This is why there can be no satisfactory substitute for yeast as a leavening agent.

In order to actually visualize the changes in the dough batch brought about by the action of yeast, let us consider the most prominent enzymes present in yeast and the function of each.

(1) Invertase

This enzyme changes cane sugar or sucrose which is not directly fermentable by yeast into a simple form of sugar known as invert sugar.

(2) Maltase

This enzyme converts malt sugar or maltose which is not directly fermentable by yeast into dextrose.

(3) Zymase

This enzyme Zymase is actually the specific fermenting enzyme in yeast. Zymase attacks the invert sugar and dextrose which previously have been produced by the action of invertase and maltase, changing them into the gas carbon-dioxide (the same gas that is used to charge soda water) and some pure alcohol along with the production of very small amounts of glycerin, succinic acid and traces of “higher” alcohols and other substances which enhance the flavor of the loaf.

(4) Proteolytic Enzymes

To this class of enzymes is attributed the power of converting insoluble proteins into soluble nitrogenous compounds, in other words the conversion of proteins which cannot dissolve in water into another form which will dissolve in water,—and in this condition can be utilized by the yeast as food. The enzymes called “proteases” belong to this group and may have a softening effect on the gluten.

(5) Additional Miscellaneous Yeast Enzymes

In addition to the above mentioned principal enzymes there are several other minor enzymes in yeast, each of which undoubtedly contributes in a small way to the sum total of the changes brought about by yeast activity in the dough. Some of these changes assist in imparting flavor and digestibility to the finished loaf.

VII. THE FUNCTION OF YEAST IN BREAD MAKING

Yeast raises and conditions the dough batch, or in other words converts the inert, heavy mass of dough into a light porous, elastic product which, when baked, is appetizing, easily digestible and nutritious. Yeast itself also adds definite food value to the loaf.

Without yeast, bread and other yeast raised products as we know them today would not be possible.

Panary fermentation brought about by the action of the yeast represents the “life process” of the dough, and upon this, the creation of bread depends. Because of its fundamental and indispensable function in the production of leavened bread, yeast has been rightly termed the “Soul of Bread.” The action of yeast in raising and conditioning the dough batch is explained more fully in Chapter XI—Section two.

VIII. HOW YEAST ENHANCES THE FOOD VALUE OF BREAD

A. NUTRITIVE VALUE OF THE YEAST ITSELF

Relatively speaking the amount of yeast in a bread dough is not large, but it is worth while noting that yeast itself is one of the finest foods known. It supplies the bread with supplementary protein of the highest order, certain valuable nutritive mineral salts and particularly Vitamin B which is so essential for growth and health of the human body. Yeast has often been called the “Vitaminizer” of bread.

B. FLAVOR AND DIGESTIBILITY OF BREAD DUE TO YEAST

It is the action of the yeast which is responsible for the conversion of the raw flour and other dough ingredients into a light well risen mass which when properly baked is practically 100% digestible.      Thus bread

is often described as the food without waste. Simultaneous with these changes is the creation of the characteristic pleasant bread flavor which, perhaps more than any other feature is responsible for the continued acceptability of bread as a standard article of diet for every meal.

IX. THE IMPORTANCE OF THE PROPER HANDLING OF YEAST BY THE BAKER

In the handling of yeast it should never be forgotten that it is a living plant and like all plants it breathes in oxygen from the air and exhales carbon dioxide. This vital process of respiration is going on all of the time, proceeding very slowly indeed at low temperature, such as should be maintained in an icebox, and very rapidly at high temperature, such as is often met with in the bake shop in summer weather. There is no food for the yeast in a pound cake and if this vital process goes on in the absence of food the yeast burns itself up, heats, digests itself and weakens. When the yeast is in a dough at 80 degrees F. the case is different, for there is plenty of food in an ordinary dough for the requirements of the yeast. It is very important, therefore, that yeast should be kept cool to retard the respiration as much as possible until it is used.

In order to prevent any deterioration of yeast, extreme care is observed during manufacture and shipment, and the same precaution should be exercised by the baker so as to insure the greatest activity of his yeast in the dough batch.

X. HOW YEAST SHOULD BE HANDLED IN THE BAKE SHOP

A. REFRIGERATION OF YEAST

1. Proper Temperature for Yeast Storage

Upon arrival at the bakery the yeast should be placed in the refrigerator immediately. The temperature of the refrigerator, for ideal yeast storage should be kept at about 45 degrees F.

2. Arrangement of Shelves in Refrigerator

Upon the delivery of yeast to the bakery, each pound package should be placed on a dry shelf in the refrigerator. The shelves should be made with open slats and the pounds of yeast placed about 14 inch apart, in order to permit the cold air to circulate around and between each pound. Yeast is delivered to the baker in pound cakes so that it can be separated properly on the shelves, thus making possible the proper circulation of cool air so that the baking strength of each pound may be retained unimpaired.

Frequently these shelves in the refrigerator are placed about six inches apart, thus permitting the pound cakes to be placed on end, so as not to occupy an undue amount of space. It is advisable to have an inch strip attached along the front edge of the shelves to prevent the yeast from falling off.

A space should be maintained between the freshest yeast and the previous shipment and the oldest yeast used first.

B. USE OF YEAST AFTER REMOVAL FROM REFRIGERATOR

The yeast required may be taken to the mixing room shortly before it is to be used—but should not be allowed to remain in the mixing room long enough to “warm up” before use in the mixer.

The wrapper should not be removed from the yeast until used, and the yeast should be dissolved in a portion of the water to be used in the dough. This water should not be much warmer than ordinary tap water. Salt should never be mixed in with that portion of the water containing the yeast.

XI. YEAST DELIVERY AND SERVICE IS OF UTMOST IMPORTANCE TO THE BAKER

Inasmuch as yeast is alive and perishable, it is of primary importance that strong uniform yeast be delivered to the baker in accordance with a definite time schedule in spite of storms or natural disaster. In this respect, yeast is different from the other materials used in the bakeshop, which can be stored for a greater period of time. Thus it is clearly seen that delivery of yeast and the service back of it is just as important as the baking strength and uniformity of the product. Therefore, from a practical commercial standpoint yeast delivery and service may well be considered as an essential quality characteristic of yeast.

At the present time, the prompt, unfailing delivery of yeast to the bakers of the United States and Canada has been developed to an extremely high state of efficiency unexcelled even by the delivery of the mails. This delivery and service has been made possible by an extremely well developed organization, operating yeast factories in about a dozen different points, as well as a huge number of agencies located throughout the United States and Canada,—thousands of men all working for a single purpose, thousands of motor cars and numerous other facilities to insure transportation and delivery,—even in the event of serious emergencies.