Yeast - A Treatise - Section II

12-25-2001

Table of Contents

Factors Effecting Fermentation

Yeast Growth
Given the essentially anaerobic environment that exists in dough once the available oxygen is used, one would expect the primary physiological activity of yeast to be that of fermentation. However, the organism also undergoes some growth and cell multiplication during the fermentative process. For example, a test dough with a yeast content of 1.67%, based on flour, and fermented at 80° F (27° C), demonstrates no significant increase in yeast-cell count during the first two hours of fermentation with the actual rise in cell numbers being on the order of 0.003%. The most vigorous yeast growth was observed during the period between the second and fourth hours of fermentation, when the yeast cell count increased by 26%. Between the fourth and sixth hours, the rate of yeast multiplication declined again to about 9%, based on the original cell count.

Other findings indicate that the smaller the original quantity of yeast in the dough, the greater the percentage increase in cell numbers during the fermentation, with all other conditions being held constant. Thus a 0.5% yeast addition to a test dough produced an 88% increase in cell count after 6 hr of fermentation, while with a 2% original yeast level the corresponding increase in cell numbers was only 29%. This is not surprising given the fact that at the lower yeast level, the competition for nutrients is far less than at the higher yeast levels. Thus, each yeast cell has access or at least the opportunity for access to greater food supplies during fermentation.

Another study found that yeast growth in a sponge fermented for 4 hr was 56%, with only an additional 1% growth by the end of the proof period. The original yeast level of 2.25% was thus increased to 3.55% in the course of the entire fermentation. In a liquid preferment made with 3% yeast, the cell count increased by only 1% in the preferment, but by 15% in the dough. This reduced growth rate of yeast in liquid ferments accounts for the general practice of using higher original yeast levels in these doughs.

Not all work in this area is in agreement with the specific findings described above. Carlin, and then Reed, found no increase in the yeast population over a 4-hr fermentation period reporting essentially the same observations. It is rather difficult to determine the actual number of cells in a dough, it is relatively easy to establish the percentage of yeast cells that have buds. Compressed yeast will normally contain about 2 to 5% budding cells, and this number increases to about 30 to 50% by the end of the sponge fermentation, with no additional increase during dough fermentation. This increase in bud formation by the yeast cells is basically a sign of incipient yeast growth.

In the case of straight doughs, there is very little budding of yeast cells during the first three rises, but a substantial increase to about 40% during the proof period. No increase in the number of yeast cells was observed in liquid flour preferments, while budding was found in only about I8% of the cells after 3.5 hr of fermentation.

Fermentative Adaptation
When yeast is first added to the sponge or dough, it is still in a relatively dormant state. A number of studies have shown that yeast requires about 45 min in a favorable environment to attain full adaptation to fermentation, although it begins to evolve carbon dioxide and ethanol in a much shorter time. During this period of adaptation yeast exhibits a high degree of sensitivity to both favorable and unfavorable environmental influences.

Adaptation is somewhat more readily accomplished in sponge-dough than in straight-dough systems. In sponges, in which the critical yeast adaptation takes place, such yeast-inhibitory ingredients as salt, and high sugar levels are normally withheld to enhance fermentation. No such amelioration of the environment for yeast is possible with straight doughs, so that in this system, the adaptive stage of fermentation represents a more critical phase. The common practice with flour-containing preferments of withholding the salt, and the bulk of the fermentable carbohydrates for  an initial period, i.e., until after the yeast has fully adapted, is also intended to provide the yeast with an optimum growth-conducive environment.

All other factors being equal, yeast adaptation is perceptibly promoted by a plentiful supply of moisture, e.g., in slack sponges and dilute preferments. Since water serves as the indispensable medium in which the metabolic processes of yeast take place, its relative abundance significantly accelerates the rate at which these processes occur. Stiff sponges and highly concentrated preferments are usually marked by delays in full yeast adaptation.

Sugar Utilization
Yeast exhibits a variable preference for different sugars. It readily assimilates four sugars, namely, sucrose (after hydrolysis to glucose and fructose by yeast invertase or sucrase), glucose, fructose, and maltose (after hydrolysis to glucose by yeast maltase). In yeasted doughs, an increase in maltose occurs during the first stages of fermentation, until the initial supply of glucose and fructose is exhausted, after which the maltose content gradually declines. Studies of the preferential utilization of sugars by yeast are documented in the literature, but this is not a topic for this discussion.

Doughs prepared only from flour, water, yeast and salt will initially contain only about 0.5% of glucose and fructose derived from the flour. This is adequate to start fermentation and to activate the yeasts adaptive malto-zymase system that is responsible for maltose fermentation. Fermentation is sustained by the action of a- and beta-amylases of flour that convert the susceptible damaged starch granules into maltose. Damaged starch results from milling and its level is normally much higher in hard wheat flours than in soft wheat flours.

Quantitative calculations show that I g of yeast will ferment about 0.32 g of glucose per hour during a normal fermentation. The disappearance of sugars in a liquid preferment-dough system to which 8% of fermentable solids in the form of glucose and maltose was added were traced, and it has been observed that the maltose content decreased somewhat in the liquid ferment, but then increased in the dough stage as a result of amylolysis in the dough. The system used up about 3% of the fermentable carbohydrates, with the remaining 5% forming the residual sugars found in the finished bread.

Since the second stage of fermentation involves the conversion of maltose into ethanol and carbon dioxide, the behavior of this sugar in the fermentation process is of some significance. This is especially the case since different yeast strains have been shown to vary in their maltase activity. Experimental results have shown that a yeast strain with low maltase activity needed 21 min longer to produce two rises in a dough than did another, high-maltase yeast. Yeast strains also differ in their maltase activity in different doughs. A single yeast strain may also exhibit variable maltase activity under different test conditions. These observations led to the hypothesis that some constituent of flour contributes in some manner to the yeast's ability to ferment maltose. The rate of maltose fermentation by yeast also has been shown to be influenced by pH to a much greater degree than is true of glucose fermentation.

Acidification
Dough fermentation, in addition to generating alcohol and carbon dioxide, also produces small amounts of a fairly large number of organic acids. These include lactic, acetic, succinic, propionic, fumaric and pyruvic, butyric, isobutyric, valeric, isovaleric and capriotic acids. Among these, the most prevalent are acetic, propionic, butyric, isobutyric, valeric, isovaleric and capriotic. Acetic acid is the most prevalent by far. The production of acetic acid is much higher in breads made with a poolish or naturally leavened than with a straight dough. Calvel speculates that acetic acid acts as a carrier for bread crumb aroma, sensitizing the taster to the other constituents of the aroma. This effect seems to be directly linked to the amount of acetic acid in the dough

As maturation progresses and fermentation is prolonged, the dough becomes richer in organic acids, and this increase becomes evident as a lowering of its pH.  The longer fermentation is allowed to continue, the richer in organic acids the medium becomes. This formation of acids is reflected in a time-dependent decrease of pH and an increase in titratable acidity in the fermenting medium. A number of factors such as aroma, and keeping quality are enhanced as a result of the development lower pH (more acidic) dough. Temperature of the dough is an important factor. Calvel demonstrates this in the graph  shown here. (Graph 2)

While progressive pH change in naturally leavened dough is relatively rapid as can be seen, the change appears to occur more slowly in dough leavened with bakers yeast.  The presence of salt in dough often masks  acetic acid.   When the dough is leavened with an unsalted preferment, the acetic acid or vinegar odor appears a little more rapidly, although it is still hardly perceptible.

The results of these evaluations of dough pH are influenced by leavening method, and are different from one another.   The pH is  ultimately related to the level of residual sugars present in the dough before baking. Thus, a discussion of pH must, by default, include discussions of residual sugars.  These residual sugars are the remainder of those that fed dough fermentation.  They fulfill important functions during the baking process. The level at which they are present plays an important role in the quality of the final loaf of bread.  Generally, a below average pH coincides with a lack of residual sugars, which translates to a deficiency in oven-spring, i.e. loaf volume, crust coloration and crust thickness, aroma, crust taste, crumb flavor, and keeping quality.

When the dough is leavened with prefemented dough which undergoes an excess of maturation or fermentation, it is good practice to remedy the lack of residual sugar in advance by adding from 0.1% to 0.2% malt extract during mixing to reestablish the proper sugar balance.
Excessive residual sugar may also occur, although this is more rare. If this phenomenon is caused by characteristics inherent in the flour it is a difficult occurrence to correct. If excessive residual sugars occur as a result of the manner in which the dough was handled, i.e. an abnormally short first fermentation, or a lack of proper dough maturation, it is more easily corrected.

The presence of an appropriate amount of residual sugars in the dough at the time of baking is extremely important.  It insures an active oven spring, assists in dough development, and  helps the loaves to reach a normal volume.  Appropriate residual sugar levels contribute to optimal crust color, which in turn, according to Professor Calvel, contributes to the exterior appearance, the aroma and the flavor of bread. 

Lactic acid, not mentioned by Calvel,  but cited as a prevalent acid in white bread by Pyler also survives at some level in the finished bread.  The accumulation of lactic acid in fermenting dough is attributable primarily to the presence of the genus Lactobacillus in both flour and compressed yeast.   Of the two acids (lactic and acetic),  acetic acid is normally found in smaller quantities. It is also weaker than lactic acid, with a lesser degree of ionization, and its effect upon the pH of the dough is correspondingly smaller.

In sourdough breads ("San Francisco Sourdough)", acetic acid represents about 50% of the total acids found, and was five to ten times that found in white (non-sourdough) breads. The pH of fermenting dough is more strongly affected by the presence of ammonium salts in yeast foods, especially if the ammonia is present as the salt of a strong acid such as hydrochloric or sulfuric acid. Yeast readily assimilates ammonia as a nitrogen source.

Yeast Tolerance to Acidity

Yeast exhibits a considerable tolerance to extremes of pH, being able to maintain an active fermentation in a 5% glucose solution in the pH range of 2.4 to 7.4, but ceasing activity at pH 2.0 or pH 8.0. For optimum results, good practice dictates that the pH of the fermenting medium be maintained within the range of about 4.0 to 6. A drop of more than 50% in fermentative activity has been observed at pH 3.5. More gradual declines in yeast activity were encountered at higher pH levels, with measurable effects showing up at pH values over 6.0.

The explanation for the yeast's ability to maintain a relatively constant activity over a 100-fold change in hydrogen ion concentration (pH 4 to 6) is found in the fact that the pH of the cell interior of the yeast remains quite constant at about pH 5.8, regardless of any relatively wide pH variations in the fermenting medium. The enzymes involved in fermentation thus operate in an optimum pH environment within the yeast cell that is largely unaffected by external changes in pH.

Fermentation Control
A prerequisite to a controlled fermentation is a fully hydrated, homogeneous dough, such as is obtained by correct mixing. The surface appearance of a sponge as fermentation progresses usually provides a reliable indication of the adequacy of its mixing. A properly mixed sponge will exhibit good gas retention that will make it rise and assume a well-rounded top. Retention of fermentation gasses allows loaves to develop properly and result in a light, well raised loaf after baking.

The surface of an under mixed sponge, on the other hand, will remain flat, which is indicative of an incomplete incorporation of the formula ingredients and an uneven fermentation. In straight doughs, mixing plays a much more critical role as the aim here is to obtain optimal physical dough development.

When a correctly mixed sponge or dough is fermented, two sets of forces come into play: gas production and gas retention. Gas production involves primarily the biological functioning of yeast on available fermentable carbohydrates, whereas gas retention is largely a measure of the mechanical and physicochemical modifications of the colloidal structure of the dough during mixing and during the course of fermentation.

The baker must control fermentation in such manner that the forces of gas production and gas retention are in proper balance. Thus, should gas production attain its maximum rate before the dough's gas retention capacity is fully developed, then too much gas will be lost to bring about maximum aeration of the dough. On the other hand, if the gas retention capacity has peaked before gas production has reached its maximum rate, then again much of the gas is unable to perform its aerating function. Hence, the aim of fermentation control is to have gas production capacity and gas retention capacity coincide both as to rate and time. As Clark (in Pyler) has stated, "When both peaks are reached at the same time there frequently is combined in one loaf the largest volume together with the best grain, texture, crust color, and other loaf characteristics which the flour in question will produce."

In the process of developing a bread dough, changes are brought about in the physical properties of the dough. In particular the dough's ability to retain the carbon dioxide gas, which will later be generated by yeast fermentation, is improved in the process. This improvement in gas retention ability is particularly important when the dough pieces reach the oven. In the entry stages of baking, before the dough has set, yeast activity is at its greatest level, and large quantities of carbon dioxide gas are being generated and released from solution in the aqueous phase of the dough. The dough is only able to reclaim the gas formed if a gluten structure with the correct physical structure is created. The baker must coordinate the timing of the development of the gluten structure with gas production. It does little good, for example, to develop bread with high carbon dioxide release due to proper fermentation processes, but without the degree of extensibility necessary to provide good gas retention.

Can one measure gas retention and gas production? The answer is "Yes - but…" Instrumentation exists which can measure both in the same dough at the same time. It is probably not available to the vast majority of home bakers and perhaps even to most commercial bakers. It is the Chopin Rheofermentometer. This is a new instrument that simultaneously measures gas production and gas retention under realistic conditions. A piece of dough is placed in a sealed chamber under a weighted piston. As the dough rises piston movements is measured to determine the rate of expansion and the dough strength. At the same time, total gas production by yeast is measured along with the amount that escapes from the dough into the chamber. Subtracting the amount released from the total gives the amount retained. All of this is controlled by a microchip that calculates the results and produces a graph depicting "development of the Dough" and "Gaseous Release". A retention coefficient is calculated by dividing the retained volume by the total volume. (Lallemand.)

Most of the desirable changes resulting from 'optimum' dough development, whatever the breadmaking process, are related to the ability of the dough to retain gas bubbles (air) and permit the uniform expansion of the dough piece under the influence of carbon dioxide gas from yeast fermentation during proof and baking.

Gas production refers to the generation of carbon dioxide gas as a natural consequence of yeast fermentation. Provided the yeast cells in the dough remain viable (alive) and sufficient substrate (food) for the yeast is available, then gas production will continue, but expansion of the dough can only occur if that carbon dioxide gas is retained in the dough. Not all of the gas generated during the processing, proof and baking will be retained within the dough before it finally sets in the oven.

What factors effect gas production and retention? These include the following that would seem to be of interest to most home bakers.

Most flours possessing adequate baking properties pass through a stage in the course of fermentation during which gas production and gas retention are in optimum balance. The time range over which this is true may properly be designated as the flour's fermentation tolerance. Since fermentation is subject to many influences that affect its course, it is evident that one and the same flour may have rather limited fermentation tolerance under one set of conditions, and good tolerance under a different set of conditions. (See The Flour Treatise.)

Sponge Doughs
Sponge doughs generally are set to ferment at temperatures of 74 to 78°F (23 to 26°C), the selected temperature depending on bread making environment. It is usually more desirable to work with cool sponges and adequate levels of yeast. With approximately 2% of yeast, fermentation in a properly formulated sponge will normally proceed quite vigorously. Full maturation of the sponge will then be reached within 3 to 4.5 hr. Fermentation involves exothermic reactions that result in a temperature increase in the dough mass. The rise in sponge temperature should not exceed 10°F (6°C) over the entire fermentation.

In actual practice, sponge fermentation times may vary from 2.5 to 6 hr and greater. Variations of relatively wide magnitude have only a nominal effect on final bread quality as long as the minimum fermentation time exceeds 3 hr. For determining the optimum length of time required by the sponge to reach proper maturity, the so-called "drop" or "break" represents a useful point of reference. Normally, a sponge will expand to about four to five times its original size and then recede in volume. This decrease in volume, referred to as the drop or break, is quite noticeable and is taken as the point from which the additional fermentation time is calculated. Depending on whether young or old sponges are desired, the drop is taken as representing the completion of 70 to 66% of the total sponge fermentation, respectively, and the sponge is then given the additional fermentation time. Generally, well-matured flours perform better with younger sponges and in this case the post-drop time is reduced to 30%. For example, if a sponge made from a fully matured flour required 3 hr to arrive at the break, it would then be permitted to stand for an additional 54 minutes The total sponge fermentation time would thus be 3 hr and 54 minutes, or about 4 hr.

The fully fermented sponge is then returned to the mixer and mixed into the final dough which then receives additional fermentation for a relatively short time. The dough will be fully matured when it has developed shortness to a sharp pull and a rather dry feel to the touch. This stage is normally reached after a floor time of 20 to 45 min under average conditions. Warmer ambient temperatures reduce the floor time and may eliminate it altogether, while cooler temperatures tend to lengthen it.

Straight Doughs
Straight doughs are normally set at slightly higher temperatures than are sponges, i.e., within a range of 77 to 79°F (25 to 26°C). The accelerating effect of the higher temperatures is desirable in this case as straight doughs contain all of the dough ingredients, some of which, such as milk solids and salt, have a retarding effect on yeast action. Dough fermentation, as a rule, proceeds at a somewhat slower rate than does sponge fermentation; hence, straight doughs take longer to reach maturity than do sponges. However, the combined time of the sponge and the sponge-dough fermentations normally exceeds that of straight dough fermentation alone.

Straight doughs differ from sponges not only in their fermentation rates, but also in their handling during fermentation. The general practice is to leave sponges undisturbed until they are ready for the return to the mixer. In contrast to this, straight doughs receive periodic punching or turning, during which a good portion of the generated carbon dioxide gas is expelled, thereby reducing the dough volume.

While the actual punching or vigorous kneading of the dough is still practiced in many bakeries, the recommended procedure is to more gently turn and fold the sides of the dough well into the center. Vigorous kneading, when well-matured flours are used, has a tendency to produce bucky doughs that will subsequently create difficulties in makeup. Folding the dough, on the other hand, avoids this problem. Moreover, this method of dough manipulation assures a more uniform fermentation by equalizing the temperature throughout the dough, minimizes a possible retarding effect by excessive carbon dioxide gas accumulation within the dough, introduces atmospheric oxygen with its stimulating effect on yeast activity, and increases the gas-retaining capacity of the dough by promoting the mechanical development of its gluten through the stretching and folding action involved in this process.

This last effect appears to be of primary significance. Gas production is not constant during fermentation, but rises at first to its maximum rate and then declines. The increase in dough volume corresponds to gas production during the first hour of fermentation only. Thereafter, there is a marked decline in the rate at which dough volume increases. A dough that is permitted to go through fermentation without folding or punching will lose a considerable amount of carbon dioxide. However, if the dough is turned and folded at the right time, its gas retention properties are improved sufficiently to prevent a significant loss of gas. Under practical conditions, the rate of dough expansion is again accelerated by then folding or punch back, and this has led to the conclusion that there has been a corresponding increase in the fermentation rate. The beneficial effects of punching or folding result essentially from the improvement in the dough's gas retention properties.

The correct time at which the dough should first be turned is usually established by the simple expediency of inserting the hand into the dough, withdrawing it quickly, and observing the dough's behavior. If the dough reshapes itself, i.e., shows only a very slight recession or indentation, it is ready to be turned and folded. This point is usually taken as the 60% completion mark of the total fermentation time. The dough is then turned again after one-half this initial time, which thus represents another 30% of the total fermentation time. During the remaining 10%, the dough is sent to the divider.

The above procedure is merely indicative of general practice and must be adapted to different conditions. For example, the quality of the flour plays an important role in determining actual fermentation times. Well-matured flours normally require a shorter fermentation and less frequent punching or folding than do so-called "green" or immature flours. The fermentation time may be shortened by the simple expediency of having the first punch represent either two-thirds or even three-fourths of the total fermentation, and omitting the second punch. This procedure will yield "young" doughs. "Old" doughs, on the other hand, are obtained by having the time to the first punch represent a lesser proportion of the total fermentation. The dough will receive a series of periodic turnings or punches during this period. This practice is normally followed with strong flours of high protein content, or with lower grade flours of longer extraction. Such flours may need four or five punches; there is the risk, however, that this may give rise to bucky doughs. Slight overmixing of the doughs or increasing the absorption somewhat will ameliorate this condition. A slight increase in the dough temperature will also act to accelerate fermentation and reduce the total time.

Adjustments in Fermentation Time
Optimum fermentation time represents that point at which the effects of interacting factors such as character of flour, yeast level, temperature, formula ingredients, degree of oxidation, etc., are in balance. Once practical experience has established the most suitable procedure for processing a given type of flour, it is generally closely adhered to in the interest of uniformity. Occasions may arise, however, when it becomes necessary to either shorten or extend the established fermentation time. To meet such exigencies, certain rules have evolved concerning changes in yeast quantity and temperature that work reasonably well, but should always be regarded only as temporary expedients. This is to say that any major deviation from an accepted procedure that has yielded good results will usually result in some loss of quality. Hence, while it is possible to shorten or lengthen the fermentation time by certain adjustments in yeast and temperature, the final product will usually not meet optimum quality standards.

There is an inverse relation between the amount of yeast and fermentation time. Thus, a reduction in the amount of yeast will result in longer fermentation times, while an increase in the amount of yeast will shorten them.  A generally accepted rule is that a I°F (0.5°C) change in dough temperature will cause a 15 minute variation in straight-dough fermentation time. Hence, a dough that comes out of the mixer I°F. warmer than normal will require about 15 minutes less fermentation under average conditions, and vice versa. Here again, practical considerations impose limits on the extent to which fermentation time may be altered; about 45 min appears to be the maximum when no other changes are involved.

Engineered & Changing Yeasts
Can yeasts be improved? Most likely work will continue on this process for as long as there are chemists, and geneticists interested in yeasts. One of the more interesting new research areas in this domain is the work on recombinant-DNA technology as it pertains to the development of newer yeast strains. This work has led to changes in formulation, ingredients and processing conditions. Some of this work has led to new strains of yeast which are more resistant to stress, produce more proteins, and more carbon dioxide. Some of the goals of this work are to increase shelf life, dough Rheology and flavor (Randez-Gil et al)

Cauvain and Young describe the changes in yeast pre and post the 1960's. They discovered that in their bread baking processes, the early yeast peaked too soon, and the resultant oven spring was much less than desired. Later yeasts were able to provide the desired gassing power at the time needed in their baking. For example, at the two percent yeast level, pre-sixties gas production from yeast activity peaked at between 70and 80 minutes, decreased between 90 and 100 minutes, then increased again until 200 minutes, finally ending at approximately 10 millimoles of carbon dioxide. Contrary to this, post-sixties yeasts, used at the same 2% level provided a smooth increase in gas production all along the time axis, peaking at 25 millimoles of gas in 140 minutes. This work was in England, and may not seem relevant to bakers in the United States. We do feel, however, that the information provided by the work there, is important to know in order to maintain a more complete picture of the effects of yeast - as well as many other ingredients - on baking.

General Considerations
Yeast produced for different needs may be single strain, hybrid, or mixed strains and propagation profiles. According to Lallemand, North American yeast is optimized for a compromise between lean and sweet dough. When compared with other countries, US and Canadian bakers prefer compressed yeast that is light in color, dry to the touch, and friable (easy to crumble). Artisan bakers in the US and Canada tend to process doughs at temperature of 75 to 90 F, as do artisan bakers in Europe. There is widespread availability of compressed and instant active dry yeast in North America, with a trend toward increasing use of instant active dry yeast.

Also, as presented by Lallemand, Lean dough requires yeast with high maltase enzyme activity, because maltose sugar from flour is the primary energy source. Also important, is the enzyme maltose permease which transports maltose into the yeast cell. Once the maltose is in the yeast cell, the maltase is able to cleave the maltose molecule into two glucose molecules. Straight dough works best with fast yeast that adapts quickly to give good oven spring. Sponge and dough methods work best with slower yeast that retains sufficient activity for the final proof. Fast strain yeast dosages customarily used for straight dough can be reduced for use in sponge and dough methods.

Bibliography & World Wide Web Links

Bibliography
Calvel, R., Wurtz, R. L. & MacGuire,J. J. , "The Taste of Bread",  Aspen, MD, 2001
Cauvain, S. P. and Young, L., "Technology of Breadmaking". Blackie, Academic & Professional,  London, 1998
Corriher, S. "Yeasts Crucial Role in Breadbaking", Fine Cooking, #43, pp 80-81, 2001
Giorilli, P., Lauri, S. "Il Pane: Un'arte una Tecnologia", Zanichelli, Franco Lucisano Editore, Milano, 1996
McGee, H. "On Food and Cooking" , NY., Collins, 1988.
Lallemand Baking Update, Vol 1, # 3, "Yeast Characteristics"
Lallemand Baking Update, Vol 1, # 4, "Yeast Dosage" 
Lallemand Baking Update, Vol 1, # 6, "Dry yeast"
Lallemand Baking Update, Vol 1, # 9 , "Yeast Production"
Lallemand Baking Update, Vol 2, # 9,  "Instant Yeast"
Personal Communication, Fleischmann Yeast Company
Personal Communication, Lesaffre/Red Star Yeast Company
Pyler, E. J.  "Baking Science &Technology" , Vols. 1 & 2, Sosland Pub.,  Kansas city, MO. 1988
Randez-Gil, F., Sanz, P., Prieto, J. A. "Engineering baker's Yeast: Room for Improvement", Tribtech, June, 199, Vol. 17, http://147.46.94.112/journal/sej/full/t0606-170608.pdf
Rosada, Didier,  "The Role of Fermentation in the Baking Process", Bread Lines, The Bread Bakers Guild of America, Vol. 6, Issue 2, Spring, 1998, pp8-9
Schunemann C. & True G., "Baking: The Art and Science", Baker Tech, Alberta, Canada, 1984
 World Wide Web Links
Fleischmann:  http://www.breadworld.com
Lallemand:     http://www.lallemand.com/BakerYeastNA/newsletter.shtm
Lesaffre:         http://www.lesaffre.com/default_eng.asp
Red Star:        http://www.redstaryeast.com 

Link to "Yeast - A Treatise" Section I

Last Updated on: 12/25/2001 11:31:30 PM