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.


Last Edited on: 12/25/2001 11:31:03 PM