Fermentation is one
of the critical and essential steps in bread baking. It is through the various
complex biochemical reactions that are caused bythe yeast cells that fermentation
achieves the following goals for the
Improves dough handling characteristics
Enhances gas retention in doughs
Enhances finished product texture
Provides desirable fermentation flavor
Extends shelf-life of final product
Critical as it is,
merely having a fermentation step does not automatically guarantee desirable
attributes in the final product. For that, proper fermentation control and
consistency is key. Given that fermentation is caused by yeast, a living cell,
the controls that need to be put in place have to be effective in influencing
the environmental factors that regulate yeast activity in that dough. It is
also worthwhile to point out that in a dough yeast is active from the point it
is mixed with the other dough ingredients in a mixer until the point it is
inactivated in the oven. The various factors that affect yeast activity and the
degree of fermentation in the baking process are:
Specific ingredients in dough formulation:
Level of water, sugar, salt and mold-inhibitor
If dough formulas remain unchanged, the factors that would have the major impact on fermentation are time and temperature. While both these factors are fairly difficult to control in an actual bakery process, many bakeries have developed effective ways to continuously monitor them at each affected process point.
One of the most effective ways of continuously monitoring process time and dough temperature is through the use of process control charts. The chart provides a basis for deciding if the variation in output is due to common causes or those out-of-control. The charts provide an excellent visual technique to guide line operators to make the necessary changes if they see their recordings shift significantly from the desired levels.
Yeast acts as a biocatalyst in the complex reactions that take place during fermentation. It is of critical importance to monitor and control its activity in doughs to ensure consistent quality of finished products.
Fermentation achieves the following goals for the baker:
The various complex reactions during fermentation produce a range of intermediate compounds. These fermentation by-products soften the dough protein structure, gluten. Long fermentation times allow for complete hydration of the gluten proteins, which also aids in its softening. The softened protein matrix allows for improved dough machinability and handling.
retention in doughs
As a direct consequence of gluten softening, the dough protein matrix is conditioned to hold more of the carbon dioxide produced by the yeast during fermentation and proofing.
Crumb texture of properly fermented bread can be appreciated the most when one compares it to that of under-fermented bread. The latter tends to have a “young” look where the crumb cell walls are thick and coarse, and are irregular in size. On the other hand, proper fermentation provides a resilient crumb, which is also soft and smooth to touch.
The fermentation process generates many volatile and non-volatile flavor precursors that create the unique fermentation flavor.
of final product
Breads that have gone through a proper fermentation process have a better shelf life than those that have not. While gluten modification definitely aids in this respect, it is possible that the action of amylases on broken starchduring the long fermentation process causes the shelf-life extension.
Yeast activity in dough is not just limited to the fermentation step or the proofing step; rather, yeast is activated right from the time it is mixed with flour, water and the other ingredients. Irrespective of whether the mixture is a sponge, a brew or a straight-dough, yeast activity does not cease until it is inactivated during the final baking process.
The various factors that affect yeast activity and the degree of fermentation in the baking process are:
This factor determines the amount of time yeast gets to act on the sugars present in the ferment, whether it be a sponge, brew, or a straight-dough. While the rate of fermentation declines with time at a constant temperature, it does not completely stop. However, the longer the fermentation time, the higher the degree of fermentation. Bakeries using flour-brew systems that chill the brew after fermentation have to be careful about this factor. Often times, depending on the available refrigeration, chilling times vary significantly. This in turn causes real fermentation time to vary as well, for yeast will retain a significant amount of activity until the brew is chilled completely.
Like any other living cell, the various enzymatic activities of the yeast cell are closely tied to the temperature of the environment. Therefore, higher ferment temperatures increase yeast activity, and vice-versa. Published literature indicates that within the range of temperatures in which yeast is operative, every one degree rise in temperature increases the rate of yeast fermentation by 3-5%. Likewise, a decrease of 1°F will cause a similar decrease in the rate of fermentation. The temperature range for optimum yeast fermentation is between 75°F-85°F. The process of fermentation also generates heat, and its measure is often used by bakeries as an effective way to monitor the degree of fermentation.
in dough formulation:
Level of water:
Generally, stiffer doughs take longer to ferment as compared to slacker ones. With additional water, the soluble solids are diluted and the osmotic pressure on the yeast cells is reduced. This causes an increase in yeast activity and the overall rate of fermentation.
Level of sugar and
It is well known that yeast fermentation is retardedin the presence of high concentrations of sugar and salt. This inhibitory effect is related to the high osmotic pressure gradient created outside of the yeast cells due to high concentrations of sugar and/or salt in dough. A measurable decline in fermentation rate is observed if the concentration of sugar exceeds 5%. This effect is more pronounced with sucrose, glucose, and fructose than with maltose.
When very little or no sugar is added, as in the case of French or Italian bread formulations, the primary source of fermentable sugars is derived from the flour. Flour contains approximately 0.5 - 1% of a combination of sucrose, glucose, and fructose, which are generally fermented within 1 - 1.5 hours. Yeast turns to maltose for CO2 production after these preferred sugars are exhausted. Once that happens, the rate of fermentation is limited by the amount of maltose being hydrolyzed (broken down) in the dough. The availability of maltose is directly related to the damaged starch content and amylase activity of the flour. Maltose is a disaccharide and is not broken down into its constituent glucose molecules until it is absorbed into the yeast cell. Therefore, it exerts a lower osmotic pressure than the monosaccharides and the readily hydrolyzed sucrose.
Salt also inhibits
yeast activity at levels above 1%. The normal usage of salt in most breads
range between 1.75-2.25% to obtain desired flavor of the product. In fact, some
bakers add higher levels of salt as a means of fermentation control.
Satisfactory fermentation rates can usually be achieved in doughs containing
high levels of salt or sugar by increasing the amount of yeast used.
The pH of doughs or preferments has little effect on yeast fermentation, unless it drops below 4.0. In general, data shows that yeast activity is fairly constant over a pH range of 4-6, which represents a 100-fold change in acidity. At the onset of fermentation, dough pH is approximately 5.5-5.8. However, during the course of fermentation, it decreases to 4.9-5.1, due to the production of carbonic acid (CO2 dissolved in water) and other organic acids. This pH drop is resisted by the buffering action of several dough ingredients. Both flour and milk are excellent buffers and help to maintain the pH range for optimum fermentation. Bakeries that use water brews add chemical buffers, such as calcium carbonate, to maintain a pH range of 4-6 during fermentation.
The reason why yeast is tolerant within the broad dough-pH range, is that the pH within the yeast cell remains quite constant at about 5.8, regardless of the pH variations in the dough. Since the various enzymes involved in yeast metabolism of sugars are located within the yeast cell, the gassing activity is relatively unaffected by external changes in pH.
One of the most effective ways of continuously monitoring process time and dough temperature is through the use of process control charts. The chart provides a basis for deciding if the variation in output is due to common causes or those out-of-control. If an out-of-control situation is detected, adjustments and/or corrective action could be taken to fix it. Thus, line staff in a bakery can use one chart per shift to record process times and dough temperatures at each of the critical processes - sponge-mixing, fermentation, dough-mixing, and proofing. These charts provide an excellent visual technique to guide line operators to make the necessary changes if they see their recordings shift significantly from the normal desired levels.
Its basic layout is provided below:
The centerline of
the chart corresponds to the mean of the process when the process is in
control. The two lines labeled UCL and LCL are important in determining if the
process is in-control or out-of-control. The values for the upper and lower
limit can be estimated through simple statistical analyses of existing data on
process time and dough temperature for any specific process. The chart can also
be divided into three zones as indicated above. Process adjustments could then
be made on basis of the following observations:
Two points, out of three successive points, on the same side of the centerline in Zone A or beyond
Four points, out of five successive points, on the same side of the centerline in Zone B or beyond
Nine successive points on the same side of the centerline
Six consecutive points increasing or decreasing
Fourteen points in a row alternating up and down
Fifteen points in a row within Zone C
Please call us at Minn-Dak Yeast Sales and Service if you need any assistance in setting up these control charts.