Saturday, 31 August 2013

The Long and Short of Fermentation.

Understanding bread fermentation can shed some light on what is needed for a gluten free dough. The reasons for mimicking glutens properties to create a dough that will trap the gases produced during fermentation and using flours that provide sugars and protein for yeast to eat, are very important when it comes to fermentation of gluten free doughs.

Fermentation, also referred to as the first rise, is the process whereby the yeast grows and produces carbon dioxide, alcohol and other compounds which enable dough to rise and modify its physical properties. The yeast digests the sugars in the flour producing alcohol and carbon dioxide (CO2). The carbon dioxide gets trapped in the pockets that result from the kneading process and causes the bread to expand or rise and develop flavor.

When the fermentation is correctly achieved, depending on the quality of flour among other things, the baker will obtain the proper external and internal characteristics (grain and texture) suitable for a determined finished product.

Fermentation begins as soon as the yeast is in contact with the mixture of water and flour. During the first fermentation (bulk fermentation), the baker lets the dough rise for a first time. During that stage, the physical properties of the dough (extensibility and elasticity) are modified, thus completing the kneading action.
This is an important stage on which depends the final quality of the bread: external and internal characteristics, taste and aroma.

Fermentation is one of the critical and essential steps in bread baking. It is through the various complex biochemical reactions that are caused by the yeast cells that fermentation achieves the following goals for the baker:
  • 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:
  • Fermentation time - 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. 
  • Fermentation temperature - 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.
  • Specific ingredients 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, salt and mold-inhibitor - It is well known that yeast fermentation is retarded in 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.
    • Dough pH - 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. 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.

Bread recipes specify the length of fermentation and how much the dough should rise. Choices for raising are:
  1. Short Rest Period - For bread recipes using instant active dry yeast: The first rise is usually a short rest period, taking about 10 minutes, and afterwards, it is divided and shaped. The dough does utilize a final fermentation period after being shaped. However, even with recipes using instant active dry yeast, a longer overnight refrigerator rise is used to develop its irregular air bubbles in its crumb, such as baguettes. A final fermentation period follows its shaping stage.
  2. Rise until (ALMOST) doubled - For bread recipes using active dry or fresh yeast, requires two rises, a first one after mixing, and a second one after shaping: Allow the dough to rise until ALMOST double in volume or bulk, although bread dough, heavy with bran and grains, will not rise much. This is tested when a fingertip is pressed into the top of the dough; fully fermented dough will retain the impression for at least 5 minutes and won’t spring back. It’s because the gluten has been stretched to the limit of its elasticity you have developed in the dough. 
  3. Multiple rising periods: Some recipes require multiple rising periods, such as a punch down and a second rise, such as in long-fermentation lean dough. As long as the dough stays in bulk(prior to dividing into smaller units), it is still in primary fermentation regardless of how many times it is punched down.
Fermentation is what gives bread flavor. But more than flavor, affects crust (thickness and color), crumb (texture), and shelf life. Good bread takes time!

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