The typical flavour from small bakeries is mainly due to the fermentation process that attracts people towards bakeries. The flavour is very pleasant and appetising. Fermentation is brought about by the action of minute forms of unicellular microorganisms which are called yeast. It is a process whereby yeast organisms feed on sugar and produce carbon dioxide gas and alcohol. Carbon dioxide gas raises the dough fabric, part of alcohol evaporates. Part is converted into acetic acid and the remaining contributes to the characteristic flavour of bread. The dough ferments more rapidly. The most favourable temperature for the action of yeast in bread dough is from 750F to 850F or about 240C to 300C. But in order to ensure uniform results in the fermentation of dough, it is necessary to hold the temperatures, and allow them to ferment in a place where the temperature is constant.
Apart from gas and alcohol, some quantities of glycerin, lactic acid, acetic acid and succinic acid are also produced. These acids and protease enzyme together have a mellowing effect on the gluten thereby imparting extensibility to it.
Goals of Yeast Fermentation
Fermentation for the baker helps to
- Improve dough handling characteristics
The various complex reactions during fermentation produce a range of intermediate compounds. These fermentation byproducts 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. - Enhances gas retention in dough
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. - Enhances finished product texture
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. Meanwhile, proper fermentation provides a resilient crumb, which is also soft and smooth to touch. - Provides desirable fermentation flavour
The fermentation process generates many volatile and non-volatile flavour precursors that create the unique fermentation flavour. - Extends shelf-life 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 starch during the long fermentation process causes the shelf-life extension.
Factors Affecting Yeast Activity and Rate of Fermentation
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. 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.
- Specific Ingredients in Dough Formulation
Level of water: Generally, stiffer dough takes longer to ferment as compared to slacker one. 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 salt: 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.
- 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 flavour 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 dough containing high levels of salt or sugar by increasing the amount of yeast used.
- Dough pH
The pH of dough or pre-ferments 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.
While preparing the yeast products it is important to select good quality raw materials, weigh the materials correctly. The baker should make sure that the yeast is fresh. It is also important to use only right quantity of salt. Care should be taken not to mix salt and yeast, add fat or egg at the beginning stage because it will control the Water Absorption Power (WAP) and will affect the yield. There is need to knead the flour thoroughly to get an elastic dough. There is need to use the correct quantity of liquid and maintain the dough’s consistency and temperature. Water should be used according to the climate condition. There is need to ensure that chilled water is used in summer and warm water during the winter months.
The dough should be kept warm and moist, and covered to avoid crust forming on the surface.
It is important to weigh the dough correctly and use clean, greased and correct size mould. When the product is arranged in a tray, there is need to sufficient space to proof. Moreover care needs to be taken not to proof in a dry or high heat. It is also not advisable to overproof or underproof as it will affect the finished product quality. There is need to give an egg wash or oil wash whenever necessary. Baking should be done at the correct temperature and time. The product should be immediately de-moulded and allowed to cool. After cooling, it should be packed immediately to prevent external contamination.
Various fermented products like types of Bread, Buns, Bread Rolls, Doughnuts, Rusk, Pav, Biscuits, Pizza, Croissants, Danish Pastry, and Mexican Roll can be prepared with fermentation.
