Technical Talk – Aug/Sept 2006

Understanding Yeast

This column is written by Dr. John Michaelides of the Guelph Food Technology Centre.

Question: How does the yeast function in baked goods, and what do I need to know about it to get the best results for my yeast-raised products?

Answer: Yeast belongs to the group of living things called fungi. The scientific name of the yeast is Saccharomyces cerevisiae, which includes all the varieties of bakers yeast. We normally buy yeast in the form of active dried, bulk or compressed type. All these types are basically manufactured the same way and they belong to the same species. The only difference is the final step of the manufacture, such as the drying, coating, and packaging, is slightly different. When they are used in the baking process, the different types may require slightly different amounts or need to be treated slightly differently. In addition, different strains of yeast (variations of the same species Saccharomyces cerevisiae) can exist and are commercially available from the manufacturers of yeast. Such strains will function better in different products such as frozen doughs, baked goods with low amounts of fermentable carbohydrates, high fibre, etc.

The basic, most important action of yeast in the baking process is the production of gas in the form of carbon dioxide (CO2), which helps in the rise of the dough and the formation of the structure that gives the baked goods their characteristic volume and shape. The production of CO2 is based on the ability of the yeast to ferment both the natural sugars found in the flour and the extra sugars which are added to the dough.

The sugar (sucrose) we normally add to the flour to produce yeast-raised baked goods is composed of two simple sugar molecules: glucose and fructose. The yeast is not capable of absorbing the sucrose into its cell in order to ferment it. For this reason, it secretes an enzyme, invertase, into the dough and, almost instantly, it hydrolyzes or inverts the sucrose into its components glucose and fructose. The yeast ferments glucose and fructose at the same rate when they are present separately. However, when they are present together, such as is the case when the sucrose is inverted, the glucose is fermented first and, when it is exhausted, the fructose fermentation follows. Studies have shown that during the normal fermentation process one gram of yeast will ferment 0.32 grams of glucose (and fructose) per hour. Other sugars will be fermented by the yeast at different rates. This characteristic activity of yeast plays a very important role in the selection of the type of sugar to be used in the different baked goods formulations. In turn, this may have an effect on the large-scale commercial production of such products.

The fermentation process converts sugars to alcohol, carbon dioxide, and other organic acids. During the fermentation, 100 grams of glucose or fructose will yield 48.6 grams of alcohol, 46.4 grams of carbon dioxide and 5 grams of other organic acids. The 46.4 grams of carbon dioxide represent a substantial volume of gas which will provide the necessary expansion of dough to give the rise and to achieve the proper volume of the baked goods. During the fermentation process, which normally occurs under the anaerobic environment of the dough, most of the yeast’s efforts are directed towards the production of the alcohol and gas, and very little, if any, growth and multiplication of the yeast takes place. When yeast is exposed to plenty of oxygen, favourable environment and proper food, growth and multiplication become the predominant activity. This can be demonstrated very easily when you consider the visible growth of yeast and moulds on the surface of spoiled baked goods (exposure to air and, therefore, oxygen). Here, most of the energy is converted into growth and multiplication of the yeast.

The yeast is a living organism and, in order to function properly, requires a favourable environment, an adequate supply of nutrients, and an appropriate energy source.

In the environment, it first requires the proper moisture level. The temperature of the dough is also very important for the yeast functionality. A range from 27 C to 35 C is normally necessary. The yeast also requires the proper pH, or acidity level. It can function in a wide range of pH from 2.4 to 7.4, but in order to optimize its activity it is a good practice to maintain the pH of the dough in the range of 4.0 to 6.0.
It is also necessary to have an ample supply of available food for the yeast in the form of fermentable carbohydrates (sugars) in order to achieve proper fermentation and production of the proper amounts of carbon dioxide. In addition, a proper nitrogen source is required for the function of the yeast cells. Salt also plays a very important role in the function of yeast in the dough as a certain amount of salt is required to control the fermentation and the growth of yeast. A salt content of over 1.5 per cent begins to inhibit the yeast activity due to its chemical action as well as changes in the osmotic pressure in the surrounding environment. All of the conditions mentioned above are required for the function of yeast in the dough in order to have the proper fermentation.

During the fermentation process, the yeast itself also changes the environment of the dough. It depletes the fermentable carbohydrates, which are used as substrate for the fermentation process. As a result, various waste products accumulate, such as alcohol and carbon dioxide, as well as various organic acids. The alcohol completely evaporates during the process of mixing, proofing and baking but it leaves its mark: in combination with the organic acids, the alcohol contributes to the characteristic aroma of the freshly baked goods. The organic acids and other compounds produced, such as esters, will add to the flavour of the baked goods. The yeast activity will also alter the pH of the dough. Furthermore, the yeast has an effect on the gluten, softening or mellowing the gluten and thus modifiying its functionality.

Yeasts are complex living organisms that have been utilized by humans for thousands of years to produce fermented foods and beverages. We now know a lot more about yeasts since the ancient discovery of bread, beer and wine making. Many types and strains of yeasts are now available to assist us with the vast variety and complex processes of mass production of baked goods. The various yeast manufacturers can provide you with the different types of yeasts that can best perform in your product.

For more information, or fee-for-service help with product or process development needs please contact the GFTC at 519-821-1246, by fax at 519-836-1281, or by e-mail at gftc@gftc.ca