Yeast is a crucial microorganism in baking, responsible for fermentation. During this process, yeasts transform sugars into ethanol and carbon dioxide. The alcohol evaporates during cooking and the carbon dioxide is responsible for making the dough or bread rise, as it is retained by the gluten mesh, creating a light and airy texture. In bread making, the most used strain is Saccharomyces cerevisiae, which was initially identified in beer.
In sourdough, in addition to Saccharomyces cerevisiae, there are other naturally present yeasts, such as Pichia, Kazachstania, Torulaspora, etc. These yeasts are distinguished from commercial yeast by their greater resistance to acidity and lower fermentative power.
Understanding yeast types and how temperature affects them during the fermentation process is essential for mastering bread making.
Types of Yeast: Fresh, Dry and Instant Yeast Compared
Yeasts can be classified into three main types:
- Fresh, pressed or baker’s yeast. It has a higher water content, which can reach 70% of its weight. It is the most used by bakers. They are usually sold in small cubes and must be stored cold.
- Active dry yeast. It has around 8% humidity. This type of yeast has the disadvantage of having to be hydrated before being used. On the other hand, it does not need cold to be preserved and can last for several months.
- Instant dry yeast. It has a water content of less than 8%. It is shaped like small noodles and is easily incorporated into the dough, since it does not require prior hydration.
How Temperature Affects Yeast Activity
The temperature of the dough is an important factor that affects the activity of both yeasts and bacteria. Both have their optimal activity point between 30 and 35 ºC.
In the case of bacteria, their activity is practically zero below 10 ºC. From 20 ºC onwards, its activity increases linearly until reaching the optimal point. Above 35ºC, its activity decreases rapidly.
Yeasts exhibit behavior similar to that of bacteria. Its activity is minimal at temperatures of 3-4 ºC, remaining similar up to 10 ºC. From 20 ºC onwards, its activity increases linearly until reaching the optimal point. Above 35ºC, its activity decreases rapidly.
Benefits of Cold Fermentation in Baking
Cold fermentation is a bread-making technique that consists of fermenting the dough at a temperature of between 3 and 8 ºC. As we have seen before, the activity of the ferments is very low below 10 ºC, so cold fermentation requires long fermentation times.
Bacteria, for their part, remain active at low temperatures, but their activity is very slow. Therefore, it is recommended to apply cold fermentation after a period of time with temperatures favorable to bacterial development, especially for sourdough breads. In this way, the bacteria will be able to develop the acids that will give the bread a more complex flavor and better conservation.
Instead, the yeasts dedicate the first 12 hours of exposure to cold to the production of sugars that, during cooking, will caramelize and react with the proteins of the crust in the so-called Maillard reaction. This will result in a darker color and more complex aromas. After 12 hours of cold fermentation, the production of carbon dioxide increases, which helps to achieve a greater final volume of the bread.