Yeast is the microorganism that, when incorporated into dough, generates carbon dioxide (CO₂), allowing the dough to increase in volume. To achieve this, yeast requires sugars, which it transforms into CO₂ and ethanol. While ethanol evaporates during baking, CO₂ remains trapped in the dough, giving it a spongy texture, provided that the dough has the ability to retain this gas, which, in most cases, depends on the gluten network.

The yeast used in baking belongs to the Saccharomyces cerevisiae genus. “Saccharomyces” because they transform saccharose, and “cerevisiae” because they are used for making beer, as well as bread and wine, among other products. These yeasts can only transform certain types of simple sugars, such as glucose, fructose, sucrose, and maltose. Other simple sugars, like lactose, are not fermentable, just like more complex sugars. Additionally, the speed at which they transform sugars varies in each case. Thus, glucose and fructose are transformed more rapidly than sucrose, and sucrose faster than maltose. In some countries, it is common to incorporate some sucrose to facilitate fermentation, but in others or for specific preparations, this practice is not as common. This is why manufacturers of bread yeasts often select yeast strains that are better at transforming maltose, which is the sugar generated from the action of amylases on damaged starch. We have discussed enzymes and starch in previous blog posts. However, yeast also requires other nutrients, such as vitamins, minerals, and amino acids. Fortunately, in bread doughs, yeast finds all these nutrients readily available. In other preparations, like winemaking, yeast may face challenges, and in such cases, it can be beneficial to incorporate proteins or amino acids to aid yeast activity.

Yeast’s role in baking is anaerobic (in the absence of oxygen). Nevertheless, yeast can also function in aerobic conditions (presence of oxygen). In fact, the presence of oxygen is recommended for yeast propagation and growth. Therefore, yeast production industries introduce oxygen into yeast growth tanks to facilitate this process. In food processes, an initial aerobic phase helps yeast acclimate and start functioning. In the wine industry, for instance, one technique used when fermentation stalls is to oxygenate the must. In baking, this oxygenation occurs during kneading. In gluten-free products, where kneading may be shorter due to the absence of gluten network development, it is advisable not to skimp on kneading to facilitate yeast action.

Yeast, like all microorganisms, has optimal pH and temperature ranges, and they function to a lesser extent outside these ranges. The optimal pH for baker’s yeast is slightly acidic, coinciding with the pH of most doughs. In highly acidic doughs, yeast may struggle to perform its function, resulting in a slowed fermentation. This can also occur if ingredients are added that increase the dough’s pH. Regarding the optimal temperature range, yeast usually functions best at temperatures close to 40°C. Hence, reducing the temperature slows down yeast activity, although yeast can still function at temperatures of 20°C or even lower, depending on the yeast strain. However, it is generally advisable to use fermentation temperatures somewhat lower than yeast’s optimal range, as longer fermentation times contribute to the development of flavours and flavour precursors, along with other enzymatic actions. These lower fermentation temperatures help create bread with more flavour and less tendency to harden. Yeast also continues to work at temperatures above 40°C. Thus, when dough is placed in the oven, the interior of the loaves gradually increases in temperature, and in the early stages of baking, if fermentable sugars remain, yeast continues to produce CO₂ and increase the dough’s volume. As the temperature continues to rise, the yeast dies and is no longer active in the finished product.

In yeast factories, a liquid with a high yeast concentration is obtained, which, after centrifugation, becomes a thicker and more concentrated liquid called liquid yeast or yeast cream. To facilitate handling and storage compressed yeast was developed. Compressed yeast is slightly more concentrated than yeast cream, as some of the water is removed. Nevertheless, compressed yeast also requires cold storage and has a limited shelf life, though it can be frozen. To address these drawbacks, dry yeast was developed. After some earlier iterations that needed to be prehydrated with warm water and had reduced fermentative activity, a dry yeast with good fermentative activity that dissolves easily and can be incorporated into the dough was successfully developed. Nowadays, liquid yeast or yeast cream is commonly used in large industrial facilities, as it better suits automated dosing systems, reducing labour costs. Many yeast manufacturers offer their expertise in developing projects and implementing automated dosing in large production lines. Compressed yeast is often the preferred choice in areas where yeast supply is reliable, and yeast is used in high volumes with quick turnover. For example, most bakeries in Spain have used this type of yeast. On the other hand, dry yeast is typically used in regions where yeast supply may be inconsistent. It is commonly used in places like the Canary Islands. It is also widely used in laboratories and research centres because it offers greater consistency and can be purchased in small packets.

In general, the lower the water content, the higher the yeast concentration, and the less yeast needs to be used. However, all drying processes may lead to a loss of fermentative power. Therefore, when using dry yeast, it is usually recommended to use one-third the amount of compressed yeast, although sometimes it is advisable to slightly increase the dosage.

In the market, yeast adapted to different types of preparations is available. For instance, regular yeast strains do not tolerate dough with high osmotic pressures, such as those with high sugar (and salt) content. For such doughs, osmotolerant yeast strains have been developed, better suited for sweet dough. Regular yeast strains also do not tolerate low temperatures well, especially once they have begun reproducing. For this reason, it is crucial to lower the temperature during mixing and resting stages in dough that will undergo cold processes, to prevent yeast from starting to act before cold application. Cryotolerant yeast strains have been developed for these processes. There are also yeast strains better suited for bread that incorporates propionate or propionic acid. This preservative, commonly used in the production of long-lasting bread, such as sandwich bread, is more effective against molds and bacteria but also affects yeast to some extent. Thus, using more tolerant yeast strains can be an advantage.

Traditionally, manufacturers of bread yeast have focused on yeast strains with high CO₂ production capacity and excellent adaptation for transforming maltose. In other processes, like winemaking or brewing, the emphasis has been on yeast’s ability to impart pleasant aromas. In recent years, research has increasingly concentrated on the ability of bread yeast to contribute to aromas in long fermentations. Investigations into yeast strains capable of fermenting other types of sugars are also ongoing. In reality, the industrial production of yeast is dominated by a few global companies, and research and development continue to adapt to the needs of bakers and industries, as well as changes in the sector, since the first yeast factories were established.