Fermentation is the process by which certain microorganisms modify a substrate, primarily in a positive manner. In the case of bread, we often refer to fermentation when discussing the processes by which yeast produces CO2 and alcohol from sugars. However, fermentation also includes the action of lactic and acetic bacteria when creating sourdough. Previous blog entries have covered yeast and sourdough. Today, we will focus on the changes that the dough undergoes and the factors to control in these processes.


When we make dough (kneading), fermentation is already underway. Microorganisms exist in the ingredients (at least water and flour), surfaces, and the environment, and they act on the nutrients present in the dough. Most of these microorganisms feed on sugars but also require other nutrients such as amino acids, vitamins, or minerals. While fermentable sugars in the dough are initially scarce, certain enzymes present in the dough act on damaged starch to produce maltose (and glucose in smaller proportions), which are fermentable sugars.

If no microorganisms are added, the existing microflora at the beginning of the process will perform its function, depending on the environmental conditions. There are both aerobic and anaerobic microorganisms. Although the dough-making conditions are aerobic (presence of oxygen), oxygen is consumed during the initial resting period, leading to anaerobic fermentation (in the absence of oxygen). Temperature is crucial, as not all microorganisms act at the same temperature. Elevated temperatures can favour the growth of butyric bacteria, producing unpleasant flavours. When making sourdough without adding other microorganisms, lactic acid bacteria usually dominate the environment, along with acetic bacteria to a lesser extent. These bacteria act by producing lactic and acetic acid, acidifying the dough and generating precursors of aromatic compounds that contribute to the flavour and aroma of the bread. During this fermentation, enzymes in the dough also play a role, partially hydrolysing dough components depending on temperature and ph.

For these types of fermentations, it’s also possible to introduce specific microorganisms. For instance, to mimic natural sourdough, lactic acid bacteria can be added. The advantage is that these bacteria are already selected, outcompeting other microflora and acting more rapidly. However, the results are slightly different due to the greater microbial complexity in natural sourdough, and shorter times may reduce other reactions occurring in the dough components, such as enzymatic reactions.

There is also the possibility of incorporating certain microorganisms with specific functionalities. A lot of research is being done on this subject, and microorganisms are being analysed that break down gluten proteins and make it possible for coeliacs to consume these products. But by breaking down the gluten, they also hinder subsequent operations and the quality of the breads obtained (there is no magic). Research is also being carried out into other microorganisms that may bring other nutritional or sensory advantages.

Fermentation with Yeast

In this entry, we will focus more on fermentation caused by yeast. Yeast can convert fermentable sugars into alcohol and CO2. If not added to the dough, naturally present yeast from flour, water, and the environment has to compete with other microorganisms, and its function is not usually prioritized. Therefore, gas generation within the dough pieces tends to be slower, both due to competition with other microorganisms and the lower yeast quantity. However, in most bread-making processes, commercial yeast is commonly added, accelerating the fermentation by yeast, the processes of CO2 and alcohol production, while reducing fermentations by other microorganisms.

This effect depends on the yeast dosage and the process conditions. It is common to use a portion of dough that has evolved without the addition of yeast, known as sourdough, and ferment with added yeast only at the end of the process when combining all ingredients. It is also possible to make bread without any yeast addition, but these will be more acidic with less volume, as the actions of lactic and acetic bacteria will be more significant, and those of yeast will be less. In this case, it is very important to control the conditions of the medium so that the presence of acetic acid is not too high and lactic acid dominates, which is much more organoleptically pleasant. In the same way, it is possible to make a bread in which the yeasts are added from the beginning and the actions of the acid lactic acid bacteria are minimised. These breads tend to have a higher pH, are less aromatic and harden more quickly, but this effect will depend very much on the fermentation time.

Apart from sourdough production, fermentation, especially yeast-dominant fermentation, can occur at different stages: bulk fermentation (after kneading), after dividing the dough, and after shaping the pieces. Although sugar transformation into CO2 and alcohol occurs in all cases, the goals of these fermentations differ. Bulk fermentations are typical for highly hydrated dough. These doughs are very sticky and difficult to work with. The fermentation, due in part to CO2 generation and a slight acidification of the environment, strengthens the dough and improves its subsequent handling. In addition, depending on the ambient humidity, a slight drying of the surface may occur, which may also help in handling, depending on how it is to be processed. Resting the dough in a ball shape is common during fermentation processes, aiming to modify the dough’s rheology, making it more manageable and extensible. Like all fermentation processes, these changes depend on yeast quantity, temperature, and time, which should be controlled. Humidity conditions should also be considered, as we will see later.

It is crucial to note that fermenting dough alters its rheology and density, affecting various processes. For instance, many dough dividers place the dough in a hopper, and these machines typically function volumetrically, obtaining equal volumes rather than equal weights. Assuming the dough has the same density, for the same volume an equal weight is achieved. However, if dough density varies due to fermentation, weights will differ. Dough rheology also affects dough slipping, where stickier dough can slide poorly, leading to heterogeneous piece weights. Thus, it is advisable to regulate parameters affecting fermentation, such as ambient temperature, yeast dosage, elapsed time (minimizing if possible), etc. This control is especially crucial with higher yeast doses or higher ambient temperatures.

The final fermentation, occurring once the piece is shaped, primarily aims to give volume to the bread. It generates CO2, and this CO2 must be retained within the pieces that have the ability to expand and gain volume. If the dough is excessively soft, gas may escape, resulting in collapse. However, if the dough is excessively tenacious, it may hinder the pieces’ expansion. CO2 formed in this phase enters the already formed holes, increasing their size. Importantly, no new cavities are created during this phase; existing ones expand. Therefore, if there are few holes in the mass, the gas formed will concentrate in these holes, increasing their size even more, and some of them may even coalesce. Conversely, if there are many holes, CO2 must distribute more, and each cavity’s size increase will be smaller, although the overall volume increase of the pieces may be similar or even greater.


An important aspect in this final fermentation, as well as in all preceding stages, is the control of various parameters. Regarding the fermentation speed, temperature is the most critical parameter. Higher temperatures accelerate yeast fermentation. However, it’s important to note that, besides gas generation, other phenomena may occur in this process, such as the development of lactic acid bacteria, enzyme action, and the formation of flavours and aromas. Studies have shown that a slower fermentation, emphasizing these secondary actions, results in bread with richer flavour, aroma, and an extended shelf life (slower hardening). Depending on the literature, fermentation can occur between 23 and 30°C or at temperatures closer to 40°C. Many Anglo-Saxon books recommend temperatures near 40°C or even higher, especially for sandwich-type bread. These bread types prioritize volume, and alveoli distribution, over flavour, and slowing hardening is less critical. This type of bread, which incorporates some other ingredient, such as oil or sugar, and does not have a crunchy crust, already has a longer shelf life, among other reasons due to the packaging it may have, and uses other techniques to slow down hardening, such as the incorporation of additives or enzymes, as we have already seen in other posts.

However, for bread where flavour primarily comes from fermentation, it is advisable to lower temperatures, at least to 30°C. The lower the temperature and the slower the fermentation, the better the organoleptic quality of the bread. While fermentation can theoretically occur at 22-23°C as a lower limit, even lower temperatures can be employed. Winemakers and brewers, using the same yeast as in bread making, often ferment at much lower temperatures to enhance aroma and flavour formation, preserving volatile substances. Although bread differs from wine and beer in that it undergoes baking after fermentation, leading to the loss of volatile substances, these lower fermentation temperatures provide insights into the possible fermentation temperatures. In the past, there were no fermentation chambers capable of maintaining temperatures below room temperature with proper humidity control. However, modern equipment allows for fermentation at 12, 14, or 16°C. Naturally, at these temperatures, fermentation time increases, and more fermentation space in the form of fermentation chambers is required, but it is worth considering.

If opting for slow fermentation, it is also crucial to regulate yeast dosage. Lower yeast doses result in slower fermentation with increased activity of other microorganisms. Conversely, higher yeast doses accelerate fermentation, reducing the activity of other microorganisms as yeast dominates the environment.

In the world of baking, where volume and speed are prioritized, yeasts with a high capacity for maltose transformation (the predominant sugar in fermentation) and high CO2 production are favoured. In contrast, in winemaking or brewing, the aromatic aspects of yeasts are crucial. This is because, beyond sugar transformation to CO2 and alcohol, secondary reactions occur, significantly influencing the final product’s aroma and taste. With a shift in mindset in some bakeries, attempts are being made to incorporate these yeast selection criteria. Some yeasts commonly used in brewing have been tested in baking, but as far as I know, these investigations are preliminary, and there is still no widespread market offering yeast selected for their ability to impart flavours and aromas.

Relative Humidity

A final but equally important factor to consider is relative humidity. In general, we do not want the dough to dry out during fermentation, nor do we want condensation to occur on it. Excessive humidity can lead to condensation, creating stains on the crust and sometimes blisters (condenses water in the form of droplets, usually not uniformly). If humidity is low, the external part of the pieces may dry out, potentially reducing their subsequent expansion capacity, and the final product’s appearance may differ, appearing more rustic, less smooth, with a matte finish and lighter tones. That said, there is no fixed recommendation, and humidity can be adjusted based on the desired effect. To prevent both drying and condensation, the ambient humidity should be similar to that of the dough, often suggested between 75% and 85%, depending on the type of dough. This also depends on temperature, and although psychrometric charts could be discussed, it would extend this entry too much. In essence, it is important to know that lower temperatures require less humidity for condensation to occur on the dough.

It’s worth noting that many books recommend excessively high ambient humidity levels. This is because these are often Anglo-Saxon books or translations based on them, primarily focused on sandwich-type bread with fermentation temperatures close to 40°C or even higher. As mentioned earlier, with lower fermentation temperatures, there is a high risk of condensation with these relative humidity levels. Additionally, ventilation or drafts can also influence this, and they should be minimized as they contribute to drying the product.

Another aspect we could discuss is delayed or controlled fermentation, the ability to stop fermentation at a certain point. However, this topic will be left for another day.

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