In the baking of bread and other products, multiple phenomena occur, encompassing chemical, physical, and biological aspects, which it is good to be aware of and understand. Let’s analyse what happens in baking. This topic is complex, and I believe addressing it in a single entry would be unfeasible (at least if we maintain the average length of entries). Therefore, we will cover the basic knowledge in two entries. It may be necessary to address specific aspects in subsequent entries.

Types of Heating

During the baking of bread, heat is transmitted through three different methods. The predominance of one over the others depends on the type of oven and its regulation. The first method is radiation, where heat is transmitted from a distance, similar to feeling the warmth when approaching a radiator without touching it. This method prevails in vaulted ovens, where the walls are preheated and transfer heat to the interior. Heat can also be transmitted through conduction, occurring when the heat source is in direct contact with the dough, such as between the oven floor and the base of the dough pieces. Conduction also takes place within the bread itself, from the hotter exterior to the cooler interior. This type of heating is crucial in deck ovens but is also present in other oven types. In fact, in these types of ovens, the heat provided by the floor and the heat provided by the top can be regulated differently. Lastly, convection heating involves establishing air currents that circulate heat inside the oven. This process dominates in tray or rack ovens, requiring trays with holes to facilitate air circulation throughout the oven. Trays without holes would impede air movement and, consequently, heat transmission.

While each type of oven has a predominant heat transmission method, all three methods coexist. In vaulted ovens, where radiation is often more significant, there is also conduction from the floor to the dough, along with some convection due to light air currents. In deck ovens and rack ovens, the heated walls also radiate heat. While deck ovens establish air currents (convection), sometimes aided by fans, rack ovens or tray ovens also experience conduction between trays and dough pieces.

Baking Temperatures

Dough enters the oven at a variable temperature, depending on the fermentation temperature. The oven is typically set at temperatures close to 200°C, so the outer part of the dough is exposed to this atmosphere upon entering. However, heat needs to penetrate the interior of the dough to reach the central zone. In other words, the outer part of the dough will increase in temperature much more rapidly than the inner zones. Therefore, when baking larger or more rounded pieces, it is advisable to use lower temperatures. This is because the central zones, further from the exterior, take longer to heat. Using very high temperatures in these cases could result in excessive baking of the outer areas, leading to a “burnt” appearance, darker colours, more bitter flavours, and increased acrylamide content (which will be discussed in another entry).

Enzymes

Generally, the enzymes present in the dough have a high inactivation temperature, but not so high that they cannot be inactivated during baking. The entire dough piece usually reaches 100°C for a minimum time, sufficient for enzymatic inactivation. The temperature at which each enzyme is inactivated depends on its type and origin. It is crucial not to use highly heat-resistant enzymes, as they might withstand the baking process and affect the final product, degrading it. This could be the case with some special amylases.

Among the enzymes crucial during baking, amylases play a vital role. As long as damaged starch exists, these enzymes act on the starch, producing dextrins and maltose, fermentable sugar. Thus, they continue generating nutrients for yeast action. Generally, the optimal action temperatures of these enzymes are higher than those during kneading and fermentation, so in the early stages of baking, they encounter their optimal working temperature, facilitating yeast action. As we will see, as the temperature rises, starch will begin to gelatinize. In this process, undamaged starch becomes accessible to the action of amylases. Therefore, if the inactivation temperature of amylases is lower than the gelatinization temperature, they will not degrade the gelatinized starch. Conversely, if there are amylases with an inactivation temperature higher than the gelatinization temperature of starch, they can degrade part of the gelatinized starch until their inactivation temperature is reached. It is essential to remember that all enzymes must be inactivated during baking. This forms the basis for using amylases with intermediate thermostability, degrading part of the gelatinized starch, minimizing retrogradation reactions during storage, and thus reducing bread hardening for this reason. However, these enzymes must be inactivated during baking.

Yeast

Baker’s yeast has an optimal operating temperature around 40°C. Typically, bread fermentation occurs at slightly lower temperatures to enhance other processes and improve the taste and preservation of the bread. Therefore, when the dough pieces are placed in the oven, and the temperature gradually increases, yeast reaches its optimum, and as long as fermentable sugars are present, it continues its function, generating CO₂, which, if the dough is in good condition, continues to be retained, increasing the volume of the dough pieces.

As the temperature continues to rise, we move away from the optimal temperature for yeast activity, and its activity decreases until lethal temperatures are reached for these microorganisms (starting from 50°C). This marks the end of their fermentative activity. For this reason, dough pieces introduced at lower temperatures have a more extended expansion phase in the oven than those introduced at higher temperatures, which can reach up to 40°C.

Alcohol

The alcohol produced during fermentation, mostly ethyl alcohol, has an evaporation temperature lower than that of water. Therefore, when this temperature is reached (around 70°C), heating stops, and the heat energy is consumed in the alcohol evaporation process. Once the alcohol has evaporated, the dough continues to heat. The amount of alcohol is very small compared to the amount of water, and this process is relatively quick. But it’s essential to know that there is no residual alcohol in the final bread, despite being produced during fermentation, as it evaporates during baking.

Leavening Agents

Regarding leavening agents or baking powders, we have discussed them in another blog entry. Essentially, they consist of a mixture of basic substances (mainly sodium bicarbonate) and acids that react when heated. The reaction temperature depends on the acid dissociation, but in general, they are formulated for this reaction to occur at elevated temperatures, during baking.

In most bread recipes, these substances are not incorporated. However, in some wheat bread, such as soda bread, and some gluten-free bread, it is common to use this type of gas-generating substance to achieve pieces with an appropriate volume. The use of leavening agents is also common in other products such as cakes or certain types of cookies.

If leavening agents are used, the most important thing is to ensure that they generate gas when the dough can retain it and expand. Gas generation when the starch has gelatinized, and the pieces are rigid could have negative consequences due to excessive internal pressure that may break the pieces. Depending on the leavening agent level, this could be more or less harmful. The increase in volume through fermentation has additional advantages that do not occur when leavening agents are used for this purpose. Additionally, using leavening agents involves the use of substances considered additives, which may be less accepted by some consumers.

Expansion

In addition to the action of yeast or leavening agents, other phenomena can contribute to the expansion of dough pieces during baking. All gases occupy a larger volume when heated (tending to expand and exert pressure on the medium). Inside the bread, there is a large amount of CO₂ formed during fermentation. Therefore, as the dough piece heats up, these CO₂ bubbles tend to expand, increasing the volume of the bread. This type of expansion, caused by the increase in the temperature of the gases enclosed in the dough, is especially important in sponge-type cakes. In fact, there are cakes that do not require the use of yeast or leavening agents and achieve significant expansion during baking.

We will continue discussing baking in a subsequent entry.