Oxidation and Reduction in Doughs

As we discussed in a previous post, wheat reserve proteins have the unique property of forming a protein network, known as the gluten network, when they hydrate and undergo mechanical work (kneading). This gluten network creates cohesive and less sticky doughs that can be stretched, rolled, folded, and more. Additionally, the gluten network allows the gases generated during fermentation to be trapped within the dough. Doughs made from gluten-free cereals lack the ability to stretch without breaking and cannot retain gases unless we introduce a gluten substitute.

The properties of this gluten network depend on the quantity and quality of proteins, but we still do not fully understand what governs this quality. It is known to be influenced in part by the ratio of low and high molecular weight glutenins, and certain protein bands obtained in electrophoretic analysis also contribute to this quality. However, a key factor in the strength of the gluten network is the presence of disulfide bonds (S=S). These bonds form between sulfhydryl groups (SH) found in specific amino acids (methionine and cysteine) under oxidizing conditions. Conversely, under reducing conditions, disulfide bonds tend to break, releasing sulfhydryl groups. As a general rule, the creation and breaking of these bonds are dynamic processes, and in doughs, they are continually forming and breaking. What happens is that under oxidizing conditions, the creation processes prevail, while under reducing conditions, the breaking processes dominate.

Oxidants

The creation of disulfide bonds leads to tougher, more elastic doughs that are less extensible but possess greater alveographic strength. In most bakery and fermented dough preparations, dough strength is a desirable property because it enhances gas retention during fermentation. Consequently, the incorporation of oxidizing agents has been a common practice in baking for many years.

Some of these oxidizing agents also contribute to the whitening of flours and doughs, as the pigments responsible for the typical cream colour of crumbs lose their colour when oxidized. This functionality is highly valued in countries where the whitish colour of crumbs is a quality factor. However, in countries like Spain, where bread with cream-colored crumbs is appreciated, this functionality is not important and may even be detrimental. In fact, a slight oxidation occurs during kneading, which can help whiten the doughs. This effect is minimized by the presence of salt, which serves as an antioxidant, as mentioned earlier. Thus, simply by delaying the addition of salt, one can achieve bread with slightly whiter crumbs, similar to those generated in salt-free bread.

Historically, some of the most widely used oxidants in the food industry included bromates, iodates, peroxides, and azodicarbonamide. Bromate was heavily used in the global baking industry because its effects were slower compared to other oxidants. This allowed doughs to be managed without becoming excessively elastic and tough, reinforcing the doughs when gas retention was needed. However, bromate has been classified as carcinogenic, and its use has been banned in most countries. The remaining oxidants are also not permitted for bread production in Spain. In other countries, it is necessary to consult the legislation.

The most commonly used substitute for bromate today is ascorbic acid, also known as vitamin C or E-300. In additive lists, ascorbic acid is classified as a reducing agent, and it indeed acts as one in the initial stages of kneading, reducing the strength of flours and allowing for somewhat faster kneading. However, the products generated in this initial phase have an oxidizing effect, reinforcing dough strength, preventing doughs from collapsing during fermentation, and enhancing gas retention from fermentation.

Ascorbic acid is used in most bread improvers today and is nearly essential when producing bread with weak flours and quick fermentations. This type of process is common in Spain, where there is a scarcity of strong wheat and strong flours are usually more expensive. Its use can be reduced or even eliminated when making bread with stronger flours or longer processes, such as sourdough bread and extended fermentations. Another possibility for replacing ascorbic acid is the incorporation of flours from products with a high percentage of ascorbic acid, such as acerola powder. It is also possible to achieve oxidation conditions through the use of glucoxidase. This enzyme, like most enzymes, deactivates during baking and does not need to be declared on the label. We will discuss enzymes in another post.

It is important to note that ascorbic acid has a limit to its effectiveness. In other words, beyond a certain point, the addition of ascorbic acid no longer has the desired effect. This should be considered in the case of flours with added ascorbic acid. Most bread improvers include ascorbic acid as a dough strength enhancer, so if the level of flour fortification is high, the ascorbic acid present in the improver may not have the expected effect. Therefore, it is preferable to work with flours without incorporated additives.

Reducers

In some cases, it may be necessary to reduce the strength of doughs. The most well-known example is Maria biscuits. Once the dough is rolled and cut, it tends to shrink due to its elasticity, losing its round shape and taking on a more oval form. These oval shapes do not fit well in cylindrical packaging formats, which would cause problems in packaging and result in irregular biscuits. To prevent this phenomenon, very weak and extensible flours are used to produce doughs that are less tenacious and elastic. However, this is often not sufficient, and a reducing agent is needed to further reduce dough elasticity.

Another scenario where the use of reducing agents may be advisable is to expedite the kneading of very strong flours. This issue is not common in Spain but can be more frequent in other countries where strong flours are used. In the past, a quick-reducing agent, acting during the kneading phase, was combined with bromate, which increased dough strength during fermentation. With the prohibition of bromate, this practice has waned. In any case, it would not make sense to use both a reducing and an oxidizing agent that act in the same process phase, as they would counteract each other. In processes where dough strength is crucial, the use of reducing agents can be quite challenging and must be done with low doses and great care.

Among reducing agents, the most commonly used in cookie production is sodium metabisulfite. The use of cysteine, an amino acid found in proteins, is also allowed. Sodium metabisulfite has a very rapid effect and is cost-effective compared to other alternatives, making it the preferred choice for manufacturers.

To reduce dough strength, proteases can also be used, as they break down proteins, weakening the gluten network. However, as mentioned earlier, we will discuss enzymes in another blog post.