Oxidation and over-oxidation in bread dough are topics that bakers don’t mention enough. To understand its importance, here’s an experience I had last year:
I had tried to make a spectacular “super bread”. I’d mixed it “hard”, bulk fermented it “long”, and gave it a nice, extended rise. I thought it would be a showstopping loaf. But in reality, the dough lost its strength and flavour. I still remember my bitter disappointment when it came out of the banneton and collapsed.
I know it did this now because of over-oxidation. So what is the truth about oxygen in bread, is oxygen required, and how can you prevent over-oxidation? Let’s take a look.
When flour is in contact with air, it absorbs oxygen. The amount of oxygen absorbed increases when the dough is kneaded. The dough uses the oxygen in the dough to strengthen the dough structure and supply the yeast. It can also have a detrimental effect on the dough.
Dough oxidation is also known as oxygenation. Dough that has too much oxygen exposure is also known as over-oxygenated. Looking through scientific papers, there is a fine line between both processes, but oxidation is the correct term.
Oxidation makes bread dough stronger. As gluten molecules are oxidated, sulfhydryl groups in some amino acids are oxidized to produce disulfide bonds. These contribute to the crosslinking of the gluten, which produces an enhanced gluten structure. It makes the dough easier to shape and rise uniformly.
Yeast has two ways it can produce carbon dioxide gas in bread making. These comprise of aerobic or anaerobic respiration. When yeast has oxygen available it respires following the citric acid cycle, but without oxygen, it takes the anaerobic pathway which leads to fermentation. The yeast fermentation process is much talked about as it produces carbon dioxide and ethanol, which supplements the growth and fermentation of organic acids. While fermentation sounds more exciting and provides flavour and bread-like features, respiration is just as important.
Yeast begins producing gas via aerobic respiration, but as the supply of oxygen is depleted, it changes its operation to anaerobic respiration. How much aerobic preparation occurs depends on the amount of oxygen incorporated into the dough.
Aerobic respiration is best for light tasting bread made with a short rise. It releases more ATP molecules which provide energy that makes the dough rise quicker. Because bread made this way undergoes little fermentation, its flavour is light.
The benefits of organic acids and ethanol provided by anaerobic respiration are favoured in artisan bread. These extra components act as natural dough conditioners and flavour enhancers to make a more exciting loaf that keeps fresh for longer.
The tricky task for home bakers is to not sit on the fence! You either want to incorporate a lot of oxygen and rise the bread quickly or a longer rise for a more fermented flavour. Well, this is not 100% correct! You can use pre-fermented flour in a yeast dough to get the best of both worlds! The pre-fermented flour adds fermented goodness to advance the flavour and structure of a quickly risen bread alongside intensive kneading.
When too much oxygen is incorporated into the flour, it destroys the flavour and appearance of the bread. The yellow/creamy lipids in the flour are the carotene pigments called carotenoids. As oxygen is absorbed, they become over-oxidated and turn pure white. If this happens, it shows that the carotenoids are damaged, and their minerals have been washed away. It is why over-oxidation is also known as flour “bleaching”.
Not only does over-oxidation wash away colour and minerals. It also removes bread-like flavours, leaving a bitter taste after baking.
The best way to see if there is too much oxygen is by looking at the colour of the crumb. If it is dull and white, there was too much exposure. Another thing to observe is the crumb structure. Too much oxygen produces irregular holes in a close-knit crumb. The aroma will also be bland.
Dough absorbs a considerable amount of oxygen as it is kneaded. Oxygen continues to be absorbed as the dough sits during the first and second rises. Long fermentation after intensive mixing is the leading cause of over-oxidation. Yet there are many causes of oxidation in bread dough, all of which are commonly found:
If a long mixing time is used, the bulk fermentation must be reduced to prevent over oxidation. It’s also a good idea to reduce the amount of oxygen that can come into contact with the dough. Little things such as using small containers for bulk fermentation and a lid to cover dough resting containers can make a big difference.
It’s also important to consider the length of time dough is fermented. Even if mixing is light, some flours can’t withstand long periods of oxygen exposure and collapse.
Incorporating too much oxygen is common where intense mixing is used. Oxidation is reduced in some bakeries by using specialist vacuum mixers that prevent oxygen from entering the dough. They use these mixers alongside adding an anti-oxidant such as ascorbic acid.
Ascorbic acid is often used to increase the amount of oxygen in the dough for home baking and many small industrial bakeries. But some industrial mixers use ascorbic acid as an anti-oxidant. This allows longer fermentation times to be achieved with heavy mixing.
Over-oxidation can also occur when stand mixers are used. Some mixers won’t develop the gluten effectively, so many home bakers increase the kneading time above 15 minutes. Alot of oxygen will be incorporated in this long mixing time, so you may find better results by increasing bulk fermentation time, adding a dough improver or kneading by hand.
A short (or zero) bulk fermentation will follow intense mixing when preparing soft-crumbed bread. Mixing times will be 2-3 minutes at slow speed and 5-7 minutes at fast speed. Ascorbic acid can be added to enhance oxygen intake.
This method pumps oxygen into the dough, which benefits from a strengthened gluten structure, a fast rise and a big oven spring without risking over-oxidation or running out of available sugars to supply the yeast.
Flour oxidation works similarly to dough oxidation but at a different stage. Typically flour is stored for 3-4 weeks before use. This ageing process lets the flour absorb oxygen which matures the protein and makes it suitable to make bread.
Nowadays, it is common for flour mills to add an oxidizing agent such as ascorbic acid to fresh flour during milling. Oxidizing agents speed up flour maturation so it can be packed and sold straight away. The process removes storage and infrastructure costs without affecting the quality of the dough.
Whole wheat flour contains yellowy-brown particles called bran, which is the outer casing of the wheat. When over-exposed to oxygen, bran takes on a darker colour. These are the black specks that are often seen on dough that have been chilled for several days.
Oxygen is not essential for the yeast to produce gas and rise, though it is beneficial for quickly risen bread. The kneading duration is best kept short for a longer fermented dough to reduce oxygen intake.