Experiment on incorporating air into the dough / aerobic yeast metabolism effects
Alright guys, just thought I'd post this up here. Basically, I was just trying to see if it actually made a difference how one kneads. Turns out, yes. Air = good.
The aim of this experiment is to determine whether or not incorporating air whilst kneading dough is on beneficence to the baked product.
Bread baking techniques have long been based on experience and tradition. Arguably, however, this has since led to misconceptions about the processes involved. Many books will tell you that you knead bread to develop the gluten in the flour, whilst others will still claim that kneading is to incorporate air.[i]
The formation of gluten from gliadin and glutelins on kneading has been confirmed. This can then provides a stable structure which can hold the CO2 produced as a by-product of yeast metabolism.[ii]
The incorporation of air then, as the cause of the dough’s increased stretchiness, is disregarded. However, air may still be important. Baker’s yeast, or saccharomyces cerevisiae, can produce CO2 by both anaerobic fermentation and by aerobic respiration (Citric Acid Cycle and Oxidative Phosphorylation).[iii] Aerobic respiration yields CO2 at a ratio of 6:1 for the same mass of substrate. As a result, the deliberate incorporation of air into the microstructure of bread dough, which we know has the capacity to store CO2, may have an effect on the final product.[iv]
- The rise of the dough with air incorporated will be faster and greater.
- The flavour difference of the breads will be insignificant in those loaves proved at room temperature. Flavours will be contrasting in retarded loaves, due to different levels of yeast stress.
- One simple white bread dough was made from the following:
500g strong white flour
375g tepid water
10g instant yeast
- This dough was mixed thoroughly and split into two smaller doughs of equal mass (to the nearest 1g)
- One dough was kneaded in a stand mixer (Kenwood Chef KM001) on slow setting with the dough hook. This was intended to incorporate as little air as possible into the mixture.
- At the same time, the other dough was kneaded by hand by slapping and folding, deliberately incorporating as much air as possible.
- When both doughs were deemed finished, they were tested for gluten development with the windowpane test. Small samples were pinched off and stretched. The length at which there was breakage was equal.
- Both doughs were then split in two equal balls, making four equal sized doughs in total; two oxygenated, two non-oxygenated.
- The doughs were left to prove for 1.5 hours.
- Each dough was shaped until of maximum tightness available by the skill of the baker and placed in floured proving baskets to prove.
- One oxy and one non-oxy dough were placed in the fridge to prove for 14 hours, whilst the others proved for just one hour before baking.
- The loaves were baked at 220 degrees Celsius for 30 minutes, next to each other in the oven (turned and switched halfway through)
- The loaves were then judged for crust and crumb, before submitted for a blind taste test by a third party.
- After kneading, although the water content was the same and the windowpane test identical, the non-oxygenated dough had a marginally looser consistency, as if it had a higher hydration
- After the first prove, the oxygenated dough had splayed out less than the non-oxy, again consistent with a lower hydration dough
- When slapped, the oxygenated dough was more resonant than the non-oxy.
- When shaped, it took less effort to get the oxygenated dough tighter than the non-oxygenated.
- After the bake of the SHORT PROVED BREAD, the size of the oxygenated bread was significantly greater.
- The crumb of the oxygenated bread was marginally more consistent, with slightly more uniform, smaller bubbles
- Due to its increased size, the oxygenated bread had a less substantial crust.
- On blind tasting, no difference could be discerned or preference made in flavour or texture
- The LONG PROVED DOUGH showed an amplification of the differences of the short proved. The oxygenated dough significantly larger in the basket.
- After the bake, it could be seen that the oxygenated bread was much larger.
- The crumb of the oxygenated bread was much superior; a more even distribution of consistently sized bubbles.
- Oven spring of the oxygenated bread was significantly greater
- Consistent with their size, the oxygenated bread had a thinner crust than the non-oxygenated.
- On blind tasting, there was an absolute and definitive preference for the oxygenated bread. There was deemed greater complexity of flavour.
Summary and Conclusion:
- Both hypotheses were correct. Therefore when making bread, hand-kneading with the aim of incorporating as much air as possible into the dough is recommended.
- The common assumption that shaping incorporates air into the bread has little support. Here, shaping was consistent and did not lead to oxygenation of the non-oxy dough during the long prove.
- If proving in the short term, oxygenation increases size of the bread (greater yield) and the consistency of the crumb (even bubbles with decreased range of diameter)
- If proving in the long term, the differences in oxygenation are amplified significantly. The yield is much greater, the crumb is much more consistent, oven spring is easier to achieve and the flavour is more complex and palatable.
- However, it must be taken into account the larger size of the bread means an inferior crust for the same baking time. Therefore, with hand kneading the baking time should be increased slightly.
Hypotheses to explain results requiring further research:
- The surface area of the bread is not increased when shaping to a level that can provide all the yeast with oxygen. Therefore
- The dough of the oxygenated bread seemed tighter and less hydrated because the increased aerobic respiration by the yeast caused increased pockets of CO2 to form, therefore increasing tension
- Flavour was increased on retardation due to increased yeast stress. When yeast is stressed it produces by-products, giving flavour. The stress of temperature change was amplified with the stress of switching from pure aerobic to anaerobic respiration once the incorporated oxygen was depleted.
- The blind taste testing was single blind, and the subjects’ conclusions could have been affected by what they saw.
- This was a small scale experiment. Therefore, stress testing equipment to determine the tensile strength and so gluten formation was not available. There may, therefore, have been small differences in gluten formation between the two kneading methods.
[i] Bertinet R; Dough; Kyle Cathie; UK 2008
[ii] Abonyi T et al; Gluten formation from flour of kernels in developing wheat grain; Cereal Research Communications; Mar 2010;38,1
[iii] Nissen TL et al; Anaerobic and aerobic batch cultivations of Saccharomyces cerevisiae mutants impaired in glycerol synthesis; Yeast; Mar 2000;30;16(5):463-74.
[iv] Gust LK; Experimental Investigation of Yeast Activity and Carbon Dioxide Production in Bread Raising; 2.671 Measurement and Instrumentation; 2010;9-12