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Chemistry


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Baker’s Yeast (Saccharomyces cerevisiae)

Freshly baked sweet rolls, together with structures of ethanol, carbon dioxide and glucose.

Freshly baked sweet rolls, together with structures of ethanol, carbon dioxide and glucose.

Hmm, freshly baked bread. Who can argue with that? Today we delve into some of the compounds giving rise to your yeasted loaf.

1. The historic use of yeast in the production of bread and alcohol stretches back many thousands of years, with the first documented instances of yeast being used in this way occurring during the time of the Ancient Egyptians. Although not aware of the chemical processes involved, or the specific compound which was responsible for the fermentation/leavening effects, the Egyptians nonetheless relied on the action of yeast in several key food products, probably even before a written language was developed. The work of Louis Pasteur in the 1860s determined that yeast was a living eukaryotic microorganism  (i.e. it has complex cells), as it has membrane-bound organelles, and enabled its isolation to a pure form. The strain of yeast that we will focus on today was the first eukaryotic organism to have its genome sequenced, in 1996 (described in Goffeau, B. Barrell, H. Bussey, R. Davis, B. Dujon, H. Feldmann, F. Galibert, J. Hoheisel, C. Jacq, M. Johnston, E. Louis, H. Mewes, Y. Murakami, P. Philippsen, H. Tettelin and S. Oliver, Science, 1996, 274, 546-567).

We have previously covered the involvement of yeast in the manufacturing process of beer in this post on Hops and there is quite a lot of detail in this blog post here by A Ph.D. in Beer, so we will focus today on a strain of yeast known as ‘baker’s yeast’ (Saccharomyces cerevisiae), which is used in the production of bread and yeasted cakes. This works with the other ingredients of flour, water and (optional) fats by producing gases which travel upwards through the dough and get trapped in its structure (you can observe this yourself, have a look at these yeast-air balloons). The trapped gas bubbles stop foods being heavy and dense, and cause bread and cake doughs to rise, as shown in the picture below.

Dough directly after kneading.

Dough directly after kneading.

The same dough, after it has been left to rise

The same dough, after it has been left to rise

Yeast can be purchased in several forms, including fresh yeast, dried yeast (where the moisture has been removed from the yeast to make it dormant and increase its shelf life), and instant dried yeast. Instant dried yeast is faster acting than ordinary dried yeast, because the hard inactive shells that surround the yeast in dried yeast have been reduced to a powder, so more of the active yeast is in contact with the process ingredients.

In order to have a well-risen bread, it is necessary for the yeast to be evenly distributed throughout the dough, which is helped by a thorough kneading process. The kneading process also causes the gluten strands in the bread to become more elastic and form a network within the dough, providing structure to the baked product. This is demonstrated in the noticeably stretchier dough in the picture on the right below, after kneading.

Stretching the dough after kneading - smoother and stretchier...

Stretching the dough after kneading – smoother and stretchier…

Stretching the dough before a lot of kneading.

Stretching the dough before a lot of kneading.

2. During the rising process, we can identify the gases and other products produced by the yeast in the dough. The yeast acts with the sugar (see our posts on sugar, part 1 and part2) and moisture in the mixture to produce carbon dioxide and ethanol (the same alcohol as found in beer and wine). Ethanol is a simple primary alcohol containing two carbons as well as the O-H functional group characteristic of alcohols, while carbon dioxide consists of small, covalently bonded molecules containing strong carbon-oxygen double bonds. At room temperature, carbon dioxide is a gas and ethanol is a liquid- it is the ethanol produced by yeast during the fermentation process that makes beer alcoholic.

However, in the case of bread making, we are interested in the carbon dioxide gas which makes the bread light and fluffy rather than the ethanol, and baker’s yeast is a strain which maximises how much of the gas is produced compared to other yeast types. Bakers usually aim to let the dough rise in conditions which cause the most carbon dioxide to be produced- typically a warm room, which can easily cause the dough to double in size, as is recommended in most recipes. Conversely, too much heat, salt or fat can cause the yeast to be killed, or the activity to be reduced by so much that it no longer works properly.

The balanced equation for the products of yeast action is shown below; note that C6H12O6 is the chemical formula for glucose and fructose, as discussed in one of our earlier posts on sugar).

C6H12O6 -> 2C2H5OH + 2CO2

Yeast can respire in this way either aerobically or anaerobically i.e. with or without oxygen. Anaerobic yeast respiration increases the amount of ethanol produced, but gives a poor yield in terms of the number of yeast cells per unit of the substrate consumed. For this reason, baker’s yeast acts aerobically and uses oxygen to respire, in order to achieve a better yield.

Strains which maximise carbon dioxide production under these conditions are used preferentially (D. Berry, Biology of yeast, E. Arnold, London, 1982). If too much ethanol is produced, the yeast organisms are killed, which is why there is a natural limit to the % alcohol content of beer and wine; after a certain point the ethanol produced by the yeast kills the yeast. However, this is not a concern in bread-making due to shorter time scales and the aerobic conditions used.

Fresh out of the oven...

Fresh out of the oven…

3. Saccharomyces cerevisiae’s compact size, suitable life-cycle for study and ability to grow on defined media are all reasons for its choice by a group of 600 scientists worldwide as the first eukaryotic genome to be sequenced (the research was published in 1996). It was found that there were almost 6,000 protein-encoding genes. Only 30% of the proteins within yeast could be classified by functions such as metabolism, energy production, DNA replication, transcription and translation at the time, showing the complication in defining the entirety of even a ‘small’ genome (Science, 1996, 274, 546-567).

Different strains of Saccharomyces cervisiae from that used for bread making have been tested for the fermentation of cane sugar, with a view to understanding and optimising the process for the production of alcohol from plants, and the kinetics of the fermentation process have been reported recently (E. Felix, O. Clara, A. O. Vincent, Open Journal of Physical Chemistry, 2014, 4, 26-31).

It seems that despite the long history of the use of yeast as a leavening agent, there is still much that we don’t understand about this small and intriguing organism.

Contributors: Eloise Hicketts (research, writing, photos and baking), Natalie Fey (structures, additional writing, editing)

 

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