How does bread yeast react to ethanol

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Baker's yeast under the scanning electron microscope magnified 12,500 times

Welcome to the first part of the chemistry of bread making. Yeasts serve as a leavening agent in a dough. You let the dough rise. You can add them specifically, as fresh or dry yeast. In addition to other microorganisms, yeasts are cultivated in a sourdough, which are found on the flour anyway. But what are yeasts, how do they work and what is better not to do to them? This article aims to provide answers to this.

Yeasts belong to the pack fungus, so they fall into the class of eukaryotes, as do plants and animals. They have a nucleus and organelles (these are the "organs" of a cell) and are unicellular. Baker's yeast, for the sake of simplicity, I will soon only be calling it yeast, is called by its Latin name Saccharomyces cerevisiae. Sucrose is the common name for perfectly normal household sugar and is included in the name because these types of fungus break down sugar, cerevisiae comes from cerevisia = beer, because these types of yeast were originally used in beer production. If you want to know how yeast are produced biotechnologically, you can read here if necessary.

In sourdough production, naturally occurring wild yeasts are grown, which are found on the flour anyway. They are not quite as effective, some have different optimal temperatures, but their metabolism works in principle in the same way as that of baker's yeast.

Yeasts can live in both oxygen and non-oxygenated environments. The biochemist calls this admirable property optional anaerobic. If oxygen is available (and the temperature is not too high), it finds Yeast breathing instead, in the absence of oxygen the Yeast fermentation (Fig. 1). The latter is probably better known and can occasionally even be observed in one's own refrigerator if carelessly. This turns sugar into ethanol and the gaseous carbon dioxide, which rise as bubbles. However, this process is not very energy-producing for the yeast. And so, as a rule, they prefer to use the oxygen present, if available, in order to eke out their existence and to adhere to the requirement “be fertile and multiply”.

Fig. 1: ATP is a biological unit of measurement for the energy available to the cells and means adenosine triphosphate. All living beings use ATP for processes in the cells. Respiration produces 6 parts of ATP per glucose molecule, and fermentation only 2 parts of ATP.

In fruit juice, the sugar comes naturally from the fruit and yeasts carry out fermentation processes when there is a high supply of sugar, even under aerobic conditions. In bread dough, sugar is created from the starch contained in the flour and is not so abundant. Starch consists of long chains of sugar molecules strung together. These chains are broken up by amylases (this is how starch-splitting enzymes are called), which are also in the grain, and are thus available to the yeast as food. This process of enzymatic breakdown of starch is also called glycolysis (Fig. 2).

Fig. 2: In glycolysis, starch is broken down into its individual components. During fermentation, these are then digested by yeasts into alcohol and carbon dioxide.

I will explain later how and why this process is also important in baking. Now let's talk about yeast and how it uses sugar as food.

During yeast aerobic respiration, sugar and oxygen also produce carbon dioxide. But this time no ethanol is produced from the pyruvate (a breakdown product of glycolysis = enzymatic breakdown of sugar), instead it is further broken down into carbon dioxide and water. And because this process delivers three times more energy than fermentation, the yeast naturally reproduces better with it. However, the process of breathing hardly plays a role in bread-making. It may be that on the surface and when air is initially trapped in the dough, the yeast prefers this route. This is exactly the effect that is sometimes desired, because if the dough is kept for a long time and the dough is kneaded repeatedly, more oxygen enters the dough. But the far greater part of the yeast will find the conditions for fermentation.

But it would be easy if it were easy. Too much oxygen is therefore not to be expected in a rather liquid and heavy bread dough. The fermentation process is therefore decisive. On the one hand, the yeasts then no longer multiply as quickly (because they have less energy available) and thus produce less carbon dioxide, but the alcohol that is formed evaporates during baking and also loosens the dough. In addition, other important aromas are created during fermentation, which give the baked goods a taste. The only stupid thing here is that the alcohol produced is not good for the yeast at all. The yeast dies from a certain concentration of ethanol. This effect is called Self-poisoning or end product inhibition. This is then the point at which the dough no longer rises. On the contrary: in this case the gas slowly escapes from the dough and it collapses. This point should be as little as possible when making sourdough and pre-dough, and it is better not to get past this point at the end of the production process.

What is often referred to in recipes is the temperature. Yeasts are not particularly squeamish about that. You may have a favorite temperature, but you can also cope with slightly different temperatures more or less well. Only then do they grow more slowly or die off. Optimal growth conditions with anaerobic fermentation for baker's yeast are around 30 ° C. Above 45 ° C the yeasts begin to die, below -7 ° C they completely stop their activity and between -6 ° C and 10 ° C their activity is strongly slowed down (see figure “Temperature dependence of yeast). You can therefore keep yeast in the refrigerator for a few days. But as soon as the yeast becomes dry and brittle, it is better not to use it anymore. The merely reduced activity is also the reason why the cold walking method still brings results, whereby the amount of yeast should be reduced so that the little helpers still have something to bite at the end of the walking time.

How much yeast goes into which dough is a question that is not at all easy to answer. Some like the yeast taste in bread, others find it annoying. It is therefore up to you how much yeast you want to add to a bread dough. As a rule of thumb: no more than 2-3% of the pure flour-water amount.

Yeasts have been well studied, but the whole process in a sourdough is very complex, since not only yeasts but also other microorganisms such as lactic and acetic acid bacteria and their breakdown products are present. The interplay of all these factors is so complicated that only yeast will be discussed specifically here.

Like all living things, yeasts have optimal "operating temperatures". They work above and below, but not as well. For yeast, as for all biochemical processes in living beings, these optimal temperature ranges are between 30 and 40 ° C. This is another reason why our body temperature is in this range. Yeast breathing only takes place at temperatures below 28 ° C. That is why it is important that temperature as possible in an optimal range. This works best with the temperature of the added water. Since the dough itself has to be warm so that the yeast can also work inside the lump, the ingredients should be at least room temperature (> 20 ° C). The temperature of the dough can be controlled by adding slightly warmer water (but not warmer than 45 ° C). If the temperature is too cold, the yeast will take longer. The diagram clearly shows that propagation and fermentation are considered separately. But before you come up with the idea of ​​heating your booth to 35 ° C: Remember, there are other organisms in a sourdough that need different temperatures. In most cases, bread is only heated to the point when it is cooked in pieces that the yeasts have their optimal operating temperature.

The temperature control of a bread dough is a separate chapter. I will only be able to go into this once I have explained (and understood!) The other micro-organisms.

Now you could also come up with the idea that Freeze fresh yeastbefore it spoils. In principle this is possible, because the driving force, i.e. the production of gases from sugars, is only due to the Zymase, the yeast enzyme. And for this this enzyme does not need a functioning cell. A large part of the cells is broken when freezing. It is comparable to a bottle filled with water in the freezer. It bursts at some point. However, the yeast needs intact cells to multiply, so after thawing it continues to generate CO2 and ethanol but formed significantly fewer new yeast cells. You can therefore easily freeze and bake ready-made baked goods and even let them rise. But it will hardly be possible to make a nice dough out of frozen yeast cubes.

In addition to water, it is mainly compounds that, when dissolved in water, disrupt the yeast metabolism. The concentration of these substances plays a decisive role in this. The various metabolic products of the microorganisms in a sourdough also have an inhibiting effect on yeast activity.

  1. Yeasts are needed for their metabolism water. The less there is, the more difficult it is for the yeast cells to multiply and digest. Because the less water the cell has available for transport reactions, the longer the substances need to get from A to B in the cell.
  2. sugar hinders yeast fermentation in sourdough with a proportion of more than 12%, based on the flour proportion. With sweet dough, this effect becomes problematic from 20%. Sugar makes the dough softer, which leads to less water and therefore less yeast activity (see point 1.)
  3. salt has a similar effect to sugar and even leads to cell death if the concentration is too high. In some doughs exactly this effect is used and a yeast-salt guide ensures that the yeasts do not multiply, but only produce fermentation gases through the enzymes that are not affected by it.
  4. alcohol is an end product of yeast fermentation. How much of this a yeast can tolerate differs depending on the strain. Special breeds for the beverage producing industry are specially designed for high alcohol content. The yeast in our sourdough preparation tolerate significantly less of this.
  5. The acid content or pH value should also be mentioned. The acid-producing bacteria or their metabolic products Lactic acid and acetic acid inhibit the yeast in its fermentation process. Wild yeasts tolerate lower pH values ​​(= higher acid content). This is one of the reasons why it makes sense to prepare your own sourdough.
  6. That your own metabolic product Carbon dioxide I have not found that slows yeast growth.
  7. fat does not inhibit directly, even if this is often said. However, the fat contained in the dough affects the amount of water and if this is reduced, it in turn affects the yeast fermentation (see point 1.)

The time is also a factor. But since the interaction of all microorganisms is also decisive here, I will go into this later. Just this much with regard to yeasts: Walking times that are too short lead to poor loosening because there is little CO2 and ethanol is formed, too long walking time leads to large, irregular pores. Too much yeast also leads to irregular pores, which is usually undesirable in bread and a measure of quality. Although it is definitely desired in some breads such as baguette or ciabatta.

Are you interested more? You can find other articles on the topic here.

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This entry was posted in Just like that and tagged amylase, fermentation, glycolysis, yeast, yeast respiration, yeast fermentation, Saccharomyces cerevisiae, sourdough, sourdough production, zymase by brittlebrick. Permanent link.

About brittlebrick

My name is Britta, I came to the vegan diet in 2014 and consider myself a part-time vegan. I have a general interest in nutrition, am married and have a son who is my main critic and who tests the recipes for family suitability. ;)