How does yeast affect bread quality?

Thursday March 07, 2013

Sourdough yeasts make an important contribution to the development of the dough flora (part 5)

From Werner Kräling

It is only in the interplay of the flour's own enzymes, lactic acid bacteria and the yeasts of the sourdough that the complex aroma profile of sourdough bread emerges, which has not yet been achieved artificially. Over 300 volatile aromatic substances can be detected in bread. However, “only” around 15 to 20 key aromas are really relevant to the taste and smell of bread. Sourdough's own yeasts make a contribution to this flavor profile that should not be underestimated, so that their reproduction (in addition to lactic acid bacteria) is also worth striving for.

Metabolism of yeast

Fermentation by yeast, also known as alcoholic fermentation, is widely known: the yeast cells ferment glucose through the yeast enzyme zymase, producing ethanol (alcohol) and carbon dioxide, which loosens the dough as a gas. During fermentation, the sourdough heats up as a by-product of metabolism is thermal energy. The liquefaction of the dough is due on the one hand to fermentation, but to a small extent also to the formation of water, which yeasts also form when they can breathe, i.e. when they find aerobic conditions. Primarily, however, the sourdough liquefies through the enzymatic breakdown of mucilage (pentosans) in rye sour and the enzymatic breakdown of the gluten (proteins) in wheat sour.

There are clear differences between commercially available baker's yeast and sourdough's own yeasts: The sourdough's own yeasts - primarily the dominant lead germ Candida humilis and also the Saccharomyces exiguus strain - can multiply in the sour environment of the sourdough, as they are relatively acid-resistant. With regard to the desired bread quality and also the aroma profile, the multiplication of the leading germs should take place in sourdoughs at temperatures of approx. 22 ° to 25 ° C, because at temperatures above 30 ° C they are partially displaced by other, particularly heat-loving yeasts. The sourdough's own yeasts absorb free amino acids and, through their metabolism, ensure the malty note of bread taste. In addition, they release fructose, which in turn is used by the heterofermentative lactic acid bacteria for metabolism, whereby the yeasts indirectly contribute to the formation of acetic acid.

Multi-stage tours

Contrary to popular belief, the added baker's yeast does not multiply in the dough and therefore hardly forms any flavoring substances. In fact, it is used almost exclusively to loosen the dough. Conversely, this does not mean that it is neither capable of reproduction nor of aroma formation. In wheat precursors with a very low dosage, it does multiply and also contributes to the taste of bread. The low dosage of 0.2 to 0.5 percent based on the ground cereal products of the pre-dough plays a role, which obviously "stimulates" them to multiply.

In the ideal case, the yeasts inherent in sourdough also multiply strongly in the sourdough and bread dough, so that the addition of baker's yeast to the main dough is not necessary. This usually requires multi-stage guides (e.g. three-step guide), as the desired yeasts develop more slowly and better with cooler guides (at the beginning of a step). This is also a reason why single-stage and multi-stage sourdough guides or cooler or warmer guides differ in the aroma profile. How these differences affect the assessment of the bread quality is explained when comparing the guides.

The art of sourdough production now consists in determining the temperature, maturing time, but also renewed supply of nutrients (flour and water of the next stage) to ensure that both the desired lactic acid bacteria and that of the yeasts multiply. Hence the term sourdough cultivation or the idioms “cultivate a sourdough”. And similar to horse breeding, you choose particularly magnificent "animals" - for example a pure-bred Lactobacillus sanfranciscensis of noble descent: a so-called pure-bred sour.

Start good as pure sourdough


The purebred cultures of the manufacturers are mostly not the use of a single strain of lactic acid bacteria, which the name might suggest, but multiple-stem cultures based on grain, which, with their metabolic activity, not only ensure the necessary acid production, but also a total produce a balanced bread aroma. The germ counts of lactic acid bacteria and yeasts in the pure breeding sour (as a starting material) are between 10 and 1000 times higher than the germ counts of the flour products - the flour's own flora is practically irrelevant, unwanted germs are effectively suppressed, the flour's own enzymes retain their activity.

The use of a starter culture therefore offers a high level of security for the start, but similar to spontaneous acidification, where the initial flora can change decisively due to the influence of the management parameters (temperature, ripening time, dough yield, growth rate as well as nutrients and dough yield) the same applies to the starting flora of a starter culture. During the fermentation and especially the repeated freshening from day to day, a completely new level of quality can even set in the microflora if it is incorrectly managed. Therefore, with continuously managed sourdoughs, compliance with the control parameters is essential for a good sourdough. Especially when bridging the weekend, mistakes can easily happen in craft businesses.

The answer to the frequently asked question of how often a sourdough should be restarted therefore has a simple answer: a weekly or 14-day restart offers a higher level of security, but if you use the guide correctly, you can keep your sourdough for years lead continuously.