Cast the first stone anyone who has never kneaded (at least their first time ever ...) with the penem canis , unloading a whole 25 grams cube of brewer's yeast in half a kilo of flour, as if he had had to leaven the bowl too ... And after a couple of hours (miracle!) find yourself munching with difficulty on the biscuity or chewy result wondering why he didn't even remotely remember a pizza ...

The "problem" is that, as already more fully explained regarding the phantom autolysis , if leavening occurs more quickly than protein synthesis , that dough loses edibility and becomes more suitable for alternative uses, for example as glue ...
leavening is not the only chemical and microbiological process that takes place in a dough but it is certainly among the most important. And if on the threshold of the "second phase" of the lockdown because of the covid-19, it is even easier to find the famous needle in the haystack (to then pass a camel in its eye) than the yeast of fresh beer in the fridge department of a supermarket I did this: flour, water (60% of the flour), a crumb of brewer's yeast that I had frozen for some time and many greetings to the moms-sandwiches who hoarded tons of brewer's yeast. Basically I'm growing the saccharomyces cerevisiae in a sort of sourdough, which here in Sicily came from the grandparents called criscenti . It works, but it's not the mother yeast . Or maybe yes?

What is mother yeast?

Theoretically it is a mixture of water and flour contaminated "spontaneously" by the endogenous yeasts of the flour. In fact, the environment in which you "work" (including laborious hands) involuntarily brings exogenous yeasts. Each refreshment (i.e. the addition of nutrients, in this case water and flour, generally to restore the original quantity) introduces unpredictable variables in the dough, promoting or disadvantaging the survival of certain species of yeasts and bacteria. Given the hectic metabolic activity of those who live in it, growing mother yeast makes sense if you plan to use it regularly (for example, at least a couple of times a week).

The role of yeasts

Yeasts are fungi ( phylum ascomycota ) which, between 4 and 45 degrees Celsius (more rapidly between 20 and 30 degrees), initiate metabolic processes (i.e. they eat, reproduce and die). The fact that the saccharomyces cerevisiae (i.e. the very common beer yeast) is able to duplicate itself no earlier than an hour and a half from the start of the metabolic activity, makes those recipes in which times are maintained implausible leavening time under an hour and a half ...
The metabolizing action of the yeasts is facilitated by hydrolysis (activated by contact with the water of the amylases and by the diastases, enzymes present in the flour) which breaks up the most complex starch molecules (which is a polysaccharide carbohydrate) in simpler monosaccharides (e.g. glucose and fructose) and disaccharides (e.g. maltose, sucrose and lactose). Yeasts, in particular saccharomyces cerevisiae, are able to survive both in the aerobic environment, by "breathing" oxygen, and in the anaerobic environment. Following the rapid depletion of oxygen present in the dough, the activity of the yeasts begins to convert into fermentation (in the case of saccharomyces cerevisiae, alcoholic) metabolic mode by which the yeasts obtain energy from organic molecules in an anaerobic environment. The sugars (monosaccharides and disaccharides) are split from the yeasts first in pyruvic acid and then, partially, in ethanol . The remaining part of pyruvic acid, oxidized by the few oxygen molecules left in the dough, is transformed into carbon dioxide . The leavening , i.e. the swelling of the dough, is due to the storage of ethanol and carbon dioxide between the glutinic mesh of the dough. The ethanol present in the dough will then evaporate during cooking.

The role of bacteria

A balanced dough hosts, in a hundred times higher quantity than yeasts , bacteria, in particular lactobacilli . The metabolic activity of bacteria occurs in an essentially anaerobic environment: it proceeds through the fermentation (in the specific case of lactobacilli, lactic fermentation and non-alcoholic as for yeasts) of monosaccharides and oxidized disaccharides, generating lactic acid and acetic. The synthesis of these acids causes the acidification of the dough , making it (within a certain limit) tastier. However, the presence of lactic acid in the dough makes the glutinic mesh more elastic, the presence of acetic acid helps to improve the flavor of the dough and both come into play, together with the sugars, in the Maillard reaction (i.e. the creation of the crust, during cooking, which takes place between 140 and 180 degrees Celsius): their complete synthesis is therefore not desirable.
Keep in mind that very hydrated doughs, long "leavening" times (between 24 and 48 hours) and high temperatures (between 30 and 40 degrees Celsius) favor the development of lactobacilli at the expense of yeasts, therefore a more important production of acid lactic compared to acetic acid. Less hydrated and "leavened" doughs at relatively low temperatures (between 25 and 30 degrees Celsius) instead favor the growth of yeasts and the predominance of lactobacilli forced heterofermentative , which instead leads to a higher presence of acid acetic.
The metabolism of bacteria is slowed down by a percentage of salt higher than 1.5 of the total weight of the dough; it is instead facilitated by the presence of oxygen. Another function of lactic bacteria is proteolysis which brings greater digestibility to proteins (but also, consequently, a weakening of gluten).

Difference between mother yeast, "criscenti" and beer yeast

Saccharomyces cerevisiae not only can naturally already be present in the flour, in the environment and in the operator's own hands but is present on the surface of many fruits including, in particular, grapes. The idea of ??stimulating the fermentation of the dough through fruit or bran increases the possibility of increasing its presence. In other words, only in a totally and permanently aseptic environment, through a careful selection of ingredients and the triggering of a specific yeast culture, could it be possible to completely exclude saccharomyces cerevisiae from the mother yeast. In mother yeast (or natural yeast or sourdough) both lactic and alcoholic fermentation takes place and not exclusively lactic fermentation as mistakenly led to believe to distinguish it from beer yeast. In the criscenti (or carry-over paste), a higher presence of saccharomyces cerevisiae than the sourdough, i.e. immediately after it has been specifically inoculated, is conceivable. However, since the criscenti is also composed of water and flour, the presence of lactobacilli is physiological . Already from the first refreshment which, as we said, is nothing more than the addition of nutrients, that is water and flour, the microbial flora of the mother yeast is distorted . It follows that no mother yeast can be considered the same as another, although made with the same ingredients, at the same exact moment but in different environments. It follows that potentially, after for example a month, the original balance between species of fungi and bacteria can be radically distorted and that new species may have been brought in by the refreshments. Therefore, what objective parameter should we refer to in order to distinguish sourdough from "criscenti"? The only certainty is that the acidic environment reduces the possibility of contamination by non-acidophilic fungal and bacterial species, that is, most of the species pathogenic to humans. Neither the mother yeast nor the criscenti are isolated and isolable systems . Talking about the longevity of a mother yeast is a logical nonsense since the entire "population" of bacteria and yeasts changes almost completely within a couple of months.
Another goes for the fresh beer yeast which is nothing more than a colony of saccharomyces cerevisiae, generated by the fermentation of the molasses (liquid obtained by centrifuging the sugar beet) with barley malt. The difference between fresh brewer's yeast and dried brewer's yeast is that in the latter the enzymes (in a nutshell, the substances that allow the yeasts to proceed metabolically by fermentation, therefore to let the dough rise) are reactivated only at contact with water.

Final thoughts

In order to obtain a good dough, it is essential to keep in mind that its maturation (which we informally distinguish from the leavening since the latter would strictly concern the exclusive action of the yeasts) relates to the metabolic processes of yeasts and bacteria: the imbalance of their action leads to dough defects which are most of the time irreversible. For example, adding a few grams of fresh brewer's yeast in a kilo of dough (instead of a whole dough) leaves space and time for the bacterial action, which we have seen to be equally important. Clearly, it will not be possible to knead in the late afternoon and bake in the same evening ... It is also true that the much advertised long leavening can also be synonymous with products so one should wonder if it is worth turning on the oven ... complete synthesis of sugars involves first of all the failure of the Maillard reaction as well as compromising the flavor of the dough. In addition, the hyper-synthesis of proteins can, as already mentioned, weaken the strength and elasticity of the glutin mesh, which is essential for the development of the alveolation. I conclude by emphasizing that the honeycomb is not a mere stylistic or aesthetic choice but optimizes the heat diffusion in the dough during cooking, so that starch gelatinization takes place in the necessarily short time to obtain a good product.