For the past few weeks, an old acquaintance has reappeared in my house. I haven’t seen kefir in years, so much so that I associate it with my teenage days. I still remember when this fermented milk first entered our house, received as a most precious gift from a family friend, with a thousand recommendations on how to take care of this ‘baby’ with almost magical characteristics.
ARTICLE INDEX
- What is fermentation?
- The different types of fermentation
- Kefir: what is it and how is it produced?
- Fermented milk products
- Sources and notes
What is fermentation?
But let’s take a step back and see what fermentation is, on a biochemical level.
Fermentation is a metabolic pathway, i.e., a sequence of chemical reactions occurring in a cell to derive energy from organic compounds such as sugars or amino acids, in the absence of oxygen. Under aerobic conditions, i.e., when there is oxygen available, the cell prefers to do cellular respiration, which is a metabolic process that goes so far as to completely burn fuel, e.g., sugar, to the point of producing carbon dioxide and water as in all combustion. This combustion, which fortunately does not generate flames or too much heat, is very efficient in producing usable energy from the cell.
However, when oxygen is in short supply or absent altogether, the cell, which must still produce energy, begins fermentation. Instead of completely burning its fuel, it only partially oxidizes it. It does not produce much energy, but it is better than nothing. The compounds that are generated by this metabolic pathway are alcohols or acids that impart characteristic aromas to fermented products (some pleasant, some unpleasant).
A totally oxygen-free environment is often not necessary to persuade cells to ferment. Brewer’s yeast, for example, when put in front of a sugar-rich feast, gets excited and starts fermenting spontaneously, even though it has oxygen available for respiration. [1]
The different types of fermentation
There are different types of fermentation. A well-known one is the alcoholic one, which is carried out mainly by yeasts and is used in the production of beer, wine, and leavened baked goods. Its characteristic is to produce alcohol and carbon dioxide. In some cases, we seek the production of alcohol (ethanol, also known as ethyl alcohol) to make drinks such as wine alcoholic. In other cases we look instead for the production of carbon dioxide, which swells bread and pizza dough (ethanol is still produced, but it evaporates during baking at high temperatures). Sometimes both are welcome, as with beer or sparkling wines.
Another popular fermentation is lactic fermentation, carried out mainly by lactobacilli. These bacteria convert sugars into lactic acid, which turns yogurt or kefir slightly acidic. Other substances produced or released during fermentation then contribute the distinctive aromas that make each type of fermented milk unique, differentiating yogurt from kefir or buttermilk from yakult.
A curiosity: our muscles are also capable of lactic fermentation; we become aware of this when we exert too much or too prolonged muscle effort and begin to feel a sense of soreness that can last for several days and be very uncomfortable. Why does this happen. If the oxygen supply to the muscle is not sufficient to do enough cellular respiration to sustain the required workload, the muscle cell must still take energy for the contraction/relaxation cycle. Guess where he gets it from? That’s right, just from lactic fermentation. It does not produce as much energy as it could have if it did only cellular respiration, but if oxygen is in short supply this is as much as it can afford. In short, it manages to keep us running a little longer, whether we are running from a bear or late to catch a train, but with the knowledge that the next day we will discount our impromptu sprint with pain and discomfort.
Kefir: what is it and how is it produced?
Kefir is a fermented milk whose consumption is common inEastern Europe and parts ofAsia. It is similar to yogurt but you cannot call it yogurt, which must be produced using two particular lactic acid bacteria, as we explained in this article .
Kefir is produced from gra ins (or granules) that look similar to the small inflorescences of cauliflower, called kefiran . These soft, gelatinous clusters are actually mini-hotels for microorganisms, harboring a huge variety of microbial species. Studies on this microbial consortium began in the 1970s, and even today new information is being discovered about the tiny inhabitants of these miniature cauliflowers. [2]
Interestingly, it turns out that it is not only bacteria that live inside kefir granules, but there are also yeasts. Their presence makes the fermentation of kefir different from that of plain yogurt. [3]
Fermented milk products
There are many different types of fermented milk products in the world. In many cases these are domestic, not industrial productions. Fermenting microbial communities, therefore, vary greatly and differ because of the microbiota typical of that region of the world, environmental characteristics, and the fermentation process used.
For example, the buttermilk , o buttermilk, comes from the processing of cream into butter and is fermented by Lactococcus lactis or Lactobacillus bulgaricus. [4] In contrast, kefir contains a very large variety of lactic acid bacteria (83-90% of the total microbial count) such as Lactococcus lactis subsp. lactis, Streptococcus thermophilus, Lactobacillus delbrueckii subsp. bulgaricus, Lactobacillus helveticus, Lactobacillus casei subsp. pseudoplantarum and Lactobacillus brevis, but also contains yeasts (10-17% of the total) such as Kluyveromyces marxianus var. lactis, Saccharomyces cerevisiae, Candida inconspicua and Candida maris. [5]
Lo yakult , on the other hand, is a Japanese probiotic beverage made from skim milk fermented by Lactobacillus paracasei Shirota, with a flavor derived from the heat treatments that occur before fermentation and the additions of ingredients that occur after fermentation. And then there is yogurt, which we are all familiar with, which can only be sold under that name in Italy if it is produced by fermentation by Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus. In short, from great varieties of microbes come great varieties of fermented milk.
Because microorganisms are present in many of the fermented milk products in very high amounts and manage to reach the intestines alive, for some of these we can speak of true probiotics. [6] Online you can find many sites extolling its beneficial, protective and in some cases even therapeutic properties. For a more in-depth look at how to weigh the many pieces of information circulating about the health effects of probiotics, we refer you to this article .
Sources and notes
[1] This behavior is called the “Crabtree effect” and is also typical of some cancer cells
[2] For a historical survey of investigations into the composition of kefir and to learn more about the microorganisms that live in it, I point to this article Edward R. Farnworth, Kefir – A complex probiotic (2005), Food Science & Technology Bulletin Functional Foods 2(1), pp. 1-17, DOI: 10.1616/1476-2137.13938]
[3] To brush up on the differences between bacteria and yeasts, you can review the article “Variety Fair ” https://ingredients.saccosystem.com/conoscere-e-riconoscere-i-microrganismi/
[4] SOURCE. https://www.livescience.com/45614-what-is-buttermilk.html
[5] SOURCE Simova E. et al, Lactic acid bacteria and yeasts in kefir grains and kefir made from them, 2002, J Ind Microbiol Biotechnol., 28(1), pp. 1-6. doi:10.1038/sj/jim/7000186
[6] See, in this regard, also the article https://ingredients.saccosystem.com/probiotici-cosa-si-cela-dietro-questo-nome/
