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I've found many sources about the positive effects of kefir for the digestive system. However I haven't found any information about the fermenting process.
What is the exact biology (chemistry?) behind the process of milk becoming kefir?
What do the kefir grains exactly consume from the milk? And what do they defecate? What happens inside the microbes?
The fermentation is relatively complex, as quite a number of different bacterial species and yeast live in the Kefir grains. These are made of of the polysaccheride kefiran which immobilizes the bacteria (in contrast for example to yoghurt). The microbes live symbiotic on this matrix.
According to the references a number of different microorganisms has been found, these include mainly Lactic acid bacteria, followed by yeasts and acetic acid bacteria. The species include: Lactobacillus paracasei, Lactobacillus parabuchneri, Lactobacillus casei, Lactobacillus kefiri, Lactococcus lactis, Acetobacter lovaniensis, Kluyveromyces lactis, Kazachstania aerobia, Saccharomyces cerevisiae and Lachancea meyersii.
In brief the lactobacilli break down lactose to produce lactic acid, while the yeasts make alcohol and carbon dioxide while the acetobacter species make acetic acid. This consumes the lactose and the fat content of the milk making it suitable for lactose intollerant people. It also lowers the pH of the kefir. The exact mixture is hard to identify as this contains strongly on the exact composition of the kefir grains.
If you are interested further, I recommend you have a look into the references, I find both articles relatively good to understand.
Well essentially the added Lactobacillus species ferment lactose and other sugars to lactic acid, thus lowering the pH of the product (they take up lactose use it as an energy source under anaerobic conditions and secrete lactic acid). This gives the kefir and other fermented milk products their sourness. Also the lower pH prevents many other bacteria from growing in the product so this is also serves as a natural preservation method. Further more the low pH can denature some of the protein in the milk/kefir resulting in a more solid form. Many of these Lactobacillus species or their relatives are natural part of our gut flora.
Fermented foods are those in which microorganisms have transformed relatively complex substances into simpler ones. 1 This simple process can change the food characteristics completely, turning grape juice into wine or milk into yogurt.
The microorganisms in charge of this transformation are called &ldquoferments&rdquo, and they are generally bacteria or yeasts. Fermentation is the natural process that the specific microorganisms use in order to obtain energy for growth and development.
What is fermentation?
Fermentation is the process of sugars being broken down by enzymes of microorganisms in the absence of oxygen. Microorganisms such as bacteria and fungi have unique sets of metabolic genes, allowing them to produce enzymes to break down distinct types of sugar metabolites. During fermentation, a variety of microorganisms are present in different proportions. The process is akin to a concert where different musicians (i.e. microorganisms) have their respective roles. Their cooperation produces beautiful music&mdashour favorite fermented food. Therefore, when the types and numbers of microorganisms are changed, the taste of fermented food can also change dramatically. That is also why food companies take extreme care to safeguard their recipes and maintain their biobanks of microorganisms.
Types of Fermentation
Based on chemicals produced during the process of fermentation, it can be divided into two types:
1. Lactic Acid Fermentation
The fermentation, in which lactic acid is the product, is called lactic acid fermentation.
- It occurs in human muscle cells when there is less availability of oxygen.
- Yogurt production is a type of lactic acid fermentation in which bacteria are used to convert milk into yogurt enzymatically.
- Lactic acid fermentation is chemically represented as:
3. Special Issue on “Yeast Fermentation”
This issue in Microorganisms aims to contribute to the update of knowledge regarding yeasts, regarding both basic and also applied aspects. Among the great contributions to this issue we have a manuscript devoted to the brewing industry and the recent isolation of the yeast Saccharomyces eubayanus . The use of headspace solid-phase microextraction followed by gas chromatography-mass spectrometry (HS-SPME-GC-MS) has contributed to the production of volatile compounds in wild strains and to compare them to a commercial yeast. All these findings highlight the potentiality of this yeast to produce new varieties of beers. Haile et al.  have explored the possibility to identify and select pectinolytic yeasts that have potential use as a starter culture for coffee fermentation. Almost 30 isolates, eight of them with the ability to produce pectinase enzymes were identified and confirmed by using molecular biology techniques. A helpful bioinformatics tool (MEGA 6) was also used to generate phylogenetic trees able to determine the evolutionary relationship of yeasts obtained from their experiments. Biofuel production by recombinant Saccharomyces cerevisiae strains with essential genes and metabolic networks for xylose metabolism has been also reported . The authors have shown that the deletion of cAMP phosphodiesterase genes PDE1 and PDE2 can increase xylose utilization. Moreover, the door is opened to provide new targets for engineering other xylose-fermenting strains. The utilization of xylose, the second most abundant sugar component in the hydrolysates of lignocellulosic materials, is a relevant issue. Understanding the relationship between xylose and the metabolic regulatory systems in yeasts is a crucial aspects where hexokinase 2 (Hxk2p) is involved . All of these processes can be damaged if contaminated. Because most fermentation substrates are not sterile, contamination is always a factor to consider. With a very interesting approach, a genetically modified strain of Komagataella phaffii yeast was used for the use of glycerol as a base substance in lactate production. Polyactide, a bioplastic widely used in the pharmaceutical, automotive, packaging and food industries was produced. The disruption of the gene encoding arabitol dehydrogenase (ArDH) was achieved, which improves the production of lactic acid by K. phaffii as a biocatalyst . Seo et al.  have developed and proposed alternative solutions to control contamination. This review includes information on industrial uses of yeast fermentation, microbial contamination and its effects on yeast fermentations. Finally, they describe strategies for controlling microbial contamination.
Sauerkraut is one of the most common forms of preserved cabbage originating in the 4th century BC. Sauerkraut is eaten frequently in Germany, but also in other European and Asian countries and the United States . Sauerkraut is produced from a combination of shredded cabbage and 2.3%𢄣.0% salt, which is left to undergo spontaneous fermentation, generally involving Leuconostoc spp., Lactobacillus spp., and Pediococcus spp. The low pH of the final product results in a preserved cabbage .
Sauerkraut (homemade and shop-bought) has been shown, through culture-dependent techniques, to contain Bifidobacterium dentium, Enterococcus faecalis, Lactobacillus casei, Lactobacillus delbrueckii, Staphylococcus epidermidis, Lactobacillus sakei, Lactobacillus curvatus, Lactobacillus plantarum, Lactobacillus brevis, Weissella confusa, Lactococcus lactis and Enterobacteriaceae [17,18,88]. Adding a starter culture of Lactobacillus casei 11MZ-5-1 produced a sauerkraut containing predominantly Lactobacillus and Lactococcus, compared to spontaneous sauerkraut which, along with Lactococcus and Lactobacillus, also contained significant Enterobacter and Pseudomonas and was more variable in microbial composition . Sauerkraut has also been shown to predominantly contain Leuconostoc and Lactobacillus spp. [18,19,90,91]. Certain Lactobacillus species isolated from sauerkraut demonstrate probiotic potential, with tolerance to low pH, adherence to Caco-2 cells and antimicrobial activity against pathogens in vitro . Lactobacillus paracasei HD1.7, commonly found in sauerkraut, has been shown to produce a broad-spectrum bacteriocin that may play a role in sauerkraut preservation .
Oral administration of sauerkraut juices in Wistar rats led to increased activity of glutathione S-transferase (GST) and NAD(P)H:quinone oxidoreductase 1 (NQO1), key liver and kidney detoxifying enzymes . Certain lactic acid bacteria contained in sauerkraut generate conjugated linoleic acid , for which there is evidence of anti-carcinogenic and anti-atherosclerotic activity in animals [96,97]. Furthermore, Lactobacillus plantarum P2 isolated from sauerkraut significantly induced TNF-α and IL-12 expression and prevented adhesion and invasion of Caco-2 cells by Salmonella enteritidis . Sauerkraut contains glucosinolate breakdown products including kaempferol, (a flavonoid) isothiocyanates, indole-3-carbinol, goitrin, allyl cyanide and nitriles . The relevance of such phytochemicals to human health is unclear, however kaempferol has been shown to have radical scavenging activity, to protect from oxidative damage and to attenuate cytokine-induced reactive oxygen species in vitro . Isothiocyanates have been shown to have antimicrobial properties, preventing the growth of a range of species, including E. coli, C. difficile, C. jejuni and C. perfringens .
Sauerkraut is one of the few fermented foods for which there is a clinical trial in functional bowel disorders. A randomised double-blind trial compared the effects of sauerkraut containing viable lactic acid bacteria (LAB) on gastrointestinal symptoms and microbiota in 58 patients with irritable bowel syndrome (IBS) of any subtype diagnosed using Rome III criteria . Patients were randomised to consume 75 g/day pasteurised (control) or unpasteurised (intervention) sauerkraut containing LAB for 6 weeks. There was a significant reduction in IBS Severity Scoring System (IBS-SSS) score between baseline and end of trial in both study groups, however there was no difference in symptoms between the diet groups 16S rRNA sequencing revealed no difference in microbiota composition between study groups or between baseline and end of trial in either group ( Table 3 ). This may suggest that the perceived health benefit of sauerkraut is independent of the live microbes. A limitation of this study is the per protocol analysis in that only patients who completed the study (n = 34) were included in the analysis of the primary outcome. Furthermore, because there was no raw cabbage arm, it is not possible to determine whether improvement in gastrointestinal symptoms was related to the fermentation-derived products or the cabbage itself.
Summary of interventions studies investigating the impact of sauerkraut, soy products and kimchi in gastrointestinal health and disease.
|Study||Fermented Food||Study Design||Study Population||Intervention||Control||Duration||Gut Microbiota||Other Findings|
|Fujisawa et al., 2006 ||Natto/miso||Uncontrolled open-label study||Healthy, |
n = 8
|200 mL miso soup containing 50 g Natto per day||-||2 weeks||Following natto-containing soup:|
Higher Bifidobacteria and Bacilli, Lower Enterobacteriaceae,
Higher acetic acid and propionic acid (all p < 0.05)
|Kil et al, 2004 ||Kimchi||Non-randomised trial||H. pylori infection, |
n = 6
|300 g of kimchi||60 g of kimchi||4 weeks||Increased Lactobacillus (p = 0.0003) and Leuconostoc (p = 0.0004)||H. pylori not eradicated in any participants (p = 0.944).|
Lower stool pH (p = 0.0001), β-glucuronidase (p = 0.0065) and β-glucosidase (p = 0.0001) activity
|Mitsui et al., 2006 ||Natto||Controlled trial||Infrequent bowel movements, |
n = unknown
|50 g/day Natto (Bacillus subtilis K-2, 3.8 × 10 9 CFU)||50 g/day boiled soybeans||2 weeks||Following Natto compared to control:|
Increased ratio of stool Bifidobacteria:total bacteria
|Following Natto compared to control:|
Higher number of bowel movements. Higher number of days with bowel movements
Higher stool quantity
|Nielsen et al., 2018 ||Sauerkraut||Randomised, double-blind controlled trial||Irritable bowel syndrome, |
n = 58
|75 g/day unpasteurised sauerkraut containing LAB||75 g/day pasteurised sauerkraut||6 weeks||No significant effects of either unpasteurised or pasteurised sauerkraut on microbiota composition||Lower IBS-SSS score following both unpasteurised (p = 0.003) and pasteurised (p = 0.04) sauerkraut|
No difference in change in IBS-SSS between groups
LAB, lactic acid bacteria IBS-SSS Irritable Bowel Syndrome Severity Scoring System.
Another study in Chinese participants suggested larger amount of sauerkraut may in fact be associated with poor health outcomes in gastrointestinal cancers. This case-control study found that the highest compared to the lowest quintile of sauerkraut intake was associated with a greater risk of laryngeal cancer (odds ratio (OR) 7.27) . One possible mechanism may relate to the high salt content of sauerkraut, although another case-control study of dietary risk factors for laryngeal cancer in China showed no associations with salt-preserved vegetables . Similarly, the high potassium content of sauerkraut is thought to counter the hypertensive effects of added salt.
Taking the limited evidence for sauerkraut into account, one trial indicates that both pasteurised and unpasteurised sauerkraut reduced IBS severity, this effect does not appear to be mediated by gastrointestinal microbiota. Further studies are required to elucidate the mechanisms of this effect on gastrointestinal symptoms. There is little evidence for effects of sauerkraut on other health conditions.
Milk fermentation: what is it?
In the case of milk, when good healthy bacteria, such as Lactobacillus delbrueckii bulgaricus and Streptococcus salivarius thermophilus, are added and a particular temperature is reached, they start to nibble at the tasty molecules of lactose, which is a sugar, transforming them into pyruvic acid.
In the absence of oxygen, the latter eventually turns into a lactic acid. Now, seeing as there are also many proteins in milk, these react with the recently formed acid by breaking up into many pieces, only to recompose themselves in a different form.
Probiotic fermented foods and health promotion
Food fermentation has, throughout much of human history, been the most common way of preserving perishable foods, thereby maintaining and in some cases even improving the nutritional value of these foods. Genesis 18:8 refers to how Abraham serves curds and milk to his guests. Not surprisingly, some of these fermented foods were perceived to be inherently healthy. The mechanism behind this preservation was not clarified until 1857, when Louis Pasteur identified “lactic yeast” as the source of lactic acid fermentation. A first “scientific” promotion of fermented food specifically as a health product came in the early 1900s with Ilya Metchnikoff, who advertised yogurt, fermented with the Bulgarian bacillus, and insisted it would contribute to longevity ( Metchnikoff, 1907 ). In the 1930s, Minoru Shirota specifically isolated a health-promoting microbe and introduced the oldest still-existing probiotic food, Yakult.
Probiotics have been defined as “live microorganisms which when administered in adequate amounts confer a health benefit on the host” ( FAO/WHO, 2002 ). Maintaining viability imposes some technological requirements on the manufacturing of the probiotic food product the minimal counts should be guaranteed until the end of shelf life. The required level of these counts is likely to depend on the probiotic strain and the intended health benefit. As a rule of thumb, a minimum of 10 9 colony-forming units (CFU)/consumption is used ( Forssten, Sindelar, & Ouwehand, 2011 ). A correct approach would be to use a minimum dose according to that used in studies documenting the given health benefit.
Although probiotics are widely consumed as dietary supplements, the focus of the present chapter is on fermented probiotic foods. Most commercially available probiotics belong to the genera Bifidobacterium and Lactobacillus strains from other genera are being marketed as well, but these rarely find application in fermented foods and will thus not be discussed here.
WHAT AFFECTS THE LACTOSE CONTENT OF MILK KEFIR?
- The amount of time that your kefir is cultured,
- What you do with your kefir after it has cultured.
By manipulating these factors you can control the lactose content of your kefir to some degree.
Milk Kefir Culturing Time
When milk is cultured into milk kefir, the culture consumes the lactose. The more time the milk kefir is given to culture, the more lactose is consumed and the more acids are produced. One of the best indications of how much lactose remains is the amount of acids in the end product. More acids present equate to a tangier milk kefir.
Maturing Milk Kefir After Culturing
Once the milk kefir has cultured for 24 hours, there is another step to take, which ensures that the lactose content is as low as possible. This step is called maturing or ripening.
Vinegar could be successfully produced from apple alcoholic must using kefir grains. This is the first study to produce kefir vinegar. The chemical analyses revealed that kefir vinegar has high contents of organic acids, which add functional value to the vinegar. Kefir vinegar had a good performance during the acetylation (∼79%), reaching the required standard for the Brazilian legislation accepts it as vinegar (4.0% acetic acid). 4 4 Brasil, Decreto n° 6.871, de 4 de Junho de 2009, Regulamenta a Lei no 8.918, de 14 de julho de 1994, que dispõe sobre a padronização, a classificação, o registro, a inspeção, a produção e a fiscalização de bebidas. Legislação Bebidas. Presidência da República. Retrieved from http://www.planalto.gov.br/ccivil_03/_Ato2007-2010/2009/Decreto/D6871.htm 2009.
http://www.planalto.gov.br/ccivil_03/_At. Microorganisms were found previously described in kefir grains and the microbiota remained standardized during the fermentation process. There was no microbial contamination during the fermentation process. A new methodology to measure the metabolic activity of kefir grains by Biospeckle Laser was presented. This fact facilitates the microbiological control over a fermentation process. Kefir grains showed efficiency in the production of apple vinegar. Kefir vinegar was well accepted by sensory analysis.
The novel technology proposed use of kefir grains for the production of vinegar was successfully done. The key point for industrial application of the proposed technology is the promotion of fermentation by an immobilized-cell biomass (kefir grains) that eliminate the use of centrifugal separators, which have a high energy demand and require high industrial investment.