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Kefir varies from other fermented products because of the specific property of its starter: The kefir grains. Kefir grains range in size from 1 to 4 cm in length and look like small cauliflower florets in shape (irregular and lobed-shaped) and color (from white to light yellow) [5]. This gelatinous and slimy structure is comprised of a natural matrix of exopolysaccharides (EPS) kefiran and proteins in which lactic acid bacteria (LA, yeasts, and acetic acid bacteria (AA co-exist in symbiotic connection [2].


The dairy and non-dairy kefir grains are quite similar to each other’s in relations of their structure, related microorganisms and their metabolic products during the fermentation procedure [4].

The fermentation procedure begins when bacteria and yeasts of the kefir grains find the proper culture requirements, which resulted in the increment of 5–7% grains’ biomass and the formation of various metabolites [18].

Non-dairy kefir is a beverage made from the fermentation of kefir grains with a sugary solution, wherein the brown sugar solution is the main alternative substrate used for kefir fermentation [23]. Other non-dairy kefir prepared from fruit juices (apple, pineapple, grape, quince, kiwi, pear, pomegranate, melon, strawberry, tomato, coconut), vegetables (ginger, onion, soybean, fennel, carrot), and molasses (sugarcane, honey) are also suitable alternative substrates for the non-milk adaptation of kefir production [4,24,25,26,27,28,29]. These adaptations came about to allow non-dairy consuming and vegan individuals to reap the benefits of drinking kefir [30]. Regular daily intake of vegetables and fruits is strongly advocated for numerous positive health effects and disease prevention [31,32,33,34]. Thus, the production of fruit or vegetable juice-based fermented kefir beverage with may be perceived by consumers as healthy and provides an extra method to boost fruit and vegetable intake [35].

The non-dairy kefir fermentation is carried out by the kefir grains consisting of a consortium of yeasts mainly Kluyveromyces, Candida and Saccharomyces, and lactic acid bacteria (LA, including the genera Lactobacillus, Lactococcus, Leuconostoc, and Streptococcus, embedded in a natural matrix of exopolysaccharides (EPS) kefiran [36,37,38]. Various species are found to have symbiotic associations and live or proliferate by sharing their bioproducts as energy supplies or growth-inducing factors, that may vary depending on non-dairy substrates used during kefir fermentation [39]. Indeed, sugary water or fruit juices contain water, sugar, and a mixture of nutrients; proteins, amino acids, vitamins and minerals that are suitable to prepare fermented beverages like kefir as they provide an ample medium for microbial expansion that could promote a fast rise of kefir grain biomass [40].

Non-dairy kefir beverages are traditionally produced by directly adding kefir grains to the pasteurized and cooled substrate and incubated for around 24 h at 25–30 °C. At the completion of fermentation, the grains are isolated from kefir by sieving, followed by washing, drying at room temperature and storage in a cooling tank for the next round of fermentation procedure [4]. The chemical composition and sensory feature of non-dairy kefir beverages vary corresponding to the substrate used, including sugars (sucrose, glucose, and fructose), organic acids (lactic, acetic, citric, tartaric, butyric, malic, and propionic acids), alcohols (ethanol, hexanol, and glycerol), and esters (ethyl propionate, ethyl hexanoate, octanoate, and decanoate). These metabolites afford distinctive flavor qualities for these products, such as revitalizing taste (due to presence of ethanol), fruity fragrance (due to presence of esters) and body and texture (due to presence of glycerol) that resulted in favorable consumer response for all analyzed products [4,35].

Currently, kefir has raised attention in the scientific group due to its various beneficial effects on health, including anti-hypertensive effects, as well as being a safe and an economical homemade food [42,43,44,45]. The symbiotic metabolic events of a number of bacterial and yeast species in kefir, which include both proteolytic and lipolytic degradation of milk constituents create numerous biologically active peptides, including ACE-inhibitory peptides [43]. ACE-inhibitors block angiotensin-converting enzyme (ACE) from converting angiotensin I to potent vasoconstrictor angiotensin II. Consequently, it inhibits the production of aldosterone, a hormone that promotes the rise of serum sodium (Na) concentration, causing a surge in blood pressure and the breakdown of bradykinin, a hormone that has vasodilating action, influencing the decrease in blood pressure [14,42,46].

The anti-carcinogenic effect of kefir and kefir fractions was studied for different cancer types, such as hematological cancers (leukemias and lymphoma), breast cancer, gastrointestinal system cancers (gastric and colorectal), and sarcoma (connective tissue tumor).

In 2002, Liu et al. [52] performed an in vivo oral treatment of milk kefir and soymilk kefir in mice inoculated with sarcoma. They found that both types of kefir have resulted in significant suppression of tumor growth through stimulation of apoptotic cell lysis in tumors and a significant rise in IgA levels in mice, proposing that both kefir have anti-cancer attributes and have enhanced the mucosal resistance to gastrointestinal infection after 30 days of consumption. Another study has shown the immunoregulatory ability of kefir or kefir cell-free fraction (KF) on the immune response in mammary glands to delay tumor growth on a murine hormone-dependent breast cancer model. The results demonstrated the relationship between immune and endocrine systems as shown by the activation of immune cells (increased the number of IgA(+) cells, and decreased the Bcl-2(+) cells) and production of cytokines (increased IL-10, and decreased IL-6) [54,55].

The anti-carcinogenic outcome of kefir and kefir fractions had been demonstrated on gastrointestinal system cancers. Gao et al. [60], studied the anti-proliferative activity of cell-free fraction of Tibetan kefir on human gastric cancer cell SGC7901 in vitro. They found that SGC7901 cells treated with kefir were impeded in the G1/S phase, and both early and late apoptotic cells could be distinguished. It was also reported that the induction of apoptosis was effected via the up-regulation of bax and downregulation of bcl-2. In addition, Khoury et al. [62] showed the ability of kefir to inhibit proliferation and induce apoptosis in HT-29 and Caco-2 colorectal cancer cells.

According to Van Wyk [11] one of the aspects of the probiotic effect of kefir is the fact that the kefir microbiota produces anti-microbial metabolites. This anti-microbial capacity may be ascribed to the presence of hydrogen peroxide, peptides (bacteriocins), ethanol, carbon dioxide, diacetyl, and organic acids (lactic and acetic acids), which inhibit pathogens, particularly in the intestinal mucosa. Kefir and kefir-associated strains have shown a multitude of anti-microbial activities as shown in Table 2.

In general, kefir showed bacteriostatic effects on Gram-negative bacteria, but it is was more effective against Gram-positive bacteria [14]. Suriasih (2011) reported kefir’s ability to exhibit antimicrobial activity against Gram-negative bacteria, Salmonella Typhi and Escherichia coli.

it has been demonstrated that a combination of kefir microorganisms exerted protection against diarrhea and enterocolitis triggered by Clostridium difficile [87]. A later study on the mixture of kefir isolated two lactobacilli, one Lactococcus, and two yeasts demonstrated protection on epithelial cells in vitro against Shigella invasion [109].

Interestingly, a recent review by Hamida et al. [117] proposed the potential of kefir and its by-products as protective agents against virus, such as Severe Acute Respiratory Syndrome Coronavirus 2' (SARS-CoV-2) that caused Coronavirus disease 2019 (COVID-19), owing to its proven antiviral mechanism against viral infections (Zika, hepatitis C, influenza, rotaviruses). Kefir and its probiotic contents were shown to regulate the immune system to overcome infections from these viruses by stimulating immune-system responses and also by suppressing the pursuit of pro-inflammatory cytokines

The study showed that the kefir peptides administration at both doses (150 mg/kg and 500 mg/kg) reduced PM4.0-induced inflammatory cell infiltration and inflammatory mediators’ expression such as TNF-a, IL-lß, and IL-4 in lung tissue by inactivating NF-?B signaling. Following the current finding in 2020, Chen et al. [122] further studied the effects of the kefir peptides against oxidative stress and inflammation and their protective ability against renal dysfunction on aged salt-induced stroke-prone spontaneously hypertensive (SHRSP) rats.

a study by Seo et al. [123] found that kefir produced extracellular vesicles (EV) that inhibit the inflammatory cytokine production by mitigating TNF-induced inflammation in intestinal cells. The study showed that treatment of each kefir-derived Lactobacillus EV (K-LEV) on TNF-a-stimulated Caco-2 cells had significantly reduced the mRNA expression and IL-8 secretion.

The results from this study demonstrated that kefir treatment had significantly decreased the ? radiation-induced hepatic function impairment, hepatic histological alterations, and dyslipidemia. The study indicated that the kefir inhibits the induced inflammation and improves the state of oxidative stress. A recent study by Tung et al. [128] reported the effects of kefir peptides on high-fat diet-induced atherosclerosis in apolipoprotein E knockout mice. The kefir peptides treatment has significantly improved the development of atherosclerotic lesion by attenuating oxidative stress, macrophage accumulation, endothelial dysfunction, aortic lipid deposition, and inflammatory immune response.

A study by Yilmaz-Ersan et al. [130] reflected that kefir samples fermented using kefir grains exhibited better antioxidant potency, which was measured by the 2,2-diphenyl-1-picrylhydrazyl (DPPH) and 2,20-azino-di(3-ethylbenzthiazolin-sulfonate) ABTS assays, than kefir samples fermented by started cultures.

Kefir was also found to have significant antioxidant effects against reactive oxygen species. Ghoneum et al. [135] studied the protective activities of a novel kefir product (PFT) on 10-month-old oxidative stress-induced mice.

Kefir has high cholesterol-lowering properties, and it has been mostly validated in animal models.

A study by Liu et al. [144] showed that milk kefir and soy milk kefir administration to male golden Syrian hamsters fed with cholesterol-enriched and cholesterol-free diet resulted in reduced total cholesterol and serum triacylglycerol levels and improved atherogenic index, indicating that kefir administration altered the endogenous cholesterol metabolism.

A randomized trial in twelve subjects with metabolic syndrome whose diets were supplemented with 180 mL/day kefir showed the modulation of intestinal microbiota, with a significant rise in Actinobacteria, as well as variations in the genera of the phyla Bacteriodetes and Firmicutes [181]. The results observed intervenes with the metabolic factors, property of metabolic syndrome, recovery in fasting insulin and insulin resistance index (HOMA-IR), and a decline in pro-inflammatory cytokines, and systolic and diastolic pressure, proving the modulation of the human gut microbiota through the kefir consumption in patients with metabolic syndrome.

Furthermore, recently there has been an immense interest in the studies of altered gut microbiota in response to mental health upon kefir consumption. In fact, the term kefir derived from the Turkish word “keyif”, indicate a good feeling and pleasure after consumption suggested its potential gut-brain axis relationship. Murray et al. [187] studied the vital role of the gut microbiota in adolescent around the fact that the probiotics consumption can alter brain chemistry and behavior. In this context, the gut microbiota regulates stress and inflammatory responses with probiotics during puberty in CD1 mice on lipopolysaccharide (LPS)-induced immune responses.