1 Introduction

The development of science and technology has shaken some empirical behaviors these last decades. It generated enormous progress in industries and is reshaping the food industry. However, this progress never signifies the disappearance of tradition, culture, and heritage [1]. The permanent increase of market needs in quantity and quality and the world population growth leads to mechanization and automation in commodities. Although this dynamic mechanism affects the house or small-scale producers, it induces improvement, innovation, creation, discipline, employment, standardization, and quality. Unfortunately, the food industry is excluded from this global change which is ongoing to provide safe and highly nutritional food to the world population.

Fermentation is a biotechnological process that has undergone tremendous changes over the decades. Although its primary function has consisted of food preservation, fermentation became one of the most critical steps in processing thousands of foods with a wide range of functions. Traditional fermented foods are obtained through spontaneous fermentation or a traditional starter culture, representing not only healthy foods and income sources but also heritage, culture, and traditions [2]. They are produced from cereals, roots, milk, non-timber forest products, vegetables, and fruits. Traditional fermented foods contain a wide range of nutrients, lactic acid bacteria, yeasts, and organic acids, which make them safe and industrialized [2, 3].

African fermented foods have probiotics that improve human gut microflora and protect against pathogens and diseases [4, 5]. Recently, Zannou et al. [2] reported the preparation ways and socio-economic advantageous of the tradtional fermented foods produced in Benin Republic. However, no overview has gethered the existing literature on microbiogical characteristics of the Beninese traditional fermented foods. Therefore, the present study will focus on the manifold health benefits of the traditional fermented foods produced in the Benin Republic and underline their microbiological characteristics. For this purpose, data published on the fermented products were collected from different sources using appropriate databases such as Google Scholar, web of science, and researchgate. Furthermore, indigenous knowledge and individual professional experience were gathered and used in the present study.

2 Concept of fermentation of traditional foods

Food fermentation is a technique that employs the growth of microorganisms, their metabolic activity, and enzymes for the processing of food materials. It is a biotechnological process involving microorganisms that convert the available hydrocarbons into organic acids, alcohols, carbon dioxide, and bacteriocins [6, 7]. Raw food materials are submitted to three different fermentation kinds: alkali, lactic acid, and alcoholic fermentation. These are resumed into two influential groups such as aerobic and anaerobic fermentation. Aerobic fermentation includes alkaline and fungal, while anaerobic fermentation comprises lactic and alcoholic. Alcoholic fermentation (ethanol and yeasts as primary metabolites) concerns alcoholic products, mainly wines and beers. In contrast, lactic acid fermentation (lactic acid bacteria) occurs with cereals, milk, and tubers, and alkali fermentation occurs in seeds and fish condiments [2, 6, 7].

Fermented foods have a significant value in the daily diet of many people worldwide, while they are generally produced through natural and spontaneous fermentation. This fermentation is cultural, cheap, energy and time-saving process. However, traditional fermentation is associated with contamination risks as it is operated in non-sterile conditions. Nonetheless, lactic acid bacteria are often predominant with other microorganisms such as yeasts and some molds. Concerning this fact, the ancient method of fermentation with the current biotechnology tools should be combined to control fermentation processes and the selection of starter cultures [8, 9]. The starter cultures are referred to the development and enhancement of inoculants containing high concentrations of live microorganisms. In most developing countries, the starter culture is added by transferring old batches to fresh raw materials. This process is known as back-slopping and looks like the batch culture transfer procedure used by microbiologists to study the evolution of microbial populations and communities in the laboratory [10]. The industrial-scale production of fermented foods ultimately depends on precise starter culture. For instance, in Tunisia, artisanal and industrial starters are employed to produce Leben [11]. Artisanal starter cultures are referred to an unknown number of undefined strains, whereas industrial starter cultures are considered well-defined strains. The use of starter cultures should allow to avoid fermentation failure, standardize the production, and guarantee quality. According to Foerst and Santivarangkna [12], starter cultures have properties that ensure fast, safe, and desired fermentation, allowing the production of fermented foods with high and constant product quality. The starter cultures for modern and industrial purposes are expected to have properties such as probiotics, bio-protection, enhancement of yield and performance, and their possible use for fermenting other foods.

The fermentation improves the digestibility of the raw products, eliminates unuseful compounds, and increases the bioavailability of proteins, lipids, hydrocarbons, minerals, and vitamins [13]. The fermented foods obtained in hygienic conditions are safe because they display low pH and contain natural antimicrobials, prebiotics, and probiotics.

3 Traditional starter cultures

The majority of traditional fermented can be prepared through spontaneous fermentation. However, the use of starter cultures emphasizes microbial action during fermentation with many advantages. Starter cultures are defined as an individual or a mixture of microorganisms strains that can transform a substrate into targeted food [14, 15]. They are added as inoculum to a raw material to aid microbial actions during the fermentation. As a result, they accelerate the fermentation process, improve nutrition quality and microbial safety, enhance organoleptic characteristics and potentiate the preservation capacities [14, 16, 17]. Two main varieties of starter cultures, such as traditional and industrial or commercial, are used to facilitate fermentation [14, 16,17,18].

Traditional starters are the most used during fermentation in developing countries as they are naturally available and affordable. The traditional starter culture can be categorized into two types. The first type is back-slopping, considered the most common starter used during the fermentation of traditional foods. Back-slopping consists of adding a portion of a previously fermented batch to a raw material prepared for the fermentation [14, 18]. In this case, the starter culture and end product are the same products. This procedure produces many food products, including sourdough, artisanal cheeses, traditional curdled milk, fermented meat, and Leben [11, 14, 18]. The second type of traditional starter is a formulated product with microbial strains capable of completing the fermentation of the desired product. Generally, this starter culture is obtained from a raw material different from the targeted product. However, it may be drifted from the same raw material but differently prepared. This second type of starter is used in the fermentation of many products worldwide, especially in developing countries.

The fermentation of some African Locust beans required the inoculation of the raw material by another product obtained from plants seeds, notably Hibiscus sabdariffa, Gossipium hirsutum, and Adansonia digitata seeds [19, 20]. In the Benin Republic, two main additives or starters, Yanyanku and Ikpiru or Iku-iru, are involved in the fermentation of African Locust beans for Sonru, Iru, and Sonru-babaru [20,21,22]. Yanyanku is prepared from H. sabdariffa, G. hirsutum, or A. digitata seeds and used as a starter for Sonru. At the same time, Ikpiru or Iku-iru is produced from H. sabdariffa and used to ferment African locust beans to Iru. The use of these starters induced an increase in the organoleptic properties, texture, and load of Bacillus strains which are the predominant microorganisms of these fermented products [20,21,22]. Interestingly, the fermentation of sorghum (Sorghum bicolor) which leads to the production of opaque African beer Tchoukoutou, demands the use of a traditional inoculum called Kpete–kpete. Even though Kpete–kpete is taken from a previous batch of Tchoukoutou, it is not the so-called back-slopping. Indeed, Kpete–kpete is a slurry of a previously sedimented Tchoukoutou. This traditional starter culture has excellent antimicrobial properties against pathogens and is predominated by lactic acid bacteria and yeasts, together with an exciting amount of bacteriocin and ethanol [23, 24]. The fermentation of cassava roots into Agbelima involves Kudeme, an artisanal starter culture. Kudeme is prepared from cassava chunks wrapped and fermented at room temperature for four days [25, 26]. Three varieties of Kudeme obtained through soaking, toasting, and blanching processing can be used to formulate Agbelima; however, these different starters affect the quality of the end product, and the blanching method revealed to be the most recommended in terms of color, texture, and flavor properties. Similar starter cultures are used in Ethiopia and India for the fermentation of enset (Ensete ventricosum) to Kocho and the bamboo shoot (Schizostachyum capitatum) to Soidon, respectively [16, 27].

Industrial or commercial starter cultures are those starters produced with well-defined and controlled conditions. The commercial starter cultures are employed in concentrated forms for direct inoculation into the food matrix, replacing the traditional methods [12]. Their development requires a long microbiological investigation, such as identifying the strains in fermented foods, their isolation, and genetic differentiation [16]. These starters regroup main lactic acid bacteria, some yeasts, and molds and are produced by specialized industries. Their use for the fermentation of traditional foods revealed to increase the dry matter, avoid fermentation failure, reduce pH levels, improve the organoleptic characteristics and general acceptability, reinforce the nutritional values and generate compounds to ensure the preservation as detoxification. In Benin Republic, industrial starter cultures are not yet used in the production of the traditional fermented.

4 Microbiological characteristics of traditional fermented foods

A wide range of traditional fermented foods is produced in Benin (Fig. 1). The cereal based traditional fermented foods include Ogi, Tchoukoutou, Gowé, Tchakpalo, and Mawè. The microbial strains in these beverages and foods comprise mainly bacteria and yeasts [28,29,30,31] (Table 1). A necessary amount and species of lactic acid bacteria encompassing Lactobacillus fermentum, Lactobacillus buchneri, Weissella confusa, Lactobacillus delbrueckii ssp. delbrueckii, Lactobacillus divergens, Lactobacillus bifermentum, Lactobacillus mucosae, Pediococcus pentosaceus, Weissella kimchii, Lactobacillus delbrueckii ssp. bulgaricus, Lactobacillus fructuvoans, Pediococcus acidilactici, Lactobacillus casei, Lactobacillus acidophilus, Enterococcus faecium, Enterococcus faecalis, Streptococcus thermophilus and Leuconostoc mesenteroides were detected in Gowé, Kpètèkpètè, Tchoukoutou, Ogi, Ikigage, Mawè, Dolo, Pito, and Burukutu. Likewise, the yeasts species such as Saccharomyces cerevisae, Torulaspora delbrueckii, Candida inconspicua, Saccharomyces pastorianus, Issatchenkia orientalis, Candida etchellsii, Kluyveromyces marxianus, Candida tropicalis, Candida krusei, Candida kunwiensis, Pichia anomala, Candida magnolia, Candida albicans, Dekkera anomala, and Candida humilis were identified [29, 31, 32]. These drinks are prized and sold on the streets in both rural and urban communities. The uncontrol handling during post-processing or selling might occur microbial contamination. For example, the presence of pathogens such as coliforms, Staphylococcus, and Salmonella was revealed in Tchapkalo sold in Ivory Coast and Benin Republic [33, 34]. Thus, strict hygienic rules should be applied during and after the production of these beverages to avoid public health problems.

Fig. 1
figure 1

Photos of Beninese traditional fermented foods; A Tchoukoutou, B Tchakpalo, C Mawè, D Gowé, E Aftin, F Lafun, G Agbelima, H Lanhouin and I D Dèguè [2]

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Table 1 Microbiological characteristics of the Beninese traditional fermented foods
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Afitin, Iru, and Sonru are three traditional condiments produced by spontaneous fermentation of African locust bean in different regions of Benin Republic. The microbiological study of afitin, Iru, and Sonru revealed that Bacillus spp are the main microorganisms involved in their fermentation [21, 22] (Table 1). These Bacilli are also reported to be responsible for the fermentation of African locust beans to produce Dawadawa, Soumbala, and Netutu. The species B. subtilis, B. licheniformis, and B. cereus are those found in the three condiments with the predominance of B. subtilis along the fermentation process [21, 22]. Some studies suggested using Bacillus spp. as starter cultures for the controlled fermentation of African locust beans to produce safe and reproducible quality condiments [21, 43]. Apart from Bacillus spp., Staphylococcus spp. has been identified in Afitin, Iru, and Sonru [21]. Staphylococcus spp. was detected at the beginning of the fermentation in the three condiments, but the number was higher in both Iru and Sonru than in Afitin. At the end of the fermentation, the initial Staphylococcus counts significantly decreased in the three condiments [22], presumably due to the modified environment, which developed at later stages and was not favorable for their growth. The presence of Staphylococcus in the fermented condiment of African locust beans could be originated from the contamination by handlers of cooked cotyledons or ingredients [20, 22]. Iru and Sonru fermentation involves using two conventional additives, Iku-iru and Yanyanku. These additives, considered a softening agent by the producers, seem to be responsible for the higher number of Staphylococci in Iru and Sonru but also enhance the initial counts of Bacillus spp. [20, 22]. It was believed that Staphylococcus spp. does not play any significant role in the fermentation of the condiments [20]. However, recent studies noticed an increase in pH and free amino acids during the fermentation of Iru produced with Staphylococcus spp., showing that these microorganisms isolated from Iru can ferment African locust beans [44]. Thus, Staphylococcus spp. plays a role in breaking down proteins in African locust beans into amino acids. Yeast, filamentous fungi, and enterobacteria were not detected in Afitin, Iru, and Sonru [22].

Root-based fermented foods are well-appreciated in Africa. The microbiologic studies of these foods aided in evaluating their safety, health benefits, and eventual uses in the food industry. Microflora of Lafun, Gari, and Agbelima is dominated mainly by Bacillus spp, lactic acid bacteria, mold, and yeasts (Table 1). B. subtilis, B. mycoides, B. pumilus, B. cereus, B. amyloliquefaciens, B. licheniformis, L. planturum, L. brevis and L. mesenteroides are the most important Bacillus and lactic acid bacteria detected in Agbelima. The main bacteria and molds identified from Gari comprise Corynebacterium species, Lactobacillus species, Bacteriodes species, Pseudomonas species, Actinomyces species, Scolecotrichum graminis, Tallospora aspera, Passalora bacilligera, Varicosporium sp., Culicidospora gravida and Diplococcium spicatum [45]. Likewise, the predominated bacteria and moulds detected from lafun are Streptococcus sp., Lactobacillus species, Listeria species, Articulospora inflate, Aspergillus niger, Aspergillus rapens, Aspergillus flavus and Lemonniera aquatica [40, 46].

Microbiological composition of traditional fermented fish showed that Bacillus spp, Staphylococcus spp, lactic acid bacteria, some yeasts, and molds are predominants [39, 47,48,49,50]. Bacillus subtilis, Bacillus licheniformis, Staphylococcus lentus, Staphylococcus xylosus. Micrococcus luteus were the most representative microorganisms isolated from Lanhouin [39], while Lactobacillus fermentum, Lactobacillus plantarum and Lactobacillus paralimentarius followed Pediococcus, Leuconostoc species represented the major lactic acid strains detected in Ajuevan and Suan yu [49, 50]. Staphylococcus and the yeast mainly consisted of Staphylococcus saprophyticus, Staphylococcus xylosus, Saccharomyces cerevisiae, and Hansenula anomala were also isolated from the traditional fermented fish [50]. Although traditional fermented fish contain important multi-useful microorganisms, they also have some pathogens which might represent public health issues [48]. The appearance of these pathogens is probably due to uncontrolled processing and post-processing handling.

Microflora of Dèguè is characterized by lactic acid bacteria, followed by yeasts and molds [51, 52]. The molecular studies of these microbial strains indicated that the lactic acid bacteria content of Dèguè is dominated by Lactobacillus, Enterococcus, Pediococcus, Streptococcus, and Weisella species. In contrast, its yeast content comprises Cyberlyndnera fabianii, Candida glabrata, Kluyveromyces marxianus, and Meyerozyma caribbica [53].

5 Health benefits

The fermentable substrates acquire many health benefits during fermentation, notably antioxidant, antimicrobial, anti-inflammatory, antidiabetic, and antiatherosclerotic [13, 54]. Many illnesses, including osteoporosis, allergies, hypertension, carcinogenesis, obesity, intestinal troubles, diabetes, atherosclerosis, high blood cholesterol, and immunity trouble, were found to be relieved with frequent consumption of fermented foods [13, 55]. The microbiologic studies of traditional fermented revealed that they contain thousands of valuable microorganisms, mainly lactic acid bacteria, Bacilli, and yeasts species which intervene in the protection of human health.

The microflora of Afitin, Iru, Sonru, and Lanhouin is prevalent in Bacillus species. Some species of Bacillus are present in root-based fermented foods such as Agbelima, Gari, and Lafun. The Bacillus spp. are largely nonpathogenic bacteria, and species of B. subtilis group have been reported to be generally regarded as safe (GRAS) by the U.S. Food and Drug Administration. The Bacilli are involved in the enzymatic hydrolysis of macromolecules producing peptides, exopolysaccharides, amicoumacin, enzymes (proteases, amylases, lipases, nucleases, and phosphatases), vitamins, carotenoids, and amino acids, which are claimed to have health benefits [56,57,58]. The Bacilli have shown probiotic properties with a resistance to heat and low pH in gastric medium. The Bacillus subtilis, together with Bacillus clausii, Bacillus coagulans, Bacillus pumilus, Bacillus licheniformis and Bacillus cereus are known as health supplements and could be used as an antibiotic, anti-diarrhea, cellular immune strengtheners, anti-allergic, anti-inflammatory, antipathogenic, and blood clotting reducer, however, Bacillus cereus might produce some toxins [59, 60]. Likewise, fermented soybeans contain many bioactive peptides which exhibit antimicrobial, antihypertensive, antioxidant, anticancer, hypocholesterolemic, chemopreventive, and antidiabetic activities [61]. It is believed in folk medicine that fermented condiments produced from African locust beans could prevent or relieve hypertension.

The cereal, milk, and root-based fermented foods described in the present study are predominated by lactic acid bacteria. Certain species of lactic acid bacteria are also figured out in the fish-based fermented condiment (Lanhouin). The experiments have proven that milk fermented by lactic acid bacteria are able to reduce considerable cholesterol level, improve the immune system and reduce diarrhea and mutagenic incidents [62]. Lactic acid bacteria are the most abundant probiotics with intense activity in the human gut. The genera Lactobacillus is the most abundant lactic acid bacteria figured out in the cereal-based fermented in the Benin Republic. In addition to Lactobacillus, these fermented products contain other lactic acid bacteria from the genera Weissella, Pediococcus, Enterococcus, Streptococcus, and Leuconostoc. These genera of lactic acid are involved in the synthesis of bioactive compounds responsible for health care, including organic acid (propionic, lactic, acetic, orotic, and citric acids), bacteriocin, enzymes (lipolytic, glycolytic and proteolytic), vitamins (B1, B2, B7, B9, and B12), GABA (γ-aminobutyric acid), hydrogen peroxide, reuterin, reutericyclin, peptides, conjugated linoleic acid and exopolysaccharides as well [4, 13]. These bioactive compounds have demonstrated antihypertensive, anti-depressant, tranquilizer, antimicrobial, anti-thrombotic, opioid, mineral binding, anti-oxidative, immunomodulatory, anti-carcinogenic, anti-atherosclerosis, anti-inflammatory, anti-osteoporosis activities, anti-adipogenic and anti-tumor properties [13, 63, 64]. Apart from lactic acid bacteria, yeasts have found in cereal-based fermented foods. Yeasts exhibit beneficial health effects and are used as food supplements. The species of yeasts discovered in these foods have shown industrial interest and are a great source of proteins, minerals, and vitamins, especially the B vitamin complex. Yeasts are involved in the synthesis of bioactive compounds responsible for health protection, such as amino acids (lysine, methionine, phenylalanine, and proline), erythritol, mannitol, glycerol, astaxanthin, organic acids (α-ketoglutaric, pyruvic, citric and isocitric, Gluconic acids), 2-phenyl ethanol, glucans, enzymes (chymosin, α-galactosidase, L-glutaminase, inulinases, invertase, lactase, lipase, phytase) and mannans [65,66,67]. Some of these yeasts synthesize, on the one hand, prebiotic compounds, mainly oligosaccharides, which promote the growth of probiotic bacteria, and on another, they secrete bioactive peptides, which showed beneficial effects for the human immune system [65,66,67]. Specific yeasts, including genera Saccharomyces Debaryomyces, Pichia, Torulaspora, Kluyveromyces, Hanseniaspora, Rhodotorula, Wickerhamomyces, Candida, and Williopsis as well have demonstrated potential probiotic activities and exhibited antioxidant, anti-inflammatory, cholesterol reducing, anti-diarrhea and human immune system reinforcement [68, 69]. Wine and fruits contain almost the same bioactive compounds, namely phenolic acids and polyphenols, which have the ability to protect human health from various ailments.

6 Enhancement of sensory properties

The taste and odor are enhanced during fermentation as the products provide strong flavor. Adding Iru, Afitin, Sonru, and Lanhouin to traditional dishes enriches the nutritional quality of foods and improves sensorial properties. Furthermore, the color and texture are distinctly upgraded as the fermented products present a pleasant and attractive aspect. The proteolytic enzymes of Bacillus in these condiments contribute to normal digestion by degrading the antinutritional factors and the allergenic compounds [70]. The metabolic properties of Bacillus through the degradation of proteins contribute directly or indirectly to the development of the texture and flavor of the fermented products [22, 71,72,73,74]. The lactic acid, ethanol, and acetic acid produced by lactic acid bacteria during fermentation are the main responsible sourdough volatile compounds, and lactic acid bacteria could also act on the production of flavor metabolites [75]. The main volatile groups such as pyrazines, ketones, aldehydes, alcohols, esters, alkanes, alkenes, benzene derivatives, pyridines, furan, volatile phenols, sulfur compounds, and terpenes of the fermented condiments obtained from African locust beans are generated by Bacillus species during fermentation [74, 76, 77]. The whiteness and thickness of Mawè increased with fermentation, which makes it better commercial.

7 Improvement of nutritional quality

The nutritional value of fermented foods is significantly improved compared to their non-fermented forms. The toxins and antinutritional agents are reduced in fermented products, and the quality of nutrients is also increased. Indeed, the intensification of the enzymatic activity during fermentation contributed to the hydrolysis of macromolecules such as proteins, lipids, polysaccharides, and phytates [78]. For instance, Bacillus spp. and enzymes such as esterase and protease involved in the fermentation of African locust beans rendered the final products digestible and nutritionally more valuable [22, 71, 72]. As a result, the levels of antinutrients are reduced, and the bioavailability of minerals, proteins, vitamins, and reducing sugars is augmented [78, 79]. The rate of crude protein was found to augment in the fermented African locust beans, while the crude carbohydrate was decreased. The protein value tended to rise in the fermented rice straw [80]. Likewise, a decrease of phytate up to 95% and an increase of phenolic compounds and iron were observed during the fermentation of the sorghum beer locally named Tchoukoutou [81]. The level of cyanogen is significantly reduced in the fermented cassava roots [82]. The bioavailability of iron is found to increase after the fermentation of carrots, beet, and sweet potato [83].

8 Preservative effects

Preservation effects are other advantages acquired by foods obtained through the fermentation process. It was observed an increase in the shelf life of foods after fermentation. The preservative effects occurring during fermentation result from natural antimicrobials (bacteriocins and organic acids) secreted by microorganisms and from inhibition properties of lactic acid bacteria and low pH on pathogen microorganisms. Recently, it has been shown that phenyllactic, caproic acid, 4-hydroxy-phenyllactic acids, bacteriocins, and reutericyclin produced during fermentation by microorganisms are responsible for a broad spectrum antimicrobial activities [84].

9 Detoxificant effects

Toxins present in some raw materials and foods represent public health issues. When the foods pass through the fermentation process, toxins are reduced. Tannin, trypsin inhibitors, and phytic acid, which constitute undesirable agents in the raw African locust beans, decreased after 72 h of fermentation [85]. The fermentation of rice straw Rhodospirillum rubrum significantly reduced the levels of toxins substances [80]. Moreover, the use of Aspergillus niger and Neurospora sitophila to ferment Jatropha curcas L. was revealed to be efficient for eliminating the antinutritional agent forbol esther [86]. Cassava toxins, namely cyanogens, were effectively reduced during heap fermentation. Therefore, fermentation is a crucial step in food manufacturing, ensuring safety and quality.

10 Conclusion

In Benin Republic, the production of fermented foods takes an important place in the daily staple diet of the population. Many of these commodities are region specific and refer to identity and culture. Technologically, the mechanism of their production is still primitive and small-scale. Most of the traditional fermented foods are prepared through spontaneous fermentation. Some of them require the use of specific traditional starter culture. The microbiological and nutritional studies on these commodities have indicated that they are a source of essential nutrients, prebiotics, and probiotics. Traditional fermented foods abound in probiotic lactic acid bacteria, Bacillus species, and yeasts, showing health and nutritional properties and potential interest in industries. These microorganisms secrete phytochemical compounds with antioxidant, anti-inflammatory, anti-diarrhea, immunomodulation, and anti-tumor activities. The intake of these fermented will help to eradicate food-induced chronic diseases. The large-scale production of these fermented foods would ensure good nutrition and food safety while reducing poverty and hunger. The conditions of processing and post-production handling should be carefully controlled.