Fermented foods are major sources of nutrition across Africa [1]. Traditional fermented food is an important part of the cultural heritage and diet that has been widely consumed since ancient times [2]. The process of fermentation involves the breakdown of organic compounds into acids or alcohol through enzymatic action of microorganisms particularly yeasts, bacteria and moulds under aerobic or anaerobic conditions [3]. It has been shown that fermentation enhances the nutritional, healthy, organoleptic and preservative properties of food [4]. As a technology that increases income sources, food availability, food diversity, reduces post-harvest losses and helps sustain food sovereignty, it plays an important role in developing economies. Most fermented foods are traditionally produced in Africa, resulting in differences in the substrate used, manufacturing conditions, packaging materials, handling and storage practices [5], and eventually significant differences in the final product.

Fermentation has been used as a means of preserving and improving the organoleptic properties of food for millennia [6]. Indeed, it has recently been revealed that more than 5000 varieties of fermented food and beverage are consumed worldwide [7]. Well-known examples include alcoholic beverages like beer and wine, dairy products such as yogurt and cheese, fermented vegetables such as pickles and sauerkraut, and fermented meat in various types of sausage. Fermentations are carried out by a variety of microorganisms: alcoholic fermentations normally involve yeast; alkaline-fermented foods normally involve a mixture of Bacillus spp.; and lactic acid bacteria are most usually found in fermentations where lactic acid is the end product while fermented legumes and cheeses involved moulds [8].

Traditional diets in Africa often lack variety and consist of large quantities of staple food (grains and tubers/root crops) with supplements of plantain, cocoyam, rice and beans, based on seasonality and availability [9] [10]. Soups eaten with staples are an essential part of the diet and may contain a wide variety of seeds, nuts, pulses and leaves [11]. The staple food supplies calories but is poor in other nutrients. Soups are key sources of protein and minerals, and one way to improve diet has been to increase the nutrient content of soups [10]. Leguminous seeds can account for up to 80% of the dietary protein and can be the only protein source for certain groups. These are consumed as meals and are commonly used as condiments to improve the taste of food in fermented form [12] [13] [14] [15]. With a high protein content, leguminous condiments can serve as a tasty supplement to sauces and soups and can be used as a substitute for fish or meat, especially for the very poor [10].

Food flavoring condiments are made using traditional methods of unregulated solid substrate fermentation resulting in substantial protein and carbohydrate hydrolysis [16] [17]. Besides increasing the lifespan and reducing anti-nutritional factors [12] [18] [19] [20]. Fermentation greatly enhances digestibility, nutritional value and raw seed taste [10].

Fermented foods are regarded as major dietary constituents in many developing countries, mainly because of their environmental quality, protection and conventional acceptability. Food fermentation as a process is at least 6,000 years old and is likely to be developed from sufficient microbial interactions. Fermentation has allowed our ancestors to survive the winter season in temperate and cooler regions and those in the tropics to survive the drought, improving shelf life and food safety. Over the years, fermentation has had a significant impact on dietary patterns and behaviors, on culture, and on the industrial processing and storage of food. Traditional fermentation processes also act as a replacement in the absence of refrigeration or other means of food safety [21].

Fermented foods can generally be defined as foods produced through controlled microbial growth and the conversion of food components through enzymatic actions. These are generally appreciated for characteristics such as good taste, flavor, texture and enhanced properties for cooking and storage. Microorganisms contribute to the development of characteristics such as taste, flavor, visual appearance, texture, shelf life and safety by virtue of their metabolic activity. Indigenous enzymes or enzymes generated by fermenting organisms may play a role in improving these characteristics [22]. Modern skills have been developed to monitor technological parameters during fermentation processes through trial and error. Research has also shown that the back-slop or inoculation of raw materials with residues from the previous batch speeds up the initial fermentation phase and facilitates beneficial improvements during the fermentation process. Reviews of considerable depth on traditional fermented foods have been published by Odunfa [12], Iwuoha and Eke [23] and Achi [10]. Against the background of the importance, benefits and improvement of traditional fermented condiments, the aim of this review is to assess the current state of knowledge of African fermented food condiments, with particular emphasis on the development of starter culture, for the production of safe, acceptable condiments capable of meeting the GMP requirements for global trade food.

In Africa, many names are used for the multitude of fermented food condiments. Table 1 indicates the variety of names attributed to the items in various parts of the continent. The precise origin of these names could be traced to 1) the region or area of manufacture 2) the type of legume or oilseed used and 3) the spelling of the product by region or area or even food use. Conventional substrates for condiment production are varied and can be produced from more

NOTE: The predominant organisms are represented in bold.

than one raw material. Almost any edible plant material may be fermented. There are numerous fermented condiments in parts of West Africa, according to the literature available [10].

Proteins, fats and carbohydrates are the major constituents of legumes and oilseeds used in the production of condiments. The organisms responsible for fermentation must therefore be capable of utilizing or hydrolysing these components [10].

Classical nutrient bioavailability evaluation methods generally involve animal models in vivo experiments. The use of proteins as a ratio of protein efficiency (PER) and the digestibility of soybean and some legumes has hardly been improved by fermentation [86] [91]. It has been documented that rats do not make better use of proteins from fermented soybean than from the cooked substrate. Nevertheless, some reports indicate otherwise. Achinewhu [85] reported that fermentation and heat treatment improved the apparent digestibility, feed quality, and protein production ratio when rats were fed on African oilseed rations diets.

Eka [17] studied the effect of fermentation on the nutrient content of locust beans and reported an increase in protein and fat when fermented, whereas the amount of carbohydrate decreased, this relative increase is due to a reduction in the percentage of carbohydrate content. Increased levels of amino acids except arginine, leucine and phenylalanine have also been reported. Similar studies have been reported for other seed legumes [12] [77] [85]. During the fermentation of melon seeds, soluble products increased and the fermented product was highly digestible. The main amino acids are alanine, lysine and glutamic acid, with small amounts of arginine and proline [13]. Improved nutritional values were attributed to increased amino acid profiles due to fermentation [10].

Starter cultures or starters are individual or mixed formulations of selected strains of microorganisms with a particular physiological capacity to turn a substrate into a food product with specific characteristics when added at specified concentrations [92] [93]. Adapted to the substrate, the typical starter facilitates improved control of the fermentation process and the predictability of its products [94]. The development, improvement and standardization of microbial starters have been driving force for the transformation of traditional food fermentations in developing countries from an “art” to a science. The development of microbial starter culture has also been a driving force for innovation in the design of equipment suitable for the hygienic processing of traditional fermented foods under controlled conditions in many developing countries.

The development of starter culture has played a pivotal role in the production of high-value products such as enzymes, microbial cultures and functional food ingredients, together with the development and improvement of bioreactor technology to control fermentation processes in developed countries. As inputs for their food processing operations, these goods are primarily manufactured in more advanced developing economies and are increasingly imported at high cost by less advanced developing countries.

Traditional starter cultures are picked either as single or multiple strains, primarily for adaptation to the substrate or raw material. Spontaneous fermentation, i.e. processes conducted without the use of starter inoculum, have been used for centuries for food preservation and have been explained by trial and error, probably over thousands of years. Many small-scale fermentations are still performed in developing countries as spontaneous processes and some industrial processes, such as sauerkraut fermentation and traditional wine production. Different types of starter cultures, even in industrialized countries, are widely used in fermentation processes. Spontaneous fermentation is typically the result of competitive activities of a variety of contaminating microorganisms [94] [95]. The process eventually dominates those that are best adapted to the food substrate and the technical control parameters. During fermentation, the development of metabolites (e.g. organic acids) that are inhibitory to other contaminating microbes (e.g. enterobacteriaceae) may provide additional benefit. Bacteria usually dominate the early stages of fermentation processes in substrates rich in fermentable sugars due to their relatively high growth rate, followed by yeasts. Content from a previous successful batch is used in many conventional processes to promote the initiation of a new process. The initial phase of the fermentation process is shortened by this method of backslopping and the risk of failure of fermentation is reduced. Repeated use of back-slopping results in the choice of best-adapted strains, some of which may have features suitable for use as starter cultures [94] [95].

It takes a relatively long time (24 - 48 h) to initiate a spontaneous fermentation process, with a high risk of failure. During this early stage, which is linked to the lag phase of microbial growth, the contamination of raw materials, utensils and the environment by microorganisms increases slowly and competes for nutrients. Through inoculation, this step can be reduced either by back-slopping or using selected start cultures. The beneficial characteristics of the substrate, consumer expectations and technical requirements largely dictate the nature of the starter culture to be used, i.e. single-strain vs. mixed-strain culture [94] [95].

In the fermented food sector, “Appropriate” starter cultures are widely used as inoculants, from household to industrial, low-income and low-middle-income economies [94]. These starter cultures are generally produced using a back-slopping process that uses samples from a previous batch of a fermented product as inoculants [95]. The inoculation belt used in traditional fermentations in West Africa serves as a carrier of undefined fermenting microorganisms, and is one example of an appropriate starter culture [94]. It usually comprises of a woven fiber or mat or a piece of wood or woven sponge, saturated with a previous fermentation batch’s “high”-quality product. To act as an inoculant, it is immersed in a new batch. The inoculation belt is used for the production of native fermented porridges, “uji” and “mawe”, as well as for the production of Ghanaian beer “pito.” Another example of an “appropriate” starter culture produced by back slopping is Iru. This starter culture is produced from concentrated fermented dawadawa (a result of fermented legumes), mixed with unfermented legumes, vegetables such as pepper, and cereals such as ground maize. It is preserved in a dried form and is used as an inoculant in dawadawa fermentations in West Africa [94]. In Asian countries, a variety of suitable starter cultures are used as fermentation inoculants, either in a granular form or as a pressed cake. Different names such as marcha or murcha in India, ragi in Indonesia, bubod in the Philippines, nuruk in Korea, koji in Japan, ragi in Malaysia, and Loog-pang in Thailand generally refer to these traditional mould starters. They generally consist of a mixture of moulds grown under non-sterile conditions.

Few specified starter cultures have been developed to be used as inoculants in commercial fermentation processes in developing countries. However, over the last ten years, a number of developing countries have witnessed the development and application of selected and pre-cultivated starter cultures in food fermentation. “Defined starter cultures” consist of single or mixed strains of micro-organisms [94]. It is possible to incorporate adjunct culture preparations that serve a function of food safety and preservation. Due to their ability to inhibit pathogenic or spoilage organisms, adjunct cultures do not necessarily produce fermentation acids or alter texture or taste, but are included in the defined culture. An inhibitory function is ascribed to the development of one or more products, such as hydrogen peroxide, organic acids, diacetyl and bacteriocin [6]. Generally defined cultures are produced by the maintenance and spread of pure culture under aseptic conditions. They are generally marketed in a liquid or powdered form or else as a pressed cake.

Defined starter cultures are also commonly imported by developing countries for use in the commercial production of dairy products such as yogurt, kefir, cheese and alcoholic beverages as well as in bakeries. Many of these cultures are tailored to produce specific textures and flavours. Most yogurt cultures also include probiotic strains in response to increasing consumer interest in achieving wellness through diet. Probiotics are currently produced in India, South Africa and several other countries for use as food additives, dietary supplements and for use in animal [94]. Methodologies used in these starters’ design and tailoring are primarily the responsibility of these starters’ suppliers. Monosodium glutamate and lactic acid, both used as ingredients in the food industry, are developed using defined starter cultures in less advanced developing countries.

Considerations for implementing household-level starter cultures must take into consideration cultural traditions, dietary habits and raw materials, and cottage-use technological conditions that vary across regions and continents. From a technical point of view, cost/benefit ratios, operational considerations and a small-scale processor’s willingness to accept new approaches are crucial in any assessment of the viability of incorporating the use of starter cultures in small-scale fermentations. Therefore, for use at the artisanal level, a minimum set of standards or performance criteria should be established for the handling and maintenance of these cultures. The possibility of applying starter cultures will become attractive to the small-scale processor only if benefits, such as reduction of costs (e.g. energy), reduced fermentation times, reduced risk of spoilage (increased shelf-life), improved process control, improved sensory quality (taste, aroma, visual appearance, texture, consistency), improved safety attributes (e.g. lower risk if diarrhoea, detoxification of cassava) and reduced preparation procedures for the final product, are perceived. These are in addition to reduced need for stringent process control. Some LAB and yeast strains associated with fermented foods, such as phytic acid and phenolic compounds, can degrade antinutritional factors. Therefore, the incorporation of these organisms into starting culture can help to improve the nutritional value of foods [94].

In recent times, there has been considerable focus on the inclusion of mycotoxin-degrading strains in starter cultures. The use of mycotoxin contaminated raw materials for fermentation in developing countries presents a particular challenge in the choice of strains that can detoxify mycotoxins [94] [95] [96] [97]. In respect of their possible contribution to improving overall health and well-being, the probiotic properties of strains involved in food fermentation are also being studied.

Spontaneous food fermentations are neither predictable nor controllable. Therefore, single-and mixed-strain cultures must be tested on the pilot scale before being used in small-scale operations. According to Holzapfel [94], the introduction of starter cultures in traditional small-scale fermentation should take into account considerations for improving the processing conditions and quality of the product by: 1) rapid accelerated metabolic activities (acidification or alcohol production); 2) improved and more predictable fermentation processes; 3) desirable sensory attributes; 4) improved safety and reduced hygienic and toxicological risks.

The primary screening is based primarily on the evaluation of candidate strains’ ability to withstand the pressure conditions imposed by fermentation processes [98]. At the beginning of the process, microbial cells are impaired by osmotic pressure due to the high concentration of solutes in the fermentation medium, such as grape sugar, hydrolyzed malt or sugar cane, milk matrix proteins and lipids and vegetable sodium [99].

Cell exposition to such hypertonic conditions leads to an efflux of water from the cell, decreasing both turgor pressure and water availability [100]. It causes main microbial metabolism enzymes to be inactivated, in addition to increasing toxic ion concentration and eventually cell death. Nonetheless, through natural selection it is possible to distinguish strains that are well suited to these stressful environments [98]. These resistant strains respond to osmotic stress by accumulating specific solutes, such as potassium ions, amino acids (e.g., glutamine, glutamate, proline, y-aminobutyrate, and glycinebetaine) and sugar (sucrose, trehalose, and glucosylglycerol), which decrease the activity of water within the cytoplasm and increase both cell volume and turgor near their prestress values [101]. When the cell adapts to the new environment and fermentation begins, other stressors become relevant, such as the accumulation of organic acids and alcohols, changes in temperature, and acidifies the environment [99]. Rising temperature and/or decreasing pH are probably the most common environmental factors that starter cultures have to deal with during fermentation processes.

Commercial starter cultures usually come from either food substrates or the processes in which they are applied. Environmental conditions, back-slopping, adaptation and repeated use of similar utensils may help select the microbial communities typical of the fermentation process. The selection of appropriate starter strains should take into account their interactions in mixed cultures, taking into account the behavior of these strains under defined conditions and within the food substrate. Other factors to be considered include: 1) competitive behaviour, viability and survival; 2) antagonism against pathogens and spoilage microbes; 3) the rate of acid or alcohol production; 4) organoleptic changes; 5) primary metabolites of fermentation; 6) degradation of antinutritive factors; 7) detoxification; 8) probiotic features [94].