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Review

Highlighting the Impact of Lactic-Acid-Bacteria-Derived Flavours or Aromas on Sensory Perception of African Fermented Cereals

by
Eugénie Kayitesi
*,
Ogheneyoma Onojakpor
* and
Siphosanele Mafa Moyo
*
Department of Consumer and Food Sciences, University of Pretoria, Private Bag X20, Hatfield, Pretoria 0028, South Africa
*
Authors to whom correspondence should be addressed.
Fermentation 2023, 9(2), 111; https://doi.org/10.3390/fermentation9020111
Submission received: 2 January 2023 / Revised: 20 January 2023 / Accepted: 23 January 2023 / Published: 24 January 2023
(This article belongs to the Special Issue Flavor and Aroma in the Fermented Food)
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Abstract

:
Sensory characteristics and flavour profiles of lactic-acid-fermented foods are influenced by lactic acid bacteria (LAB) metabolic activities. The flavour compounds released/produced are directly linked to the sensory characteristics of fermented cereals. African fermented cereals constitute a staple, frequently consumed food group and provide high energy and essential nutrients to many communities on the continent. The flavour and aroma characteristics of fermented cereal products could be correlated with the metabolic pathways of fermenting microorganisms. This report looks at the comprehensive link between LAB-produced flavour metabolites and sensory attributes of African fermented cereals by reviewing previous studies. The evaluation of such data may point to future prospects in the application of flavour compounds derived from African fermented cereals in various food systems and contribute toward the improvement of flavour attributes in existing African fermented cereal products.

1. Introduction

Superseding the significance of nutrition and health in influencing consumer food choice is the flavour and aroma attributes of food [1]. Flavour and aroma components in turn have an impact on the sensory perception of food [2]. Cereals such as sorghum, millets, rice, maize and wheat are the staple foods of a majority of the African population, with millets, sorghum and teff being recognised as indigenous crops [3,4]. The consumption of these cereals is widespread due to the fact they are affordable and easily available to both rural and urban communities. These cereals are also a major component of traditional weaning foods in Africa. The dietary significance of these cereals is their carbohydrate, vitamin, mineral, dietary fibre and bioactive phytochemical content [5]. However, the presence of anti-nutrients, low protein digestibility, low iron bioavailability and inadequate quantities of organoleptic-active compounds [5,6] necessitates processing techniques such as fermentation. With challenges relating to accessibility to sophisticated food processing and preservation technologies, fermentation remains a dominant food processing technique in the developing world, particularly in Africa. Fermentation is known to provide numerous benefits, including the enhancement of nutritional and sensory quality of food products, as well as increased shelf life and the production of bioactive compounds [7].
Cereal fermentations can be affected by a combination of bacteria and yeast metabolism [8]. Traditionally, the fermentation of cereals in Africa is spontaneous, activated by the presence of microorganisms in raw materials. Fermentation can be carried out in homes or on a micro industrial scale. Back-slopping, which introduces previously fermented material as an inoculum during fermentation, is a common technique [9]. However, controlled fermentation is gaining popularity due to the need to improve efficiency and ensure consistent product quality. During lactic acid bacteria (LAB)-based fermentations, catabolic and anabolic reactions take place to break down the substrate constituents and synthesise various compounds which influence the sensory qualities of cereals [5,10]. Different cereal substrates, fermentation conditions and the time and type of LAB strain/s used may lead to varying concentrations and types of flavour metabolites produced [5]. Flavour perception is influenced by taste (mainly non-volatile compounds), aroma (mainly volatile compounds), tactile and temperature factors [1,5,11]. In addition, non-volatile compounds may act as flavour precursors for the development of volatile flavour compounds [12].
Volatile flavour compounds are characterised by their vaporisation properties in the release from food and can be detected by the olfactory system during human consumption, which activates odour receptors [1,5]. They include compounds such as some carboxylic acids, ketones, esters, alcohols and aldehydes [13]. Non-volatile flavour compounds include salts, flavonoids, sugars, amino acids, peptides and some carboxylic acids, which interact with taste buds located in the tongue [14]. They are perceived during the gustation process by five taste attributes (sweet, sour, salty, umami and bitter) [14]. Cereal substrate constituents such as fatty acids, amino acids, carbohydrates, organic acids and other compounds released by LAB during fermentation are metabolised to various flavour compounds [5].
Several studies have investigated the sensory perception/consumer acceptability of African fermented cereals [15,16,17,18,19,20]; some have reported data on the volatile and non-volatile compounds produced as a result of LAB fermentation [21,22,23,24,25,26]. A few studies have identified aroma profiles and related odour descriptors [27,28,29,30]. Therefore, this review describes the LAB metabolic pathways that are responsible for the production of various flavour metabolites in non-alcoholic African fermented cereals and formulates a correlation between LAB-produced flavour metabolites and the sensory perception of the corresponding fermented cereals. The evaluation of such data will provide an understanding of the type of flavour compounds associated with the sensory perception of various African fermented cereals. These correlations could assist in the development of new products, the application of flavour-derived compounds from African fermented cereals in various food systems and the improvement of flavour attributes in existing African fermented cereal products.

2. An Overview of Significant African Fermented Cereal-Based Foods

Cereals are among the most popular fermentation substrates in Africa. Maize, millet, sorghum and, to a lesser extent, rice and wheat are fermented to produce a wide array of thin and thick porridges, beverages and bread with distinct sensory characteristics [31]. The products produced depend on the region and the most readily available cereal crops; products from different regions can also use similar substrates while differing in some areas of the production process, such as the temperature and duration of fermentation, and the use of additional processing steps such as malting. The cereals may be fermented individually, or they may be composited with other cereals [32,33], legumes or other food ingredients before or after fermentation. Traditionally, most cereal fermentation processes rely on uncontrolled fermentation techniques such as spontaneous fermentation and back-slopping. The microbial populations predominantly involved in the fermentation process are LAB and yeasts [34]. Commercially available starter cultures, which can be differentiated as single-strain (one strain of a species), multi-strain (more than one strain of a single species) or multi-strain mixed cultures (strains from different species), are mostly LAB-based strains [35]. LAB remain the most popular and frequently studied bacterial strains used in the fermentation of cereal-based foods [36]. While a pure lactobacillus spp. inoculum has been used in some studies, others have used mixed inocula which incorporate other organisms such as Candida spp., Saccharomyces cerevisiae, Streptococcus spp. and Acetobacter spp. Lactic-acid-based fermentation is also gaining popularity due to an increased need for the production of probiotics with specific health and/or nutritional benefits [37,38], such as the increased production of folate in injera fermented with L. plantarum compared to spontaneously fermented injera [39].
Examples of African fermented cereal food are presented in Table 1. Akamu/ogi is a porridge that can be consumed in a thin or thick constituency in Nigeria; it is commonly consumed as a weaning food [40]. Uji is a thin maize gruel popularly consumed in Kenya [32]. Ting is a fermented porridge made in Botswana, which may be consumed in a thin (motogo) or thick (bogobe) consistency [25]. Maheu/mageu is a non-alcoholic maize-based beverage that is popular in Southern Africa. Togwa is a beverage or gruel made in Tanzania from maize, millet, sorghum, cassava or their composites [41]. Kunun-zaki is a non-alcoholic beverage popularly consumed in Northern Nigeria made from millet or a combination of millet, rice and wheat [17].

3. Lactic Acid Fermentation Pathways Involved in Flavour/Aroma Compound Production

Lactic acid bacteria belong to the Gram-positive, non-sporulating type of microorganisms in the phylum of Firmicutes and the order Lactobacillales [1,11]. The genera Lactobacillus, Lactococcus, Leuconostoc and Pedicoccus are mainly predominant in African fermented cereals [7]. The main metabolic pathways involved in flavour development during fermentation are carbohydrate metabolism, degradation proteins and fats [48]. However, the dominating pathway depends on the type of substrate. In the case of cereals, carbohydrate metabolism is dominant, while lipid hydrolysis is low [11].

3.1. Carbohydrate Metabolism during LAB Fermentation of Cereals and Its Link to Flavour and Aroma Compounds Production

As the main constituent of cereals, LAB use carbohydrates to generate ATP via fermentation, coupled to substrate-level phosphorylation [11]. Different LAB possess various abilities to utilise the type of carbohydrates present. Bacteria use the permease (PM) and phosphotransferase systems (PTS) to transport carbohydrates into the cell. The PTS ensures the conversion of glucose, sucrose, and so on into their corresponding phosphoesters during transport [49]. In cereal fermentations, homofermentative LAB can be found in association with heterofermentative LAB [36]. Homofermentative LAB produce lactic acid by metabolising hexoses via the Embden–Meyerhof pathway or pentoses via the pentose phosphate pathway to form lactate, while heterofermentative LAB metabolise hexoses and pentoses via the phosphoketolase pathway to form lactate, acetic acid, ethanol and carbon dioxide [11], as demonstrated in Figure 1. Homofermentative LAB prefer glucose as a substrate, while heterofermentative LAB prefer fructose, sucrose and maltose as their substrate, which speaks to their high levels in cereal porridges and sourdough, as these types of carbohydrates are available in abundance [36]. Cereals are a major source of complex carbohydrates such as starch, which may be hydrolysed during the initial stages of fermentation due to the presence of amylolytic LAB (Lactobacillus fermentum, L. acidophilus, L. plantarum, L. pentosus, L. paracasei, P. acidilactici, Lc. lactis, Leuc. mesenteroides and W. confusa) [24,50,51]. Amylolytic LAB can provide fermentable sugars to non-amylolytic LAB, depending on the catabolic repression characteristics of the bacteria in the presence of glucose [50,51]. Additionally, other enzymes such as sucrase, galactosidase, fructosidase and so on may be released to catalyse the respective substrates such as sucrose, raffinose and fructooligosaccharides [52,53].
In African fermented cereals, homofermentative LAB such as Pediococcus acidilactici, Lactococcus lactis, Enterococcus, Streptococcus, Lactobacillus casei, Lactobacillus acidophilus and Lactobacillus helveticus have been isolated. Heterofermentative LAB such as Lactpbacillus planturum, Lactobacillus brevis, Lactobacillus fermentum, Lactobacillus reuteri and Leuconostoc mesenteriodes have also been noted (Table 1 and Table 2). Lactate has been identified as the main non-volatile flavour compound in fermented cereals such as mawe or ogi (maize/corn), koko (millet) and mahewu (maize/corn), due the presence of mainly homolactic LAB. In products such as ting (sorghum) and bushera (sorghum), lactate and acetate have been identified due to the presence of heterofermentative LAB. Further, LAB such L. paracasei subsp. paracasei MINF98, L. plantarum MINF227 and L. brevis MINF226 have been associated with the production of acetylaldehyde, which is a volatile flavour compound produced via the heterofermentative pathway as an intermediate for ethanol synthesis [5,24]. Acetoin, butanediol and diacetyl, which contribute to the aroma of fermented cereals, are produced as a result of citrate metabolism via LAB such as Leuconostoc mesenteriodes and Pediococcus acidilactici [11,24]. During the spontaneous fermentation of the Ghanaian maize dough kenkey, the presence of diacetyl was related to the presence of LAB [28]. Citrate metabolism may be expressed by LAB during the late exponential growth phase, when acidic conditions persist due the presence of lactic acid [54]. A low-pH environment shifts the metabolism of carbohydrates to the utilisation of amino acids [36].

3.2. Protein Metabolism during Lab Fermentation of Cereals and Its Association with Flavour and Aroma Compound Production

LAB require amino acids for nitrogen and carbon metabolism [55]. Some of the nitrogen sources are available as free amino acids in the substrate, while some have to be released from the parent polypeptide, which indicates the presence of a proteolytic system. The proteolytic mechanisms of LAB involve the degradation of proteins, various peptide transport systems, the degradation of peptides, and amino acid catabolism (Figure 2) [56]. LAB such as Lb. plantarum, Lb. fermentum and L. deldrueckii isolated from ting, bushera and maheu in studies have been reported to possess a proteolytic ability that enables them to hydrolyse proteins into peptides and amino acids [57]. This protein degradation may be initiated by cell envelope proteinases (CEPs) as well as substrate proteases activated by a low-pH environment [56,58,59]. The efficiency of the proteolytic system and pattern of protein hydrolysis may be affected by CEP differences (gene mutation or gene expression) and enzyme optimal conditions [59].
The presence of large hydrophobic peptides produced by the proteases is related to the bitter taste of fermented products. In fermented foods, the peptide His-Leu-Arg-Phe-Ile-Lys-Val has been associated with a bitter taste, while Tyr-Phe-Ala has been described as sour [11]. Ala-Gly-Thr-Ser-Pro-Hyp and Glu-Asp have been reported to have a sweet and umami/salty taste, respectively [11]. The produced peptides are internalised into the bacterial cell for further degradation into amino acids [58]. The utilisation of amino acids via LAB varies according to strains (genetic and adopted differences) as well as the availability of nutrients from the substrate [55]. The amino acids produced may be required for the survival of LAB in a hostile environment or for toxin synthesis [55].
The amino acids present in the bacterial cell can be further metabolised to produce important aroma compounds such as aldehydes, alcohols and esters via the transamination and elimination of amino acid catabolic pathways [11]. Branched-chain amino acids (isoleucine, leucine and valine), aromatic amino acids, (phenylalanine, tryptophan and tyrosine) and sulphur amino acids (methionine and cysteine) may be degraded via the transamination route, whereas methionine may be converted to methanethiol [60]. The transaminated amino acids are converted to α-keto acids, which can also be further metabolised to aldehydes. The aldehydes are metabolised to produce either alcohols, via alcohol dehydrogenase, or carboxylic acids. Esters are then synthesised from carboxylic acids via esterases [60]. Acetaldehyde, which has been detected in ogi (maize/corn), may also be produced via LAB from threonine catabolism [24,29]. Volatile compounds such as ethanol, propionic acid, 1-Propanol, acetic acid, 3-Methyl-1-butanol and hexanoic acid, all isolated from fermented maize products [28,29], may be metabolic products of Ala, Gly, Leu and Lys [61].

3.3. Other Compounds which Can Contribute to the Flavour/Aroma of Fermented Cereals

Cereals generally have a low concentration of lipids. However, lipids can be hydrolysed by LAB, particularly the Lactococcus and Lactobacillus species, to release free fatty acids [11]. Free fatty acids such as butanoic acid, hexanoic acid and octanoic acid have been identified in fermented maize [28,30]. LAB can release esterases to produce ethyl butanoate and ethyl hexanoate from the esterification of ethanol with butyric and hexanoic acids, respectively. Free fatty acids can be oxidised to produce hydroperoxides and, subsequently, aldehydes. In addition, compounds such as alkanes, methyl ketones, esters, secondary alcohols and lactones can be formed as a result of free fatty acid oxidation [11]. Cereals are good sources of phenolic phytochemicals such as ferulic acid. Although these compounds are known to have some inhibitory activity against bacteria, some LAB (Levilactobacillus brevis, Limosilactobacillus fermentum and Lactiplantibacillus plantarum) have been found to have phytochemical degradation activities by decarboxylation and reduction reactions [56,62]. LAB may produce enzymes such as β-glucanase, which hydrolyses the β-glycosidic bond of conjugated phenolic compounds or ferulic acid esterase, to release a free form of ferulic acid [63]. Flavour compounds such as 4-vinyl phenol and 4-vinyl guaiacol may be produced via the decarboxylation of p-coumaric and ferulic acids [62]. Vinyl phenol has been detected after the spontaneous fermentation of maize, with LAB such as L. pentosus strain CE56.28.1, CE56.19, L. plantarum strain 28.19 E, NWAFU1572, P. acidilactici strain AA106, P. pentosaceus strain OO1, L. fermentum strain CAU:235, YLc37C, BB101, Enterococcus faecium strain AA109, Weissella confusa strain JBA12 and Weissella confusa partial being identified, among other microorganisms present in the substrate [29]. Guaiacol has been identified in sorghum fermented with L. fermentum FUA 3165 [64]. A reduction in vinyl phenols may lead to the production of flavour metabolites such as ethyl phenol and ethyl guaiacol [62].

4. Relationship between the Sensory Characteristics and Flavour/Aroma Compound of African Fermented Cereals

Fermentation results in biochemical changes in the substrate, which in turn determine the sensory characteristics of the fermented product, and thus influence the perception of consumers. The organoleptic properties of fermentation-derived metabolites have a significant impact on the appearance, aroma, texture and taste/flavour of the resultant products. The metabolites, and thus sensory perception produced, depend on the nature of the substrate, the inoculum used or lack thereof and other fermentation conditions such as duration and temperature [65]. Although the appearance and texture of fermented products are important, their aroma and flavour are more valuable studies, as these are distinguishing characteristics. To this end, several studies on African fermented cereals have attempted to determine the aroma and flavour metabolites produced by the fermentation of different cereals. This is essential to better understand how the fermentation process impacts the consumer acceptance of products. Table 2 depicts the flavour compounds identified in various African fermented cereals and their sensory descriptors. In general, LAB-fermented products are characterised by a sour/acidic flavour [66], which has been linked to the production of acetic acid from the breakdown of sugars [28,30,67]. The presence of alcohols in fermented maize has been associated with fruity (mild and ether), alcoholic, sweet-smelling, pungent, buttery or creamy scents, as well as the scent of rubber, boiled potatoes or a choking alcohol odour, as depicted in Table 2. The presence of alcohol is linked mainly to yeasts in spontaneously fermented cereals, while LAB contribute to a lesser extent [28]. Diacetyl, a carbonyl compound, has also been detected in fermented maize and is associated with a butter or butterscotch flavour.
Various types of esters have been identified in fermented maize, pearl millet and sorghum products [21,22,30]. Esters are usually associated with a fruity flavour [68] and can mask or reduce the flavour intensity of free-amino-acid-derived notes that are usually unpleasant [69]. The perception of a flavour compound may change with its increased concentration in a product [70]. Therefore, the overall sensory characteristics of African fermented cereals is ultimately determined by the interaction and balance between different flavour compounds. Fermentation, in conjunction with low pH, contributes to the sensory perception of the product. This is evidenced by the absence of desirable flavours associated with fermentation after the reduction in pH by the addition of lemon juice in non-fermented cereals [71]. Akamu (ogi) fermented with L. plantarum was better accepted compared to an unfermented, artificially acidified sample or unfermented, unacidified akamu [31]. L. plantarum-fermented akamu was characterised by flavour notes reminiscent of honey and apples [31].
Even though LAB are important players in spontaneous fermentation, the aroma profiles of products from controlled LAB fermentation differ from their spontaneous fermentation counterparts. For example, uji produced via LAB fermentation was dominated by acidic aromatic compounds, while the same spontaneously fermented product was dominated by aromatic esters [32]. The authors of this particular study suggested that the former was generally described as sour and unpleasant, and the latter was related to fruity aromas and was thus more likely to be accepted by consumers. Spontaneously fermented products sometimes develop off-flavours due to the presence and growth of undesirable microorganisms during the fermentation process [32]. Fermented products have also been negatively characterised as rancid or pungent by some consumers [30]. This is likely to have occurred due to the presence of butanoic acid, hexanoic acid and octanoic acids, which have been variously described to have rancid and cheesy aromas, a pungent aroma, and what has been pronounced to have a waxy, soapy, goaty, musty, rancid or fruity flavour, respectively. However, several factors may affect the odour and flavour profiles of fermented products.

5. Correlation between Sensory Characteristics and Consumer Preference of African Fermented Cereals

In general, African fermented foods provide a unique culinary experience which makes them sought after. Sensory perception plays a significant role in consumer acceptance. Therefore, it is important for studies on the fermentation of cereals to evaluate the sensory perception of products. Sensory evaluation entails the objective (examining the descriptive sensory analysis) and/or subjective assessment (such as hedonic studies) of the intensity and human preference towards the appearance, aroma, taste/flavour, texture and aftertaste of food products, following standard procedures that reduce bias and unwanted/erroneous variation [72].
Most studies reviewed for this work which have carried out sensory evaluations for controlled LAB-fermented products employed hedonic studies to determine the acceptability of the products by consumers [43]. These hedonic studies involved the use of a ‘liking scale’ of 7 to 11 points (pts), ranging from dislike extremely (lower end of the scale) to like extremely (upper end of the scale) [73]. Ideally, over 100 (minimum 50) consumers who regularly consume the same or similar products should be used for a study. Fermented products were generally liked, as indicated by a rating from ‘like slightly’ to ‘like extremely’ on a hedonic scale [17,33,74]. Many of the studies reviewed reported the use of marginal sensory methods; for example, most of the studies used less than 50 people and sometimes included staff and students who were working on the project, which may have led to the introduction of bias. For example, Banwo et al. [43], Obinna-Echem [31] and Tamene et al. [39] used 30 people, while Adesokan et al. [33] and Wakil and Kazeem [6] used as few as 10 people. The studies did not report whether or not these were regular consumers of the products.
Some studies compared the impact of spontaneous versus controlled LAB fermentation on the acceptability of the products [31,39]. Other studies compared LAB-fermented products to their unfermented counterparts. The flavour of LAB-fermented akamu was moderately liked in a study by Obinna-Echem [31], with the controlled LAB-fermented akamu being preferred to the back-slopped fermented variant, while the unacidified, unfermented variant was neither liked nor disliked, and the artificially acidified variant was disliked [31]. Consumer preference for the controlled LAB-fermented akamu may have been due to its honey or fruity notes, which are a result of the production of alcohols and esters during the microbial fermentation of maize [28,30].
Sensory evaluation was used to compare different inocula. In a study, Maheu fermented via back-slopping from an original preparation of maheu inoculated with L. rhamnosus yoba was compared to a spontaneously fermented variant [38]. As expected, the authors reported a drop in the pH and an increase in the titratable acidity of both maheu slurries during fermentation. Although both maheu were generally liked, with scores above 5 on a 9 pt scale, the back-slopped maheu had significantly higher acceptability than the control in all the attributes measured at Day 0, with these being appearance, taste, colour, sourness and overall acceptability, but it only had higher acceptability for appearance, taste and colour by Day 20. The sourness and overall acceptability of both maheu preparations decreased between Day 0 and Day 20. This indicates that the overall acceptability of the maheu preparations decreased with increasing acidity. A similar study conducted by Mamhoud et al. [9] reported that the addition of spontaneously and control-fermented sourdough (wheat) significantly increased the intensity of the acidic flavour in the bread [9]. The bread made with the control-fermented sourdough was saltier and less sweet than that made from the spontaneously fermented sourdough. The authors linked these findings to the production of acids and saltiness due to the proteolysis of wheat proteins.
Only a few of the studies linked the consumer acceptability of products with their objective sensory perception, with one study only carrying out the latter [9,73]. Objective sensory evaluation entails the use of methods such as descriptive sensory evaluation, where a trained sensory panel generates sensory attributes (descriptors) and assesses the intensity of the sensory attributes in each product. In this study, Maheu/mageu was neither particularly liked nor disliked, as consumers rated it between 4.7 to 5.4 on a 9 pt scale [73]. A comparison with data from the trained sensory panel revealed that consumers in the study generally preferred a sweeter product, and the consumers recommended that the taste and flavour of the products should be improved. In another study, the sensory quality of bread made from wheat sourdough fermented via Pedicoccus acidilactici O1A1, Pediococcus pentosaceus OA2 and L. curvatus MA2 was assessed using 10 trained panellists [73]. The attributes assessed included the colour of crust and crumb, acid taste and flavour, sweetness, elasticity, and dryness. The fermented wheat sourdough breads were characterised by an acidic flavour and salty taste compared to the baker’s yeast bread. The acidic flavour and salty taste were likely a result of the LAB carbohydrate and protein metabolic activities, as discussed in Section 3. Sensory evaluation was also carried out in some studies to determine the storage stability of fermented products [40].

6. Conclusions and Future Perspectives

Fermentation remains the dominant food processing technology in the developing world, and cereals are some of the most popular fermentation substrates in Africa. LAB and yeasts are the predominant microorganisms during spontaneous fermentation. However, LAB activities during cereal fermentations (spontaneous or controlled) contribute significantly to the production of flavour-active/aroma compounds and flavour precursors important to the sensory perception of fermented products. This review presented the biochemical pathways (carbohydrate, protein, lipid and polyphenol metabolism) related to the production of various flavour compounds found in African fermented cereals. Although it is recognised that lactic acid is dominant in homofermentative fermentations and acetic acid is dominant in heterofermentative fermentation, other fermentation metabolites produced also contribute to varied sensory perceptions of these African fermented cereal products. The flavour consortia of metabolites present in substrates (pearl millet, maize and sorghum) varies, suggesting that an elaborate flavour interdependency exists between the presence of LAB and substrate components. In order to understand the comprehensive flavour characteristics of African fermented cereals, sensory evaluation in conjunction with instrumental flavour/aroma analysis is required. As depicted in Table 2, more sensory lexicons should be developed to appropriately describe these fermented cereals and to identify compounds that influence consumers’ preferences of these products. In the studies reviewed, the acceptability of products made from controlled LAB fermentation depended on the substrate or compositing of substrates, the inoculum used and the duration of fermentation.
In the future, African fermented cereal products should be designed to meet the changing consumer preferences. The product portfolio should be diversified to provide more flavour options to meet every consumer’s preference. Microorganisms can be selected and used in controlled fermentation to impact the desired flavour in the products. For instance, some alcohols and esters produced via fermentation have a characteristic fruity flavour [28,29,30]; thus, microorganisms that produce such metabolites during the fermentation of cereals may be selected. Artificial flavourings and/or sweeteners may also be used. Furthermore, the fermentation time and temperature may be adjusted to obtain the desired flavour and aroma; for instance, the fermentation time may be adjusted to limit the production of acids and reduce the acidic flavour. Sensory evaluation techniques should also be applied to better improve the acceptance of these products. Future studies should endeavour to use many consumers (at least 100) who regularly eat the products studied to determine the sensory drivers of consumer preference. The objective characterisation of these products using a trained sensory panel and hedonic studies can be used to develop and tailor African fermented cereal products for non-African communities.

Author Contributions

Conceptualisation, E.K., O.O. and S.M.M.; writing—original draft preparation, E.K., O.O. and S.M.M.; writing—review and editing, E.K., O.O. and S.M.M.; visualisation, E.K., O.O. and S.M.M.; supervision, E.K.; project administration, E.K.; funding acquisition, E.K. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Research Foundation of South Africa (NRF), grant number 129619, and the Alliance for African Partnership (AAP) as part of the Partnerships for Innovative Research in Africa (PIRA) grant programme, grant number AID855.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Fermentation 09 00111 g001 550
Figure 1. LAB carbohydrate and organic acid metabolism and flavour compounds produced. Phosphotransferase system (PTS), permease (PM), pentose phosphate pathway (PPP), Embden–Meyerhof pathway (EMP), phosphoketolase pathway (PKP). Flavour compounds labelled in red.
Figure 1. LAB carbohydrate and organic acid metabolism and flavour compounds produced. Phosphotransferase system (PTS), permease (PM), pentose phosphate pathway (PPP), Embden–Meyerhof pathway (EMP), phosphoketolase pathway (PKP). Flavour compounds labelled in red.
Fermentation 09 00111 g001
Fermentation 09 00111 g002 550
Figure 2. Protein metabolism via LAB in cereal substrates. Cell envelope proteinases (CEPs), oligopeptide permease (Opp), ion-linked transporter (DtpT), ABC transporter (Dpp). Flavour compounds labelled in red.
Figure 2. Protein metabolism via LAB in cereal substrates. Cell envelope proteinases (CEPs), oligopeptide permease (Opp), ion-linked transporter (DtpT), ABC transporter (Dpp). Flavour compounds labelled in red.
Fermentation 09 00111 g002
Table
Table 1. Selected African fermented cereals and the microorganisms used for fermentation.
Table 1. Selected African fermented cereals and the microorganisms used for fermentation.
CerealProductMicroorganismsReference
MaizeDogik (ogi)L. pentosus and L. acidophilus[40]
Akamu/ogiL. plantarum[31]
Akamu/ogiL. plantarum/ L. cellobiosus[42]
MilletKunun-zakiL. plantarum/L. cellobiosus[17]
Koko (ogi)Lb. fermentum KL4 and C. tropicalis MKY6[42]
Kenkey [43]
PitoL. plantarum (LpTx)[44]
SorghumKisraLactobacillus fermentum, Lactobacillus amylovorus, Lactobacillus brevis and Saccharomyces
cerevisiae		[45]
TingL. reuteri FUA3168, L. harbinensis,
L. fermentum FUA3165, L. coryniformis, L. casei FUA3166,
L. plantarum FUA3171 and L. parabuchneri,		[25]
RiceIdli [45]
Dhokla [25]
Ice creamL. acidophilus LA-5 wer[39]
Maize/sorghum/millet/rice/ wheat/other materialsKunun-zakiLactobacillus plantarum, L. fermentum and Lactococcus lactis
MaheuLb. rhamnosus yoba[17]
TogwaL. fermentum, L. brevis, L. plantarum, L. cellobiosus,
and Pediococcus pentosaceus) and yeasts		[46]
TogwaLactobacillus plantarum, Pediococcus
pentosaceus, Lactobacillus casei and Lactobacillus fermentum		[47]
UjiLactobacillus plantarum,
L. brevis, L. buchneri, L. paracasei ssp. paracasei and Pediococcus
pentosaceus.
SourdoughPedicoccus acidilactici
O1A1, Pediococcus pentosaceus OA2, and Lactobacillus curvatus MA2		[41]
Breadfruit–corn S. thermophilus, L. bulgaricus and L. acidophilus
Table
Table 2. Organoleptic properties of aroma and flavour metabolites identified in African fermented cereals.
Table 2. Organoleptic properties of aroma and flavour metabolites identified in African fermented cereals.
MetaboliteLactic Acid Bacteria IdentifiedFermented CerealAssociated Sensory CharacteristicReference
Acids
Lactic AcidL. rhamnosus GR-1 and S. thermophilusPearl milletUnknown[15]
Acetic acidL. fermentum LB-11 *
L. pentosus strain CE56.28.1, CE56.19, L. plantarum strain 28.19 E, NWAFU1572, P. acidilactici strain AA106, P. pentosaceus strain OO1, L. fermentum strain CAU:235, YLc37C, BB101, Enterococcus faecium strain AA109, Weissella confusa strain JBA12 and Weissella confusa partial
L. rhamnosus GR-1 and S. thermophilus		Maize
Pearl Millet		Sour odour, vinegar odour, pungent smell
Unknown		[28]
[29]
[15]
Propionic acidL. fermentum LB-11 *
L. pentosus strain CE56.28.1, CE56.19, L. plantarum strain 28.19 E, NWAFU1572, P. acidilactici strain AA106, P. pentosaceus strain OO1, L. fermentum strain CAU:235, YLc37C, BB101, Enterococcus faecium strain AA109, Weissella confusa strain JBA12 and Weissella confusa partial		MaizeSharp, rancid odour[28]
[29]
Pentanoic acidL. fermentum LB-11 *MaizeBurnt rubber odour[28]
Hexanoic acidL. fermentum LB-11 *MaizeUrine-like odour, hay odour[28]
Heptanoic acidL. fermentum LB-11 *MaizeRubber odour[28]
Octanoic acidL. fermentum LB-11 *MaizeUrine-like odour[28]
Nonanoic acidL. fermentum LB-11 *MaizeRoast nut odour, chemical odour[28]
Hexadecanoic acidL. pentosus strain CE56.28.1, CE56.19, L. plantarum strain 28.19 E, NWAFU1572, P. acidilactici strain AA106, P. pentosaceus strain OO1, L. fermentum strain CAU:235, YLc37C, BB101, Enterococcus faecium strain AA109, Weissella confusa strain JBA12 and Weissella confusa partial
Not identified		MaizeNearly odourlessGreen/fatty/
waxy flavour		[29,30]
tetradecanoic acidNot identifiedMaizeGreen-fatty/waxy flavour[30]
(E)-octadec-11-enoic acidNot identifiedMaizeUnknown[30]
(E)-hex-2-enoic acidNot identifiedMaizeAnimal[30]
hexanoic acidNot identifiedMaizeAnimal[30]
Alcohols
3H-pyrazol-3-oneLeuconostoc mesenteroides and Pediococcus pentoseaceusPearl milletUnknown[21]
3,4-furandiol, tetrahydro-,transLeuconostoc mesenteroides and Pediococcus pentoseaceusPearl milletUnknown[21]
EthanolL. fermentum LB-11 *
L. pentosus strain CE56.28.1, CE56.19, L. plantarum strain 28.19 E, NWAFU1572, P. acidilactici strain AA106, P. pentosaceus strain OO1, L. fermentum strain CAU:235, YLc37C, BB101, Enterococcus faecium strain AA109, Weissella confusa strain JBA12 and Weissella confusa partial		MaizeAlcohol, fruity (mild, ether)[28,29]
PropanolL. fermentum LB-11 *
L. pentosus strain CE56.28.1, CE56.19, L. plantarum strain 28.19 E, NWAFU1572, P. acidilactici strain AA106, P. pentosaceus strain OO1, L. fermentum strain CAU:235, YLc37C, BB101, Enterococcus faecium strain AA109, Weissella confusa strain JBA12, Weissella confusa partial		MaizeAlcoholic, fruity, gum[28,29]
2-Methyl-1-propanolL. fermentum LB-11
L. pentosus strain CE56.28.1, CE56.19, L. plantarum strain 28.19 E, NWAFU1572, P. acidilactici strain AA106, P. pentosaceus strain OO1, L. fermentum strain CAU:235, YLc37C, BB101, Enterococcus faecium strain AA109, Weissella confusa strain JBA12, Weissella confusa partial		MaizeSweet smell, fruity[28,29]
2-PentanolL. fermentum LB-11 *MaizeGum, fruity[28]
1-ButanolL. fermentum LB-11 *MaizePungent, rubber[28]
1-Penten-3-olL. fermentum LB-11 *MaizeBoiled potatoes[28]
2-Methyl butanolL. pentosus strain CE56.28.1, CE56.19, L. plantarum strain 28.19 E, NWAFU1572, P. acidilactici strain AA106, P. pentosaceus strain OO1, L. fermentum strain CAU:235, YLc37C, BB101, Enterococcus faecium strain AA109, Weissella confusa strain JBA12, Weissella confusa partialMaizeAroma of tuber[29]
2-Methoxl phenolL. pentosus strain CE56.28.1, CE56.19, L. plantarum strain 28.19 E, NWAFU1572, P. acidilactici strain AA106, P. pentosaceus strain OO1, L. fermentum strain CAU:235, YLc37C, BB101, Enterococcus faecium strain AA109, Weissella confusa strain JBA12, Weissella confusa partialMaizeSweet odour[29]
2-Methyl propanolL. pentosus strain CE56.28.1, CE56.19, L. plantarum strain 28.19 E, NWAFU1572, P. acidilactici strain AA106, P. pentosaceus strain OO1, L. fermentum strain CAU:235, YLc37C, BB101, Enterococcus faecium strain AA109, Weissella confusa strain JBA12, Weissella confusa partialMaizeButtery or creamy[29]
3-Methyl-butanolL. fermentum LB-11 *
L. pentosus strain CE56.28.1, CE56.19, L. plantarum strain 28.19 E, NWAFU1572, P. acidilactici strain AA106, P. pentosaceus strain OO1, L. fermentum strain CAU:235, YLc37C, BB101, Enterococcus faecium strain AA109, Weissella confusa strain JBA12, Weissella confusa partial		MaizeVegetables, green odour
Choking alcohol odour		[28,29]
1-PentanolL. fermentum LB-11 *MaizeUnknown[28]
1-HexanolL. fermentum LB-11 *MaizeUnknown[28]
1-Octen-3-olL. fermentum LB-11 *MaizeUnknown[28]
HeptanolL. fermentum LB-11 *L. pentosus strain CE56.28.1, CE56.19, L. plantarum strain 28.19 E, NWAFU1572, P. acidilactici strain AA106, P. pentosaceus strain OO1, L. fermentum strain CAU:235, YLc37C, BB101, Enterococcus faecium strain AA109, Weissella confusa strain JBA12, Weissella confusa partialMaizeAlcohol[28,29]
1-OctanolL. fermentum LB-11 *MaizeOrange, sweet, fruity aroma[28]
2-Undecen-1-olL. fermentum LB-11 *MaizeUnknown[28]
NonanolL. fermentum LB-11 *MaizePopcorn aroma, odour of vitamin pill[28]
3-NonenolL. fermentum LB-11 *MaizeUnknown[28]
1,2-ButandiolL. rhamnosus GR-1 and S. thermophilusPearl milletUnknown[15]
Amino acid
TyrosineL. fermentum FUA 3165 and L. fermentum FUA 3321SorghumUnknown[22]
Amide
2-hydroxypropanamideNot IdentifiedMaizeUnknown[30]
Carbonyls (ketones and aldehydes)
2,4-Dihydroxy-2,5-dimethyl-3(2H)-furanoneSpontaneous, L. fermentum FUA 3165 and L. fermentum FUA 3321SorghumUnknown[22]
ValerophenoneSpontaneous and L. fermentum FUA 3321SorghumUnknown[22]
3-Acetoxy-2-methyl-pyran-4-oneL. fermentum FUA 3165 and L. fermentum FUA 3321SorghumUnknown[22]
2′,6′-Dihydroxy-3′-methylacetophenoneL. fermentum FUA 3165 and L. fermentum FUA 3321SorghumUnknown[22]
2-Hydroxy-1-(10-pyrrolidiyl)-1-buten-3-oneSpontaneous, L. fermentum FUA 3165 and L. fermentum FUA 3321SorghumUnknown[22]
Benz[cd]indol-2(1H)-oneL. fermentum FUA 3165 and L. fermentum FUA 3321SorghumUnknown[22]
[22]
Ethanone, 1-(3,4-dimethoxyphenyl)-Spontaneous, L. fermentum FUA 3165SorghumUnknown[22]
4-Amino-7-diethylamino-chromen-2-oneSpontaneous, L. fermentum FUA 3165SorghumUnknown[22]
6-Ethyl-4,7-dimethyl-2,3-benzofurandioneL. fermentum FUA 3165 and L. fermentum FUA 3321SorghumUnknown[22]
DiacetylL. fermentum LB-11 *MaizeUnknown[28]
PentanalL. fermentum LB-11 *MaizeUnknown[28]
HexanalL. fermentum LB-11 *MaizeGreen, grass, pine odour[28]
2-PentenalL. fermentum LB-11 *MaizeUnknown[28]
2-HeptanoneL. fermentum LB-11 *MaizeBad, unpleasant odour[28]
HeptanalL. fermentum LB-11 *MaizeGreen, sour green[28]
FurfuralL. fermentum LB-11 *MaizeUnknown[28]
NonanalL. fermentum LB-11 *MaizeAquarium, green, fatty[28]
2-OctenalL. fermentum LB-11 *MaizeUnknown[28]
2,4-HeptadienalL. fermentum LB-11 *MaizeHot, potato, silage[28]
BenzaldehydeL. fermentum LB-11 *MaizeHot, potato, silage[28]
2-NonenalL. fermentum LB-11 *MaizeOld, musty, onions[28]
BenzeneacetaldehydeL. fermentum LB-11 *MaizeUnknown[28]
2-DecenalL. fermentum LB-11 *MaizeUnknown[28]
2,4-NonadienalL. fermentum LB-11 *MaizeFermented, deep frying[28]
2-UndecenalL. fermentum LB-11 *MaizeFruity, cooked, rice[28]
2,4-DecadienalL. fermentum LB-11 *MaizeDough, fatty, soup[28]
gamma.-nonalactoneL. fermentum LB-11 *MaizeUnknown[28]
VinylphenolL. pentosus strain CE56.28.1, CE56.19, L. plantarum strain 28.19 E, NWAFU1572, P. acidilactici strain AA106, P. pentosaceus strain OO1, L. fermentum strain CAU:235, YLc37C, BB101, Enterococcus faecium strain AA109, Weissella confusa strain JBA12 and Weissella confusa partialMaizeColour of grape wine[29]
Ethyl caprylateL. pentosus strain CE56.28.1, CE56.19, L. plantarum strain 28.19 E, NWAFU1572, P. acidilactici strain AA106, P. pentosaceus strain OO1, L. fermentum strain CAU:235, YLc37C, BB101, Enterococcus faecium strain AA109, Weissella confusa strain JBA12 and Weissella confusa partialMaizeWaxy odour[29]
Ethyl caprateL. pentosus strain CE56.28.1, CE56.19, L. plantarum strain 28.19 E, NWAFU1572, P. acidilactici strain AA106, P. pentosaceus strain OO1, L. fermentum strain CAU:235, YLc37C, BB101, Enterococcus faecium strain AA109, Weissella confusa strain JBA12 and Weissella confusa partialMaizeWaxy odour[29]
2-methyl-naphthaleneL. pentosus strain CE56.28.1, CE56.19, L. plantarum strain 28.19 E, NWAFU1572, P. acidilactici strain AA106, P. pentosaceus strain OO1, L. fermentum strain CAU:235, YLc37C, BB101, Enterococcus faecium strain AA109, Weissella confusa strain JBA12 and Weissella confusa partialMaizeUnpleasant odour[29]
1-methylnaphthaleneL. pentosus strain CE56.28.1, CE56.19, L. plantarum strain 28.19 E, NWAFU1572, P. acidilactici strain AA106, P. pentosaceus strain OO1, L. fermentum strain CAU:235, YLc37C, BB101, Enterococcus faecium strain AA109, Weissella confusa strain JBA12 and Weissella confusa partialMaizeMothball smell[29]
2-3-ButanediolL. pentosus strain CE56.28.1, CE56.19, L. plantarum strain 28.19 E, NWAFU1572, P. acidilactici strain AA106, P. pentosaceus strain OO1, L. fermentum strain CAU:235, YLc37C, BB101, Enterococcus faecium strain AA109, Weissella confusa strain JBA12 and Weissella confusa partialMaizeOdourless[29]
Ethyl isovalerateL. pentosus strain CE56.28.1, CE56.19, L. plantarum strain 28.19 E, NWAFU1572, P. acidilactici strain AA106, P. pentosaceus strain OO1, L. fermentum strain CAU:235, YLc37C, BB101, Enterococcus faecium strain AA109, Weissella confusa strain JBA12 and Weissella confusa partialMaizeStrong odour[29]
2-methoxy-4L. pentosus strain CE56.28.1, CE56.19, L. plantarum strain 28.19 E, NWAFU1572, P. acidilactici strain AA106, P. pentosaceus strain OO1, L. fermentum strain CAU:235, YLc37C, BB101, Enterococcus faecium strain AA109, Weissella confusa strain JBA12 and Weissella confusa partialMaizePleasant, spicy odour[29]
Ethyl dec-a-enoateL. pentosus strain CE56.28.1, CE56.19, L. plantarum strain 28.19 E, NWAFU1572, P. acidilactici strain AA106, P. pentosaceus strain OO1, L. fermentum strain CAU:235, YLc37C, BB101, Enterococcus faecium strain AA109, Weissella confusa strain JBA12 and Weissella confusa partialMaizeOdourless[29]
Ester
3-deoxy-d-mannoic lactoneLeuconostoc mesenteroides and Pediococcus pentoseaceusPearl MilletUnknown[21]
Pentyl acetateNot identifiedMaizeFruity—tropical[30]
Prop-2-enyl hexanoateNot identifiedMaizeFruity—tropical[30]
ethyl methyl carbonateNot identifiedMaizeFruity—sweet[30]
pentyl 2-methylpropanoateNot identifiedMaizeFruity—sweet[30]
pentyl butanoateNot identifiedMaizeFruity—tropical[30]
ethyl butanoateNot identifiedMaizeFruity—tropical[30]
ethyl nonanoateNot identifiedMaizeFruity—tropical[30]
pentyl 2-methylbutanoateNot identifiedMaizeFruity—apple[30]
1,2-Ethanediol, diacetate 1,2-Ethanediol,Spontaneous, L. fermentum FUA 3165 and L. fermentum FUA 3321SorghumUnknown[22]
l-Proline, N-methoxycarbonyl-, heptadecyl esterSpontaneous, L. fermentum FUA 3165 and L. fermentum FUA 3321SorghumUnknown[22]
4-Methoxyphenol, acetate
Tetrahydropyran		L. fermentum FUA 3165 and L. fermentum FUA 3321SorghumUnknown[22]
Salicylic acid, methyl ester BromoaceticL. fermentum FUA 3165SorghumUnknown[22]
Methyl 2-oxo-1-pyrrolidine acetateSpontaneous, L. fermentum FUA 3165 and L. fermentum FUA 3321SorghumUnknown[22]
1-Ethyl-2-pyrrolidinecarboxylic acid, methyl esterL. fermentum FUA 3165 and L. fermentum FUA 3321SorghumUnknown[22]
Proline, 1-acetyl-, methyl esterSpontaneous, L. fermentum FUA 3165 and L. fermentum FUA 3321SorghumUnknown[22]
Phthalic acid, 4-chloro-3-methylphenyl ethyl esterSpontaneous, L. fermentum FUA 3165 and L. fermentum FUA 3321SorghumUnknown[22]
Hydrocinnamic acid, 4-hydroxy-3-methoxy-, methyl esterSpontaneous, L. fermentum FUA 3165 and L. fermentum FUA 3321SorghumUnknown[22]
3-Cyclopentylpropionic acid, 2-dimethylami- noethyl esterL. fermentum FUA 3165 and L. fermentum FUA 3321SorghumUnknown[22]
Phthalic acid, di(hept-2-yl) esterL. fermentum FUA 3165 and L. fermentum FUA 3321SorghumUnknown[22]
Ethyl acetateL. fermentum LB-11 *MaizeFlowery, ester odour[28]
Ethyl propionateL. fermentum LB-11 *MaizeFruity, gum, sweet[28]
Isoamyl acetateL. fermentum LB-11 *MaizeFruity[28]
Ethyl hexanoateL. fermentum LB-11 *MaizeFruity, plant, pear[28]
Hexyl acetateL. fermentum LB-11 *MaizeGreen odour, onion aroma[28]
Ethyl lactateL. fermentum LB-11 *MaizeFruity[28]
Ethyl heptanoateL. fermentum LB-11 *MaizeFlowery[28]
Heptyl acetateL. fermentum LB-11 *MaizeOnion, green[28]
Ethyl nonanoateL. fermentum LB-11 *MaizeUnknown
Fruity—tropical		[28]
[30]
Ethylphenyl acetateL. fermentum LB-11 *MaizeUnknown[28]
Ethyl
dodecanoate		L. fermentum LB-11 *MaizeUnknown[28]
Myristic acid, methyl esterL. fermentum FUA 3165 and L. fermentum FUA 3321SorghumUnknown[22]
9,12-Hexadecadienoic acid, methyl esterSpontaneous, L. fermentum FUA 3165 and L. fermentum FUA 3321SorghumUnknown[22]
cis-13-Octadecenoic acid, methyl esterSpontaneous, L. fermentum FUA 3165 and L. fermentum FUA 3321SorghumUnknown[22]
n-Hexadecanoic acid Pearl MilletUnknown[21]
trans-9-Octadecenoic acid, pentyl esterL. fermentum FUA 3165SorghumUnknown[22]
* LAB identified during spontaneous fermentation.
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Kayitesi, E.; Onojakpor, O.; Moyo, S.M. Highlighting the Impact of Lactic-Acid-Bacteria-Derived Flavours or Aromas on Sensory Perception of African Fermented Cereals. Fermentation 2023, 9, 111. https://doi.org/10.3390/fermentation9020111

AMA Style

Kayitesi E, Onojakpor O, Moyo SM. Highlighting the Impact of Lactic-Acid-Bacteria-Derived Flavours or Aromas on Sensory Perception of African Fermented Cereals. Fermentation. 2023; 9(2):111. https://doi.org/10.3390/fermentation9020111

Chicago/Turabian Style

Kayitesi, Eugénie, Ogheneyoma Onojakpor, and Siphosanele Mafa Moyo. 2023. "Highlighting the Impact of Lactic-Acid-Bacteria-Derived Flavours or Aromas on Sensory Perception of African Fermented Cereals" Fermentation 9, no. 2: 111. https://doi.org/10.3390/fermentation9020111

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