1. Introduction

The main natural sources of human nutrition come from a great diversity of plant species able of providing water, proteins, lipids, carbohydrates, minerals, trace elements and vitamins. Consumed plants sometimes have therapeutic properties against pathologies including metabolic diseases such as Allium sativum (Liliaceae) [1], Combretum micranthum (Combretaceae) [2] and Piper nigrum (Piperaceae) [3]. However, other plant species such as Panax ginseng (Araliaceae) and Pilocarpus microphyllus (Rutaceae) can be toxic at some doses due to the presence of toxic chemical compounds [4-6]. This toxicity can also be linked to the methods of preparation, preservation and storage conditions inducing degradation processes of chemical constituents [7,8]. Furthermore, several cases of intoxications associated to plant derived products have been reported and related to species confusion [9].

Herbal preparations show growing interest around the world. This is particularly true in France, where the market for supplement herbal food represents approximately 29% of sales, with an annual growth of 5 to 15% [10]. In Burkina Faso, more than one million tons of medicinal plants are sold each year and the same quantity is exported [11]. These sales generate a turnover of more than ten billion CFA francs [11]. In addition, the World Health Organization (WHO) recommends the integration of traditional medicine and associated plant-based recipes in primary health care in order to achieve universal health coverage [12,13]

Thus, to meet these needs, many herbal products are on sale in stores. These products are quite often found in grocery stores, food stores and herbal specialty stores. However, consuming plants and their derived products is not always without any risk for human health. These risks are related to poor quality, toxicity and possible interactions [9,10,14].

However, in Burkina Faso in particular, data on the plants used, food distribution channels, quality of raw materials, safety and traceability of marketed herbal preparations are not available. The main objective of this study is therefore to contribute to providing factual information on herbal products consumed by populations of the city of Ouagadougou. More specifically, it involves assessing the supply of herbal products in food stores in the city of Ouagadougou and studying pharmacognostics characteristics of raw materials used to manufacture targeted products.

2. Materials and Methods

2.1 Plant-based products offer

Data on the plant-based products offer in food stores were collected from 30 stores in all the 12 districts of the city of Ouagadougou. These data were collected using a detailed informative form. All plant-based products found in visited structures were taken into account. Data was analysed using a descriptive statistical approach. Frequencies were calculated and acceptability threshold was set to more than 5%.

2.2 Pharmacognosic characteristics study raw materials

2.2.1  Products sampling and preparation for assays: Combretum micranthum, Moringa oleifera and Hibiscus sabdarifa were selected for more pharmacognosic investigations based on the frequency of use as raw materials and their disponibility in Burkina Faso. Then, four food-products containing Combretum micranthum, four others with Moringa oleifera, and three with Hibiscus sabdarifa were sampled for testing. All selected products raw materials were submitted to macroscopic observation. Then hot water extraction (HWE) based on the manufacturer's preparation instructions, were performed before phytochemical screening. When products were packaged in a tea bag form, raw material were directly collected and used for HWE. In other cases, HWE was performed after grounding.

2.2.2 Macroscopic and organoleptic observations: Selected product specimens were submitted to macroscopic observation for fungal contamination, presence of impurities or non-plant particles, and organoleptic characteristics as colour, odor and texture.

2.2.3 Phytochemical screening: Phytochemical screening was carried out on HWE from selected samples for terpenoids, phenolic compounds and alkaloids. Analyses were carried out respectively using the Salkowski test (chloroform-sulphuric acid) for terpenoids [15,16], ferric chloride (10% FeCl₃ solution) for phenolics [17,18] and the Dragendorff test (bismuth sub-nitrate, potassium iodide in concentrated hydrochloric acid) for alkaloids [15,19].

2.2.4 Thin layer chromatography analysis coupled to densitometric analysis: Ten (10) μL of HWE from samples were applied onto a TLC silica (Silica gel 60 F254, Merck®) glass plate (20 × 10 cm, Merck) previously dried in an oven at 110 °C, as 8 mm bands from 10 mm from the lower edge and 10 mm between them, by using an automatic CAMAG®-TLC auto-sampler. Then, plates were developed in a saturated Twin-Trough chamber with three different mobile phase systems (Table 1) to 10 mm from the top of plates. Developed TLC plates were then air dried and images were captured under white light, UV at 254 and 366 nm. Derivatization were performed by spraying with sulfuric vanillin reagent (for terpenoids) or Neu reagent (for flavonoids and phenolic acids).

To perform densitometric data collection, CAMAG Thin Layer Chromatography scanner IV with reflectance absorbance mode and Win CATS software and tungstun lamp was used  after derivatization to obtain chromatograms [20,21].

Table 1 : TLC mobile phase according to phytochemical groups.

3. Results and Discussion

3.1 The supply of plant-based products

Samples were collected from 30 food stores in Ouagadougou. A total of 1020 plant-based products were identified. These products were mainly presented in manufactured form (81.37%) and were available at least in two stores (31.30%)

Plant parts in tea-bag for infusion represented (49.76%) of plant-based food products. Leaves were most plant part frequently found (49.47%) compare to roots (6.32%). This could be explained by their easy  access and preparation [11]

Used plant identity was available for 842 (82.55%) products and allowed to identify fifty-five plant species belonging to twenty-eight  plant families. Lamiaceae, Asteraceae and Rosaceae were the most represented (Table 2).

Table 2 : Plants recorded by families and species.

These plants were used both for food and therapeutic purposes. Five plants are common to 85,98% of the 842 products (Table 3). Three of them (Combretum micranthum, Hibiscus sabdarifa and Moringa oleifera) are produced locally.

Table 3 : Main plants found in collected products.

Collected product labellings did not include plant scientific names for 69.10% of cases and none indications were mentioned in 78.83% of products. When some information was mentioned it concern care benefit for digestive disorders, high blood pressure and the purgative effect. Preparation instructions and precautions were found only for 12.72% of the products. Labels also show that the products come mainly from France for 27.07% of products, Burkina Faso for 12.78% and China for 10.53%.

 Furthermore, the labeling of surveyed products must be improved to meet regulatory requirements in Burkina Faso for prepackaged foodstuffs [22,23]. These include country of origin, indications, conditions of use and precautions for use. The absence of theses items for some products can lead to misuse, sometimes with negative consequences on consumer’s health. The lack of information on the country of origin does not allow good traceability of product for risk management.

In order to improve the safety of use of prepackaged food products, rules have been edicted by the Codex Alimentarius, and have been integrated as Burkina Faso national standard NBF01-117:2009. The objective of these rules is to facilitate quality control and to ensure traceability to better inform consumers [22,23]. For herbal products, it is important to have an effective risk management policy because some plants can cause various allergic reactions and/or interact with other substances or medications when taken concomitantly.

For example, excessive or chronic use of Panax gingseng can cause various disorders, including neurological, cardiovascular, haematological and/or skin rashes. Described interactions associated with Panax gingseng use, include alcohol, warfarin, and phenelzine [6,24]. Allium sativum, exhibit enzyme-inducing properties that influence some drug bioavailability eg. antiretrovirals (ARVs) [25,26].

3.2 Study of raw materials characteristics

Plant based products containing Combretum micranthum, Hibiscus sabdarifa and Moringa oleifera were selected for their frequency of use and the presence of their raw materials in Burkina Faso.

3.2.1 Macroscopic observation: In plant-based products containing Moringa oleifera, that had been coarsely ground, leaves were light green or green-brown. Samples contained petioles in similar size and colour (Figure 1A,1B).

Observed browning leaves may be relie to manufacturing process and/or storage conditions, such as high exposure to sunlight during drying or bad storage conditions.For the fourth sample in tea bags, plant parts were finely ground and contained woody elements (Figure 1C).

Figure 1: Macroscopic aspects of plant-based products raw material.

 It is therefore difficult to conclude that this fourth sample contains Moringa on based on raw materials appearance. Then, macroscopic analysis revealed shortcomings related to quality and manufacturing process, such as browning and presence of wood in leaf samples. In addition, macroscopic observation is not effective to identify and/or to characterize plant powders.

Products containing raw Combretum micranthum, that had been coarsely ground, leaves were green and petioles brown (Figure 1A). Observed samples present a strong similarity when compared. This may suggest some similar storage conditions. In the tea bags specimens, leaves were finely ground with similar colour to each other. No foreign particles were detected on macroscopic observation.

Products containing raw Hibiscus sabdarifa, plant material had bright red and came from flowers calyxes. Tea bags specimens contain coarsely ground calyxes. All selected products present similar acidic smell.

Characterization and identification of plant powders require to combinate other analytical assays. To this end, various usefull analytical techniques that are simple, effective and economical, such as microscopy and infrared spectrometry, are insufficiently exploited [27,28]. This highlights the need of development and strengthenning analytical methods to standardize and authenticate raw materials for safe use in medicines and foods [29,28]. Analytical techniques applied by microscopy give excellent results that could be validated with multicentric samples. Also, infrared spectrometry applied in metabolomics approach on plant powders enabled good characterization and optimal discrimination of samples based on their spectral profile [30,31].

3.2.2 Phytochemical screening: All samples are positive to ferric chloride and Salkowski test, indicates the presence of phenolics and terpenoids compounds respectively. Draggendorf test was negative and indicates absence of alkaloids. Alkaloids are sources of plant actives substances responsible for most plant associated toxicity [4]. However, Bassene et al. and Mogue Ingrid et al. had reported presence of some nitrogenous compounds in Combretum micranthum [9,14]. Further investigations are need to precise types of nitrogenous compounds present in Combretum micranthum.

3.2.3 TLC analysis: Visual observations: TLC allow chemical profile comparison of hot water extract (HWE) obtained from selected products. Derivatization with vanillin sulfuric showed low intensities of spots for terpenoids detection This means low concentration of terpenic compounds in these extracts. In contrast, screening for phenolics using Neu's reagent reveal the presence of significant concentration (Figure 2) of phenolic compounds.

The extraction method could explain the concentration of different compounds. Indeed, with hot water infusion most terpenoids are weakly extract except oxygenated polar triterpenes and triterpenoids compared to organic solvent as acetone and ethanol coupled with hexane or ethyl acetate [32,33]. In contrast, phenolics compounds are best extracted in polar solvents such as water, notably with increased temperature. Extraction yields are improved when polar solvents such as acetone-water or ethanol-water are combined [34,35].

Figure 2: TLC profile of aqueous infusions from plant-based products.

Chromatographic profile: TLC profile describe on literature data and using derivatization with Neu's reagent, products containing Moringa oleifera organs showed intense yellow-orange spots (Rf: 0.21 and 0.48) for flavonols [15], greenish yellow (Rf: 0.58) for flavones [16] and light blue (Rf: 0.44) for phenolics acids [16]. Several molecules belonging to these three families have already been characterized in Moringa oleifera leaves including quercetin, rutin and kaempferol as flavonols, chlorogenic acid as phenolic acids and isorhamnetin as flavones. These compounds possess biological and physiological properties that have been extensively evaluated [36-38].

Products containing Combretum micrantum, showed two (02) dominant spots coloured in light blue (Rf of 0.48) and greenish yellow (Rf of 0.38), with however lower intensities for one of the products. These spots are characteristic of the presence of phenolics acids [16], and flavone type flavonoids respectively [16]. However, spots in the fourth sample differed completely from the others. Presence of phenolic acids and flavones such as gallic acid, hydroxybenzoic acid, chlorogenic acid, syringic acid and various flavones including vitexin, isovitexin, orientin and homoorientin in Combretum micrantum have been documented by several authors  [39-41].

All products containing Hibiscus sabdarifa calyx, showed intense blue light colour (Rf: 0.44) characteristic of phenolics acids [17]. The purplish tendency in visible is characteristic of anthocyanins [18]. Among phenolic acids present in the calyxes several authors have already found chlorogenic acid, caffeic acid and protocatechuic acid [42,1].

Densitometric profile: Densitometric analysis was carried out under visible light after vaporisation with Neu's reagent. Spots from Moringa oleifera based products showed qualitative identical profiles for three while the fourth profile shows an additional peak, probably due to degradation product. Also, a density difference existed between similar profiles. This may be consistent to degradation state detected with macroscopic observations (Figure 3)

Figure 3: Densitometry densitometric profiles of aqueous infusions of plant-based products after TLC.

For plant foods containing Combretum micranthum, only samples with identical spots under UV light at 366 nm were processed by densitometry. Images showed qualitative similarities between three samples and suggest that these products come from the same species. However, contents were relatively low in infused samples (Figure 3) which could be explained by the extraction method from raw leaves. For Hibiscus sabdarifa containing products, TLC profiles were qualitatively similar which differences in semi-quantitative profile (Figure 3). The variations in chromatographic and densitometric profiles can be explained by the degradation of compounds or possible adulteration by mixing with other plants or non-plant materials [10,43].

These potential risks of adulteration require the development of quality control methods for natural plant-based products. These methods integrate multidisciplinary approaches to detect and characterize chemical markers of plant species. In the case of this study, for a given species, the TLC profiles of each sample show the presence of similar spots that could constitute chemical markers for species identification. Characterization of these substances could be a first step towards the identification of potential chemical tracers for Combretum micranthum, Hibiscus sabdarifa and Moringa oleifera.

Conclusions

 The objective of this work was to document the availability of food and / or medicinal plant-based products found in supermarkets in the city of Ouagadougou, and to assess their pharmacognosic quality.

A total of 1020 products containing around 55 plant species from 28 families were identified. Five plant species were frequently used, include three from Burkina Faso's flora. Among these products, those containing Moringa oleifera Lam., Combretum micranthum G. Don., And Hibiscus sabdarifa L. were selected for pharmacognostics analysis.

Effective characterisation of plant speciy specific coumpounds could improve the quality control and traceability of food products containing these plants. This research should be pursued using multidisciplinary analytical methods coupled with high-throughput multivariate data analysis like chemometric tools.

Conflicts of interests

No conflicts of interest have been registered on this work to the best of our knowledge.

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