Re-developing African traditional foods for industrial production, improved quality and safety

Abstract

There is a need to re-develop African traditional foods through innovation, research and development, and institute food safety risk assessment and science-based safety standards and quality assurance. Development of starter cultures for controlled fermentations; development and enforcement of standards and regulations for food packaging materials; use of extrusion cooking and other appropriate technologies are among the innovations needed to re-develop African traditional foods for industrial production, improved quality and safety.

Introduction

Africa is the most diverse continent with the largest number of countries (54), and hundreds of ethnic groups and tribes, with different cultures and a great diversity of traditional foods that are part of Africa’s rich cultural heritage that have served the people so well for centuries in terms of food and nutrition security¹. Regrettably, Africa is now the most food insecure continent, where poverty, hunger and malnutrition are widespread with devastating effects on productivity, intellectual development and health, particularly maternal and infant health². Over 250 million people in Africa were undernourished in 2019³. Africa accounts for a substantial number of the world’s malnourished children, with an estimated 64 million African children under 5 years of age affected by stunting (43% of the global share), with adverse consequences for their cognitive potential. Another 3 million African children were affected by severe wasting (22% of the global share), with an increased risk of death². Sub-Saharan Africa (SSA) and South Asia have the greatest burden of maternal mortality, accounting for 85% of maternal deaths worldwide, and SSA is still far from achieving the United Nations Sustainable Development Goals that state that by 2030 the global maternal mortality rate should be lower than 70 per 100,000 live births^4,5. Africa’s population is growing at a very rapid rate (up to 3% per annum in many countries), and is projected to reach 2.4 billion by 2050 from the current 1.5 billion⁶. High population growth rate, low food production, high post-harvest losses, weak supply chains, poor healthcare, social inequalities, climate change and conflicts are among the primary factors responsible for food and nutrition insecurity in Africa^7,8.

There is a need to revisit African traditional foods and practices that have sustained the continent in the past, and come up with new initiatives and approaches for increasing food production, reducing post-harvest losses and strengthening food value chains for sustainable food security in SSA⁸. African traditional foods are key to sustainable food security⁹. Indigenous African fruits and vegetables, roots and tubers, cereals and legumes, mushrooms, oilseed crops, and edible insects are major constituents of traditional African diets that provide sustenance to the people and contribute to alleviating under-and over-nutrition, promoting health and wellness in SSA on account of their unique nutritional and nutraceutical properties^10,11. Part of Africa’s rich cultural heritage are traditional food processing techniques developed since ancient times, over many centuries, to transform Africa’s indigenous crops, wild food plants, fish and livestock into safe, nutritious food products using skills acquired empirically through observation and experience^12,13. The objectives, main features and limitations of the traditional techniques for the processing of diverse foods in West Africa have been documented and discussed^11,12. The objectives of this review are to highlight the main challenges of African traditional foods that relate to engineering and technology, quality assurance, safety and marketing, and discuss progress made in re-developing African traditional foods through innovation, research and development to facilitate their industrial production, improve their quality and safety, with particular reference to fermentation, traditional beverages and extrusion cooking. The review also highlights the critical roles of modern food pilot plants, strong industry-academia partnership, and adequate government funding in facilitating industrial production and advancing African traditional foods into contemporary, modern products for urban lifestyles, as has been done in many other parts of the world, including India, Japan, and South Korea^14–16.

Dietary diversity, nutritional, and nutraceutical benefits

There is great dietary diversity in Africa, a reflection of the rich food culture of the world’s most diverse continent¹. Traditional and ethnic diets in SSA are enriched by hundreds of indigenous, lesser-known, underutilized crops and wild food plants with high nutritional value that are rich in health-promoting plant bioactives^1,10,11. African traditional food processing techniques such as salting and curing, smoking, roasting, sun-drying and fermentation are used to transform perishable agricultural raw materials into diverse nutritious processed foods with desirable sensory properties cherished by consumers⁹. Fermentation is one of the most important traditional food processing techniques in Africa used for the production of numerous fermented foods and beverages from different types of agricultural raw materials (substrates), including fruits and vegetables, cereal grains and legumes, roots and tubers, fish, meat and milk (Table 1). These foods are naturally fermented by a mixture of cultivable and non-cultivable microorganisms that are derived from the raw materials, processing facilities and environment^9,17. Laboratory studies have been carried out on many of these fermentations; the nature of the substrates, the fermenting microorganisms, and the microbiological and biochemical changes during the fermentations have been documented^9,17–19. The advantages of African traditional fermentations for food preservation, food safety and security, nutrition and health have been documented^9,17,18. Fermentation transforms plant food items such as African locust bean and African oil bean that are inedible in their natural state into edible products through extensive hydrolysis of their indigestible components and development of desirable organoleptic quality attributes⁹. Lactic acid bacteria and yeasts involved in African traditional fermentations are probiotics that confer numerous health benefits, including improved digestion, enhanced immunity, and potential protection against certain diseases. Other advantages of fermentation include increased nutrient content through microbial synthesis, especially B-vitamins; increased digestibility of proteins through hydrolysis to amino acids; increased bioavailability of minerals such as calcium, phosphorus, zinc and iron through hydrolysis of complexing agents such as phytate and oxalate; elimination of naturally occurring toxicants such as cyanide in cassava. Research on the microbial ecology of African food fermentations can lead to the identification of biomarkers for evaluating the quality of the products, and the development of optimal starter cultures for controlled fermentation, leading to better quality products while still maintaining their inherent cultural characteristics^17,20,21.

Table 1.

Selected traditional fermented foods of SSA

Fermented food (Country/Region)	Raw material (Substrate)	Predominating bacteria & yeasts	Nature & uses
Garri (Nigeria)	Cassava pulp	Leuconostoc mesenteroides, Leuconostoc lactis, Bacillus cereus, Staphylococcus spp	Granular product, dietary staple
Fufu (West & Central Africa)	Whole cassava roots	Lactobacillus plantarum, Leuconostoc mesenteroides, Lactobacillus cellobiosus Saccharomyces cerevisiae	Dough, dietary staple
Lafun (West Africa)	Cassava chips	Lactobacillus fermentum, Lactobacillus plantarum Weissella confusa Saccharomyces cerevisiae	Dough, dietary staple
Ogi (West Africa)	Maize, sorghum Millet	Lactobacillus fermentum, Lactobacillus amylolyticus Lactobacillus delbrueckii subsp. bulgaricus Lactococcus lactis Bacillus spp. Weissella confusa	Breakfast pudding, weaning food
Injera (Ethiopia)	Teff	Pichia fermentans Pichia occidentalis Candida humilis Saccharomyces cerevisiae	Pancake-like flat bread, dietary staple
Iru/dawadawa (West & Central Africa)	African locust bean (Parkia biglobosa), soybean	Bacillus amyloliquefaciens Bacillus licheniformis Bacillus pumilus Bacillus subtilis	Condiment in stews, soups, & sauces
Ogiri (Nigeria)	Melon, fluted pumpkin, castor oil seeds	Bacillus safensis Bacillus altitudinis	Condiment in stews, soups, & sauces
Kpaye/okpehe (Nigeria)	Prosopsis africana (algarroba or mesquite)	Bacillus subtilis Bacillus amyloliquefaciens Bacillus cereus Bacillus licheniformis	Condiment in stews, soups, & sauces
Ugba/ukpaka (Nigeria)	African oil bean (Pentaclethra macrophylla)	Bacillus cereus Lysinibacillus xylanilyticus Bacillus clausii Bacillus licheniformis	Delicacy usually consumed with stock fish or dried fish
Palm wine (West Africa)	Palm tree sap	Saccharomyces cerevisiae Lactobacillaceae Leuconostocaceae Acetobacteriaceae	Alcoholic beverage
Burukutu/pito/otika/dolo (West Africa)	Sorghum, millet, maize	Saccharomyces cerevisiae Lactobacillus fermentum Lactobacillus delbrueckii Pediococcus acidilactici	Alcoholic beverage
Obushera (Uganda)	Sorghum, millet	Streptococcus gallolyticus Streptococcus infantarius Lactobacillus fermentum Lactobacillus delbrueckii Weissella confusa	Non-alcoholic beverage
Gowe (Benin)	Sorghum, maize	Lactobacillus fermentum Lactobacillus mucosae Pediococcus acidilactici Weissella confusa	Non-alcoholic beverage
Nono/nunu (West Africa)	Milk	Lactobacillus fermentum Lactobacillus plantarum Leuconostoc mesenteroides Saccharomyces cerevisiae	Beverage or converted to butter
Lait-caille (Burkina Faso, Senegal)	Milk	Leuconostoc mesenteroides Pediococcus pentosaceus Weissella paramesenteroides Lactococcus lactis	Beverage
Mursik (Kenya)	Milk	Saccharomyces cerevisiae Lactobacillus kefiri Lactobacillus casei Lactobacillus paracasei Candida krusei	Beverage
Fura da nono (Nigeria)	Millet, milk	Lactobacillus plantarum Lactobacillus casei Enterococcus lactis Saccharomyces cerevisiae	Milk-cereal mixture, beverage, weaning food
Lanhouin (Benin)	Fish	Bacillus subtilis Bacillus licheniformis Staphylococcus lentus Staphylococcus xylosus	Condiment in stews, soups, & sauces
Momoni (Ghana)	Fish	Bacillus subtilis Bacillus licheniformis Staphylococcus saprophyticus Lactobacillus plantarum Pediococcus pentosaceus	Condiment in stews, soups & sauces

Changing dietary patterns due to “westernization” and urbanization

Africa is the most rapidly urbanizing continent in the world, with an average annual urban growth rate of 3.5%, and the urban population is expected to grow from 395 million in 2010 to 1.4 billion in 2050²². Urbanization adversely affects food systems and biodiversity in Africa. As Africa is the fastest urbanizing continent, it is also the most food insecure, and future urban area expansion is predicted to result in a substantial reduction in food production, loss of biodiversity, increased agricultural methane emissions, and dietary shifts²³. Dietary patterns are changing in SSA, especially in the urban areas, with dire consequences for nutrition and health^8,24. Rural diets are replaced by a Western-oriented diet when people move to urban areas in SSA. People living in rural areas still have a traditional and rural lifestyle, whereas “westernization” of diet and lifestyle is now a major feature of life in the urban areas²⁵. Traditional diets of the rural areas are plant-based and are rich in fresh fruits and vegetables, legumes, cereals, roots and tubers, whereas the “western diets” of the urban areas are energy-dense and are high in saturated fats, trans fat, cholesterol, sugar and salt. Urban diets are characterized by high intakes of fat, meat, especially fatty meat, and low intake of fiber and plant-based foods^8,24. Animal protein intake and sugar intake were higher in families living in urban areas, while fiber intake was higher in those living in rural and semi-urban areas in Burkina Faso²⁵. In a study of urban household characteristics and dietary diversity in Accra, Ghana, similar dietary changes were reported²⁶. Processed and packaged foods are more widely available in urban areas than in rural areas, and supermarkets and restaurants in urban areas provide greater access to a variety of foods, such as the so-called ultra-processed foods and sugary beverages that are rich in sugar, salt and saturated fats²⁵.

A recent study indicated that higher intakes of fruits, vegetables, whole grains, unsaturated fats, nuts, legumes and low-fat dairy products were positively correlated with healthy aging, whereas higher intakes of trans fat, sodium, sugary beverages and red or processed meats (or both) were inversely associated. The study suggested that dietary patterns rich in plant-based foods, with moderate inclusion of healthy animal-based foods, may enhance overall healthy aging²⁷. The changes in dietary patterns due to urbanization and “westernization” coupled with physical inactivity from sedentary lifestyles are contributing to the increase in the incidence of diet-related non-communicable diseases (NCDs) such as obesity, diabetes, hypertension and cardiovascular diseases or heart disease in SSA^8,24. SSA now faces the double burden of malnutrition in which undernutrition coexists with overnutrition (overweight, obesity and NCDs) in the midst of pervasive poverty. SSA also has the double burden of disease with NCDs coexisting with infectious diseases, including lower respiratory tract infections, HIV/AIDS and diarrheal diseases, further stressing the fragile healthcare system²⁸. There is a need for advocacy, education, industrial production and aggressive promotion and marketing of traditional foods to reverse the trend of changing dietary patterns due to urbanization and “westernization” and their adverse consequences on nutrition and health.

Regulation, safety, process control, and quality assurance

Despite their superior nutritional quality and nutraceutical benefits, safety remains a major concern of African traditional foods associated with foodborne illnesses due to pathogenic bacteria, viruses, parasites, mycotoxins, chemicals, and naturally occurring toxicants^29,30. Food safety concerns with African traditional foods arise mainly from lack of adherence to good manufacturing processes and good hygienic practices from the procurement of raw materials and ingredient formulation, through processing, packaging, distribution, and marketing^9,18,29. Production of fermented beverages from cereals such as maize, millet, and sorghum that are heavily contaminated with multiple mycotoxins due to poor farm and post-harvest handling practices and improper storage is a major food safety concern^31,32. In addition, pathogenic bacteria have been reported in several cereal-based fermented beverages produced in different parts of SSA. Foodborne pathogens, including Staphylococcus aureus, Salmonella spp., and Escherichia coli, were reported in fura da nono from Nigeria^33–35. Gowe from Benin was contaminated with E. coli and Enterobacteriaceae³¹. Similarly, obushera from Uganda did not meet microbial safety standards and had populations of coliforms and Staphylococcus spp above the recommended minimum levels³⁶. Bacillus cereus, Staphylococcus aureus, and Listeria monocytogenes are among pathogenic bacteria reported in African traditional fermented foods¹⁸. The presence of pathogenic bacteria and indicator microorganisms that are resistant to antibiotics in some traditional fermented foods, including dairy, cereal, vegetable, and oil seed products from different parts of Africa raises serious concern with foodborne antimicrobial resistance (AMR) that reduces the options for treating human and animal diseases; underscoring the need for monitoring and surveillance of AMR in food chains in Africa^9,18.

Key to re-developing African traditional fermented foods and improving their safety, quality, and production capacity is control of the fermentation process. The use of natural spontaneous fermentation processes and backslopping involving a multitude of microorganisms that may include spoilage and pathogenic organisms should be replaced by controlled fermentation and the use of starter cultures. Re-developing African traditional fermented foods will involve the following steps: isolation, identification, preservation and storage of the microorganisms involved; determination of the roles of the various microbial isolates; selection among the most promising microbial isolates and/or genetic improvement; optimization of the fermentation through process control and/or equipment design and fabrication; improvement in the quality of the fermented food; laboratory simulation of the production of the fermented food using the microbial isolates (starter culture) and any specially produced small-scale or laboratory equipment; pilot plant production of the fermented food incorporating the development of appropriate machinery; industrial scale production of the fermented food^24,37. While substantial progress has been made in identifying and characterizing the microorganisms involved in many traditional African food fermentations (Table 1), relatively little progress has been made on the development of starter cultures, and even less progress on their commercial production and utilization. However, there are a few notable exceptions, such as South African mahewu and amasi. Mahewu is a traditional South African non-alcoholic beverage made from white maize or sorghum flour and produced on an industrial scale through controlled fermentation using starter cultures such as Lactobacillus bulgaricus var delbrueckii and Lactobacillus brevis³⁸. Amasi is a traditional South African fermented milk produced on an industrial scale under controlled conditions using starter cultures of Lactobacillus lactis subsp. lactis and Lactobacillus lactis subsp. cremoris³⁹. This underscores the fact that with adequate investment, adoption of modern food processing technologies, and robust regulation, good quality, safe African traditional foods can be produced profitably on an industrial scale, contributing to food and nutrition security, and improving livelihoods.

There are many nonfermented, nonalcoholic beverages produced using traditional methods from lesser-known, underutilized indigenous African fruits and vegetables that are important sources of essential minerals and vitamins such as roselle (Hibiscus sabdariffa), baobab (Adansonia digitata), black plum (Vitex doniana) and tamarind (Tamarindus indica). These products, such as zobo (Nigeria) or sobolo (Ghana), buoye (Senegal), and tsimi/tsamia (Nigeria), are very popular among the vast majority of low-income earners in many parts of SSA on account of their affordability, as they are cheaper than carbonated and non-carbonated soft drinks produced by food transnational corporations and other companies^9,40,41. The traditional household methods for their production vary from locality to locality but invariably involve extraction/expression, straining/sieving/filtration, ingredient formulation and packaging, with numerous opportunities for product contamination from raw materials, processing utensils, processing environment, personnel, and packaging materials⁹. These traditional beverages that are hawked in the streets, especially in urban areas, and sold in the open in a tropical environment without refrigeration, are packaged in discarded containers such as plastic bottles previously used for the packaging of other products, retrieved from garbage bins, drains, and dumpsites with other domestic and industrial wastes; a major source of contamination with serious implications for food safety and public health^42,43. There are no regulations governing the reuse of plastic bottles for food packaging, including specifications for cleaning and sanitization. High levels of microbial contamination, including fecal and other coliforms, were reported in used bottles sold in three major markets in Accra, Ghana, underscoring the need to develop and enforce standards and regulations for packaging materials used by producers and vendors of traditional foods and beverages in SSA to safeguard public health⁴². Another major food safety concern in the collecting and recycling systems of bottles reused for traditional food packaging, even after proper cleaning of the bottles, is microbial biofilms containing clusters of microorganisms, including pathogens, formed on inert materials such as plastic bottles that are very difficult to remove even with disinfection^43,44. In a cross-sectional study in 43 randomly selected neighborhoods in Yaounde, Cameroon, involving 162 producers and street sellers of traditional foods who used recovered bottles for food packaging, microbial biofilms were detected in all the bottles irrespective of the cleaning method used; even when the cleaning processes included soaking and disinfection, which were more efficient in biofilm removal⁴³.

Improved simple processing procedures, suitable for small-scale commercial production, have been developed for nonalcoholic beverages from lesser-known and underutilized indigenous African fruits and vegetables such as African star apple (Chrysophyllum albidum), hog plum (Spondias mombin), roselle and tamarind involving mechanical juice extraction using a pulper/finisher (where appropriate), pasteurization, and packaging in previously sterilized glass or polyethylene terephthalate (PET) bottles^40,45,46. To a very limited extent, zobo is now produced on a commercial/industrial scale and is available in supermarkets, but by and large, most of the zobo produced and sold in Nigeria is still at the household level and the informal food sector using the traditional technology with no regulation. Measures that can improve the quality and safety of African traditional foods and beverages include the adoption of good agricultural practices, good hygienic practices, good manufacturing processes and Hazard Analysis Critical Control Points (HACCP). There is a need for food safety risk assessment, science-based safety standards, and quality assurance for African traditional foods. The production operations and processes need to be harmonized with international standards, including those of Codex Alimentarius. To this end, government regulatory agencies in African countries should be strengthened and empowered to develop and enforce food safety regulations and quality standards for traditional foods.

Extrusion processing, scaling, and food pilot plants

Extrusion processing, a physical food processing technique, that accomplishes a number of functions including grinding, mixing and homogenization, shaping, hydration, expansion, texturization, partial hydration, heat treatment, protein denaturation and starch gelatinization, is used for the production of a variety of food products notably noodles and other pasta products, weaning (complementary) foods, texturized vegetable proteins, snacks and convenience foods. Versatility, high productivity, low costs, high product quality, and no process effluents are among the attractive advantages of extrusion processing. Extrusion cooking that utilizes a combination of shear, high pressure and inter-particulate frictional heat to attain high temperatures, commonly 100–200 °C, in the extruder barrel in a very short time leads to microbial destruction, enzyme inactivation, and elimination of naturally occurring toxicants; producing a cooked, dry product with low water activity that is microbiologically safe and shelf stable at non-refrigeration temperatures (Fig. 1). Extrusion cooking is increasingly being adopted as a processing technology for producing nutritionally dense foods from traditional cereals⁴⁷. Extrusion cooking eliminates the drawbacks of African traditional food processing and offers unique advantages for re-developing African traditional foods, especially cereal- and legume-based products, when cooking or gelation, and shaping are required⁴⁸. Extrusion cooking has great prospects for producing a wide range of nutritious, safe, ready-to-eat products for all age groups from local and indigenous cereals enriched with legumes, including high protein-energy complementary foods for alleviating protein-energy malnutrition in children in SSA^48,49. Table 2 highlights selected African traditional food product development studies using extrusion cooking.

Fig. 1. Laboratory pilot plant extrusion cooking of cereal-based products.

Laboratory pilot plant extrusion cooking of cereal-based products; showing a twin screw extruder on the left, and theextruded dried products on the right.

Table 2.

Selected African traditional food development studies using extrusion cooking

Product	Extrusion cooking conditions	Reference
Sorghum-based porridges fortified with baobab and moringa powder	Twin screw extruder; barrel temp 60–140 °C; 20% feed moisture content	53
Snack from aerial yams and African breadfruit seed	Single-screw extruder; barrel temp 120 °C; 17.5% feed moisture content	54
Nakiya, a Nigerian snack made from rice and peanuts	Pilot scale co-rotating twin screw extruder; barrel temp 80–120 °C; 18–22% feed moisture content	55
Senegalese infant formula from pearl millet, peanut and cowpea	Single-screw mini-extruder with 7.5 HP electric motor; 48.2 Hz, 138 °C	56
Breakfast cereal from sorghum and bambara groundnut	Single-screw extruder; barrel temp 120–160 °C; 20–25% feed moisture content	57
Sorghum malt and bambara groundnut-based extrudates for breakfast cereal or snack	Twin screw extruder; barrel temp 100–130 °C; 20–30% feed moisture content	58
Snack from cocoyam, a dietary staple in Nigeria, the leading producer	Single-screw extruder; barrel temp 200–240 °C; 22–26% feed moisture content	59
Complementary food from acha and cowpea	Single-screw extruder; barrel temp 120–160 °C; 18–25% feed moisture content	49,60
Fura, a Nigerian staple from pearl millet and grain legumes	Single-screw extruder; barrel temp 150–170 °C; 30% feed moisture content	61,62
Puffed maize-based snack enriched with soybean	Single-screw extruder; barrel temp 200 °C; feed moisture content 20%	63
Extruded fermented maize-finger millet blend for uji, an East African porridge	Single-screw extruder; barrel temp 150-180 °C; feed moisture 19%	64,65
Soyabari snack sticks based on Nigerian snack (kokoro)	Single-screw extruder; barrel temp 150–170 °C; 20% feed moisture content	66

In the developed economies of the world, there is a vast array of diverse, safe, nutritious foods made possible by developments in food processing and preservation technologies. Regrettably, advances in food processing and preservation have not made the same impact in developing countries. Even though a considerable amount of research has been done on improving the quality of African traditional foods, they remain largely at the laboratory bench stage, and the foods are still largely produced in the home and the unregulated informal food sector with little improvement in quality and process techniques. Lack of modern food pilot plants, needed to move from bench scale to commercial production, is one of the most important factors constraining the commercialization of research findings in re-developing African traditional foods. Food pilot plants enable the scaling of production capacity from a few kg/day (bench scale) to a few tons/day, facilitating detailed technical and economic feasibility studies that are required before adoption by the industry. For example, Cornell University’s food research facilities include, among others, the state-of-the-art Food Venture Center Pilot Plant, renovated in 2018 with a $13 million grant from the New York State Government⁵⁰. The facility located at Cornell AgriTech in Geneva, New York, has a 10,000 sq ft main processing floor and supports more than 500 companies and delivers about 2000 products to the market every year⁵⁰. Industry-academia partnerships are very weak in Africa, constraining the commercialization of research findings from universities and research institutes and minimizing their impacts on society. Strong industry-academia partnerships are not only mutually beneficial but synergistic, providing huge benefits to universities, including among others: financial support for the teaching, research and community services missions of universities; assisting in identifying significant, relevant problems that can be addressed through research; promoting the commercialization of research findings; enhancing the contribution of universities to economic development⁵¹. Poor funding severely constrains research and development in Africa. The present paltry expenditure on research and development, less than 0.5% of GDP in most sub-Saharan African countries, should be increased substantially to at least 1–2% of GDP⁵². Given the high cost of modern food pilot plants, the establishment of zonal/regional food pilot plants in African universities and research institutes by governments in partnership with the private sector will promote the commercialization of research findings in re-developing African traditional foods, with huge benefits for food and nutrition security, food safety, and wellness⁹.

Conclusion

Engineering and technology, quality assurance, safety and marketing are the main challenges of African traditional foods. There is a need to re-develop African traditional foods through innovation, research and development, and institute food safety risk assessment and science-based safety standards and quality assurance. There is also a need for the development of optimal starter cultures for controlled fermentation, leading to better quality products, and to develop and enforce standards and regulations for packaging materials used by producers and vendors of traditional foods and beverages to safeguard public health. Extrusion cooking offers unique advantages for re-developing African traditional foods. Lack of modern food pilot plants, weak industry-academia partnership and poor government funding constrain research and commercialization of research findings in re-developing African traditional foods.

Author contributions

O. C. Aworh is the sole author.

Data availability

No datasets were generated or analyzed during the current study.

Competing interests

The author declares no competing interests.