Stevia is a sweetener and sugar substitute extracted from the leaves of the plant species Stevia rebaudiana, native to Brazil and Paraguay. Dr. Ibrahim Hussaini Dogo, the Director-General of the Raw Material and Research Development Council (RMRDC), in this interview, explained why Nigeria must fully embrace its production for bio sugar extraction. Excerpts:
RMRDC intends to go into production of stevia, which I doubt many Nigerians know much about. Can you give us a vivid description of the plant and its cultivation potentials in Nigeria?
Stevia rebaudiana is a natural sweetener with zero calories that has recently been found in widespread use in food and pharmaceutical industries globally. It is a natural sweetener with high medicinal and commercial importance.
Stevia is a subtropical perennial herb, belonging to the family, asteraceae. It has annual, subligneous, more or less pubescent stems with extensive, fibrous and filiform root system. The cultivated stevia plant grows vigorously, giving a branched bushy shrub-like appearance. Stevia is native to Paraguay and Brazil and is often referred to as sweet herb, horny leaf, honey yerba and candy leaf. The leaf has been used for centuries as a sweetener to counteract the bitter taste of various plant medicines.
In 1964, this crop was cultivated commercially for the first time in Paraguay and later introduced to a number of countries. It is now successfully growing under different cultivation conditions and climatic locations of the world. Stevia seeds remain viable for a limited period and have very low germination rate because of their small size. Being a highly heterozygous specie, there is also variation in the plants raised from the seeds. They do not produce true-to-type plants, and therefore constant re-selection for type is required in mother-seed plots.
Stevia is mainly propagated vegetatively by stem cuttings, although it is a labour intensive procedure. Careful selection of the plant is required to get more productivity and profitability.
Stevia prefers a well-drained fertile sandy loam or loam soil high in organic matter. It prefers lighter acidic to neutral (pH 6-7) soil for better growth. It requires consistent supply of water, but excessive irrigation in water logging soils can cause stem rot disease. It requires partial shade during very hot and long sunny days.
Fertiliser requirement for stevia is moderate and varies according to the environment and soil type. The plant prefers low levels of nitrogen, but high levels of phosphorus and potassium. Under average climatic conditions and soil type, 70kg Nitrogen, 35kg Phosphorus and 45kg Potassium per hectare has been recommended.
Why does the council intend to go into production of stevia when Nigeria has so many plant species that are yet to be developed?
This is a very brilliant question. It is true that Nigeria has more than 500 plant species. A lot of work has been done on plant biodiversity locally. It is also true that the industrial potentials of some of them have not been adequately studied. Let me, however, say that global interest in stevia is because of its special attributes. Today, stevia is being promoted in most parts of the world for use in bio-sugar production. The World Health Organisation (WHO) is of the opinion that within the next few years, 20 per cent of the sugar consumed in most countries would be replaced by bio-sugar. Emphasis is being shifted from sweeteners produced from chemical components as they cannot control the incidence of diabetes that is ravaging the world.
Do you think it can be commercialised in a country like Nigeria?
Stevia can be commercialised in developing countries in view of its inherent value. Presently, it is commercially cultivated in China, Japan, Brazil, Canada, USA, UK, Spain, Belgium, Australia, South Korea, Thailand, Israel and Taiwan. China and Japan are the world’s major producers and exporters of diterpene glycosides. Japan has approved the use of stevioside in many food products, including cereals, teas and soft drinks.
In India, stevia was introduced in the last decade as a result of high demand due to its huge diabetic population. It has been successfully cultivated in many Indian states like Rajasthan, Maharashtra, Punjab, Kerala and Orissa. High demand for natural sweeteners as compared to artificial ones drove the farmers in India towards large-scale stevia cultivation.
In these economies, stevia cultivation and processing are fast developing into multi-million dollar enterprises. If adequately promoted, I am sure a number of pharmaceutical companies will latch unto it. Presently, the National Agency for Food, Drug Administration and Control (NAFDAC)-registered imported bio-sugar is sold in many supermarkets in the country.
Considering the high rate of post-harvest losses in Nigeria; which is about 30 to 70 per cent on annual basis, how do you intend to ensure that this initiative does not go the same way?
Stevia leaves are ready for first harvesting after four months of planting, and subsequent harvesting can be done after every three to four months. On an average, one can get three to four commercial harvests in a year, depending on land type, variety and climatic conditions. Young actively growing shoot sections and leaves have high glycoside content and over-matured leaves showing chlorosis have less glycoside content.
Shoot tips can also be clipped off along with leaves while harvesting as they contain same amount of diterpene glycosides as leaves. The total glycoside content of the leaves starts decreasing with onset of flowering, so harvesting should be done before the onset of flowering or immediately after flower bud formation.
Drying is the most important activity in post-harvest handling of stevia. The council has developed various types of driers. Some of these have been deployed commercially. This is important as freshly harvested leaves of stevia contain high moisture content and deteriorate if not dried properly. Drying of leaves should be completed immediately after harvesting by placing leaves on a screen or net. Sun drying method can also be used. This can be done in moderately warm conditions in about 12 hours. Proper aeration with low density of loading is required for quick drying in the sun. It can also be dried in a simple dryer by passing hot air just above room temperature. It has also been reported that the quality of leaves dried at 50°C in hot air dryer for six hours is better in terms of colour, sweetness and nutrients. Dried leaves with three to five per cent moisture content should be packed in air-tight containers and stored in cool and dry places.
How do you intend to process the leaves in view of the low level of technology advancement in Nigeria?
Extraction of steviol glycosides from leaves involve only conventional processes and purification procedures. It is somewhat similar to the extraction process used in sugar mills. Extraction of glycosides from leaves of stevia involves aqueous or solvent extraction, ion exchange purification, precipitation, filtration, crystallisation and drying. Extraction protocol involves dissolving leaves in hot water or alcohol.
Also, the leaves can be treated with non-polar solvents such as chloroform to remove oil, lipids, chlorophyll and other non-polar substances. It has been revealed that methanol is the best solvent for the extraction of Rebaudioside-A from the leaves in terms of component yield. Also, ethanol and aqueous acetone have also been found suitable to extract Rebaudioside-A, but yield may be less compared to methanol. Extracts would then be clarified by precipitation with salt or alkaline solutions, concentrated and re-dissolved in methanol for crystallisation of the glycosides.
The common steps involved in the extraction procedure are soaking of the leaves in warm water to dissolve the glycosides, precipitation and filtration of the resultant solution, concentration by evaporation, ion exchange purification, spray drying and crystallisation to produce white powder/crystals.
Today, most of the commercial processing of stevia leaves for the extraction of steviol glycosides are mainly concentrated in China and Japan where factories are located near stevia cultivated areas.
As a new initiative, has relevant bodies approved the consumption stevia products globally?
Yes; many biological and toxicological investigations have been carried out on steviol compounds of stevia in the last 50 years. European Commission’s Scientific Committee on Food (SCF) evaluated safety-related issues of this natural sweetener in 1985 and 1999 and raised questions about lack of acceptable purity specifications.
In 2004, the Joint FAO/WHO Expert Committee on Food Additives (JECFA) established tentative purity specifications which were later made permanent. JECFA established the Acceptable Daily Intake (ADI) of 4mg/kg bodyweight/day for purified steviol glycosides in 2008 and validated its use as a sweetener in food and beverages.
The plant and its extracts have also been used for several years as a sweetener in South America, Asia, Japan, China and in different countries of the European Union. In Brazil, Korea and Japan, stevia leaves, stevioside and highly refined extracts are officially used as low calorie sweeteners.
Presently in the US, leaf or extracted forms of stevia are used as a dietary supplement.
How do you intend to ensure the success of this initiative considering that there are no stevia agronomists in Nigeria?
The council has been collaborating with Kong Associates Shaghai (China) Limited to promote the development of different varieties of the plant. The company has introduced more than 100 improved castor seed varieties from China to the Ahafo Region of Ghana for multi-locational trials.
In November, 2017, the trials were finalised and Kong Associates selected the most adaptable varieties to the tropics for commercial castor seed plantation development. In view of extant collaboration between RMRDC, Kong Associates and the Castor Producers, Processors and Marketers Association of Nigeria (CPPMAN); Kong gave Nigeria appreciable quantity of the seeds for multi-locational trials in Nigeria in 2017.
Kong is also promoting safflower and stevia in Ghana. The company has agreed to give us improved stevia varieties for multi-locational trials in the country. It is also planning to send stevia agronomists to the country to direct planting operations. The company is interested in buying the leaves from and farmers that participate in the programme. This will guaranteed the off take for the produce.