TRADITIONAL VEGETAL SALTS AS SUBSTITUTE FOR COMMON SALT
Okoli, Ifeanyi Charles
The numerous health concerns associated with excessive sodium intake have rekindled a preference for potassium-rich vegetable salts, especially in Africa, where it has been a traditional alternative to common salt. Indeed, their usage has historically been shown to be pantropical, especially in places where common salt was not readily available in Africa, Asia, Oceania, and South America, dating back to several decades before Christ. In sub-Saharan Africa, especially in West, Central, and East African countries, indigenous plant salts are still traditionally produced and consumed either for nutritional or medical reasons. They are specifically used in the preparation of indigenous vegetables, legumes, cereals, and meat, and are therefore major ingredients in several local dishes. Traditional vegetable salts have also been used as a substitute for common salt in the treatment of stomach upset, jaundice, and cough and for lowering blood pressure. The generally adopted protocol for vegetable salt production at these locations is the volarization of biomass wastes such as grasses, ferns, fruit peels, herbs, shrubs, palms, leaves, barks, shoots, stems, or even the whole plant and tree to produce ash and leaching the ash with water to obtain a potassium-carbonate-rich crude bio-extract.
According to Mianpeurem and colleagues, these functional food salts are made up of cations and anions with the major cations being sodium or potassium, while the major anions are generally carbonates, bicarbonates, sulfates, and chlorides. Their functional properties have been attributed to the alkalinity of their aqueous solution. They are therefore used as emulsifiers, tenderizers, thickeners, seasoning, potentiating adjuncts, and preservatives in the preparation of a variety of foods. In Africa, traditional vegetable salts are increasingly being preferred to trona, an earthy sodium sequicarbonate dihydrate as a cheaper, safer, less-toxic, and readily available biomass-derived salt. In addition to increasing the sodium content of foods, several studies have associated the intake of trona with cardiac failure in foetuses, and stomach upset and diarrhoea in consumers. Excessive intake of trona has also been associated with severe kidney and reproductive problems. Therefore, the replacement of trona with a potassium-rich substitute has health benefits, especially in populations generally known for high sodium and low potassium intake. Indeed, Dickie and Malcolm in their study with rats reported that when used in suitable quantity, traditional vegetal salts contain enough NaCl for the physiological needs in higher animals and humans despite containing large amounts of potassium ions. Gopalakrishnan however maintains that based on available data, traditional vegetal salts cannot act as salt substitutes because of their very low sodium content, indicating the need for some form of sodium augmentation.
Production of Traditional Vegetal Salt
Several authors have reported essentially similar methods of producing vegetal salts from plants in Africa, South America, India, and Papua New Guinea, with little variations in plants, tools, and equipment used. In general, plant materials are dried, and burnt, the ashes are lixiviated through the filters, and the resulting brine is slowly dehydrated to obtain the dry salt. In Asia and Oceania, dried plant material may also be soaked in saline water collected from salt springs followed by burning and evaporation of the filtrate. The synoptic diagram of the production of traditional vegetal salt is shown in figure 1, while a typical traditional extraction technology setup is shown in figure 2.
Fig. 1: Synoptic diagram of the production of traditional vegetal salt
(Source: Ngoualem et al., 2019)
Traditional vegetal salt extraction set-up (Source: Babayemi et al., 2011)
Different types of plants and plant parts have been used to produce vegetal salts in various places probably based on tradition, accessibility, and abundance. For example, Echeverri, and Román-Jitdutjaaño documented 57 plant species used by the Witoto Indians for making salt. Gopalakrishnan listed 16 plants including cocoyam and sweet potato in three provinces of Papua New Guinea. In Chad, West Africa, Mianpeurem and colleagues reported that the plants used are mostly waste parts of cultivated crops such as stems of maize, millet, sorghum, and some abundant plants like the Hygrophila species. In other parts of West and Central Africa, pawpaw and banana trunks, banana peels, cocoa pods, and several others are being used. Kubkomawa and colleagues also reported that in northern Nigeria, the major raw materials used in producing vegetal salts include plant parts such as cowpea husk, maize stover, and maize combs, and animal parts such as cow, goat, and sheep dungs. In palm-producing countries, oil palm, coconut, nappia, and sago palm tree parts are extensively used in producing vegetal salts. Enokakuiodo and coworkers also reported that the Huitoto Indians from the Colombian Amazon prepare vegetable salt using the bark, flowers, buds, or other plant material from 30 different species. In India and other Asian countries, several halophyte plants are also used to extract plant salts, while in East Africa reeds.
According to Gopalakrishnan, in Papua New Guinea most of the plants used are usually found near river streams, swampy and marshy lands and are cultivated by the people living in the area, and when matured, they are harvested, gathered, heaped up, and sun-dried for 10 hours before processing. It was estimated that about 5 - 6 tons of the plants will generate about 5 kg of salt after processing with specialized indigenous equipment like pots, ladles, pipes, filters, and containers. Researchers at the University of Bamenda, Bambili, Cameroon were able to obtain an ash yield of 10.62 ± 0.12 and 7.10 ± 0.05 percent, with the raw samples of banana and plantain peels combusted at 2500C and 3000C respectively, and a pH of range 10.95 ± 0 to 12.01 ± 0.056. The potash or ash salt content ranged from 32.45 ± 0.905 to 72.29 ± 1.31 percent, with the highest and lowest values obtained from the raw sample combusted at 2500C and 3500C respectively. The salt produced using these traditional methods usually contains impurities responsible for color variations and may also influence their taste and functional properties when used in food preparation. There is therefore the for further refining to standardize the ecstatic, physicochemical, and functional properties of these salts.
Vegetal salts from northern Cameroon (Image Source: Ngoualem et al., 2019)
Physicochemical Characteristics of Vegetal Salts
According to Echeverri and Roman-Jitdutjaaño, the physicochemical and functional properties of traditional vegetal salts are influenced by the type of plants and processing conditions. Processing factors such as the degree of compaction of banana/plantain peels and stalks during combustion, number of cycles, and duration of filtration have been reported to affect some properties of the salt. Therefore, there is the need to understand the particular characteristics of salts produced from different biomass substrates and even regions of the world. Most studies on the elemental compositions of vegetal salts agree that the common cations present in these salts are Na, K, Mg, and Ca in the form of macro-nutrients, with potassium being the dominant ion in most cases. An array of micro-nutrients such as Fe, Mn, Cu, Zn, Ni, Cr, Br, and Mo among others have also been assayed. The rich abundance of minerals at most tropical locations where the plants used in making these salts grow is probably responsible for their presence in the salt samples.
According to the reports of studies carried out by Gopalakrishnan, in Papua New Guinea, Echeverri and Roman-Jitdutjaaño in Colombia, and Mianpeurem and colleagues in Chad, the dominant anions in vegetal salts are chlorides, sulfates, carbonates, bicarbonates, and phosphates, with the chloride and carbonates being the highest ions in most cases. The pH of the salts is usually alkaline (9 - 10+) due to the presence of bicarbonate, carbonate, oxide, and hydroxide ions. Based on published research results, Echeverri and Roman-Jitdutjaaño developed a table of the chemical composition of vegetal salts produced in Africa, Central America Papua New Guinea as shown in table 1.
Table 1: Chemical composition of vegetal salts from different parts of the world
Source: Echeverri and Roman-Jitdutjaño, 2011
The potassium content of the African salts was very high with the minimum value being the 28.76 percent KCl recorded in Salvadora persica from Chad. Many of the salts contained high levels of KCl, and K2SO4, while a few others also contained relatively lower levels of K3CO2 indicating that the potassium salts occur mostly in their chloride and sulfate forms. The NaCl content ranged from 0.85 to 67.60 percent recorded in the Salvadora persica from Chad, with five of the salts recording more than 30 percent NaCl value. The American salts were also generally high in their potassium content (33.60 - 86.70 percent), while the only result on NaCl was the 50.40 percent recorded in Mourera sp. at British Guyana. The salts produced in Papua New Guinea were generally low in their sodium content and ranged from 0.06 to 7.90 percent. This observation has been confirmed by a more recent study by Gopalakrishnan that reported a value range of < 0.001 to 20.100 percent in vegetal salts produced in the area. The potassium values ranged from 31.30 to 57.30 percent and occurred mostly in their chloride, carbonate, and sulfates forms, similar to the African salts. Therefore, the essential difference between the vegetal salts produced in Africa and Papua New Guinea is in their sodium content which is generally low in the latter.
Dietary Substitution of Sodium Chloride with Vegetal Salts
It is now a scientific fact that increasing potassium intake and reducing sodium intake are additive in lowering normal and high blood pressure. Potassium-induced reductions in blood pressure have been associated with lower incidence of stroke, coronary heart disease, myocardial infarction, and other cardiovascular diseases. Increased potassium intake has also been associated with reduced urinary calcium excretion, which has beneficial effects on bone health. It has specifically been reported that increased potassium intake of 90 – 120 mmol/ day results in a reduction in systolic blood pressure of 5.82 mmHg and diastolic blood pressure of 3.52 mmHg. Based on this information, the World Health Organization (WHO) in 2012 recommended a dietary K: Na ratio of 1:1. The global dietary K: Na ratio for most countries is however lower than this, mostly due to higher sodium and insufficient potassium intakes. Potassium substitution of 25 - 30 percent usually in the form of KCl has therefore been generally recommended as ideal in most food products and home cooking and is an effective and acceptable means of controlling blood pressure.
Current methods of producing low sodium salts are based on production from halophytes (salt tolerant plants), and seaweeds, chemical extraction from bittern, and the ashing of plants. Low sodium salt obtained from bittern has been shown to contain 53.58 - 76.1 percent NaCl and 21.5 - 44.52 percent KCl, in addition to calcium, magnesium, and sulphur oxide. Crude low sodium salts obtained from halophytes contained 70.0 percent NaCl, and 6.0 percent KCl, while the refined salt contained 81 percent NaCl, and 11.0 percent KCl in addition to traces of Fe, Mn, Zn, and Cu. These salts are currently being promoted and sold as low-sodium salts in many countries. The data in table 1 also show that six of the vegetal salts of African origin have a high percentage of sodium chloride with a range of 9.62 - 67.6 percent, while the potassium chloride and potassium sulphate are in varying proportions, with a minimum value of 28.76 percent.
A detailed study by researchers at the University of Kabianga, Kericho, Kenya on two Kenyan reed-derived salts showed that Typha latifolia-derived salt had a sodium content of 3943.8±7.34 mg/kg and potassium content of 4635.8±0.01 mg/kg, while for the Cyperus papyrus derived salt, it was 4635.8±0.01 and 8813.5±7.33 mg/kg respectively, compared to commercial herbal and common salt value ranges of 18913.6±59.07 - 23855.1±33.94 mg/kg sodium and 3819.0±0.01 - 10047.0±13.4 mg/kg potassium respectively. The Na: K ratios of these vegetal salts were 0.85:1, for T. latifolia and 3.2:1 for C. papyrus. Compared to the 4:1 recommended by WHO (2006), the two salts could serve as good low-sodium salts, although the C. papyrus-derived salt is better. T. latifolia and C. papyrus reed salts also recorded slightly higher iodine (2.0 mg/kg) values than the Recommended Dietary Allowances (RDA) for iodine (0.015 mg/kg), while the iron values are within the recommended limit (8 mg/kg).
A lump of Kenya reed salt (Source: Business Insider)
Vegetal salts containing adequate levels of potassium and sodium have also been extracted from banana and plantain wastes such as the pseudo-stem and peels. In a study described by Neog and Deka, a post-harvest banana trunk or pseudo-stem was collected, subjected to size reduction, sun-dried for several days in the open air, and then burnt completely into ash. The ash was mixed thoroughly with distilled water at 1:20 ration and then filtered and the residue was washed with distilled water. The light yellow filtrate was evaporated and analysed for its sodium, potassium, and other essential characteristics. The vegetal salt was on the analysis found to contain 2.61 percent sodium, and 46.04 percent potassium, which were mostly in their chloride form (48.22 percent). Essential trace metals such as magnesium (2.77 mg/100g), manganese (0.11 mg/100g), zinc (2.90 mg/100g), iron (7.77 mg/100g), copper (0.68 mg/100g), and nickel (0.97 mg/100g), among others were also detected. Empty palm fruit bunch, palm inflorescent, palm kernel shell, and banana and plantain peel ash filtrates and evaporites have also been used extensively in West and Central Africa as salt substitutes, especially in the preparation of indigenous meals. This natural combination of rich potassium and low sodium salt could serve as an ideal common salt substitute in the diets of persons suffering from and/or prone to high blood pressure due to high sodium intake.
Quality and Safety Concerns
Several studies have highlighted the critical effects of plant types, production methods, storage period, and environmental conditions as well as packaging materials, on the functional properties of vegetal salts. For example, Ngwasiri and co-workers reported that the degree of compaction of banana/plantain peels and stalks during combustion determines the colour of the salts produced from them with low compaction giving a greenish salt, while high compaction produces black or white salt. Researchers at the University of Kabianga, Kericho, Kenya have also investigated the stability of important trace minerals such as iodine and iron (II) in the indigenous salts obtained from Cyperus papyrus and Typha latifolia reed plant species as affected by the preparation methods, packaging materials, storage conditions and time. They reported that iodine and iron (II) losses depended on plant species, method of preparation, packaging material, and storage period and conditions. Reed salts, especially Cyperus papyrus salt prepared by the complete evaporation method and packaged in a low-density polyethylene film bag for 3 months storage period was recommended in other to preserve their trace mineral values. Lima and colleagues in their study on the shelf life of Salicornia ramosissima vegetable salt stored in cylindrical aluminium boxes with or without adhesive tape around the lid predicted a shelf-life of 35 and 80 days at 25OC and 75 percent relative humidity and 19 and 63 days at 35OC and 90 percent relative humidity respectively.
Again, the taste has been a major concern in the promotion of salt substitutes such as vegetable salts. According to Echeverri and Román-Jitdutjaaño anions play a minor role in determining the salt taste while cations have a major influence. Saltiness arises only when sodium cations are combined with chloride anions. Therefore, vegetal salts containing very low levels of sodium and chloride ions as shown in the Papua New Guinea salts will not taste as salty as the African ones such as the reed salts from Kenya. Several vegetal salts such as the Chambira salt from Columbia are currently used more as a spice to add flavour to traditional meals than for salting. Elsewhere in Africa, they are used more as emulsifiers, tenderizers, thickeners, seasonings, potentiating adjuncts, and preservatives in traditional meals than for salting. Therefore, before promoting vegetal salts as ideal low-sodium salt substitutes, there is a need for more data on the taste and preservation qualities of the final products relative to full salt products. One of the few reports published on the subject showed that a plant salt substitute made from aqueous extracts of Salornia heracea (saltwort), Laminaria japonica (sea tangle), and Lentinus sedodes (mushroom) had 57 percent similar saltiness as sodium chloride.
Several chemical analysis results show that most of the elements present in vegetal salts are not harmful to human health, except when the potassium content exceeds the recommended dietary intake or there are high levels of trace or heavy metals such as lead, mercury, arsenic, and cadmium among others. Unrefined vegetal salts may indeed contain varied and unspecified elemental concentrations, which could pose health problems to consumers, especially if they contain hazardous contaminants. Excessive potassium intake has been shown to cause stomach upsets, intestinal problems, and heart disorders, while iron intakes > 20 mg may cause stomach upset and constipation or dark faeces. Therefore, since most of the vegetal salts produced in Africa and few from Papua New Guinea recorded marginally higher potassium values than the recommended dietary intake value of 3 500 mg, there is a need to regulate their intake to prevent untoward health consequences.
Although vegetal salt and edible ash production remain an active rural cottage industry activities in many African countries, the case is different in other parts of the world where their production and consumption have gradually diminished and may finally disappear due primarily to urbanization, westernization of values and the desire for an easy life. The public health-driven reassessment of global sodium consumption habits has however kindled the search for salts with better health attributes. Vegetal salts from Asia and Africa are beginning to catch the attention of the world as essential nutritional and healthy products. For example, in 2003, researchers at India's Central Salt and Marine Chemicals Research Institute at Bhavnagar in Gujarat announced the production of salt from a halophyte plant, Salicornia brachiate. Named Saloni salt, it is currently used for cooking worldwide, it contains several important nutrients not usually found in sea salt and is therefore promising as a health salt. According to an article in Times of India, a hectare of Salicornia brachiata farm produces 1200 - 1500 kg seeds for oil extraction and livestock feed, in addition to 12 to 15 tonnes of biomass, which could be processed into 3 - 4 tons of vegetable salt. The technology could be transferred to rural farmers and coastal dwellers as a livelihood improvement option.
Again, in October 2014, the reed salt from Kenya emerged as the best indigenous salt at the Salone del Gusto and Terra Madere Conference in Italy, ranking higher than products from other countries. In Kenya, the demand for the reed salt cuts across local consumers and five-star hotels especially because of its soft texture, sharp taste, and medicinal value. Interestingly, several commercial vegetable salts are now sold both online and in the markets. Many of these products are branded Saloni vegetable salts probably imitating the Indian Solani and have claims on their labels such as reduced Na intake and hence blood volume, supplementation of the potassium lost in the urine, antioxidant properties, much saltier than normal salt and control of pregnancy-induced hypertension. To verify these claims, Yin and colleagues determined the sodium contents in 87 commercially available low-sodium salts sold in 47 countries made up of 28 high-income,13 middle-income, and 6 low-income countries. A total of the 51 low-sodium salts studied recorded both sodium and potassium levels, with variations across countries. Specifically, less than 20 percent of the sodium chloride was replaced in the Indian products, while in North American, Middle Eastern, and South American countries about 50 percent of the sodium was replaced. Sodium-free substitutes were available in North America. These results highlight the need for stricter regulations on label declarations. The promotion, production, sale, adoption, and sustainability of low-sodium salts, especially those produced using simple cost-effective rural technologies should be encouraged by health authorities since this will help to promote the government's goal of reducing population-level dietary sodium consumption.
Traditional vegetable salts derived from plant ash are used as emulsifiers, tenderizers, thickeners, seasoning, potentiating adjuncts, and preservatives in the preparation of a variety of foods in many tropical locations that originally had limited access to common salt. These salts are potassium-rich crude bio-extracts that also contain substantial amounts of sodium and other trace minerals in their chloride, carbonate, and sulphate forms. This natural combination of rich potassium and low sodium salt could serve as an ideal substitute in the diets of persons suffering from and/or prone to high blood pressure due to high sodium intake. The public health-driven reassessment of global sodium consumption habits has kindled the search for salts with better health attributes. While vegetal salts have received some attention, there is the need to streamline their production methods, storage conditions, and packaging, to standardize their functional properties and safety. It is therefore recommended that the production, sales, adoption, and promotion of vegetal salts be encouraged by governments and health authorities towards reducing dietary sodium consumption at individual, family, and community levels.
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