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Food science

Fermented foods and cottage industries in Nigeria
Effects of a brief, intense infrared radiation treatment on the nutritional quality of maize, rice, sorghum, and beans
Stability of iodine in iodized salt used for correction of iodine-deficiency disorders

Fermented foods and cottage industries in Nigeria

Abstract
Introduction
Fermented foods of Nigeria
Development of fermented foods in Nigeria
Finished product development
Research and development
Acknowledgements
References

G. O. Latunde-Dada

G. O. Latunde-Dada is affiliated with the Department of Chemical Sciences in the College of Natural Sciences at the University of Agriculture in Abeokuta, Nigeria.

Abstract

Fermentation contributes significantly to food technological processes in developing countries such as Nigeria. Fermentation extends the shelf life, adds a variety of flavours, and enhances the nutritional value of processed foods. The traditional fermented foods in Nigeria are derived from the following agricultural commodity groupings: roots and tubers (gari, lafun, and fufu), cereals (ogi), legumes (dawadawa and iru), milk (local cheeses), and beverages (palm wine and pito). Despite extensive research on isolating and characterizing the wide range of micro-organisms involved in the fermentation of these foods, the techniques employed in the cottage industries of Nigeria remain traditional. Simple and non-sterile materials are used, and there is great reliance on the use of natural inocula under uncontrolled fermentation conditions. Other features of this system include contamination, varied sensory characteristics, unattractive packaging and presentation, and short shelf life of the products. It is imperative that modern biotechnological techniques be integrated into the traditional processing procedures to upgrade their efficiency and productivity. The production and use of starter cultures and the maintenance of optimum conditions for microbial activity will enhance the qualities of fermented foods produced in the cottage industries of Nigeria. This paper reviews the processing procedures of indigenous fermented Nigerian foods and presents information on the enhancement of the iron content of some of these foods.

Introduction

Fermentation processes play important roles in food technology in developing countries. In traditional fermentation processes, natural micro-organisms are employed in the preparation and preservation of different types of food. These processes add to the nutritive value of foods as well as enhancing flavour and other desirable qualities associated with digestibility and edibility. The fermentation techniques are often characterized by the use of simple, non-sterile equipment, chance or natural inoculum, unregulated conditions, sensory fluctuations, poor durability, and unattractive packaging of the processed products [1]. Nigeria is endowed with a wide range of fermentable indigenous staple foods that serve as raw materials for agro-allied cottage industries. These industries utilize small-scale equipment and provide alternative equipment for rural communities while adding value to such local produce.

Fermented foods of Nigeria

The fermented foods in Nigeria can be classified into groups according to the substrates or raw materials employed [2]. These include the following:

» tubers (cassava products): gari, lafun, fufu
» cereals (maize, sorghum, millet): ogi, pito, burukutu
» legumes (locust beans, soya beans): iru, dawadawa
» fruit (melon): ogiri
» beverages: palm wine
» animal proteins (milk): cheese

Development of fermented foods in Nigeria

Raw material development
Starter development
Development of fermentation processes
Nutritional enhancement of fermented foods

Good-quality raw materials that have been efficiently graded and sorted, simple equipment, optimum conditions, and attractive packaging are the key requirements of a food fermentation industry.

TABLE 1. Fermented foods of Nigeria

Substrate	Micro-organism	Product	Shelf life
Cassava	Streptococcus lacticus Geotrichum candidum Corynebacterium manihot Lactobacillus spp Leuconostoc spp	Gari granules Fufu paste Lafun powder	3 mo 1 wk 3 mo
Cereals: maize, sorghum, millet	Saccharomyces cerevisiae Lactobacillus spp Fusarium spp Candida mycoderma Penicillium spp	Ogi Agidi	2 wk
	Rhizopus oryzae Aspergillus flavus Penicillium funiculosum Geotrichum candidum Candida spp	Pito Burukutu	Days
Legumes: locust, beans, soya beans	Coryneform bacteria Bacillus subtilis B. licheniformis B. cereus Staphylococcus saprophyticus Pseudomonas aeruginosa	Iru (dawadawa)	1 mo
Melon (Citrullus vulgaris)	Bacillus spp Escherichia spp Proteus spp Pediococcus spp	Ogiri	1 mo
Oil palm sap	Saccharomyces cerevisiae Candida tropicalis C. utilis Lactobacillus brevis Bacillus spp Streptococcus spp	Palm wine	Days
Milk	Lactic acid bacteria	Local cheese	Days

Raw material development

Agricultural practices that ensure increased production of improved varieties of the produce should be encouraged. Biotechnology represents the latest tool employed in the ongoing scientific quest to solve the problems of food production. In the case of cassava, the International Institute of Tropical Agriculture (IITA) and the Centro Internacional de Agricultura Tropical (CIAT) are actively engaged in research aimed at reducing cyanide content, improving storage properties, improving starch and protein content, increasing the efficiency of photosynthesis, increasing resistance to insects and viruses, and developing propagation from seeds [4]. The development of genetically engineered bacteria capable of producing cyanoglucoside-destroying enzymes used in the fermentation of cassava will contribute immensely to reducing the toxicity of the cassava products [5]. Attempts are being made to use cloned genes to increase the level of detoxifying enzymes in cassava tubers.

Improved-quality maize protein with an increased level of lysine, a limiting amino acid, is now popular in some parts of Africa. Brady [6] surveyed recent research on maize and projected a significant increase in productivity by the year 2000 (table 2).

In the case of dawadawa, a fermented food obtained from locust bean, the production bottleneck associated with the crop’s seasonality and perenniality might be overcome easily by substituting soya bean, an annual produced on a larger scale, for locust bean.

TABLE 2. Contributions of agrobiological research to increased maize production in the year 2000

Research area	Year of first significant contribution	Predicted contribution to maize yield in 2000 (kg/ha)
Increase in photosynthetic rate	1995	497
Cell and tissue culture	1990	195
Growth regulators	1994	988
Biological fixation of atmospheric nitrogen	1996	142

FIG. 1. Flow chart of cassava processing (source: ref. 9)

Starter development

Most traditional fermentations employ the whole gamut of natural microflora that could function under the varied environmental and non-sterile conditions presented by the different processes. Such fermentations are characterized by:

» numerous micro-organisms of varying functions that could be beneficial or detrimental to the fermentation processes;

» mixed cultures that produce the blend of rich flavours and aromas of the product;

» lactic acid bacteria creating unfavourable environment for pathogenic bacillaceae and enterobacteriaceae;

» some micro-organisms that could accelerate spoilage, particularly in the finished products.

The use of starter cultures, as practised in Asia for the production of koji and raji tempe, would contribute significantly to improving the quality of some Nigerian fermented foods. After isolation, selection, and collection of the microbial strains responsible for food fermentation, the next step is the maintenance and propagation of such cultures. Subsequent strain-improvement programmes should employ mutation and genetic manipulation techniques for desirable characteristics such as enhanced enzymatic activity, nutritive value, and flavour.

Just as amylases and other enzymes are used in the fermentation of alcoholic beverages, the use of linamarase is advocated in the fermentation of cassava for the production of gari [7]. This will be facilitated by the successful characterization and purification of linamarase and other cyanoglucoside-detoxifying enzymes in cassava [8].

Development of fermentation processes

Traditional fermentation processes at the cottage level use simple operations and equipment. Fermentation may be carried out on a solid substrate or under submerged conditions. The temperature, pH, and aeration are not optimized for efficient output. The fermentation processes for ogi, gari, and palm wine have been upgraded and modernized (fig. 1) [9]. Iru is one of the traditional fermented condiments used to flavour soups and stews in Nigeria. The traditional alkaline fermentation of locust beans using Bacillus subtilis [10] was modernized in the production of dadawa cubes. The greatest advance, however, is the production on an industrial scale of iru (dawadawa) as cubes by Cadbury Nigeria (fig. 2). The advertisement of dadawa cubes utilizes attractive packaging to appeal to the consumer’s preference and taste. This also enhances saleability, particularly among sophisticated housewives. For other products, however, the main problem is the scaling-up of the fermentation processes, which involves heavy capital investment in equipment design, defining regulatory conditions of moisture, pH, temperature, oxygen transfer, aeration, and agitation, and calculating microbial growth characteristics.

The fermentation of local cheese is perhaps the most rudimentary of the fermentation processes. The availability of milk products in Nigeria is limited, and the warm climate is unsuitable for the preparation of cheese. Fermented cheeses are usually the soft variety. Cheeses take a long period to mature. The milk is heated in pots, and the juice of Calotropis procera or pawpaw leaves is introduced to curdle the milk. The curdled milk is then heated for at least 20 minutes and allowed to drain before it is moulded into different shapes.

FIG. 2. Packaging of iru by traditional (left) and modern industrial (right) methods

Nutritional enhancement of fermented foods

Traditional fermented protein-rich foods offer excellent opportunities for improving the diets of people in tropical countries. Various attempts have been made to increase the protein level of cassava-based products, particularly gari. Growth of the fungus Aspergillus niger for 24 to 30 hours on cassava flour with an initial content of 2% to 3% protein and 80% to 90% carbohydrate resulted in a product containing 18% to 20% protein and 30% to 35% carbohydrate [11]. Another approach is the supplementation of cassava with protein-rich foods, for example, supplementation of gari with soya protein. Soya-ogi, a combination of maize and soya beans, was developed by the Federal Institute of Industrial Research, Oshodi, to increase the protein level of ogi.

The germination and fermentation of cereals enhance the availability of elemental iron [12], the deficiency of which is responsible for the high incidence of anaemia in tropical countries (table 3).

Finished product development

Quality control
Packaging

Quality control

Fermented products should be hygienically and microbiologically safe for human consumption. Dehydration of home-produced products is desirable to extend their shelf life. Gari and lafun are sold in a dried, powdered form, but ogi and fufu are sold wet. Ogiri and iru can be preserved by salting.

TABLE 3. Effect of germination and fermentation on iron levels and availability of some tropical foods (mean ± SEM)

Food	Iron (mg/100 g)	Dialysable iron (% of total)
Defatted soya bean	6.1 ± 0.03	5.0 ± 0.32
Fermented soya bean	4.9 ± 0.02	13.6 ± 0.15
Germinated soya bean	4.5 ± 0.01	11.0 ± 0.50
Germinated roasted maize	3.2 ± 0.13	8.2 ± 0.74
Ungerminated roasted maize	4.2 ± 0.14	3.4 ± 0.31

Packaging

Indigenous fermented food products are usually exposed to environmental contamination. The presentations are often unattractive and unappealing to sophisticated consumers. Consumer appeal and sale-ability may be enhanced by packaging fermented foods in simple polyethylene bags. Dadawa cubes are attractive and more appealing than iru packaged in broad leaves (fig. 2).

Research and development

Coordination between research, industry, and development is important for the advancement of research and development capacity in biotechnology. Modern biotechnology may be applied to research and development to improve the raw materials of fermented foods, develop appropriate bioreactors and optimum conditions for fermentation processes [6], and enhance the efficiency of the inocula. Large-scale production involving the use of heavy machinery, however, is expensive and demands high capital input. This in turn results in the production of foods that are too expensive for most people in a developing country such as Nigeria. The problem with the industrial production of gari is that there is little or no prospect of profit because of competition from cottage producers, who do not incur the high costs associated with investments in machinery, land, and agricultural equipment [13].

Universities and research institutes should sell patented results to medium-scale food manufacturers who need to scale up the results of pilot studies. This is perhaps one of the greatest limiting factors militating against the upgrading of traditional indigenous fermentation processes. Moreover, industrial processes sometimes have difficulty in copying a traditional product without losing some of the flavour, aroma, and other peculiar characteristics. Many indigenous traditional technologies are not easily adopted by transnational companies without altering the methods of preparation and perhaps ending up with a product of altered flavour and unacceptability [14]. Since indigenous peoples are sources of knowledge of traditional food systems, interinstitutional initiative will be more likely to contribute to the development of these resources if indigenous people are encouraged to participate [15]. The conservative food habits of consumers are an advantage for small-scale indigenous cottage industries. In spite of current threats to the integrity of traditional food systems, significant activities are under way to document this traditional knowledge [14].

The Board of Science and Technology for International Development (BOSTID) [16] proposed the following goals for a biotechnological research and development program in traditional fermentation:

» improving understanding of the biological and physical processes involved in local practices in the management of micro-organisms, for example, making an inventory of local knowledge of genetic resources, isolating and preserving active micro-organisms, and understanding the specific cultural context of local practices;

» refining and improving the processes involved;

» improving the utilization of locally produced products, for example, improving packaging;

» development of local capacities through workshops or training for rural people [15].

These developmental processes will be enhanced significantly through the assistance of international biotechnological public organizations.

Acknowledgements

The support of the International Foundation for Science (IFS), Sweden, is gratefully acknowledged.

References

1. Nout MJR. Upgrading traditional biotechnological processes. In: Prage L, ed. Proceedings of the IFS/UNU workshop on the development of indigenous fermented foods and food technology in Africa, Douala, Cameroon. Stockholm: International Foundation for Science, 1985: 90-9.

2. Odunfa SA. African fermented foods. In: Wood BJB, ed. Microbiology of foods. London: Elsevier, 1985: 155-91.

3. Oyewole OB, Odunfa SA. Characterisation and distribution of lactic acid bacteria in cassava fermentation during fufu production. J Appl Bacteriol 1990; 68: 145-52.

4. Burley T. Application of biotechnology in forestry and rural development. Commonwealth Forestry Rev 1987; 66: 357-67.

5. Hughes M. Making cassava safer for consumers. Spore 1995; 60: 11.

6. Brady NC. Chemistry and world food supplies. Science 1982; 218: 847-53.

7. Ikediobi CO, Onyike E. The use of linamarase in gari production. Proc Biochem 1982; 17: 2-5.

8. Okafor N, Ejiofor MAN. Rapid detoxification of cassava mash fermenting for gari production following inoculation with a yeast simultaneously producing linamarase and amylase. Proc Biochem 1990; 25: 82-6.

9. Kuboye AO. Traditional fermented foods and beverages of Nigeria. In: Prage L, ed. Proceedings of the IFA/UNU workshop on the development of indigenous fermented foods and food technology in Africa, Douala, Cameroon. Stockholm: International Foundation for Science, 1985: 224-36.

10. Steinkraus KH. African alkaline fermented foods and their relation to similar foods in other parts of the world. In: Wesby A, Reilly PJA, eds. Traditional African foods: quality and nutrition. New York: Marcel Dekker, 1991; 87-92.

11. Ramboult M. Fermentation en milieu solide: croissance de champignons filamenteux sur substrat amylase. Doctoral Thesis, Université de Toulouse, France, 1980.

12. Latunde-Dada GO. Some physical properties of ten soyabean varieties and effects of processing on iron levels and availability. Food Chem 1991; 42: 89-98.

13. Heys G. Industrialization of gari fermentation. In: Steinkraus KH, ed. Industrialization of indigenous fermented foods. New York: Marcel Dekker, 1989: 76-84.

14. Onyekwere OO, Akinyele IA, Koleoso OA. Industrialization of ogi. In: Steinkraus KH, ed. Industrialization of indigenous fermented foods. New York: Marcel Dekker, 1989: 34-45.

15. Kuhnlein HV, Receveur 0. Dietary change and traditional food systems of indigenous peoples. Annu Rev Nutr 1996; 16: 417-42.

16. Board of Science and Technology for International Development (BOSTID). Applications of biotechnology to traditional fermented foods. Report for an ad-hoc panel of the Board of Science and Technology for International Development. Washington, DC: National Academy Press, 1992.

17. Dirar HA. Commentary. The fermented foods of Sudan. Ecol Food Nutr 1994; 32: 207-18.

Effects of a brief, intense infrared radiation treatment on the nutritional quality of maize, rice, sorghum, and beans

Abstract
Introduction
Materials and methods
Results and discussion
Conclusions
Acknowledgements
References

E. L. Keya and U. Sherman

E. L. Keya is affiliated with the Department of Food Technology and Nutrition in the University of Nairobi, Kenya. U. Sherman is affiliated with Anglo-Swiss Bakery Ltd. in Mombasa Kenya.

Mention of the names of firms and commercial products does not imply endorsement by the United Nations University.

Abstract

Maize, sorghum, rice, and beans were subjected to a temperature of 22,000°C for 0.5 minute in an infrared radiator 3 m long. The uncharred grains exited the radiator at a temperature of 140° C and were cooled to room temperature. The moisture content dropped to 3% to 7%, thus affecting the proximate composition of the grain components. The digestibility of cereal starch remained high and unchanged (71%-84%), whereas protein digestibility was reduced by 7%, 21%, and 25% in rice, sorghum, and maize, respectively. The caloric values of the cereals remained unaltered. Anti-trypsin factor in beans and haemagglutinins in both beans and sorghum were inactivated. Tannin in sorghum was reduced extensively. The traces of aflatoxin in sorghum and maize were completely destroyed. It was concluded that since there is always some loss of protein quality in thermal food preservation processes, the slight decline in protein digestibility did not have practical significance in comparison with the benefit of radiation as a brief, high-temperature preservation technique that could be adopted as a pre-processing treatment to destroy anti-nutrients and dehydrate food to the very low water activity desirable for long shelf life.

Introduction

Infrared radiation is thermal energy defined by its wavelength, frequency, and level of energy in the electromagnetic spectrum. The spectrum on one side of infrared consists of visible light, ultraviolet light, X-rays, and gamma rays, in order of decreasing wavelength and increasing energy. On the opposite side of infrared are microwaves and radio waves, with longer wavelengths but less energy than infrared radiation [1]. Infrared radiation has wavelengths between 0.7 and 500 µm. Radiation with wavelengths just below 0.7 µm is visible light, whereas radiation with wavelengths just above 500 µm is microwave radiation [2].

Infrared radiation with shorter wavelengths transmits more thermal energy to foods in shallow-bed radiators designed for in-depth cooking [3]. Such radiators are equipped with glass-encapsulated heaters operating at about 3, 000 kW. Alternatively, dielectric or microwave heating may be used to achieve the same heating effect [4].

When infrared radiation is used to heat and dry moist materials, the radiation strikes the surface of the material, penetrates it, and is converted to heat. Heat and mass transfers occur in the radiation chamber, on the surface, and inside the materials being irradiated [5]. Mass transfer involves mainly moisture and volatile substances that are lost from the material. The process is most efficient if the food materials are fairly coarse granules or grains thinly spread out as they pass through the radiator. The heat flux through the materials is greatly enhanced if they are agitated to expose new surfaces for thermal reception [4]. The heating effect of infrared radiation is highest when food is fried [6].

Careful process control is necessary to avoid charring, burning, and smoking, which are responsible for the formation of polycyclic aromatic hydrocarbons that have been linked to some food-borne cancers [7]. Because infrared radiation has no food-ionizing characteristics, as do X-rays, gamma rays, beta rays, and cathode rays, infrared radiation cannot induce any radioactivity in the foods treated with it [1].

Maize is the staple food for the majority of Kenyans. Wheat, rice, sorghum, millets, and beans are also widely consumed. Maize, wheat, rice, and most millets have no serious anti-nutritional problems apart from occasional traces of aflatoxin and low levels of lysine. Sorghum contains tannin and variable amounts of haemagglutinins, which reduce its nutritional quality. However, because it is tolerant of drought, sorghum is the cereal of choice in the semi-arid regions that constitute more than two-thirds of the area of Kenya and are inhabited by 8 to 10 million people. Sorghum is not only consumed by adults but is a weaning food for children [8-10]. Many efforts have been made to improve the quality of sorghum by grain dehulling [11-13], mailing [14-17], and fermentation [18].

Beans contain trypsin inhibitors and haemagglutinins that reduce their nutritional quality, especially protein utilization. Beans also require long cooking before they are safe for consumption, which expends a lot of energy.

Thus any advances in processing to destroy any anti-nutrients will facilitate processing and cooking and improve the nutritional quality of cereals and legumes, especially sorghum and beans. The resulting savings in energy and time would be beneficial at both the domestic and the industrial levels.

The objective of this study was to investigate the effects of a very brief (30 seconds) but intense treatment with infrared radiation at 2,000 °C on some of the nutritional and anti-nutritional qualities of maize, rice, sorghum, and beans.

Materials and methods

Materials
Methods

Materials

Brown sorghum (Sorghum bicolor, cv. Serere), white maize (Zea mays), rice (Oryza sativa, type bismati), and beans (Phaseolus vulgare, cv. Rose coco) were purchased from the municipal market in Mombasa. Analytical-grade chemicals were purchased from Kobian Chemicals Limited in Nairobi.

Methods

Infrared radiation

The cereal grains and beans were irradiated for 30 seconds at 2000° C in an infrared radiator built by U. Sherman of Anglo-Bakery, Mombasa, Kenya, and Nurley Oy of Finland. The radiator was a simple structure 1.5 m high, 3.0 m long, and 30 cm wide. The radiation tunnel was 1.0 m from the floor. The metallic table that served as the floor on which the grains were moved forward was fitted inside the vertical sides of the tunnel and was hoisted on a shake-able structure operated by two motors mounted below the table.

When the two motors were turned on, they set the table into short-pitched, very high-frequency vibrations. These vibrations spread any particles placed on the table into a single layer, displacing them forwards at the same time, thus simulating a stifled fluidized-bed effect.

A panel of quartz glass-encapsulated heaters operating at 3, 000 kW was fixed under the roof of the radiation channel. The panel could be moved up and down manually by a lever to control the distance of the radiation source from the surface of the metallic table, in order to avoid charring the materials being irradiated. The radiation and the operation of the motors were synchronized by electronic controls mounted at one side of the equipment. The grains were let into the radiator through an opening at one end and came out at the other end 30 seconds later.

Proximate composition and mineral contents

The proximate composition of the samples was determined according to the AOAC Approved Methods of Analysis [19]. Three samples each from the control and treated lots were analysed. The lot was mixed thoroughly before weighing a 100-g sample of grain that was then ground, in a laboratory hammer mill, into a meal on which the proximate analysis was done. Sample mineral composition was determined by the AOAC atomic absorption spectrophotometer method [19].

Protein and starch digestibility

In vitro protein digestibility was determined by the method of Mertz et al. [20]. Three samples were used in each case. In vitro starch digestibility was determined by timed enzymatic digestion of the samples, followed by determination of the freed sugars by the phenol-sulphuric acid method of Duboise et al. [21].

Energy

Energy values were determined in an adiabatic bomb calorimeter (Model IKA Kalorimeter C400, Adiabatic 2800, Bremen, Germany). The gross heat of combustion was measured with water and compared with that of benzoic acid of known energy capacity. The energy obtained was not adjusted for reduced protein digestibility, indigestible carbohydrate residues, fibre, and mineral content, as would be the case with in vivo digestibility, where a net energy estimate would be necessary.

Tannin

Tannin in sorghum was estimated according to the method of Folin and Denis [22] using three samples prepared as explained above.

TABLE 1. Proximate composition of control and infrared-irradiated grains (mean percentage ± SD)^a

Component	Sorghum		Maize		Rice
	Control	Treated	Control	Treated	Control	Treated
Moisture	8.1 ± 0.3	3.9 ± 0.4	12.1 ± 0.2	2.8 ± 0.2	12.4 ± 0.5	7.3 ± 0.5
Crude protein	10.6 ± 0.4	11.3 ± 0.3	8.8 ± 0.3	9.4 ± 0.2	5.6 ± 0.3	5.7 ± 0.4
Crude fat	3.5 ± 0.3	3.7 ± 0.3	4.4 ± 0.3	4.9 ± 0.4	0.4 ± 0.1	0.4 ± 0.2
Crude fibre	1.4 ± 0.2	1.8+ 0.2	0.9+ 0.1	1.2 ± 0.1	0.6 ± 0.1	1.0 ± 0.1
Ash	1.0 ± 0.1	1.2+ 0.2	0.7 ± 0.3	0.8 ± 0.2	0.3 ± 0.1	1.1 ± 0.1
Soluble sugars	2.3 ± 0.3	2.3 ± 0.2	2.6 ± 0.3	2.4 ± 0.2	0.6 ± 0.1	1.1 ± 0.2
Total carbohydrate	75.3 ± 2.0	78.4 ± 4.0	73.1 ± 2.5	82.5 ± 4.6	80.1 ± 2.3	81.9+ 1.5

a. Three control and three replicate samples were analysed.

The presence or absence of trypsin inhibitors was demonstrated by the reaction of the food sample or its extracted crude protein with gelatin in the presence of trypsin enzyme. Crude protein was extracted by 0.1 N sodium acetate. The sample or its extracted crude protein was incubated at 37°C for two hours in sodium phosphate buffer at pH 7.5 containing dissolved trypsin enzyme, and then cooled in a refrigerator at 4°C. The samples containing trypsin inhibitor solidified in 15 minutes, since the gelatin was unaffected, whereas samples without trypsin inhibitor remained liquid indefinitely because of the hydrolytic activity of the uninhibited trypsin on gelatin. Three samples were also used.

Haemagglutinins

The presence or absence of haemagglutinins in crude protein extracted from samples with 0.1 N sodium acetate solution was demonstrated by precipitating the crude protein in the 0.1 N sodium acetate solution with 6.0 M sodium hydrogen sulphate solution and redissolving it in 0.1 N sodium acetate solution. The precipitate in the solution was reacted with a suspension in 0.5% EDTA (ethylenediaminetetraacetic acid tetra sodium salt) of rabbit red blood cells previously separated from the serum by centrifugation. Precipitation and lysis of the red blood cells from the suspension indicated the presence of haemagglutinins. If no precipitation and lysis of the red cells occurred, the sample or its crude protein did not contain haemagglutinins.

Aflatoxin

The thin-layer chromatography method of Stutz [23] was used to detect and quantify aflatoxins B₁, B₂, G₁, and G₂.

Results and discussion

Proximate composition
Digestibility and energy
Anti-nutrients

Proximate composition

The results for proximate composition of untreated and infrared irradiated brown sorghum, maize, and rice are shown in table 1. The corresponding mineral contents are given in table 2. The results indicate that the brief (0.5 minute) but very high-temperature (2, 000° C) infrared irradiation of the grains resulted in extensive loss of moisture content to levels of 3% to 7%. The implication was that such grains had quite low water activity and had the potential to be stored for a long time with limited deterioration. The increases in protein, fat, fibre, ash, and total carbohydrate of the irradiated samples were attributed to the reduction in moisture content. Similar trends were observed in mineral contents, indicating that the results did not differ significantly from those reported by others [24, 25].

Digestibility and energy

The digestibility of protein and starch in vitro is compared in table 3, and a similar comparison for energy values is presented in table 4. Irradiation reduced protein digestibility by about 25%, 21%, and 7% in maize, sorghum, and rice, respectively. These results suggest that maize protein is more susceptible to intense thermal treatment than sorghum protein and that rice protein is the most stable. These differences may be attributed to differences in composition of the protein fractions of the different grains. Rice protein has lower concentrations of globulin, albumin, and prolamins and higher concentrations of glutelins than maize and sorghum. Maize is much richer in globulin, albumin, prolamin, and glutelins than sorghum [24].

TABLE 2. Mineral contents of control and infrared-irradiated grains (mean mg/100 g dry matter ± SD)^a

Component	Sorghum		Maize		Rice
	Control	Treated	Control	Treated	Control	Treated
Iron	9.4 ± 2.1	9.4 ± 1.8	8.9 ± 1.1	5.2 ± 0.6	0.4 ± 0.01	0.36 ± 0.01
Calcium	220.0 ± 3.3	20.0 ± 3.3	3.5 ± 0.4	5.5 ± 0.2	1.2 ± 0.10	1.20 ± 0.05
Magnesium	190.0 ± 10.0	220.0 ± 17.0	160.0 ± 11.0	130.0 ± 12.5	4.9 ± 0.30	15.10 ± 2.45
Phosphate	130.0 ± 12.5	140.0 ± 6.5	130.0 ± 7.2	110.0 ± 14.5	4.8 ± 0.25	12.75 ± 1.55
Copper	0.7 ± 0.1	0.5 ± 0.1	0.6 ± 0.1	0.6 ± 0.1	0.04 ± <0.01	0.05 ± <0.01
Zinc	4.9 ± 0.8	4.5 ± 0.4	3.8 ± 0.7	3.4 ± 0.2	0.10 ± <0.01	0.31 ± 0.02
Sodium	8.8 ± 0.5	9.0 ± 1.3	7.8 ± 0.9	8.8 ± 0.7	0.35 ± 0.05	0.39 ± 0.03
Potassium	310.0 ± 15.4	330.0 ± 14.2	300.0 ± 20.3	230.0 ± 18.1	4.25 ± 0.24	20.95 ± 3.50

a. Three control and three replicate samples were analysed.

Grain	Digestibility
	Protein	Starch
Control sorghum	70.2 ± 2.1	71.1 ± 1.5
Irradiated sorghum	49.5 ± 3.4	72.0 ± 2.5
Control maize	80.5 ± 1.5	74.4 ± 1.5
Irradiated maize	56.0 ± 1.4	73.7 ± 1.3
Control rice	66.0 ± 2.2	80.6 ± 2.6
Irradiated rice	58.7 ± 1.9	84.0 ± 1.5

a. Pour control and three replicate samples were analysed.

Grain	Energy
Irradiated sorghum	4.5 ± 0.2
Control sorghum	4.3 ± 0.4
Irradiated maize	4.5 ± 0.1
Control maize	4.1 ± 0.4
Irradiated rice	3.9 ± 0.3
Control rice	4.3 ± 0.3

a. Three control and three replicate samples were analysed.

Our unpublished results further demonstrated that cereal and legume grains treated by this process were not attacked by weevils until six to seven months later, whereas untreated control samples stored under the same conditions at room temperature (20-25° C) were attacked by weevils within the first month of storage. Furthermore, when treated beans were cooked and consumed, they did not produce flatulence, unlike the control samples. The infrared irradiation technique may thus be used carefully to quick-dry cereals and legumes after harvest to preserve them, reduce post-harvest spoilage, and control flatulence.

Apart from some deterioration in protein quality, starch digestibility and grain energy values were practically unaffected by the process. The energy values presented in table 4 have not been corrected for any loss in protein quality, nor have they been corrected for the calories arising from the fibre and indigestible carbohydrate residues in the samples. This was not found necessary in this study, as it would be if net energy was required, as in digestion studies in vivo.

Anti-nutrients

The anti-nutrients studied are shown in table 5. Rice had no anti-nutrients. Maize had traces of aflatoxins B₁, G₁, and G₂, which were destroyed by irradiation. The tannin in sorghum was extensively denatured. Sorghum contained traces of haemagglutinins that were also destroyed. Anti-trypsin and haemagglutinins in beans were fully destroyed. Thus, infrared irradiation of beans, sorghum, and maize was effective in improving the nutritive value of these foods.

TABLE 5. Content of anti-nutrients in control and infrared-irradiated foods”

Anti-nutrient	Sorghum		Beans		Maize
	Control	Treated	Control	Treated	Control	Treated
Trypsin inhibitor	-	-	+	-	-	-
Haemagglutinins	+	-	+	-	-	-
Tannin	1.7%	<0.15%	-	-	-	-
Aflatoxin
B1	T	-	-	-	T	-
B2	-	-	-	-	-	-
G1	T	-	-	-	T	-
G2	-	-	-	-	T	-

a. Three control and three replicate samples were analysed. +, Present; - absent; T, trace. No anti-nutrients were present in rice.

Conclusions

Intense infrared irradiation of maize, sorghum, and rice for only 30 seconds produced extensive dehydration, resulting in a moisture content as low as 3% to 7%. This technique could be used to dry grains at the community and industrial levels.

Although the proximate compositions, starch digestibility, and energy values of the grains were not significantly affected, there was some deterioration in protein quality. This disadvantage was considered to be of little practical significance in comparison with the benefits of this process for food preservation and increasing the wholesomeness of the grains as foods.

A literature survey indicated that grains such as maize and sorghum, which are high in albumin, globulin, and prolamin, are more susceptible to denaturation by thermal treatment than rice, which has a significantly lower content of these proteins.

No anti-nutrients were found in rice. The tannin in sorghum was extensively denatured by the process. The traces of aflatoxin in maize and sorghum and the anti-trypsin factors in beans were completely in-activated. Any haemagglutinins originally present in sorghum and beans were also fully destroyed by the process.

The results implied that the long cooking times required to denature the haemagglutinins and trypsin inhibitors in beans and related foods would not be necessary after irradiation of the grains for 30 seconds with infrared energy. This would be environmentally friendly, especially in developing countries where women and children spend much time and effort collecting wood for cooking fuel. With the availability of this technique, a reasonable proportion of their time and energy would be redirected to other spheres of individual and social development.

It was concluded that a brief, intense, but well-controlled infrared irradiation of maize, brown sorghum, rice, and beans was a beneficial preprocessing treatment with the potential to increase the wholesomeness and average nutritive value of foods derived from such grains.

Acknowledgements

The funds for carrying out this research were provided by Mr. U. Sherman Farouk of the Anglo-Swiss Bakery Limited, Mombasa, Kenya, who also provided the infrared radiator. His useful remarks and comments are also noted. Mr. Simon Mwaura and the technical staff of the Department of Food Technology and Nutrition of the University of Nairobi, who carried out the laboratory analytical assignments, are also thanked for their contribution.

References

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2. Potts WJ. Chemical infra-red spectroscopy. New York: John Wiley and Sons, 1963.

3. Tore H, Oskar K. Physical, chemical and biological changes in foods caused by thermal processing. London: Applied Science Publishers, 1977.

4. Lorentzen J. Freeze drying: the process, equipment and products. In: Thorne IS, ed. Developments in food preservation. Vol. 1. London: Applied Science Publishers, 1981: 153-75.

5. Ginsburg AS. Application of infra-red radiation in food processing. London: Leonard Hill, 1969.

6. Magnus D. Time-temperature relationships in industrial cooking and frying. In: Tore H, Oskar K, eds. Physical, chemical and biological changes in foods caused by thermal processing. London: Applied Science Publishers, 1977: 77-100.

7. Belitz HD, Grosch W. Food chemistry. 2nd ed. Berlin: Springer-Verlag, 1987.

8. UNICEF. The state of the world’s children. Oxford: Oxford University Press, 1987.

9. Seenappa M. Sorghum and millet in East Africa with reference to their use in weaning foods. Nairobi, Kenya: UNICEF, 1987.

10. Oniang’o R, Alnwick DJ. Weaning foods in Kenya: traditions and trends. In: Alnwick DJ, Moses S, Schmidt OG, eds. Improving young child feeding in eastern and southern Africa. Household-level food technology. Ottawa: International Development Research Centre, 1987: 76-80.

11. Chibber BAK, Mertz ET, Axtell JD. Effects of dehulling on tannin content, protein digestibility and quality of high and low tannin sorghum. J Agric Food Chem 1978; 26: 679-83.

12. Hoseney RC. Overview of sorghum and pearl millet quality, utilisation and scope for alternative uses. In: de Wet JMJ, Preston TA, eds. Biotechnology in tropical crop improvement. Patancheru, India: International Institute for the Semi-Arid Tropics Centre, 1988: 127-31.

13. Mwasaru MA, Reichert RD, Mukuru SZ. Factors affecting the abrasive dehulling efficiency of high-tannin sorghum. Cereal Chem 1988; 65: 171-4.

14. Bhise VJ, Shaven JK, Kadam SS. Effects of mailing on proximate composition and in vitro protein and starch digestibility in grain sorghum. J Food Sci Technol 1988; 25: 327-9.

15. Marero LM, Paymo EM, Aguhoda AR, Hommas RI. Nutritional characteristics of weaning food produced from germinated cereals and legumes. J Food Sci 1988; 53: 1399-1402.

17. Hemanalini GK, Padma V, Jamuna RR, Saraswathi G. Nutritional evaluation of sprouted ragi. Nutr Rep Int 1980; 22: 271-7

18. Mbugua SK, Ahrens RA, Kigutha SN, Subramanian V. Effect of fermentation, malted flour treatment and drum drying on nutritional quality of uji. Ecol Food Nutr 1992; 28: 271-7.

19. Association of Official Analytical Chemists. Approved methods of analysis. 14th ed. Washington, DC: AOAC, 1984.

20. Mertz MM, Mohammed C, Whittern AW, Kerleis TU, Axtell L. Pepsin digestibility of protein in sorghum and other major cereals. Proc Natl Acad Sci USA 1984; 81: 1-2.

21. Duboise M, Gilles KA, Hamilton JK, Rebers PA, Smith F. Calorimetric method for the determination of sugars and related substances. Anal Chem 1956; 28: 350-6.

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23. Stutz W. Official Swiss method for analysis of aflatoxin B₁, B₂, G₁ and G₂ in peanuts. Mitteil Geb Lebensmittel Hyg 1982; 73: 506-13.

24. Kent LN. Technology of cereals. 3rd ed. New York: Pergamon Press, 1983.

25. Jambunathan R, Subramanian V. Grain quality and utilisation of sorghum and pearl millet. In: de Wet JMJ, Preston TA, eds. Biotechnology in tropical crop improvement. Patancheru, India: International Crops Research Institute for the Semi-Arid Tropics Centre, 1988: 133-9.

Stability of iodine in iodized salt used for correction of iodine-deficiency disorders

Abstract
Introduction
Materials and methods
Results and discussion
Conclusions
Acknowledgements
References

L. L. Diosady, J. 0. Alberti, M. G. Venkatesh Mannar, and T. G. Stone

L. L. Diosady and J. O. Alberti are affiliated with the Department of Chemical Engineering and Applied Chemistry in the University of Toronto, Canada. M. G. Venkatesh Mannar is affiliated with the Micronutrient Initiative, International Development Research Centre in Ottawa, Canada, and T. G. Stone is with PATH (Programme for Appropriate Technology in Health) Canada in Ottawa.

Mention of the names of firms and commercial products does not imply endorsement by the United Nations University.

Abstract

The food supply of more than 1.6 billion people is lacking in adequate iodine, resulting in the widespread prevalence of a spectrum of iodine-deficiency disorders. The virtual elimination of iodine-deficiency disorders in the world through universal iodization of salt by the year 2000 has been set as a goal at several international forums. The stability of iodine in salt and the levels of iodization are questions of crucial importance to national planners and salt producers, as they have implications for programme effectiveness, safety, and cost. The purpose of this study was to assess the effect of humidity and packaging materials on the stability of iodine in typical salt samples from countries with tropical and subtropical climates, under controlled climatic conditions typical of these countries. High humidity resulted in rapid loss of iodine from iodized salt, ranging from 30% to 98% of the original iodine content. Solid low-density polyethylene packaging protected the iodine to a great extent. The highest losses occurred from woven high-density polyethylene bags, whereas losses from open containers were intermediate. By using packaging with a good moisture barrier, such as low-density polyethylene bags, iodine losses can be significantly reduced, and in most cases salt can be produced that has relatively stable iodine content for at least six months. The findings of this study indicate that to ensure the effectiveness of local salt iodization programmes, countries should determine iodine losses from local iodized salt under local conditions of production, climate, packaging, and storage.

Introduction

Iodization level
Stability of iodine
Packaging materials
Objectives

The food supply of more than 1.6 billion people is lacking in adequate levels of iodine, resulting in the widespread prevalence of a spectrum of iodine-deficiency disorders. This public health problem may be corrected by the regular delivery of small doses of iodine to the population in commonly eaten foods or condiments. Salt is an excellent carrier for iodine, as it is consumed at relatively constant, well-defined levels by all people within a society, independently of economic status. Over the past decade, as part of the Universal Salt Iodization (USI) initiative, a large number of developing countries have taken steps to ensure that all salt for human and animal consumption is iodized. The virtual elimination of iodine-deficiency disorders in the world by the year 2000 has been set as a goal at several international forums, including the World Health Assembly [1, 2] the World Summit for Children [3], and the International Conference on Nutrition [4].

Salt is iodized by the addition of fixed amounts of potassium iodide or iodate as either a dry solid or an aqueous solution at the point of production. The actual availability of iodine from iodized salt at the consumer level can vary over a wide range as a result of:

» variability in the amount of iodine added during the iodization process;

» uneven distribution of iodine in the iodized salt, within batches and individual bags;

» the extent of loss of iodine due to salt impurities, packaging, and environmental conditions during storage and distribution;

» loss of iodine due to food-processing, washing, and cooking processes in the household.

Iodization level

There is no universal specification for the level of iodine to be added to salt to achieve recommended intakes of iodine. Until recently, the level of iodine to be added to salt for a given population was determined on the basis of the severity of iodine-deficiency disorders, the average per capita salt consumption, and anticipated losses of iodine during distribution. Specifically, Mannar and Dunn [5], on the basis of their earlier experience, recommended that the level of addition be based on the assumption of 50% iodine loss between iodization and consumption. The International Council for Control of Iodine Deficiency Disorders/World Health Organization/ UNICEF [6] have described desirable average levels of iodine in salt at various points in the salt distribution chain, taking into account the level of salt intake, climatic conditions, and packaging. As an example, based on ICCIDD/WHO/UNICEF recommendations, to achieve the required iodine addition level for persons with daily salt consumption of 10 g, 25 fig of iodine per gram of salt is required at the point of consumption. In a warm, humid climate, with an expected loss of 50% during transport and storage, this would require the addition of 50 fig of iodine at the iodization facility. On the basis of field experience, WHO and UNICEF are currently revising their recommended levels of iodization.

The stability of iodine in salt and levels of iodization are questions of importance to national planners and salt producers, as they have implications for programme effectiveness, safety, and cost. Higher levels of iodine may be added to compensate for higher losses due to known high levels of impurities in salt or the use of lower-grade packaging; this may result in higher costs to the producer and consumer, reducing sustainability. Recent evidence has also been presented for isolated cases of iodine-induced thyrotoxicosis in populations in a few sub-Saharan African countries where salt iodization has been introduced. In these areas, the iodine levels in salt at the point of consumption were higher than would have been predicted from the ICCIDD/WHO/UNICEF tables and may be related to the high urinary iodine levels found (F. Delange, personal communication, 1996).

Stability of iodine

Elemental iodine readily sublimates and is then rapidly lost to the atmosphere by diffusion. Potassium iodide can be oxidized to elemental iodine by oxygen or other oxidizing agents, especially in the presence of catalysts such as metal ions and moisture. Potassium iodate can be reduced to elemental iodine by a variety of reducing agents in the salt, such as ferrous ions. Moisture is naturally present in salt or is abstracted from the air by hygroscopic impurities such as magnesium chloride. The pH of the condensed moisture on the salt is very much influenced by the type and quantities of impurities present, and this may in turn affect the stability of the iodine compounds. Thus, the stability of iodine is affected by the level of impurities in the salt and the moisture content. As with most chemical reactions, an increase in temperature increases the rates of the reactions that form elemental iodine and increases its evaporation rate.

The moisture content, levels and types of impurities, and pH of salt produced for human consumption vary widely, depending upon the source and the production process. Salt is extracted from a variety of sources, including sea water, rock salt deposits, and lake sediments. Extraction methods include solar evaporation of brines and dry solution mining of rock salt deposits. Refining processes range from technologically unsophisticated physical washing methods to large-scale, mechanized, vacuum-evaporation systems with trained operators and rigorous quality assurance. As a result, the salt that is available for iodization may contain, in addition to sodium chloride, impurities such as mud and organic matter, salts of magnesium, calcium, carbonates, and sulphates, insoluble matter, and moisture. Physically, the grains may range from large, crude crystals, white or off-white in colour, of varying size (5-25 mm), to a refined, dry, uniform-grain powder.

On the basis of the chemistry, losses of iodine were not unexpected. A review of the literature showed that iodate is superior to iodide in terms of stability as a fortificant in salt. However, the studies undertaken between 1923 and 1996 are difficult to compare. Inherent variation between the rates of iodine loss from salt samples reflects variations in impurity profiles, moisture content, and processing methods before iodization. Conditions of packaging, humidity, and temperature also affect the final iodine content of the salt, yet these factors were not always clearly defined in earlier studies. The sample sizes and the reproducibility of the results were not always reported, making it more difficult to asses the statistical significance of the results.

Salt blocks, iodized with potassium iodate and exposed to wind, rain, and sun under typical Canadian conditions, lost up to 33% of their iodine after eight weeks [7].

A comprehensive review of the literature by Kelly [8] concluded that the stability of iodine in salt is determined by the moisture content of the salt and the humidity of the atmosphere, light, impurities in the salt, alkalinity or acidity, and the form in which the iodine is present. Kelly concluded that the iodine content will remain relatively constant if the salt is packed dry in a container with an impervious lining and kept dry, cool, and away from light. He recommended that iodate be used under adverse conditions, such those found in developing countries where the salt being iodized is crude, unprocessed, and usually not dried sufficiently.

Arroyave et al. [9] showed that potassium iodate mixed with calcium carbonate was stable when added to crude local sea salt stored in hemp fibre sacks. After eight months at ambient temperatures and relative humidities between 70% and 84%, an average of 3.5% of the added iodine was lost. There was no significant migration of iodine within the sacks, probably because of the low solubility of iodate.

A 1992 study by Chauhan et al. [10] compared the stability of iodine over 300 days in common salt iodized with iodate and packed in 5-kg solid high-density polyethylene bags or left in open heaps. The relative humidity varied from 41% to 83% (median, 52%) and the temperature from 30° to 39°C. Both the salt packed in high-density polyethylene bags and the salt left in the open lost 9% to 10% of added iodine within the first month, after which the values remained practically constant.

Other evidence of the magnitude of iodine losses from iodized salt comes from studies assessing the efficacy of stabilizing compounds in local salt. Findings from control samples (without stabilizers) suggest considerable variation in iodine stability, in spite of differences in experimental design.

Ranganathan and Narasinga Rao [11] found that coarse salt iodized with iodate at “normal” room temperature and humidity showed iodine losses of 15% at 3 months, increasing to 20% at 12 months. There was virtually no difference in iodine losses between salt iodized with iodide and salt iodized with iodate. Samples containing a stabilizer (calcium carbonate) did not appear to lose any iodine after 18 months under these conditions. Unfortunately, the type of packaging used by the researchers was not stated, and the samples appeared to be small.

In a later study, analysis of five types of Indian salt (including powder and crystal) iodized with iodate showed losses of 28% to 51% after 3 months, 35% to 52% after 6 months, and up to 66% after 12 months. The losses from powdered salt appeared to be lower. No information was given on packaging.

The findings demonstrate the utility of sodium carbonate as a stabilizer [12].

In Hubei, China, the iodine losses from refined solar salt packed in open 1-kg plastic film bags, heated to 130°C for 2.5 hours (to simulate drying) and stored at ambient temperature, were 5.7% after 12 months and 11% after 3 years [13].

Although the literature and North American practice indicate that iodine loss is reduced by stabilizers such as carbonates, these are not used in most developing countries, and thus we chose not to use them in this study.

Packaging materials

Salt may be sold to the consumer packaged or in bulk. Packaging materials in wide use in developing countries include paper, high- and low-density polyethylene, and woven bags made of jute, straw, or high-density polyethylene. The solid, non-woven polymer bags are the best moisture barriers, and if properly sealed and kept intact will maintain the moisture level in the salt throughout the distribution system, thus minimizing the loss of iodine resulting from the absorption of moisture and the subsequent chemical reactions.

Objectives

The purpose of the present study was to assess the effects of humidity and packaging materials on the stability of iodine in typical salt samples from countries with tropical and subtropical climates, under controlled climatic conditions. In the short term, the results may be useful for assessing the potential losses of iodine from salt between the points of production and consumption. The overall goal of the study was to determine the range and timing of the iodine losses that may be expected from salt iodized with iodate under typical conditions, and to define cost-effective means of controlling or compensating for these losses, ensuring that populations at risk for iodine-deficiency disorders receive effective amounts of iodine in iodized salt.

Materials and methods

Materials
Sample treatment
Packaging materials
Storage conditions
Analytical methods

Materials

Potassium iodate, analytical reagent grade, was obtained from BDH Canada, Toronto. Samples of non-iodized salt consumed by low-income populations from eight countries (Bolivia, China, Ghana, India, Indonesia, the Philippines, Senegal, and Tanzania) were obtained through UNICEF country offices from routine production runs of local salt producers and shipped by air to Toronto. A sample of non-iodized refined Canadian salt was obtained from Toronto Salt Chemical Co., Toronto. The Toronto sample was used as a reference.

Sample treatment

Salt samples with particles less than 2 mm in diameter were used without pre-grinding. Because wet salt could not be sieved, salt containing larger particles than this and with a moisture content of more than 3% were dried in a forced-convection oven at 70° C overnight, ground with a mortar and pestle, and passed through a 10-mesh sieve. The water content was then reconstituted to the original moisture level.

Two-kilogram samples of salt from each source were fortified to contain about 50 mg/kg iodine using potassium iodate added as a 30 g/L solution. The mixtures were blended to ensure uniformity using a 5-L ribbon blender (LeRoy Somer-LSTronics, Montreal PQ, Canada).

Packaging materials

For each salt sample, three packaging methods were tested. For each treatment condition, three 500-g samples were prepared in solid, continuous-film, low-density polyethylene bags 0.07 mm thick, in open plastic containers, and in woven high-density polyethylene bags 0.15 mm thick. Both low-density and high-density polyethylene are clear transparent or translucent plastic materials that are extruded as a thin sheet. High-density polyethylene has a much higher tensile strength because it is made of longer molecular chains. Low-density polyethylene bags were made by folding the sheets into the appropriate shape and welding the seams by heating. High-density polyethylene bags were made by cutting the sheets into thin strips 1.5 to 2.5 mm wide and weaving them into a cloth, which was then sewed into bags. Although high-density polyethylene does not absorb water, the woven bags readily allow the passage of water through the weave.

Storage conditions

The packages were stored under two conditions: elevated temperature (~40°C) and high humidity (100%), and elevated temperature (~40°C) and medium humidity (~60%). High-temperature, high-humidity conditions were maintained by using a controlled-temperature oven in which the air was saturated with water vapour by exposure to a tray of water. High-temperature, medium-humidity conditions were maintained in an environmental chamber manufactured by Associated Environmental Systems Division of Craig Systems Corporation, Toronto.

Analytical methods

Sampling

Packages of salt were sampled at the start of the experimental series and after 1, 2, 3, 6, and 12 months of storage. To obtain a representative and reasonably homogeneous sample for analysis, the complete solid salt contents of a bag were split into two equal subsamples by pouring them through a two-stemmed powder funnel. The splitting of the sample was repeated until only about 15 g of salt had been collected. This subsample was used for the analyses.

Some salt samples with hygroscopic impurities stored in open containers collected sufficient moisture at 100% relative humidity to develop a liquid layer on top of the solid salt. The liquid was filtered off from these samples, its volume was measured, the liquid was volumetrically sampled, and the contents of iodine and solids were determined. The solid phase was analysed separately. The overall iodine concentration in the combined liquid and solid sample was then calculated on the basis of the weight and iodine concentration of each phase.

Moisture

The moisture content was determined gravimetrically. Samples of salt were weighed, then dried at 110°C for 16 hours and reweighed.

Iodine

The iodine content was measured by neutron activation analysis or titration.

1. Approximately 1.25 g of salt was accurately weighed into a polyethylene vial. To decrease the interference due to the presence of a high concentration of chlorine in the sample, the sample was shielded with cadmium.

2. The vials were irradiated at 1 kW power with a neutron flux of 5.0 x 10¹¹ cm^-2 s^-1 for 3 minutes in the University of Toronto’s SLOWPOKE nuclear reactor.

3. The samples were removed from the reactor and left for 6 minutes.

4. After 6 minutes, the gamma emission at 443 keV was measured with a hyperpure-germanium-based gamma-ray spectrometer.

5. The iodine content was calculated using a calibration curve established by a series of spiked samples that covered the range from 5 to 250 mg of iodine per kilogram of salt. The relative standard deviation of the analysis was determined to be 2%.

1. Ten grams of salt was dissolved in approximately 100 ml of water. The pH was adjusted to 2.8 with 0.6% HCl.

FIG. 1. Effect of relative humidity on the stability of iodine in salt from India stored in woven high-density polyethylene bags at 40° C

2. Thirty milligrams of KI powder was added to convert all of the iodate present to elemental iodine.

3. The liberated iodine was titrated with 0.004 N freshly prepared sodium thiosulphate solution. Starch was used as the end-point indicator.

Results and discussion

Relative humidity
Packaging material
Country of origin

All salt samples lost iodine over the 12-month sampling period. The losses ranged from less than 10% to 100% of the original iodine in the sample from a starting value of 50 µg/g+ 5%. The rate of iodine loss was influenced by the origin of the salt, the packaging material, and the relative humidity during storage.

Relative humidity

In all cases the samples stored at 60% relative humidity lost iodine at a lower rate than those stored in saturated air (100% relative humidity). After six months of storage at 60% relative humidity, the loss ranged from ~0% to 20%, which is lower than might be expected on the basis of the ICCIDD/WHO/UNICEF tables.

At high humidity, the losses were more dramatic. Iodine losses over six months of storage ranged up to 98.5%, indicating that effectively all of the iodine added to the sample disappeared within six months. At high humidity, the samples stored in containers that allowed contact with the air lost two-thirds or more of the added iodine within the year. The results demonstrate the large effect of ambient humidity on the stability of iodine. Although it would be desirable to study the effects of other values of humidity on iodine loss, the results at the selected extremes of humidity give a good indication of the expected range of iodine loss from these samples in the field. Even under moist tropical conditions, it is unlikely that the relative humidity would remain at this extreme level for six months. However, within bags exposed to sunlight or in storage facilities heated by the sun, the high humidity will be retained, once moisture is absorbed into the bag’s contents, and temperatures may readily rise to over 60° C. Only one month of exposure of the samples in open containers to 40°C at 100% relative humidity resulted in the loss of more than 25% of the iodine from most of the containers. The typical effect of relative humidity is illustrated in figure 1.

Packaging material

Packaging affected the levels of moisture absorbed. Solid low-density polyethylene provided an excellent moisture barrier that maintained the total water content of the salt near the level at the time of packaging. Some absorption of moisture was possible because the bags were not sealed sufficiently to prevent some diffusion of air containing water, iodine, or both in and out of the bags (tables 1 and 2).

TABLE 1. Stability of iodine in salt stored in low-density polyethylene film bags at 60% relative humidity and 40°C

Source of salt	Initial iodine(mg/kg)a	% of original iodine remaining after storage for:
		0 mo	1 mo	3 mo	6 mo	12 mo
Bolivia	55.6 ± 0.8	100.0	89.2	95.3	85.3	68.7
Ghana	54.1 ± 0.9	100.0	93.7	98.9	100.0	88.9
India	55.1. ± 1.1	100.0	100.0	98.7	99.1	87.4
Indonesia	54.3 ± 1.2	100.0	98.9	93.7	96.5	79.0
Philippines	55.3 ± 0.7	100.0	100.0	100.0	100.0	82.5
Senegal	54.7 ± 0.8	100.0	93.8	93.2	93.8	76.1
Tanzania	50.8 ± 0.6	100.0	95.1	97.8	83.9	70.3
Canada	55.1 ± 0.8	100.0	95.6	94.2	94.9	77.3

a. mg/kg = ppm.

Source of salt	% of original iodine remaining after storage for:
	0 mo	1 mo	3 mo	6 mo	12 mo
Bolivia	100.0	91.7	95.0	67.8	41.4
Ghana	100.0	95.9	100.0	72.8	17.9
India	100.0	100.0	100.0	91.5	56.8
Indonesia	100.0	93.2	91.2	77.5	46.2
Philippines	100.0	99.3	99.3	89.0	62.4
Senegal	100.0	92.5	93.4	87.6	63.8
Tanzania	100.0	88.0	89.6	72.2	22.4
Canada	100.0	100.0	96.0	92.0	66.6

TABLE 3. Stability of iodine in salt stored in high-density woven polyethylene bags at 60% relative humidity and 40°C

Source of salt	% of original iodine remaining after storage for:
	0 mo	1 mo	3 mo	6 mo	12 mo
Bolivia	100.0	92.1	95.0	98.4	68.7
Ghana	100.0	92.2	90.8	99.6	81.0
India	100.0	100.0	99.1	98.0	75.7
Indonesia	100.0	95.9	94.3	98.2	78.5
Philippines	100.0	98.6	98.4	99.6	78.8
Senegal	100.0	91.8	90.5	90.3	77.1
Tanzania	100.0	89.4	91.5	89.2	69.3
Canada	100.0	100.0	94.9	94.9	76.4

Woven high-density polyethylene bags allowed ready access of air to the salt, but they also allowed any condensed moisture to drip out of the bag in the form of a saturated salt solution containing iodate (tables 3 and 4). The woven bags behaved similarly to the open containers at 60% relative humidity, although at high humidity the open containers retained significantly more iodine than the woven high-density polyethylene bags in all but two cases.

TABLE 4. Stability of iodine in salt stored in high-density woven polyethylene bags at 100% relative humidity and 40°C

Source of salt	% of original iodine remaining after storage for:
	0 mo	1 mo	3 mo	6 mo	12 mo
Bolivia	100.0	85.8	64.9	9.2	0.0
Ghana	100.0	92.2	90.9	61.4	3.9
India	100.0	98.7	93.3	28.1	1.1
Indonesia	100.0	91.3	27.1	7.2	0.0
Philippines	100.0	92.0	36.7	3.4	0.0
Senegal	100.0	91.2	14.1	1.6	0.0
Tanzania	100.0	51.0	12.0	2.4	0.0
Canada	100.0	89.7	72.2	4.2	2.0

TABLE 5. Stability of iodine in salt stored in open containers at 60% relative humidity and 40°C

Source of salt	% of original iodine remaining after storage for:
	0 mo	1 mo	3 mo	6 mo	12 mo
Bolivia	100.0	98.7	94.8	93.0	70.3
Ghana	100.0	98.2	96.3	95.4	79.7
India	100.0	100.0	96.7	98.7	75.7
Indonesia	100.0	88.0	87.5	92.8	76.2
Philippines	100.0	100.0	97.8	95.3	74.7
Senegal	100.0	85.2	83.0	81.4	70.7
Tanzania	100.0	100.0	99.0	87.4	69.3
Canada	100.0	100.0	90.6	90.6	80.9

TABLE 6. Stability of iodine in salt stored in open containers at 100% relative humidity and 40°C

Source of salt	% of original iodine remaining after storage for:
	0 mo	1 mo	3 mo	6 mo	12 mo
Bolivia	100.0	60.4	33.8	17.8	81.1
Ghana	100.0	74.1	30.7	22.1	10.6
India	100.0	91.7	48.3	30.3	11.1
Indonesia	100.0	73.8	43.1	28.4	11.2
Philippines	100.0	84.1	53.5	36.5	19.2
Senegal	100.0	72.6	32.7	31.8	26.1
Tanzania	100.0	62.2	30.1	18.5	8.2
Canada	100.0	81.1	54.1	36.3	17.8

Open containers of course allowed free contact between the air and the salt sample. Any absorbed or condensed moisture would remain in the sample as a liquid and contribute to the instability of iodine (tables 5 and 6).

As expected, the best results in terms of iodine stability were obtained with the solid low-density polyethylene bags. The typical effect of the container type is presented graphically in figure 2.

Country of origin

The physical characteristics of salt samples resulting from the profile of impurities and the extent of processing at the source had a major effect on the stability of the salt. The salt samples received varied in colour from very bright white to dark grey or rusty brown. The particle sizes ranged from ~100 µm to 15 mm, with great variability in the homogeneity of particle size.

FIG. 2. Effect of packaging on the stability of iodine in salt from India stored at 100% relative humidity and 40°C. HDPE, Woven high-density polyethylene bags; LDPE, solid low-density polyethylene bags; Open, open containers

As expected, Canadian salt, which was of high purity and contained very little moisture or hygroscopic impurities, was relatively stable. At low humidity, the iodine loss was less than 10% after six months of storage and less than 25% after a year. At 100% humidity, the protection of the polyethylene bag maintained iodine losses at less than 8%, whereas in woven high-density polyethylene bags and open containers, 63% and 95% of the iodine were lost, respectfully.

Some salt samples with lower levels of purity and higher levels of moisture were as stable as the Canadian sample. Salts from Canada, India, the Philippines, and Senegal lost less than 15% of added iodine, even at 100% relative humidity, when stored in low-density polyethylene bags. Salts from Tanzania, Indonesia, and Ghana retained more than 70% of added iodine, and salt from Bolivia retained more than two-thirds of added iodine for the first six months, yet the Tanzanian salt retained only 22% of the iodine after a year under the same conditions.

At low humidity, the salt from Ghana lost very little iodine. At high humidity, the iodine content of Ghanaian salt remained high for the first three months and dropped sharply during the next three months, losing 83% after a year.

Conclusions

The study clearly indicates that moisture plays a critical role in the stability of iodine. In particular, when salt is stored at temperatures characteristic of the storage and distribution conditions in many developing countries, moisture absorbed by hygroscopic impurities contributes to the rapid loss of iodine.

Although the use of highly purified salt would improve the stability of iodine in most cases, this would be expensive and probably not technically or economically feasible in the short term in many developing countries.

By packaging salt in an effective moisture barrier, such as solid low-density polyethylene bags, iodine losses can be significantly reduced. With solid low-density polyethylene packaging, the loss of iodine from salt stored for up to six months can be kept in the range of 10% to 15%. Unfortunately, woven high-density polyethylene bags are necessary for the bulk packing of salt to ensure adequate mechanical strength. Fitting woven high-density polyethylene bags with an impervious liner of high-density polyethylene or low-density polyethylene would appear to be an effective, low-cost method of improving the stability of iodine in iodized salt. However, because the loss of iodine exceeded 25% in many samples after 12 months, even with solid low-density polyethylene packaging, the time required for distribution, sale, and consumption should be minimized to ensure efficient and effective use of the added iodine.

The results indicate that the control of the moisture content of iodized salt throughout manufacturing and distribution by improved processing, packaging, and storage is critical to the stability of the added iodine. In order to make allowances for the probable losses of iodine, countries must determine iodine losses from local iodized salt under local conditions, as these will be greatly affected by the source and quality of the salt and the way it was processed.

To evaluate the range of differences between salt sources, the study will be expanded to other countries with determination of the details of salt composition in an effort to identify the effect of impurities on the stability of iodine.

Acknowledgements

This work is dedicated to the memory of Dr. Timothy R. Stone, Executive Director of PATH Canada, who was tragically killed while on a humanitarian mission in the crash of the hijacked Ethiopian plane in the Comoro Islands in November 1996. The project was a collaborative effort between PATH Canada, the Department of Chemical Engineering and Applied Chemistry of the University of Toronto, and the Micronutrient Initiative. We gratefully acknowledge the financial assistance of the Micronutrient Initiative and the assistance of UNICEF and UNICEF field offices in procuring representative salt samples.

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