EFFECT OF STORAGE ON THE QUALITY AND SAFETY
OF GRAINS IN THARAKA DISTRICT, KENYA

Kung'u JK*¹, Muroki N¹ and A Omwega¹

ABSTRACT

The contribution of grains to food security is limited by deterioration during storage and as such it was necessary to document the effect of storage on their quality and safety. The objective of this study was to determine the effect of storage on the quality and safety of grains in Maragua and Gikingo locations, Tharaka District, Kenya. Proximate composition and aflatoxin content were used as indicators of quality and safety respectively, and their determination was done on a total of 36 samples of grains that were randomly collected from different storage methods at two time periods, two months after storage (T₁) and seven months after storage (T₂).

The moisture content of the grains was below the maximum moisture content for safe storage of cereals and legumes (<13%). There was an increase in moisture content in all the grains analysed between T₁ and T₂, except in cowpeas, where there was a decrease. However, only sorghum and green grams showed a significant difference (p<0.05) in the increase. Only green grams showed a significant difference (p<0.05) in increase in protein content between T₁ and T₂. There was a significant (p<0.05) reduction in the fat content between T₁ and T₂ in millet, sorghum and green grams. In cowpeas, the decrease was not significant (p>0.05). The fibre content showed a significant (p<0.05) increase in all grains, except in green grams that showed a significant (p<0.05) decrease. There was also a significant (p<0.05) increase in the ash content of all the grain samples. However, there was a decrease in the available carbohydrates in all grains analysed between T₁ and T₂, except in cowpeas that showed a significant (p<0.05) increase. Similarly, the energy content showed a significant (p<0.05) decrease in all the grains, except cowpeas. There was an increase in the aflatoxin content in millet (from 0 to 0.35 µg/kg), green grams (from 0 to 0.35 µg/kg) and sorghum (from 0 to 0.48 µg/kg) grains with duration of storage. This increase was significant (p<0.05) only in sorghum.

In conclusion, storage caused a decrease in the fat, carbohydrate and energy contents. The aflatoxin content increased during storage while apparent increases were noted in the protein, fibre and ash contents. These results suggest an association between the variation in nutrient composition in the storage duration and the storage methods and type of grain stored.

Keywords: Storage of grains; Quality of grains; Safety of grains

French

EFFET DE LA CONSERVATION SUR LA QUALITE ET LA SECURITE DES GRAINS DE CEREALES DANS LE DISTRICT DE THARAKA, KENYA.

RESUME

La contribution des céréales à la sécurité alimentaire est limitée par la détérioration qui s’opère au cours de la conservation ; c’est pourquoi il a été nécessaire de publier l’effet de la conservation sur leur qualité et leur sécurité. L’objectif de cette étude était de déterminer l’effet de la conservation sur la qualité et la sécurité des grains de céréales dans les localités de Maragua et de Gikingo, dans le District de Tharaka au Kenya. La composition approximative et la teneur en aflatoxine ont été utilisées comme indicateurs de la qualité et de la sécurité respectivement, et leur détermination a été faite sur un total de 36 échantillons de céréales qui ont été collectés au hasard au moyen de différentes méthodes de conservation pendant deux périodes, deux mois après le temps de conservation (T₁) et sept mois après le temps de conservation (T₂).

Le degré d’humidité des grains de céréales était au-dessous du degré maximum d’humidité pour la conservation des céréales et des légumineuses (<13%). Il y a eu une augmentation du degré d’humidité dans toutes les céréales analysées entre T₁ et T₂, sauf dans les petits pois où il y avait une diminution. Cependant, seuls le sorgho et les pois chiches verts ont montré une grande différence (p<0,05) dans l’augmentation. Seuls les pois chiches verts ont montré une grande différence (p<0,05) dans l’augmentation de la teneur en protéines entre T₁ et T₂. Il y avait une forte réduction (p<0,05) de la teneur en graisses entre T₁ et T₂ dans le millet, le sorgho et les pois chiches verts. Dans les petits pois, la diminution n’était pas considérable (p>0,05). La teneur en fibres a montré une forte augmentation (p<0,05) dans toutes les céréales, sauf dans les pois chiches verts qui ont montré une grande diminution (p<0,05). Il y avait également une grande augmentation (p<0,05) dans la teneur en cendres dans toutes les céréales prises comme échantillons. Néanmoins, il y a eu une diminution dans les glucides disponibles dans toutes les céréales analysées entre T1 et T2, sauf dans les petits pois qui ont montré une forte augmentation (p<0,05). D’une manière similaire, la teneur en énergie a montré une grande diminution (p<0,05) dans toutes les céréales, sauf dans les petits pois. Une augmentation de la teneur en aflatoxine s’est manifestée avec la durée de conservation dans les grains de millet (de 0 à 0,35 µg/kg), dans les pois chiches verts (de 0 à 0,35 µg/kg) et dans les grains de sorgho (de 0 à 0,48 µg/kg). Cette augmentation n’était considérable (p<0,05) que dans le sorgho.

En conclusion, la conservation a causé une diminution dans les teneurs en graisses, en glucides et en énergie. La teneur en aflatoxine a augmenté pendant la conservation tandis que des augmentations apparentes ont été remarquées dans les teneurs en protéines, en fibres et en cendres. Ces résultats suggèrent une association entre la variation dans la composition des nutriments pendant la durée de la conservation et les méthodes de conservation et le type de céréales conservées.

Mots-clés: conservation des céréales; qualité des céréales; sécurité des céréales

INTRODUCTION

Cereals and legumes constitute the major staples in developing countries and are critically important to food security. Cereals occupy first place as source of calories and proteins, followed by food legumes [1]. In developing countries, cereal grains comprise more than 75% of the basic staples by weight and provide more than 60% of the per capita energy intake [2; 3]. It is reported that the average smallholder in the rural areas of Kenya obtains a large share (61%) of his/her daily energy intake from cereals [4]. Most cereal grains are important as food, because of their excellent keeping qualities, tolerance to drought and adaptation to dry tropical and subtropical ecosystems throughout the world [5]. Legumes are important sources of dietary protein in countries where animal proteins are scarce and expensive or are not consumed for religious or cultural reasons [6]. Dry legumes, like dry cereal grains, keep well for months or years when properly stored but, on the other hand deteriorate rapidly when exposed to high temperatures, moisture and insect infestation. The losses and changes in nutritive value are related to conditions of storage [7].

Grain storage occupies a vital place in the economies of developed and developing countries. Massive investments have been made to raise production levels of grains to meet the food needs of growing populations. Once the grain has been harvested it must be held in storage and care should be exercised to reduce, if not completely eliminate, quantitative and qualitative post-harvest losses caused by various physical, biological and mechanical factors [5, 8]. The contribution of cereals and legumes to food security is limited by deterioration during storage.

The rural poor have traditionally relied upon agriculture-based savings in terms of food stocks [4]. The main purpose of storing grains is to secure household food supplies [8].

Traditional storage methods are the products of decades of development and improvements by users and their ancestors. This maxim is generally upheld as true and would-be ‘developers’ should employ utmost respect for traditional methods when endeavouring to introduce ‘improvements’. These methods are usually well adapted to both the types of grains and the environment in which they are employed [8]. Consequently, quantitative storage losses are to a large extent minimal. However, losses in the intrinsic properties (qualitative losses) of these grains remain unknown and unattended to by the farming households.

According to the Food and Agriculture Organization (FAO) estimates 25% of the world food production is lost at post harvest level [9]. Of these losses, 10% occur in cereal grains with peaks for some less developed countries reaching 30-50% of the nutritional value of some crops. The main reasons for these losses, especially in developing countries, is the lack or inadequacy of storage facilities, permitting insects, rodents and birds free access to the produce; the dangerous influence of high humidity or even rain, causing mold growth and poor storage practices [10]. Products can be stored in many different kinds of storage containers varying from earthen gourds, baskets and cribs to big metal or ferro-cement silos, which may have different effects on losses. Depending on financial possibilities, available material and external factors such as climate, one can choose any storage method [11].

The basic objective of good storage is to create the appropriate environmental conditions, which provide sufficient protection to the product to maintain its quality and quantity, thus reducing product and financial loss [12]. Numerous examples show that losses are lower in the traditional systems than in modern structures [13]. It is, therefore, necessary to continue, at village level with the form of storage that has proved it’s worth, while at the same time making improvements to matters of detail and guaranteeing better protection of foodstuffs by introducing insecticidal treatment in cases where it proves necessary [13].

Inadequate storage facilities and poor household storage practices influence the quantity of foods available in the households. However, the adequacy of diets should not be considered only in quantitative terms (caloric sufficiency), but also in qualitative terms [14]. During storage, food materials can undergo both losses in quantity and quality. Data obtained in loss assessment studies in Zambia, indicate that quality aspects in loss assessment should not be overlooked [15]. Nutritional losses of food may occur during storage, thus lowering its qualitative value. These losses represent a reduction in the food value of the grain as a result of lowering of its protein and hydrocarbon content mainly. Pests prefer to consume certain parts of the grain, thus lowering its nutritive value [13]. The FAO loss estimates, in addition, include losses caused by mold and mycotoxin contamination [9].

The qualitative losses in food grains can be measured in terms of mycotoxin content, nutritive value and the depletion of the nutritive constituents among other things [5]. The present study was carried out to determine the quality changes (change in the proximate composition) during the duration of storage of sorghum, millet, cowpeas and green grams and also to determine the safety, specifically the aflatoxin levels during storage.

MATERIALS AND METHODS

Study area
Tharaka District is located in Eastern Province of Kenya. It borders Meru District to the north, Nyambene District to the northeast, Mwingi District to the southeast, Embu and Mbeere districts to the south, and Kirinyaga, Nyeri and Meru South Districts to the west. The district has a bimodal rainfall pattern with the rains falling during the months of March to May and October to December with the highest precipitation being received in October to December. The annual rainfall ranges from 500-700mm. Temperatures range between 21 - 27 °C and may reach a peak of 37 °C. Tharaka District is designated as arid and semi-arid land (ASAL) and is disadvantaged in many ways due to harsh natural conditions. It experiences severe and frequent drought that limits agro-pastoral performance [16]. Also, it is a new district with almost no history of development projects. For a long time it has had poor access to development opportunities. The Intermediate Technology Development Group (ITDG) Kenya Marginal Farmers’ Project in Tharaka District is based in Maragua and Gikingo locations in Tharaka North Division of the district. Tharaka North Division, where the study was carried out, is the largest division in Tharaka District, with an area of 838 km², which is about 54% of the area of the district.

Determination of proximate composition of the grains
The most common varieties of cereals and legumes, namely sorghum, millet, cowpeas and green grams, were randomly collected and used for laboratory analysis after two months (T₁) and then seven months of storage (T₂). These were the months of July and November 2000 respectively.

A total of 36 representative samples of the grains were obtained from randomly selected households. Similar samples of respective grains from three households were bulked together. The grains were prepared for laboratory analysis by first milling them into flour. The quality of the grains, specifically the proximate composition of the flour samples, was determined according to the official methods of analysis of the Association of Official Analytical Chemists [17] in duplicate as follows:
Moisture: Air oven method 14.004
Ash: Direct method 14.006
Crude fat/Ether extract: Direct extraction with petroleum ether14.018
Crude fibre: Ceramic fibre filter method 14.020
Crude protein: Kjeldahl method14.026
The percentage nitrogen of the samples was multiplied by a nitrogen-protein conversion factor of 6.25 to calculate the crude protein content assuming 16% nitrogen content of the protein in the grains. Carbohydrate content was estimated by the difference between 100 and the sum of the percentages of water, protein, fibre, fat, and ash in the food sample. Energy was calculated using Atwater factors, using the formula, Energy content (kcal) = [carbohydrate (g) x 4 kcal/g] +[fat (g) x 9 kcal/g] + [protein (g) x 4 kcal/g] [18; 19].

Determination of aflatoxin levels in the cereal and legume grains
The safety of the grains, as indicated by the aflatoxin content of the flour samples (in duplicate) was determined according to the official Swiss method for analysis of aflatoxins B₁, B₂, G₁ and G₂ [20]. Thin layer chromatography was used to separate aflatoxins B₁, B₂, G₁ and G₂ which were then individually quantified by in-situ fluorescence screening.

Statistical analysis
To find out whether there was a significant difference (p<0.05) in the proximate composition of grain samples obtained at time 1 (T₁) and time 2 (T₂), statistical analysis using independent samples t-test was done. Further analysis was done using both analysis of variance (ANOVA) and independent samples t-test to find out if there was a significant difference in the different storage methods from where the grains were obtained.

RESULTS

Proximate composition and aflatoxin content of grain samples
The proximate composition and aflatoxin content of the grain samples are as shown in Table 1. There was an increase in moisture content in all the grains analysed between T₁ (two months after storage) and T₂ (seven months after storage, except in cowpeas where there was a decrease. However only sorghum, and green grams showed a significant increase (p<0.05). In millet, there was no significant difference (p>0.05) in the increase in moisture content between T₁ and T₂. Millet, sorghum and green grams showed an increase in the protein content between T₁ and T₂, however, only green grams showed a significant increase (p<0.05). In cowpeas, there was a decrease in the protein content with duration of storage, although it was not significant (p>0.05). The fat content decreased in all the grains with duration of storage, with a significant (p<0.05) reduction in millet, sorghum and green grams. In cowpeas, the decrease was not significant (p>0.05). The fibre content showed a significant (p<0.05) increase in all the grains except in green grams that showed a significant (p<0.05) decrease. There was also a significant (p<0.05) increase in the ash content of all the grain samples. However, there was a decrease in the available carbohydrates in all grains analysed between T₁ and T₂, except in cowpeas which showed a significant (p<0.05) increase. The decrease in available carbohydrates was significant (p<0.05) in sorghum and green grams. Similarly, the energy content showed a significant (p<0.05) decrease in all the grains, except in cowpeas where the decrease was not significant (p>0.05). Although millet, sorghum and green grams showed an increase in the aflatoxin content with duration of storage, this was significant (p<0.05) only in sorghum.

The millet grain analysed for proximate composition was obtained from three storage methods namely: mururu (A reed woven large covered basket whose airspaces may sometimes be sealed with cow dung), granary and sack storage. Statistical analysis based on the different storage methods showed that the mean difference was significant (p<0.05) only in the levels of protein, carbohydrates and moisture content (Table 2). In protein level, there is a significant (p<0.05) decrease between mururu and granary and between mururu and sack storage. The mean difference was more between mururu and sack storage. The highest mean protein value in the three storage methods was in mururu, followed by granary then sack storage. In carbohydrate content, there is a significant (p<0.05) increase between mururu and granary and between mururu and sack storage. The mean difference was more between mururu and sack storage. The highest mean carbohydrate value in the three storage methods was in sack storage followed by granary, then mururu. In moisture content, there is a significant (p<0.05) decrease between mururu and granary, with the mean moisture content in granary being lower than in the mururu.

Sorghum grain was obtained only from two storage methods namely; sack storage and granary. Statistical analysis of the data showed that there was no significant difference (p>0.05) between the two storage methods in levels of all proximate constituents, except in protein level. Sack storage had a higher mean protein value than the granary.

Cowpeas were obtained from two storage methods namely; chemical preservation and ash preservation. Statistical analysis of the data showed that there was no significant difference (p>0.05) between the two storage methods in the levels of all proximate constituents, except in fat content, which showed a significant difference (p<0.05) with cowpeas stored using ash preservation having a higher value than those stored using chemical preservation.
Green grams were obtained from only one storage method, the granary, so no statistical analysis based on storage methods has been done (Table 2).

DISCUSSION

Storage period affects the nutritive value of grain. When dried to moisture contents below the safe moisture level, cereals and legumes can be stored for periods of a year or more under a wider range of temperatures, provided that during storage the moisture level does not rise, and precautions against insects are taken [11]. The drying stage is thus all important to reduce attack and damage from insects and fungi [14]. The moisture content of the analysed grains was below the moisture content for safe storage of cereals and legumes [14].

Grains are relatively stable when properly stored and do not undergo substantial changes in the protein composition [21]. Protein analysis data is in conformity with this, because there was no significant decrease (p>0.05) with the period of storage.

Both the fibre and ash contents showed significant increases (p<0.05) with duration of storage in most instances. This is contrary to the FAO report [5], which indicates that little change in mineral content is expected during storage of grain under sound condition.

The apparent increases in the protein, fibre and ash contents are most likely due to infestation during storage. Pests consuming certain parts of the grain may leave debris that may be analysed alongside the grain thus overstating the contents of some proximate constituents. Alternatively, loss of grain components for a given nutrient may result in an apparent increase in a nutrient that does not change or changes little with storage duration.

The fat content decreased in all the grains analysed with duration of storage. FAO [5] reports that the fat content may decrease, particularly when there is a mold attack in the grain. The fat decrease in this study may not be explained by this observation, since there was only a little increase in the aflatoxin levels, which would imply low mold attack.

The findings of this present study also show that there was a significant decrease (p<0.05) in the available carbohydrate in all grains analysed, except cowpeas. The decrease could be attributed to the lowering of the hydrocarbon content of grains expected during storage [13].

The storage duration also has an effect on the safety of grains in terms of the aflatoxin content. Grains analysed at T₁ had no detectable levels of aflatoxin. The levels of aflatoxin as B₁ obtained at T₂ (0.35 mg/kg in millet and green grams and 0.48 mg/kg in sorghum) are below the acceptable levels of mycotoxin contamination [8]. However, this does not mean that the mycotoxin contamination should be ignored, since their presence indicates imminent danger. Analysis for aflatoxin levels has been reported in various studies. Sinha and Ranjan [22] found that the toxin levels in gram and cowpea samples were considerably lower while Madsen and Rasmussen [23] found that aflatoxin B₁ occurred most frequently.

In conclusion, fat, carbohydrate and energy contents decreased in all the grains with duration of storage while the protein, fibre, ash and aflatoxin contents increased in most instances. From the present study, it is evident that the recommended method for storage varies with the cereal or legume grain and the nutrient component in question.

ACKNOWLEDGEMENTS
We acknowledge the University of Nairobi and the Intermediate Technology Development Group (ITDG) for facilitating and funding this study.

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*Corresponding author Email : JKKungu@hotmail.com
1 Applied Nutrition Programme, Department of Food Technology and Nutrition, University of Nairobi, P. O. Box 442 Uthiru, Nairobi, Kenya.