CHAPTER FOUR

4.0 RESULTS AND DISCUSIONS

4.1 PHYSICOCHEMICAL PROPERTIES OF THE VEGETABLE OILS

The quality of sunflower, peanut, and sesame oils were analyzed by determining the physicochemical properties such as viscosity, density, peroxide, iodine, acid, and saponification values according to the methods of (AOAC, 1990). Results are presented (Table 4.1).

Table 4.1: Physicochemical parameters of vegetable oil samples.

4.1.1 Viscosity and Density

Viscosity and density are used to monitor compositional quality of vegetable oils. Oils with lower values of viscosity and density are highly appreciable to consumers (Ceriani et al., 2008; Mousavi et al., 2012). Results tabulated in (table 4.1) revealed that at 27^oC, the viscosities ranged from 50.05cP in sunflower oil to 55.78cP in peanut oil. Richard (1986) reported the viscosity of crude sunflower oil at 25^oC as 50cP. Murwan (1994) reported viscosity of sesame oil in the ranged of 18.90 – 26.43cP at 40^oC. Balla (2001) reported viscosity of peanut oil in the range of 46.0 – 52.43 cP at 30°C.

Fazal et al. (2015) reported that vegetable oils are a mixture of triglycerides and their viscosity depends on the nature and arrangement of the fatty acids on the glycerol backbone of the triglyceride molecule, and oil viscosity has a direct relationship with degree of unsaturation and chain length of the fatty acids in lipids, and it increases with increasing degree of saturation. Kim et al. (2010) also indicated that viscosity and density decreases with increase in level of unsaturation, and also increases with high levels of saturation and polymerization. Prasad and Dutt (1989) reported that viscosity of oil is influenced by temperature changes, and it generally decreases exponentially with increase in temperature. Wakeham (1954) also reported that hydrogenation increases oil viscosity.

Based on data presented in (table 4.1) at 27^oC, the density values covered a small range of 0.85 – 0.86g/mL, with density value being lowest in peanut oil and highest in sunflower and sesame oils. The small range in the values of density of the vegetable oils analyzed could be attributed to a small difference in their composition and content of fatty acids. Wiess (1983) reported that density of peanut at 15°C was 0.917 – 0.921 and at 25°C was 0.910 – 0.915. While the relative density of peanut oil recommended by joint FAO/WHO (2019) ranges from 0.914 to 0.917 at 20°C.

The density of sesame oil reported by Bernardini (1986) ranged between 0.889 and 0.894 at temperature 60°C, which is higher than density of sesame oil (0.86) of the present study at 27^oC. Bailey (1996) reported that density of sesame oil between 0.915 and 0.924 at 20^oC, which is higher than density of sesame oil of the present study. The recommended density standard for sesame oil at 20°C/ water at 20°C by the joint FAO/WHO (2019) is 0.915 – 0.923. Hui (1996) reported relative density of regular sunflower oil in the range of 0.909 to 0.915 (20^oC /water at 20^oC), which is higher than the density of sunflower oil (0.86) of the present study at 27^oC. The joint FAO/WHO recommends a relative density of regular sunflower oil in the range of 0.918 to 0.923 (20^oC /water at 20^oC). Therefore, the density of all the analyzed oils is lower than the values recommended by the joint FAO/WHO (2019). The differences in the oil densities could be attributed to fatty acid compositions, minor components in the oils, and temperature (Fakhri and Qadir, 2011).

4.1.2 Iodine Value (IV)

Iodine value (IV) measures the degree of unsaturation in fats or vegetable oils. It determines the stability of oils to oxidation, and allows the overall unsaturation of oils to be determined qualitatively (AOCS, 1999; Asuquo et al., 2012). Onyeike (2003) indicated that the higher the iodine value, the greater degree of unsaturation, and the lower the stability, and the more susceptible the oil to oxidation. The iodine value obtained for the analyzed oils (table 4.1) showed 126.59mg I₂/100g for sunflower oil, 106.55mg I₂/100g for sesame oil, and 79.64mg I₂/100g for peanut oil. Matola et al. (2015) found that the iodine value for sunflower oil ranged from 124.8 to 125.7mg I₂/100g, which is in agreement with result of the present study of 126.55mg I₂/100g. Mohammed and Hamza (2008) reported iodine value of sesame oil in the range of 103 – 116mg I₂/100g, and this value is in agreement with the findings of the present study.  Sulaiman et al. (2012) reported iodine value of peanut oil in the range of 43.72 to 45.12mg I₂/100g, and this finding is not in agreement with result of the present study. Therefore, the difference in iodine values may be attributed to fatty acid composition of the different oils (Atasie and Akinhanmi, 2008).

The recommended ranges of iodine value for different vegetable oils are 110 – 143, 105 – 120, and 7.7 – 107mg of I₂/100g of fat for sunflower, sesame, and peanut oils respectively (FAO/WHO, 1999). The iodine values obtained in the present study are within the recommended range of iodine values for edible oils specified by joint FAO/WHO (2019). The low iodine value of peanut oil obtained in the present study demonstrates that it has greater oxidative storage stability. The oxidative and chemical changes of oil during storage are characterized by a decrease in the total unsaturation of oil (Perkin, 1992).

4.1.3 Peroxide Value (PV)

Peroxide value (PV) is used as a measure of the extent to which rancidity reactions have occurred during storage, and it is used as a good criterion for the prediction of the quality and stability of oils (Nangbes et al., 2013). The peroxide values tabulated in (table 4.1) for all oils range from 1.42-9.25meq/kg, with sunflower oil at (9.25), peanut oil at (3.39), and sesame oil at (1.42). Jar Elnabi (2001) reported the peroxide value for refined and crude sunflower oil as 4.4 and 6.1meq/kg respectively, and these peroxide values are lower than the peroxide value (9.25) for sunflower oil of the current study.  SSMO (2002) reported the peroxide value of unrefined sesame oil as 15meq/kg and refined sesame oil as 10meq/kg, which is higher than the peroxide value of sesame oil (1.42) of the current study.  Balla (2001) reported the peroxide value of crude and refined peanut oils in the range of 0.7 to 1.2meq/kg and 2.9 to 4.1meq/kg, respectively, and these values are not in agreement with peroxide value particularly for crude peanut oil (3.39) of the present study. The joint FAO/WHO (2019) recommended maximum peroxide value of 10meq/kg for all vegetable oils.  Mohammed and Ali (2015) indicated that high peroxide value could be due to high degree of unsaturation, and found to increase with the storage time, temperature, light, and contact with atmospheric oxygen.

Sunflower oil exhibited higher value than the rest. Therefore, sunflower oil is relatively more susceptible to oxidative rancidity than the other oil samples. However, the values for all the samples were within the recommended range of the joint FAO/WHO (2019). In general, the analysis showed that sesame oil, peanut oil and sunflower oil samples exhibited excellent, good, and acceptable qualities, respectively.

4.1.4 Acid Value (AV)

Acid value represents the weight of KOH in mg needed to neutralize the free fatty acids present in 1g of fat, while free fatty acid is the percentage by weight of a specified fatty acid such as the percentage of oleic acid in oil (Amos et al., 2012). An increment in the amount of free fatty acid in a sample of oil or fat indicates hydrolysis of triglycerides, and such reactions occur by action of lipase enzyme, which is an indicator of inadequate processing and storage conditions like high temperature, moisture, and tissue damage (Othman and Ngaasapa, 2010).  Acid value therefore, is a good indicator of oil degradation caused by hydrolysis or enzymes, which is the also an indicator of level of rancidity and edibility of oils. The joint FAO/WHO (2019) recommenced maximum acid value of 4mgKOH/g for all vegetable oils.

The acid values tabulated (table 4.1) for all the analyzed oils of the present study range from 2.42 – 4.31mgKOH/g, with 2.42mgKOH/g for sesame oil, 3.11mgKOH/g for sunflower oil, and 4.31mgKOH/g for peanut oil. As it is seen in (table 4.1), except peanut oil, the acid values for oil samples analyzed were below the maximum value of 4mgKOH/g recommended by the joint FAO/WHO (2019). Negash et al. (2019) reported the acid value of refined sunflower oil as 1.2mgKOH/g, and this value is lower than the acid value for sunflower oil (3.11) of the current study. Mohammed and Hamza (2008) reported the acid value of white and brown sesame oil as 0.5 and 0.45mgKOH/g respectively, and these values are lower than the acid value for sesame oil (1.42) of the present study.  Akhtar et al. (2014) reported acid value for peanut oil in the range of 0.6 to 0.99nmgKOH/g of oil, and these values are also lower than the acid value for peanut oil (3.39) in the current study.

The high acid values of the analyzed oils mainly for peanut oil could be attributed to high moisture content, poor extraction techniques, use of damaged seeds, and incorrect or lengthy storage that can be accelerated by light and temperature. Nevertheless, the most common factor for high acidity is the nature of unrefined oils which easily hydrolyses under storage (Rajko et al., 2010; Fazal et al., 2015). According to the results, edibility of sesame oil, sunflower oil and peanut oil is excellent, good, and acceptable, respectively.

4.1.5 Saponification Value (SV)

Saponification measures the average chain length of the fatty acid that makes up the oil. In other words, saponification values are useful in providing information as to the quantity, type of glycerides, and mean weight of the acids in a given oil sample (Mohammed and Ali, 2015; Fazal et al., 2015). The lower the saponification value, the larger the molecular weight of fatty acids in the glycerides, and the lower the mean molecular weight (Musa et al., 2012). The saponification value obtained for the oil samples in (table 4.1) are 191.68mgKOH/g for sunflower oil, 180.24mgKOH/g for sesame oil, and 175.19mgKOH/g for peanut oil. Sulaiman et al. (2012) reported saponification value of sunflower oil in the range of 188.9 to 189mgKOH/g, Mohammed and Hamza (2008) reported saponification values of white and brown sesame seed oils as 189 and 191mgKOH/g, respectively. Matola et al. (2015) reported saponification value of peanut oil as 189.4. Therefore, the findings of the present study for saponification value of all the analyzed oils are not in agreement with those reported from other studies. The difference in the saponification values could be attributed to the chain length or molecular weight of the fatty acids in the triglycerides.

The recommended ranges of saponification value for different vegetable oils are 187 – 196mgKOH/g for peanut oil, 185 – 193mgKOH/g for sesame oil, and 188 – 194mgKOH/g for sunflower oil (FAO/WHO, 2019). The saponification values of the current study are within the recommended value of the joint FAO/WHO (2019).

All the vegetable oils analyzed showed relatively high saponification values characterized by presence of relatively high concentration of low molecular weight fatty acids which can be used as valuable raw materials for soaps and cosmetics (Nangbes et al., 2013).

4.2 FATTY ACID COMPOSITION

The fatty acid composition of analyzed oils is shown (table 4.2 and table 4.3) respectively. The mean of total saturated fatty acid (SFA), monounsaturated fatty acids (MFA), polyunsaturated fatty acids (PUFA), and the nutritional indexes (P/S) are shown (table 4.3).

Table 4.2: Fatty acids (%) in vegetable oils analyzed

Data are expressed as percentages of total fatty acid methyl esters; values are means of triplicate determinations, values followed by the same letter with in each row are not significantly different (P>0.05), nd: not detected

Table 4.3: Total fatty acids (%) and liquid health index (P/s) of the vegetable oils

SFA: Saturated fatty acid; MUFA: monounsaturated fatty acid; PUFA: Polyunsaturated fatty acids; P/S : nutritional index, which is ratio of polyunsaturated to saturated fatty acids

Results of fatty acid composition (Table 4.2) show that 8 fatty acids were present in all the analyzed vegetable oil samples, but only four fatty acids, oleic, linoleic, palmitic and stearic fatty acids make up about 95% of total fatty acid content. Linolenic, arachidic, behenic, and lignoceric fatty acids accounted for only <5%. This finding is close to the data by Ahmed and Young (1982) who reported that palmitic, stearic, linoleic, and oleic acids accounted for 95% of total fatty acid content in peanut and sunflower oils. There were no significant differences (P>0.05) in the levels of different fatty acids based on location or type of oil seeds.

In Table 4.2, There is an inverse relation between oleic and linoleic acids in the analyzed oils and this finding is consistent with data by Hassan and Ahmed (2012), Achola et al. (2017), Asubo et al. (2008), Ashish (2013) and Azharudheen et al. (2013) who reported inverse relationship between oleic acid and linoleic acid in sunflower, peanut, and sesame oils. This could be due to action of desaturase enzymes, encoded by three genes which converts linoleic acid to oleic acid, and hence altering the fatty acid composition in plant seeds.

Nutritional quality of any vegetable oil is based on the proportions of SFA, MUFA, and PUFA in the oils. Vegetable oils and fats are perceived as a dietary component associated with risk of diseases, the problem is not the oil but their overconsumption, especially in the unbalanced intake of fatty acids (Babinská and Béderová, 2012).

The results from this study presented (Table 4.3) show that the percentage of total saturated fatty acids (SFA) ranged from 24.37% to 25.91% in all the analyzed oils with predominant presence of palmitic and stearic acids. Peanut oil showed the highest saturated fatty acid content of 25.91%, comprising of palmitic 13.07%, stearic 7.37%, behenic 2.77%, arachidic 1.61, and lignoceric 1.09%. The saturated fatty acid content of sunflower oil (24.59%) and sesame oil (24.37%) showed similar pattern of fatty acid composition, except behenic and lignoceric contents which were significantly different at P<0.05. According to FAO/WHO report of an expert consultation on fats and fatty acids in human nutrition, the recommended human consumption of SFA should cover <10% of the available energy from food (Velíšek and Hajšlová, 2009). Keresteš et al. (2011) reported that SFA with longer chain (above 14 carbons) have a tendency to agglomerate with negative effect to the cardiovascular disease occurrence. Studies done in both mice and humans indicate that high levels of long chain saturated fatty acids such as palmitic acid in the diet may adversely affect mood, and reduce physical activities (Kien et al., 2003).  High levels of palmitic acid also raise the levels of LDL (bad) cholesterol, which is associated with increased risk of heart diseases and cardiovascular complications. Myristic acid is a long chain saturated fatty acid, and its consumption raises the LDL cholesterol more than other fatty acids (Zock et al., 1994). Stearic acid lowers the LDL cholesterol slightly as such it may be healthier than other fatty acids (Zock et al., 1994). Medium chained saturated fatty acids such as capric, caprylic, and caproic acids are unique because they promote weight loss, increase in insulin sensitivity, and reduce risk of seizers (Dullo et al., 1996). Studies show that replacing SFAs with other macronutrients particularly, PUFA has a favorable effect on the blood lipid profile, including lowering of LDL cholesterol levels (WHO, 2016).

Besides the negative impact of SFA on human health, the human body uses SFA up to the length of 12 carbon atoms, mainly for the energy production.  Saturated fatty acids (SFA) are also chemically very stable and change either by prolonged heating or at high temperatures. Saturated fatty acids are also recommended to people with digestive problems or liver disorders, because they are not stored in body as fat and also due to their easy digestibility.

All the analyzed oils showed similar patterns of unsaturated fatty acid composition (Table 4.2 and 4.3). The total unsaturated fatty acid content ranged from 74.35 to 75.99% in all the analyzed oils, with the predominant presence of oleic acid, a monounsaturated fatty acid (MUFA). Sesame oil showed the highest percentage of total unsaturated fatty acid of 75.99%, comprising of oleic acid (MUFA) 44.14%, linoleic acid (omega-6) 31.51%, and then linolenic acid (omega-3) 0.36%. This finding is close to data by Yoshida et al., (2000) who reported FA composition of sesame oil as 44% oleic acids, 34% linoleic acid 10% palmitic acid and 7% stearic acid.              The total unsaturated fatty acid content of sunflower oil (75.66%) and peanut (74.35%) had almost equal amounts of oleic (MUFA) and linoleic (omega-6) acids, except linolenic acid (omega-3). This finding is similar to data by Musimenta et al., 2019) who reported oil from peanut in Uganda contained 39.71% to 55.89% oleic acid, 20.21 to 35.5% linoleic acid and 11.91-17.16% palmitic acid. Liu et al.  (2017) reported that unsaturated fatty acids have a wide range of biological roles, and cellular functions which include formation of phospholipid bilayer of cellular membranes, transport of proteins and cellular receptors for hormones and neurotransmitters, and used as substrates for eicosanoid synthesis.

Glick and Fischer (2013) reported that human body is capable of producing all the unsaturated fatty acids, except the essential fatty acids like linoleic acid (omega-6 fatty acid) and alpha-linolenic acid (omega-3 fatty acid).  These fatty acids are necessary for growth and development, and are also used as starting materials for the manufacture of other fatty acids (e.g. arachidonic acid (AA) is formed from LA) (Elleuch et al., 2007).

FAO/WHO (1998) recommended that human consumption of linoleic (omega-6) should be within 4-8% on average of total energy intake of food. The recommended intake of linolenic acid (omega-3) is about 0.6-1.2% of daily energy intake or 1-2 g per day (Frej, 2014; Nitrayová et al., 2014).

Various studies have revealed that vegetable oils like sunflower, soybean, peanut, sesame contain abundant dietary unsaturated fatty acids that can supply the nutritional needs of humans (Achola et al., 2017; Kowalski et al., 2004; Kamal, 2006). Through selective breeding and manufacturing processes, oils of differing proportions of unsaturated fatty acids can be produced (Fernandez-Martínez et al., 2004). Based on fatty acid composition, three types of sunflower oils exist with varying levels of oleic and linoleic acids. High oleic type, which contains nearly (80%) oleic acid, high linoleic type contains (70%) linoleic acid, and mid oleic type contains (42% to 65%) (Fernandez-Martynez et al., 2004). The results presented (table 4.3) show that sunflower oil in the current study contained oleic acid 44.07%, linoleic acid 31.26%, and linolenic acid 0.33%. The findings of this study show that the sunflower oil analyzed is mid oleic type. Thambugala et al. (2013) reported that fatty acid biosynthesis in plant cells is controlled by a group of genes identified as sad1, sad2, fad2a, fad2b, fad3a, fad3b, which are collectively known as desaturase. Research studies have also reported that differences in fatty acid composition of oils could be due to differential expression of desaturase genes during seed development and maturation (Baud and Graham, 2006). Morrison et al. (1995) reported that fatty acid composition of sunflower is dependent on where the crop is grown. Cooler climates produce higher amounts of linoleic acid compared with warmer climate, where oleic acid is more dominant. Fernandez-Martynez et al. (2004) indicates that mid oleic type retains high enough levels of linoleic acid to remain an excellent dietary source, but the relatively high levels of oleic acid make it less prone to rancidity. Various research studies have reported that oleic and linoleic acids are hypocholesterolemic, although linoleic acid is an essential fatty acid, oils rich in oleic acid (MUFA) are preferred as it combines the hypocholesterolemic effect (Mensink and Katan, 1989), and a greater oxidative stability than linoleic acid (PUFA) (Yodice, 1990). However, high linoleic acid oils have alternative nutritional advantages such as the production of conjugated linoleic acid (CLA), associated with a wide range of positive health benefits (Belury, 1995; Ip, 1997).

Sunflower and sesame oils in the current study showed similar presence of linolenic, ranging from 0.33% for sunflower to 0.36% for sesame oil. Linolenic acid is an omega-3 PUFA that plays an important role in regulation of biological functions, prevention and treatment of a great numbers of human diseases such as heart and inflammatory diseases. The low content of linolenic acid (PUFA) in both sesame and sunflower oil makes them resistant to oxidation and suitable for human consumption.

The total unsaturated fatty acid content of peanut oil is 74.5%, with predominant presence of oleic acid 44.06%, followed by linoleic acid 30.29%, (Table 4.3), this findings were consistent with data by Achola et al. (2017) who reported oleic acid and linoleic acid contents of S.7T peanut variety in Uganda as 43.19% oleic acid, and 33.44% linoleic acid. The differences in fatty acid composition may be attributed to differences in genotype, growing season (Singkham et al., 2010), location and planting date (Andersen and Gorbet, 2002), soil nutrient, soil temperature, and maturity (Dwivedi et al., 1993).

Sesame oil is a superior vegetable oil, and has a pleasant flavour. It ranks second after olive oil with regard to nutritional value. Worldwide, fatty acid composition of sesame oil is variable among the different cultivars of sesame seeds such as black, brown and white, and in this study, sesame oil was extracted from brown sesame seeds. Sesame seeds contain mainly mono, and polyunsaturated fatty acids accounting for almost 85% of total fatty acid (Unal and Yalcin, 2008). Unal and Yalcin (2008) reported that sesame is a rich source of proteins and several health promoting compounds such as phytosterols, tocopherols, and lignans, and this peculiar biochemical composition makes sesame oil the most resistant oil against oxidation though it is highly unsaturated.

Table 4.2 and Table 4.3 show that the sum of mono- and polyunsaturated fatty acids accounted for 75.99% of the total fatty acid content of sesame oil, with predominant presence of oleic acid with 41.12%, followed by linoleic acid 31.51%, and linolenic acid 0.36%. The findings of the present study are similar to the results reported by Yoshida et al. (2000) who indicated that oleic acid and linoleic acid contents were 44% and 34%, respectively.  Contrary to our findings, Thakur et al., (2017) reported oleic acid content of 41.36%, linoleic acid content as 41.25%, and linolenic acid content of 0.35%. This implies that Fatty acid composition of sesame oil depends on different factors such as climatic conditions, soil conditions, and ripeness of plants (Rahman et al., 2007).

The nutritional index (P/S) is the relationship between saturated (SFA) and polyunsaturated fatty acid (PUFA) content, which is an important parameter for determining nutritional value of vegetable oils. The vegetable oils shown in (table 4.3) covered a small range of nutritional index from 1.17 to 1.31, with the P/S value being highest for sesame oil, and lowest for peanut oil.

The polyunsaturated to saturated fatty acid ratio (P/S) measures the tendency of the diet to influence the incidence of coronary heart diseases (Simat et al., 2015) this ratio is also important in determining cholosteromic effect of dietary lipids.

Foods with P/S ratio above 0.45 (FAO, 2010) are considered beneficial due to their potential to lower serum cholesterol (Kostik et al., 2013). The beneficial effect is even more significant when the PH ratio are >1. The P/S ratios of the vegetable oils analyzed ranged from 1.17 to 1.31 (Table 4.3).

Johnson et al., (2009) obtained PS ratios of 1.8 and 2.28 for peanut oil. The high P/S ratios in this study suggest that consumption of diet rich in peanut, sesame and sunflower oil is beneficial for human health.

Oils with high P/S ratios are of high nutritional value than the ones with less (Kostik et al., 2013).

Ramprasath et al., (2012) reported that the serum cholestoral concentration are linked with diets rich in SFA while the opposite effect is provided by diets containing high levels of BUFA.

4.3 HEAVY METAL CONTENTS

In table 4.4, all the vegetable oils were contaminated with substantial amounts of iron (Fe) and lead (Pb) metals, while cadmium (Cd) and zinc (Zn) metals were not detected. The concentration of iron ranged from (0.263mg/kg to 14.982mg/kg). sesame oil from Nebbi had the lowest iron level, whereas sesame oil from Yumbe had the highest.

The concentration of lead (Pb) ranged from 0.585mg/kg to 2.035mg/kg. Peanut oil and sesame oil from Nebbi had the lowest and same levels of lead, whereas sunflower oil from Zombo had the highest level of lead.

Table 4.4:  Average amount of heavy metal in vegetable oils (mg/kg)

(“nd” means below detection limit)

Table 4.5: Linear regression equations, coefficient of determination, limit of detection

Various studies have reported the presence of heavy metals such as iron, nickel, lead, cadmium, copper and Arsenic in vegetable oils (Mendil et al., 2009; Juszczak 2008). The presence of heavy metals in vegetable oils may depend on several factors, which includes uptake by the plant roots from soils contaminated with metals, the metals may also be introduced during the refining processes such as bleaching, hardening, refining, and deodorization (Zeiner et al., 2005; Jamali et al., 2008). The metals may also be introduced into the vegetable oils by contamination during oil extraction, storage, and transportation (Leonardis et al., 2000).

Iron (Fe) is considered an essential mineral because it is needed to make hemoglobin, a part of blood cells which is responsible for carrying oxygen in the body. Therefore, sufficient amount of iron in the diet is necessary for manufacture of hemoglobin in order to reduce incidence of anaemia (Ashraf and Mian, 2008). Anemia is also commonly associated with a decrease in working power, and damaged intellectual development (Ettle et al., 2007). High levels of iron in the body has also been associated with some negative effects such as tissue damage, as well as formation of free radicals (Schümann et al., 2007). The Maximum Permissible Limit (MPL) of iron is 5.0mg/kg in all vegetable oils (WHO/FAO, 2003).

The concentration of iron (table 4.4) in most samples exceeded the maximum permissible limit. The high iron content in this study may be attributed to uptake of iron by roots from contaminated soil, reaction between the relatively high-unsaturated portion of the oil with the surface of iron containers used during transportation, storage, and processing of vegetable oils (Duran et al., 2013).

Pelivan et al. (2009) reported low iron content in different vegetable oils in the range of 0.0039 – 0.0352mg/kg.  Mendil et al. (2009) also reported the content of iron in different vegetable oils in the range of 52.0 – 291.0µg/g. The difference in the iron content reported to that in the present study could be attributed to oil extraction methods, storage and processing, and soil factors (Leonardis et al., 2000).

Lead (Pb) is a toxic heavy metal which serves no useful purpose in human body. Its presence in the body may cause chronic and acute poisoning, which may lead to failure of heart and liver, and other health disorders such as tiredness, sleepiness, hear, and weight loss (Needleman et al., 2002). The maximum permissible concentration of lead is 0.1mg/kg for all vegetable oils (FAO/WHO, 2019).

From table 4.4, lead level was highest in sunflower oil from Zombo (2.035mg/kg), and lowest in peanut oil from Nebbi and sesame oil from Yumbe (0.585mg/kg). Sunflower oil samples from Nebbi and Arua, sesame oil from Arua, peanut oil from Zombo and Yumbe, did not contain lead.

The reported concentration of lead in different vegetable oils from Saudi Arabia ranged from 0.007-0.015mg/kg (Ashraf, 2014). Asemave et al. (2012) reported concentration of lead in palm oil, peanut oil, and soybean oil from Nigeria as 0.1780mg/kg, 0.1631mg/kg and 0.1631 mg/kg, respectively. Findings of the present study reveals higher levels of lead than in the reported result. Research studies have reported that contamination of vegetable oils with lead may be from soil through mineralization by crops, oil processing or environmental contamination, as in the application of agricultural inputs such as fertilizers and pesticides, which are in common use in farms (Onianwa et al., 2001; Mendil et al., 2009). Contamination of vegetable oil with lead may also be due to industrial emission, combustion of fuel in refinery process, and from packaging materials such as stabilizer and colorant in plastic (Dugo et al., 2004).

Zinc (Zn) is an essential element known to be involved in most metabolic pathways, and is regarded as an important cofactor for many enzymes that participates in metabolism. Zinc deficiency is the most ubiquitous micronutrient deficiency in crops. Zinc is essential for both plants and animals because it is a structural constituent and regulatory cofactor in enzymes, and proteins involved in many biochemical pathways (Alloway, 2009). Millions of hectares of crop land are affected by zinc deficiency, and approximately one third of human population suffers from inadequate intake of zinc. The main soil factors affecting availability of zinc to plants are low total zinc content, high pH, high calcite and organic matter content, and high concentrations of sodium, calcium, magnesium, bicarbonate, and phosphate in the soil solution.

Zinc was not detected in all the analyzed vegetable oils (table 4.4). This result is in agreement with the findings by Alloway (2009) who revealed that zinc deficiency to crops is a major problem in the world, and affecting millions of hectares of crop land and one third of human population. Other studies elsewhere have reported Zinc levels in the range of 9.1 – 31.8mg/kg (Ewuzie and Nnorom, 2015), and 0.0484 – 0.287mg/kg (Pehlivan et al., 1998). High levels of zinc in vegetable oil could be attributed to absorption by plants from soil or contamination of the vegetable oils during refining process, storage tank and packaging materials.

Cadmium (Cd) is a highly toxic heavy metal with a natural occurrence in soil, but it spreads in the environment due to human activities. Needleman et al. (2002) reported that excessive exposure to cadmium may lead to renal and reproductive effects. Table 4.4 shows cadmium was not detected in all the analyzed vegetable oil samples. Reported concentration of cadmium in different vegetable oils from Turkey were in the range of 0.09-4.57µg/kg (mendil et al., 2009). Zhu et al. (2011) reported the levels of cadmium in peanut oil, sesame oil, and sunflower oils from China as 3.81, 3.44 and 3.51µg/g, respectively.

The absence of cadmium in the present study may be attributed to low levels of pollution or crop variety. High levels of cadmium in vegetable oils of the previous studies may be attributed to uptake by plants from contaminated soils, contamination through refining process, storage tank or the packing material such as a colorant or stabilizer in plastics (Mendil et al., 2009).