CHAPTER FOUR
4.0 Results and Discussion
4.1 Physicochemical Properties of Vegetable Oils
The quality of sunflower, peanut, and sesame oils was evaluated by analyzing key physicochemical parameters including viscosity, density, peroxide value, iodine value, acid value, and saponification value using standard analytical procedures (AOAC, 1990). The results are presented in Table 4.1.
Overall, the results indicate noticeable variations in physicochemical properties among the oil types and across different sampling locations, suggesting differences in composition and quality.
4.1.1 Viscosity and Density
Viscosity and density are important indicators of the compositional quality of vegetable oils. Oils with relatively lower viscosity and density are generally preferred by consumers.
The results showed that at 27°C, viscosity ranged from 50.05 cP in sunflower oil to 55.78 cP in peanut oil, indicating that peanut oil is comparatively more viscous. These variations are largely attributed to differences in fatty acid composition, particularly the degree of saturation and chain length. Increased saturation and longer fatty acid chains tend to increase viscosity, while higher unsaturation reduces it. Temperature also plays a significant role, as viscosity decreases with rising temperature.
Density values ranged narrowly between 0.85 and 0.86 g/mL, with peanut oil exhibiting the lowest density and sunflower and sesame oils showing slightly higher values. The small variation in density suggests minimal differences in overall composition among the oils. However, all density values obtained were lower than internationally recommended standards, which may be due to differences in temperature conditions, fatty acid composition, and minor constituents present in the oils.
4.1.2 Iodine Value (IV)
The iodine value is an indicator of the degree of unsaturation in oils and their susceptibility to oxidation. Higher iodine values correspond to higher unsaturation and lower oxidative stability.
The mean iodine values recorded were:
- Sunflower oil: 126.59 mg I₂/100 g
- Sesame oil: 106.55 mg I₂/100 g
- Peanut oil: 79.64 mg I₂/100 g
These values fall within the recommended ranges for edible oils. Sunflower oil exhibited the highest degree of unsaturation, while peanut oil showed the lowest, indicating greater oxidative stability and longer shelf life. Variations in iodine values are mainly due to differences in fatty acid composition across the oils.
4.1.3 Peroxide Value (PV)
Peroxide value measures the extent of primary oxidation in oils and is a key indicator of rancidity.
The peroxide values ranged from 1.42 to 9.25 meq/kg, with sunflower oil showing the highest value, followed by peanut and sesame oils. Despite these variations, all samples remained within the maximum permissible limit of 10 meq/kg, indicating acceptable quality.
The relatively higher peroxide value in sunflower oil suggests greater susceptibility to oxidative deterioration, likely due to its higher unsaturation. Factors such as exposure to light, oxygen, storage duration, and temperature may also contribute to increased peroxide levels.
4.1.4 Acid Value (AV)
Acid value reflects the level of free fatty acids present in oil, which indicates hydrolytic degradation and overall quality.
The acid values ranged from 2.42 to 4.31 mg KOH/g, with peanut oil recording the highest value. While sesame and sunflower oils were within the recommended limit of 4 mg KOH/g, peanut oil slightly exceeded this threshold.
Elevated acid values may result from poor storage conditions, high moisture content, enzymatic activity, or the use of damaged seeds during oil extraction. Overall, sesame oil demonstrated the best quality, followed by sunflower oil, while peanut oil showed comparatively lower quality.
4.1.5 Saponification Value (SV)
Saponification value provides insight into the average molecular weight and chain length of fatty acids in oils.
The results showed:
- Sunflower oil: 191.68 mg KOH/g
- Sesame oil: 180.24 mg KOH/g
- Peanut oil: 176.44 mg KOH/g
These values fall within recommended standards, indicating that the oils contain relatively shorter-chain fatty acids. High saponification values suggest suitability for industrial applications such as soap and cosmetic production. Differences observed among samples may be attributed to variations in fatty acid composition.
4.2 Fatty Acid Composition
The fatty acid profiles of the analyzed oils (Tables 4.2 and 4.3) revealed the presence of eight fatty acids. However, four major fatty acids—oleic, linoleic, palmitic, and stearic acids—accounted for approximately 95% of the total fatty acid content. Minor fatty acids such as linolenic, arachidic, behenic, and lignoceric acids contributed less than 5%.
No significant differences were observed in fatty acid composition across locations or oil types (P > 0.05). An inverse relationship between oleic and linoleic acids was observed, which is consistent with known biochemical processes involving desaturase enzymes.
In terms of composition:
- Saturated fatty acids (SFA) ranged from 24.37% to 25.91%
- Monounsaturated fatty acids (MUFA) averaged around 44%
- Polyunsaturated fatty acids (PUFA) ranged from 30.29% to 31.87%
Peanut oil had the highest saturated fat content, while sesame oil had the highest unsaturated fatty acid content.
Nutritionally, unsaturated fatty acids—especially oleic and linoleic acids—are beneficial due to their roles in cardiovascular protection, membrane structure, and metabolic processes. However, excessive intake of saturated fats is associated with increased risk of cardiovascular diseases.
The polyunsaturated-to-saturated (P/S) ratio ranged from 1.17 to 1.31, exceeding the recommended minimum value of 0.45. This indicates that all analyzed oils are nutritionally beneficial and may help in reducing serum cholesterol levels when consumed appropriately.
4.3 Heavy Metal Content
Analysis of heavy metals revealed the presence of iron (Fe) and lead (Pb) in varying concentrations, while cadmium (Cd) and zinc (Zn) were not detected in any samples.
Iron concentrations ranged from 0.263 to 14.982 mg/kg, with several samples exceeding the recommended limit of 5.0 mg/kg. High iron levels may result from soil contamination, processing equipment, or storage conditions. While iron is essential for human health, excessive amounts can lead to oxidative damage and health complications.
Lead concentrations ranged from 0.585 to 2.035 mg/kg, exceeding the permissible limit of 0.1 mg/kg in most samples. Lead is highly toxic and poses serious health risks, including organ damage and neurological disorders.
The presence of these metals may be attributed to environmental contamination, agricultural inputs such as fertilizers and pesticides, industrial emissions, and processing or packaging methods.
The absence of cadmium and zinc suggests minimal contamination from these elements, possibly due to low environmental levels or limited uptake by the plants.
Summary of Findings
- Physicochemical properties of oils were largely within acceptable limits, though some variations were observed.
- Fatty acid composition showed high levels of beneficial unsaturated fatty acids.
- Heavy metal analysis revealed concerning levels of iron and lead in some samples.
- Overall, sesame oil exhibited the best quality, followed by sunflower oil, while peanut oil showed relatively lower quality in some parameters.
