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 American Journal of  
Food Science and Technology (AJFST)

Quality and Safety of  Soybean Oils in Makurdi: Physicochemical Properties, 
Contaminants, and Residues

Abah Joseph Onoja1, Muyong Gordon Muyong2, Ndi Betrand Bongjo3*

Volume 4 Issue 2, Year 2025
ISSN: 2834-0086 (Online)

DOI: https://doi.org/10.54536/ajfst.v4i2.4592
https://journals.e-palli.com/home/index.php/ajfst

Article Information ABSTRACT

Received: February 20, 2025

Accepted: March 26, 2025

Published: August 08, 2025

The study aimed to investigate the physiochemical properties, heavy metals, total aflatoxins, 
and solvent residues in soybean oils produced in some parts of  the Makurdi metropolis. The 
oil samples were collected from “Seraph” and “Hule and Sons” vegetable oil factories in 
Makurdi. Samples include: Seraph crude soybean oil (A1), Seraph Refined soybean oil (A2), 
Hule and Sons crude soybean oil (B1), and Hule and Sons refined soybean oil (B2). Analysis 
showed that the physicochemical properties varied between the refined and unrefined 
samples. Specific gravity increased from 0.807 to 0.923, 0.822 to 0.931 for A1 and A2 and 
B1 and B2 respectively. The peroxide value reduced from 9.71 to 9.6 (mEq.O2kg-1) after 
refining. Iodine value and moisture content also reduced with refining. Cadmium, arsenic, 
and lead were detected only in the crude soybean oil (A1 and B1) copper content reduced 
from 5.3 to 2.6 mg/kg, and nickel decreased from 0.55 to 0.01 mg/kg upon refining. Crude 
soybean oil content (A1 and B1) total aflatoxins of  6.05 ppm and 7.10 ppm respectively. 
No aflatoxin was detected in the refined oil samples. Major chemical compounds and 
solvent residues in refined soybean oil (A2) composed of  21.10% 2,4–Decadienal, 18.16% 
n-Hexadecanoic acid, 4.66% Bicyclo [3.1.1] heptane, 4.45% 1,6,10,14–Hexadecatetraen-3-
ol, 4.17% p-xylene, 4.22% cyclohexane, 5.05% cyclopentane, 1.69% 9-Eicosyne and 3.88% 
toluene. While B2 composed mainly of  2,4-decadienal (27.92%), 2-pentene (17.42%), 
Bicyclo [3.1.1] heptane (2.10%), 9-octadecyne (1.45%), 2-methyl-E-7-octadecane (1.31%), 
2-Heptenal (2.17%), Cyclohexane (2.33%) and Hexanal (2.90%). Refining has a significant 
role in the physicochemical properties of  soybean oil. Refining greatly reduces heavy metal 
content and aflatoxin in crude vegetable oils.

Keywords

Aflatoxins, Heavy Metal, 
Physicochemical Properties, 
Refining, Soybean Oil

1 Department of  Climate Change, Ministry of  Water Resources, Environment and Climate Change, Makurdi, Benue State, Nigeria
2 Center for Food Technology and Research, Benue State University Makurdi Benue State, Nigeria
3 Fobang Institutes for Innovations in Science and Technology (FINISTECH), Yaounde, Cameroon
* Corresponding author’s e-mail: betrandbongjo@gmail.com

INTRODUCTION
Soybean oil belongs to the popular vegetable oils used in 
food, cosmetic, and pharmaceutical industries (Mohdaly 
et al., 2017). It is considered a good source of  the essential 
fatty acids and lipid-soluble vitamins. Predominantly, 
soybean oil is made up of  unsaturated fatty acids like 
oleic, linoleic and linolenic acids (Makni et al., 2015). 
Soybean seeds are one of  the dominant oilseeds in the 
world. Its high-quality protein yield and edible oil give 
soybeans peculiar characteristics. This vegetable oil is 
rich in nicotinic acid, riboflavin, thiamine and minerals. 
Oleo-chemical application includes; cosmetic, soap 
manufacture, biodiesel production and methyl esters. 
Refining vegetable oils is essential to ensure the removal of  
gums, phosphatides, pigments and getting rid of  odorants 
(Mohdaly et al., 2017). Crude soybean oil contains a large 
amount of  non-glyceride materials consisting chiefly 
of  phosphatide and other impurities. Crude oil has a 
lighter amber colour which upon refining is reduced to 
the light yellow colour of  most vegetable oils. Refined 
oil is usually clear, odourless, and resistant to rancidity. 
Reports state that refining crude vegetable oil affects the 
physicochemical properties of  the oils. Changes have been 
observed in oxidation indices, phenolic compounds and 
fatty acid composition during refining of  vegetable oils. 
Nevertheless, refining increases the value of  soybean oils, 
but refining could modify the chemical constituents of  

these oils to the point which could be injurious to health 
(Mohdaly et al., 2017).
Heavy metal contamination of  vegetable oils has been a great 
concern related to human health problems (Gu et al., 2019). 
Studies have shown that these metals contaminate soils 
and trace levels above required limits have been estimated 
in oil-extracting seeds (Asemave et al., 2012; Onakpa et al., 
2018). Trace elements found in edible oils can be a result 
of  environmental contamination, extraction and refining 
process of  the oil (Kalappurayil & Joseph, 2017).
Aflatoxin contamination in foods is a global food safety 
issue (Aghemwenhio et al., 2017; Karunarathna et al., 2019). 
Environmental conditions, conventional agricultural 
practices, and illiteracy are the main factors which favour 
the production of  AFs in food (Waqas et al., 2019). There 
are a number of  related fungal metabolites found mostly in 
cereals, maize grains, peanuts and animal fodders mainly by 
the fungi Aspergillus flavus and Aspergillus parasitica (Idris 
et al., 2010). Aflatoxin B1 is a lethal naturally occurring, 
it is a potent liver carcinogen (Negash, 2018). Food and 
Agriculture Organization of  the United Nations (FAO) 
states that about 25% of  the world’s crops are contaminated 
with mycotoxins (Aflatoxin) during the growth or storage 
period (Pankaj et al., 2017). 
The extraction of  vegetable oils usually employs the 
use of  n-hexane. Reports state that solvent residues in 
consumable food are undesirable. Generally in edible 



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Am. J. Food. Sci. Technol. 4(2) 9-14, 2025

oils, hexane residues could be considered as undesirable 
substances when the maximum residual limits are 
exceeded (Yousef  & Hosseini, 2017). Even though the 
recovery process of  these compounds is ensured, a good 
percentage of  solvent is lost in the process which may 
either be retained in the oil or undergo other reactions to 
produce other compounds. These derivatives are known 
to be toxic at certain dosages (Cerutti et al., 2017). 
There is a paucity of  information on the heavy metal 
content and the level of  aflatoxins in crude and refined 
soybean oil produced and sold in Makurdi. Therefore, 
there is a great necessity to investigate the physicochemical 
properties, some heavy metals, aflatoxins contents and 
solvent residues in soybean oil from “Seraph” and “Hule 
and Sons” oil factories in Makurdi.

MATERIALS AND METHODS
Reagents and chemicals include; Hydrochloric acid (HCl), 
Sodium hydroxide (NaOH), Potassium hydroxide (KOH), 
Idobromine (IBr), Sodium thiosulphate (Na2S2O3), 
Potassium iodide (KI), Acetic acid (CH3COOH) Nitric acid 
(HNO3) and, Hydrogen Peroxide (H2O2). All reagents were 
of  analytical grade. Instruments used includes1; Watman 
No 1 filter paper, ELISA READER, atomic absorption 
spectrophotometer (AAS), GC-MS (Agilent USA5975C), 
refractometer, electric Oven, water bath.
Seraph and Hule and Sons Vegetable oils (soybean oils) 
were purchased at their factories in Makurdi and Wannune 
respectively. The oil samples were stored at room 
temperature away from direct lights. Samples were coded 
A1, A2, B1 and B2: Seraph crude vegetable oil, Seraph 
Refined vegetable oil, Hule and Sons crude vegetable oil 
and Hule and Sons refined vegetable oil. 

Determination of  Physicochemical properties
The Specific gravity, refractive index, the peroxide value, 
saponification values, the moisture content, of  the 
samples was determined using the method of  (AOAC, 
2015). The acid value was measured by titration (Bello, 
2015). The smoke point of  the oils was determined as 
described in literature. The temperature at which 10 mL 
of  the sample gave off  a thin bluish smoke continuously 
was recorded as the smoke point (Kenechi et al., 2017). 

Determination of  Heavy Metals Contents
Heavy metals assay of  the samples was done using 

atomic absorption spectrophotometer (AAS) following 
the Methods described by (AOAC, 2015). The lamps 
were allowed to warm up for 15 minutes and the 
monochromator was positioned to a wave length 
corresponding to the metals needed (217.0 nm for lead, 
232.0 nm for nickel, 324.7 nm for copper, 213.9 nm for 
iron, 228.8 nm for arsenic and 328.4 nm for iron

Determination of  Total Aflatoxins
Determination of  total Aflatoxins was done by ELISA 
method (Murshed et al., 2019). Extraction of  aflatoxins 
was done using 10 mL of  oil sample in 200 mL of  70% 
ethanol in an orbital shaker for 3 hours. 

GC-MS Analysis of  Solvent Residues in Refined 
Soybean Oil 
A gas chromatography- mass spectrophotometer 
(Agilent USA 5975C) with triple axis detector equipped 
with an auto injector (10 µL syringe) was used to 
analysed the solvent residues. Helium was the carrier gas. 
Capillary column specifications includes: length; 30m, 
internal diameter 0.2 µm, thickness; 250 µm, treated with 
phenyl methyl silox. Other GC-MS conditions are ion 
source temperature (EI), 250 oC, interface temperature; 
300 oC, pressure; 16.2 psia, out time, 1.8 mm, 1 µL 
injector in Split mode with split ratio 1:50 with injection 
temperature of  300 oC the column temperature started 
at 35 oC for 5 min and changed to 150 oC at the rate of  4 
oC/min. The temperature was raised to 250 oC at the rate 
of  20 oC/min and held for 5mins. The total elution was 
47.5minutes. Mass spectrophotometer solution software 
provided by supplier was used to control the system and 
to acquire the data. Identification of  the compounds was 
carried out by comparing the mass spectra obtained with 
those of  the standard mass spectra from NIST library 
(NISTII).

Statistical Analysis
Values were reported in mean ± standard deviation of  
triplicate determinations. Mean values were compared 
using One Way analysis of  Variance (ANOVA) at p<0.05 
using statistical package for social sciences (SPSS) 
software version 22. Duncan multiple range test (DMRT) 
was used for mean separation

RESULTS AND DISCUSSION

Table 1: Physicochemical properties of  crude and refined soybean oil
Physicochemical properties Samples

A1 A2 B1 B2

Specific gravity 0.807b±0.02 0.923a±0.01 0.822b±0.01 0.931a±0.01
Refractive index at 40oC (oBx) 1.482a±0.01 1.468b±0.03 1.489a±0.01 1.470b±0.02
Acid value (mgKOHg-1) 2.04a±0.01 0.58b±0.01 2.11a±0.01 0.60b±0.01
Saponification value (mgKOHg-1) 191.00b±0.02 193.06a±0.03 192.50b±0.04 194.64a±0.02
Peroxide value (mEq.O2kg-1) 9.71a±0.01 9.67b±0.02 9.70a±0.02 9.61b±0.01
Iodine value (Wijj’s) 146.07a±0.01 136.94b±0.01 145.87a±0.02 137.54b±0.01



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Am. J. Food. Sci. Technol. 4(2) 9-14, 2025

Moisture (%) 1.02a±0.01 0.18b±0.02 1.26a±0.01 0.20b±0.03
Smoke point (oC) 216.0b±0.01 234.0a±0.01 215.0a±0.02a 234.0a±0.01
Ester value(mgKOH/g) 188.96a±0.01 192.48b±0.01 190.39a±0.01 194.04b±0.01

KEY: A1 = Seraph crude vegetable oil, A2 = Seraph Refined vegetable oil, B1 = Hule and Sons crude vegetable oil. B2 = Hule and 
Sons refined vegetable oils. Means with different superscript on same column are significantly different at p < 0.05.

Table 2: Heavy metals content of  crude and refined soybean oil 
Metals Samples (mg/kg)

A1 A2 B1 B2

Cadmium 0.82±0.01 ND 0.86±0.01 ND
Arsenic 0.46±0.01 ND 0.50±0.01 ND
Copper 5.36a±0.01 2.64b±0.01 5.35a±0.01 2.65b±0.01
Nickel 0.55a±0.01 0.01b±0.01 0.57a±0.01 0.01b±0.01
Lead 0.08±0.01 ND 0.09±0.01 ND
Iron 6.23a±0.01 4.95b±0.05 6.31a±0.01 4.97b±0.01

A1 = Seraph crude vegetable oil, A2 = Seraph Refined vegetable oil, B1 = Hule and Sons crude vegetable oil`, B2 = Hule and Sons 
refined vegetable oil, MPL = maximum permissible limits, ND=Not Detected. Means with different superscript in the same column 
are significantly different at p < 0.05.

Table 3: Aflatoxin content of  soybean oils
Oils Sample Total aflatoxin concentration
Soybean oil

A1 6.05±0.01
A2 ND
B1 7.10±0.01
B2 ND

A1 = Seraph crude vegetable oil, A2 = Seraph Refined vegetable oil, B1 = Hule and Sons crude vegetable oil`, B2 = Hule and Sons 
refined vegetable oil, ND=Not Detected. 

Table 4: Major Chemical constituents of  refined soybean oil (GC-MS) (GCMS)
Compound Retention time % composition

Sample A2 (E-E) 2,4-Decadienal 10.341 21.10
n-Hexadecanoic acid 14.426 18.16
1,6,10,14-Hexadecatetraen-3-ol 14.173 4.45
Bicyclo [3.1.1] heptane 13.722 4.66
p-xylene 4.651 4.17
Cyclohexane 13.947 4.22
Cyclopentane 2.285 5.05
9-Eicosyne 11.581 1.69
Toluene 3.496 3.88

Sample B2 (E-E) 2,4-Decadienal 10.342 27.92
2-pentene 2.285 17.42
Bicyclo [3.1.1] heptane 13.722 2.10
9-octadecyne 11.581 1.45
2-Methyl-E-7-octadecane 13.102 1.31
2-Heptenal 5.750 2.17
Cyclohexane 2.510 2.33
Hexanal 3.834 2.90

A2 = Seraph Refined vegetable oil, B2 = Hule and Sons refined vegetable oil



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Physico-Chemical Properties of  Crude and Refined 
Soybean Oil in Benue State
Result of  the physicochemical properties of  crude and 
refined soybean oil is presented in Table 1. The specific 
gravity of  the oil increased with refining from 0.807 for 
A1 to 0.923 A2. It also increased from 0.822 in crude 
Hule and Sons vegetable oil (B1) to 0.931 in refined Hule 
and Sons vegetable oil. This trend was also reported in 
refined oils by Gando et al. (2014). Rather, others reports 
that specific gravity decreased after refining (Mohdaly 
et al., 2017). Refractive index, Acid value, and peroxide 
decreased progressively with refining. The refractive 
index decreased from 1.482 oBx to 1.468 oBx and from 
1.489 oBx to 1.470 oBx for Seraph and Hule and Sons 
vegetable oils respectively. 
It is often stated generally that the refractive indices 
of  oils increase with increase in the number of  double 
bonds (Mohdaly et al., 2017). The acid value of  seraph 
oils decreased from 2.04 mgKOH-1 to 0.58 mgKOH-1 and 
2.11 mgKOH-1 (sample B1) to 0.60 mgKOH-1 (sample 
B2) in Hule and Sons vegetable oils. Peroxide values 
ranges from from 19.71 mEq.O2kg-1 to 9.67 mEq.O2kg-1 

and 21.01 mEq.O2kg-1 to 12.21 mEq.O2kg-1 from crude 
to refined oils. Refining increases the oxidative stability 
of  the oil (Mohdaly et al., 2017). The moisture content 
reduced by 17% when the oil was refined. Most of  the 
physicochemical properties of  soybean oil reduce upon 
refining. 

Heavy Metals Content of  Crude and Refined Soybean 
Oil in Benue State 
Heavy metals content of  crude and refined soybean oil 
is presented in Table 4. Crude soybeans oil from Seraph 
and Hule and Sons oil contained 0.82 mg/kg and 0.86 
mg/kg of  cadmium, respectively, which were all higher 
than the maximum permissible limit (0.1 mg/kg) of  the 
metal as stipulated by CODEX (Codex Alimentarius 
Commission, 2009). The high cadmium content may be 
attributed to soil contamination on which the soybean 
was cultivated. Reports highlight that the frequent use of  
phosphorous fertilizers (Lagerwerff  & Aspecht, 2017), 
and the leaching of  paints into the farm could be the main 
coursed of  cadmium contamination to the environment. 
However, cadmium was not detected in the refined 
soybean oil samples. It has earlier been reported that 
the refining process removes heavy metals in vegetable 
oil (Lagerwerff  & Aspecht, 2017; Ikebueze et al., 2009). 
Cadmium is a toxic element. Its presence in consumable 
food products is highly undesirable as it creates serious 
health concerns on the consumer’s health.
Measures of  0.46 mg/kg and 0.50 mg/kg of  arsenic were 
recorded in Seraph and Hule and Sons crude soybean oils 
respectively. These values were higher than the 0.1 mg/
kg maximum permissible limits stipulated by NAFDAC 
(Ohimain et al., 2013). Arsenic occurs naturally in the soil 
but the major source of  Arsenic in farm fields is through 
the use of  herbicides (Gando et al., 2014). It was observed 
that the refining process reduced the arsenic content 

to a point of  not detected. This result highlights the 
importance of  refining vegetable oils for consumption 
and expressed the effectiveness of  the refining process 
in removing heavy metals like arsenic in oils refined for 
human consumption.
Copper content in the crude oils was 5.36 mg/kg and 
5.35 mg/kg for Seraph and Hule and Sons respectively. 
Refining these crude oils significantly (p<0.05) reduced 
the copper content to 2.64 mg/kg in Seraph refined oil 
and 2.65 mg/kg in Hule and Sons refined oil. Values of  
copper in refined oil recorded in this study also agreed 
with some earlier reports (Andersson & Lingnert, 2008). 
Lead measures from 0.08 mg/g and 0.09 mg/g in crude 
Seraph and Hule and Sons oils respectively but the metal 
was not detected in any of  the refined oil samples. Iron 
content reduced from 6.23 mg/g in the crude Seraph oil 
to 4.95 mg/g in the refined oil. Crude Hule and Sons oil 
had an iron content of  6.31 mg/g which reduced to 4.97 
mg/g after refining of  the oil. 
The 0.55 mg/kg and 0.57 mg/kg of  nickel recorded in 
crude soybean oil from Seraph and Hule and Sons oils 
were slightly higher than the stipulated limit. Refining the 
crude vegetable oils reduced the nickel content to 0.01 
mg/kg in both Seraph and Hule and Sons oils which were 
all lower the FAO/WHO maximum permissible limit of  
0.5 mg/kg. The nickel content of  the refined oils reported 
in this study was safe and competes favourably with other 
oils like Power oil and Kings Oil which were all reported 
to contain 0.02 mg/kg nickel (Mehmood et al., 2012).

Total Aflatoxin Content of  the Oils
Crude soybean oil from Seraph and Hule and Sons 
recorded total aflatoxin contents of  0.05 and 0.10 
respectively which implies the oils were not safe from 
consumption. No aflatoxin was recorded in the refined 
oils which implied that the oxidizing agent (sodium 
bisulfite) added to the oils during the refining process 
could significantly reduce the aflatoxin. Low level of  
aflatoxins recorded in the extracted soybean oil is an 
indication that the soybean was properly handled at the 
harvesting and post-harvest stages of  the production line 
as soybeans are hardly contaminated by aflatoxins during 
cultivation in the fields. Aflatoxins, toxic metabolites 
of  Aspergillus flavus and Aspergillus parasiticus fungi, 
are naturally occurring contaminants of  food (PACA, 
2015). Consumption of  oils contaminated with aflatoxins 
is highly dangerous as it causes aflatoxicosis. Severe 
conditions can lead to death (Sowley, 2016).

Major Chemical Constituents and Solvent Residues 
of  Refined Soybean Oil 
Major chemical components of  refined vegetable oil 
from Seraph and Hule and Sons are presented in Table7. 
(E-E) 2,4-Decadienal was the major chemical component 
of  both oils having constituted 21.10% in Seraph oil and 
27.92% in Hule and Sons oil. This compound is the major 
component of  refined vegetable oils. It occurs naturally in 
oil seeds and is a natural flavouring component contained 



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in soybeans (Zhang et al., 2020; WHO, 2019). Other 
compound detected includes Bicyclo [3.1.1] heptane and 
cyclohexane. Bicyclo [3.1.1] heptane has been reported 
to be a product of  the isomerization of  n-hexane that 
causes eye damage at doses above 10%. The result 
showed Seraph oil and Hule and Sons oil contained 
4.66% and 2.10% Bicyclo [3.1.1] heptane respectively 
which were all lower than the 10% reported (Okechalu 
et al., 2011). Cyclohexane was common to both oils 
Seraph oil contained 4.22% cyclohexane whereas Hule 
and Sons oil contained 2.33%. Cyclohexane is known to 
be toxic at concentrations above 10 ppm. Even though 
most of  the n-hexane used as a solvent in oil extraction is 
recovered, a reasonable amount of  the solvent undergoes 
aromatization to produce undesirable products like 
cyclohexane, bicycle [3.1.1] heptane and many others as 
oil constituents (WHO, 2019). Bicycle [3.1.1] heptane, 
cyclohexane, p-xylene and Toluene have been reported in 
refined vegetable oils (WHO, 2019). n-hexane lost during 
processing of  the oil has also been reported Srbinovska 
et al.,2020). 2-pentene (17.42%) was recorded in Hule and 
Sons refined oil. This compound does not occur naturally 
in soybean and its presence in the refined soybean oil may 
also be traced to the reactions of  lost n-hexane that occur 
in the refining line. The presence of  2-pentene however 
could not be of  much concern as no health risks have 
been linked to the compound (WHO, 2019). Other studies 
reported 21% of  2-pentene in cottonseed oil extracted 
using n-hexane as solvent (Attah & Ibemesi, 1990).

CONCLUSION
The physicochemical properties of  crude and refined 
soybean oils from Seraph and Hule and Sons oil 
factories varied significantly at p>0.05. Findings revealed 
that refining had a significant role in enhancing the 
physicochemical properties of  the oils. The heavy metals 
and aflatoxins content were greatly reduced upon refining. 
The refined oil contained some residual chemicals 
like (E-E) 2,4-Decadienal, Bicyclo [3.1.1] heptane and 
cyclohexane, 2-pentene, p-xylene and Toluene. However, 
these residues were found at levels that could not pose an 
immediate harm to health.

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