Nigerian Journal of Agriculture, Food and Environment. 9(3):12-15
Isong et al., 2013
Published September, 2013
PHYSICOCHEMICAL PROPERTIES OF AFRICAN WALNUT
(Tetracarpidium conophorum) OIL AND ITS SUITABILITY FOR
DOMESTIC AND INDUSTRIAL USES
Isong, N., Alozie*, Y. E., and Ekwere, Y.
ABSTRACT
Department of Human Ecology, Nutrition & Dietetics, Faculty of Agriculture, University of Uyo, AkwaIbom State, Nigeria
*Corresponding Author :Email Address: [email protected] Phone number: +2348039385198
This study examined the potential of African walnut as an oil seed. Mature walnuts were purchased from IkpeAnang market in
EssienUdim Local Government Area, of Akwa Ibom State, Nigeria. Walnuts were dehulled, sorted, dried, weighed and milled
to powdered form. The oil was extracted using soxhlet extraction method and characterized for physicochemical properties
using standard methods. Oil yield of walnut kernel was 57.5%. Characterization of the oil suggests that it is a non-drying oil
suitable for paint making (high iodine value), soap making (as saponification value was 176.87mgKOH g-1) and industrial
purposes as unsaponifiable matter was greater than five (>5). The result also revealed that the oil has potential as a biofuel
with value of40.84MJ L-1 which compare favourably with commercial grade diesel. Free fatty acid content was 9.47% and
ester was 89.74mgKOH g-1 suggest its suitability also as edible oil while the high acid (87.22mgKOH g -1) and low peroxide
(9.67meqO2 g-1) content were indicatives of high susceptibility to rancidity and low antioxidant levels. These suggest the
suitability of walnut oil for industrial and domestic purposes.
Key words: walnut, oil, physicochemical, industrial, domestic
INTRODUCTION
African walnuts (Tetracarpidium conophorum) a member of Juglandaceae family, is one of the finest nuts of the
temperate regions. It is the oldest cultivated fruit in the world (Caglarirmak, 2003).It is a perennial woody,
climber commonly found in low bush especially in Africa (Dalzier, 2000). In Nigeria, it is widely cultivated in the
rainforest belt of Nigeria and called Ukpa in Igbo language, and awusa or asala in Yoruba language (Dalzier,
2000). The oil yielding crop plants are very important for economic growth in the agricultural sector. The oil
seeds containing uncommon fatty acids which are industrially important, as they are used in protective coating,
dispersants, pharmaceuticals, cosmetics and a variety of synthetic intermediates as stabilizers in plastic
formulations (Hosamani and Sattigeri, 2003, Eganathan et al., 2006).
Walnut is one of the several high nutrient density foods with the presence of oxalates, phytates, tannins as well as
protein, fiber, carbohydrate and vitamins (Savage et al., 2001).The vitamin content is useful for the treatment of
common cold and other diseases like prostate cancer. Other minerals though in trace amount are essential for body
metabolism (Okwu and Okele, 2003; 2004). Walnut is a rich source of mineral elements such as calcium,
magnesium, sodium, potassium, and phosphorus (James, 2009). Walnut when eaten, have a bitter taste usually
observed upon drinking water immediately. This is attributed to the presence of chemical substances such as
alkaloids (Ayolele, 2003).
Ripe walnuts are mostly consumed fresh, toasted or used in cakes, desserts and confectionaries. These seeds are
edible even when raw. Walnuts are rich in fat. A diet supplemented with walnuts has a beneficial effect on blood
lipids and lowering of blood cholesterol (Salvage, 2001). Walnut kernel generally contained about 60% of oil
(Prasad, 2003) however this varies from 52 to 70% depending on the cultivar, location grown and irrigation rate
(Ozkan and Koyuncu, 2005). The consumption of 68g of walnuts per day reduced the total and low density
lipoprotein cholesterol by five percent and nine percent respectively. These reductions would have some positive
effect in reducing the risk of coronary heart disease (Abbey, 1994). This study was carried out to determine the
potentials of walnut as an oilseed and to establish its suitability for industrial use as compared to other seed oils.
MATERIALS AND METHOD
Sample collection
Mature walnut was purchased from Ikpe Annang Market in EssienUdim Local Government Area of AkwaI bom
State. Walnuts were first sorted to remove the unwholesome ones after cracking and removal of kernels.
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Nigerian Journal of Agriculture, Food and Environment. 9(3):12-15
Isong et al., 2013
Published September, 2013
Sample preparation
Extraction of oil
Walnut kernels were grounded into powdered form with a manual grinder. Thereafter, the oil was extracted from
the resulting powder by soxhlet extraction method (AOAC, 2010).
Physicochemical characterization
Walnut oil was evaluated for saponification, peroxide, free fatty acid, refractive index and acid values using
methods described by AOAC (2010), while iodine value was determined according to the method of Pike
(2002).The methods of Morris (1999) and Osborne and Voogts (1978) were used to determine moisture and
specific gravity respectively. The unsaponifiable matter was determined using cylinder method described by
AOAC (2010), while the ester value was determined by subtracting the acid value from the saponification value.
The equation according to Aigbodion et al. (2004) was used for the determination of biofuel potential (Hu) of the
oil sample.
Biofuel potential (HU) = (47645 -4.187I -38.315S) MJ/L
Where I – Iodine; S – Saponification value
Data analysis
Physicochemical characteristics data were analysed using simple percentage and descriptive statistics
RESULTS AND DISCUSSION
Table 1 shows the physicochemical characteristics of walnut oil.
Table 1: Physicochemical characteristics of walnut oil
Parameter
Oil yield (%)
Moisture (%)
Specific gravity (g cm-1)
Saponification value (mg KOH g-1)
Iodine value (gI 100g-1)
Free fatty acid (%)
Acid value (mg KOH g-1)
Peroxide value (meqO₂ g-1)
Ester value(mg KOH g-1)
Refractive index
Unsaponifiable matter (%)
Biofuel potential (Hu) MJ L-1
Colour
Composition
57.50 ± 0.06
1.59 ± 0.04
0.91 ± 0.02
179.87 ± 0.3
7.31 ± 0.02
9.47 ± 0.1
87.13 ± 0.05
9.67 ± 0.1
89.74 ± 0.01
1.42 ± 0.03
12.26 ± 0.05
40.84 ± 0.01
Yellow
Mean ± SEM of three determinations
Walnut oil obtained in this study was yellowish in colour. The oil content of walnut was relatively high (57.50%)
which was in the same range (52 – 70%) reported by Ozkan and Koyuncu (2005) for walnut kernel. Walnut
weights vary depending on the cultivar, location grown and irrigation rate (Caglarirmak, 2003; Ozkan and
Koyuncu, 2005). The oil content of walnut obtained in this study compares favorably with other oil bearing seeds
such as Q. undulata seed (wavy leaf oak) (56%) (Louppe et al., 2008).The low moisture content of walnut oil
(1.59%) showed that walnut kernel has low moisture content hence can be preserved for a long time. The value
obtained is lower than those reported for castor (8%), rubber seed (8.6%), and sheer butter (10%) oils (Asuquo,
2008). Specific gravity is the ratio of the mass of a given volume to the mass of an equal volume of water. The
specific gravity value obtained in this study was found to be 0.9g cm-1, indicating that the oil is less dense than
water. The specific gravity of walnut oil was in the same range, 0.86 to 0.98, reported by Karmakar et al., (2010)
for landolphia seed oil. Saponification value is a measure of the average molecular weight or chain length of all
the fatty acids present. The saponification value obtained for walnut oil in this study was 176.87 mg KOH g-1
which was lower than 194 mg KOH g-1 of liquid red palm oil (Osita, 2007) and 213 mg KOH g-1 in neem seed oil
(Akpan, 2000) but higher than 159.33 mg KOH g-1 reported for Dennettia tripatala fruit oil (pepper fruit)
(Nwinuka and Nwiloh, 2009) and 143.76 mg KOH g-1 of African pear oil which was reported to be good for soap
making (Ikhuoria and Maliki, 2007). The value was also similar to the range reported by Ogunniyi (2006) for
castor seed oil (177 – 182 mg KOH g-1). This suggests the suitability of walnut oil for industrial soap making
since its saponification value falls within this range of oils currently used for the same purpose. The iodine value
is used to determine the degree of unsaponifiable matter of fats and oils. It has been reported that lowering the
iodine value improves the stability and good yield of oil (Nkafamiya et al., 2010). The iodine value of walnut oil
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Nigerian Journal of Agriculture, Food and Environment. 9(3):12-15
Isong et al., 2013
Published September, 2013
was found to be 7.31gI 100 g-1. Oils with iodine value less than 1.30 are non drying oil and are not suitable for
paint making (Hilditch and Seavell, 1980). Walnut oil thus has the potential of being a suitable raw material for
paint industries. Free fatty acid value is an important variable in determining the suitability of the oil as edible oil.
The free fatty acid value obtained (9.47%) in this study was lower than the value of ginger bread plum (15.10%)
(Ajayi, 2010). However, it was higher than that reported for sesame oil (0.82%) (Elleuch et al., 2007) and 2.35%
for camelina oil (Zubr, 1997) which were reported to be good edible oils. Oils intended for human dietary purpose
should not contain high free fatty acid, value obtained for walnut oil in this study suggests it suitability as edible
oil. A low acid value is an indication of its susceptibility to rancidity while high acid value is an indication of
deterioration. The acid value obtain in this study was 87.13 mg KOH g-1 which was higher than the value obtained
from rubber seed oil with 15.03 mg KOH g-1 (Jumat and Bashar, 2009) and cashew kernel oil (10.7 mg KOH g-1)
(Akinhanmi et al., 2008). The high acid value of the walnut oil indicates rapid deterioration of the oil.
Peroxide value measures the initial stages of oxygen absorption in oil and is considered satisfactory at values ≤
10. Peroxide value also suggest low levels of antioxidant (Kyari, 2008).The peroxide value obtained in this study
(9.67 meq O₂ g-1) was less than ≤ 10 thus can be classified as satisfactory and indicative of low antioxidants level.
This correlate with acid level in that it is susceptible to rancidity. Oils having higher ester value are more intact
and therefore more suitable for consumption (Nkafamiya et al., 2010). It is obtained as the difference between the
saponification value and acid value. The ester value (89.74mgKOH g-1) obtained in this study was lower than that
obtained from castor oil (174.09 mg KOH g-1) (Asuquo, 2008). However, it was higher than that (22.44 mg KOH
g-1) reported for bay laurel oil (Sayyah, 2003) indicating it to be suitable for consumption. The refractive index
obtained for walnut oil was 1.42 which was similar to that reported for soybean oil (1.466 – 1.470) and palm
kernel oil (1.449 – 1.451). The high refractive index of this oil seems to conform to the high number of carbon
atoms in their fatty acids (Falade et al., 2008). Oils with high unsaponifiable matter (>5) adds value to the oil in
terms of industrial purposes. The unsaponifiable matter content was 12.26% which was lower than that of
terebinth fruit (15.7%) (Ozcan, 2004) and higher than that of peanut kernel oil (0.27 – 0.99%) (Ozcan and Steven,
2003). However, it was greater than five which is suggestive of its usefulness for industrial purposes. The biofuel
potential value (40.84 MJ L-1) obtained in this study was higher than that of soya oil (39.71 MJ L-1) and red palm
oil (39.95 MJ L-1) (Osita, 2007). Walnut oil has the potential of being a biofuel if improved upon as the biofuel
values did not differ much compared to the biofuel value of commercial grade diesel (44.95 MJ L-1)(Osita, 2007).
CONCLUSION
The oil content of walnut compares favourably with existing oils and is similar to other existing edible oils in
physicochemical properties. The result of the physicochemical properties suggest the suitability of the oil for
domestic use (cooking), soap making, paint making and useful for other industrial purposes. The biofuel potential
of Walnut oil observed in this study suggests that it could be explored and improved for utilization a biofuel for
certain machines and equipment since the biofuel potential (Hu) compared with that of commercial grade diesel.
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