To evaluate the effectiveness of Urtica dioica
(Urticaceae) extract as a natural antioxidant and compare with the most commonly used synthetic antioxidants, butylated hydroxyanisole (BHA) and
butylated hydroxytoluene (BHT).
Three different U. dioica
extracts, viz, chloroform, methanol (80 %) and water extracts, were
prepared. The antioxidant activity of the extracts were evaluated by 1,1-diphenyl-2-picryl hydrazyl
(DPPH) radical scavenging and soybean oil models. Varying concentrations of the extracts (200, 500
and 800 ppm), BHA and BHT (100 and 200 ppm) were separately added to soybean oil and stored in
the oven (60±1 °C) for 25 days. Peroxide and thiobarbituric acid (TBA) values were measured at various
heating periods for the oil samples. Total phenolic and flavonoid contents of the extracts were
determined using Folin–Ciocalteu and aluminium chloride methods, respectively, while the aerial parts
of the plant were also phytochemically screened.
Analysis of the chemical composition of U. dioica
aerial parts showed they contain crude
proteins (21.78 %±0.11), crude lipids (1.66 %±0.03), total soluble carbohydrates (37.19 %±0.21), crude
fibers (19.62 %±0.14) and ash (19.75 %±0.17). The aqueous extract contained the highest level of total
phenolic contents (7.89 ±0.38, mg g-1
of powder) while the choroform extract contained the highest level
of flavonoid contents (15.40 ±0.53 mg g-1
powder). The half-maximal concentration (IC50
) values for
chloroform, methanol (80 %) and aqueous extracts in respect of DPPH radical scavenging activity were
77.53±0.99, 199.71±1.02 and 159.88±1.57 μg ml-1
, respectively. Mixing soybean oil with 200 - 800 ppm
of extract decreased oil oxidation and formation rate of TBA reacting substances at a level that is almost
equivalent to the synthetic antioxidant, BHT, at a concentration of 200 ppm.
These findings demonstrate that mixing soybean oil with U. dioica
extract can improve the
quality of the oil during frying process.