ANTIOXIDANT ACTIVITIES OF EXTRACTS OF SCREENED FRUIT PEELS AND VEGETABLES ON EDIBLE OILS

Abstract

The indiscriminate consumption of edible oils stored for long time has been identified as the cause of most cardiovascular diseases due to deterioration that might have set-in; hence there arises the need to use safer means of preventing oil deterioration by the use of plant extracts rather than synthetic chemicals that are toxic and carcinogenic. The edible oils used for this study were refined soybean (RSBO), refined palm kernel (RPKO) and crude groundnut oils (CGNO). RSBO and RPKO were obtained from Jof Ideal Family Farms Limited, Owo, Ondo-State, while CGNO was obtained from a local producer at Ore, Ondo-State. The plant materials used were (i) vegetables: Chaya (Cnidoscolus aconitifolius), Fireweed (Crassocephalum crepidioides), Wild lettuce(Launae taraxacifolia), Tusil (Ocimum gratissmium), Fluted Pumpkin (Telfaira occidentalis), Woolflower/Red Soko (Celosia trigyna), Plumed cockscomb/Green Soko (Celosia argentea) and Copper leaf /Red Acalypha (Acalypha wilkesiana) and (ii) fruit peels: Sweet Orange (Citrus sinensis), Mango (Magnifera indica), Banana (Musa septium) and Plantain (Musa paradisiaca)). The powdered samples were analysed for proximate composition and mineral contents using standard methods. Bioactive extracts of each powdery sample were obtained using acetone, chloroform, ethylacetate, methanol and water. Qualitative and quantitative determination of phytochemicals (flavonoids, phenol, ascorbic acid, saponin, tannin, alkaloids, oxalate, phytate and carotenoids) using appropriate reagents were carried out on the powdered samples and sample extracts by standard methods. The antioxidant properties such as reducing power, ferrous ion chelating activity, 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging activity and 2,2-azinobis(3-ethylbenzothiazoline-6-sulphonic acid) (ABTS) of samples were determined on water extract. Physico-chemical properties of the oil samples were determined using standard methods. Thereafter methanol and water extracts of the vegetables and fruit peels were added to the oil samples at varying concentrations of 0.02g/100ml (200ppm) to 0.10g/100ml (1000ppm) oil samples contained in transparent plastic bottles of equal capacity (500ml). 0.02g butylated hydroxyl toluene (BHT) per 100ml of oil samples as well as control (oil samples without any additive) were equally set up under the sample conditions. The colour and refractive index of these oil samples were immediately determined while free fatty acid, acid value and peroxide value were determined each month for a period of twelve months. The lipid content was the lowest while carbohydrate content was the highest in all the plant samples. The food energy value (Kcal/ 100g) ranged between 287.46±1.02 – 324.05±1.52. The ranges of mineral contents of K, Ca, Na and Mg were between 15.31±0.00mg/Kg and 791.40±1.08mg/Kg while Fe, Zn, Mn, Pb, Cu and Se were between 0.01±0.00mg/Kg and 5.66±0.06mg/Kg for all the fruit peels and vegetables analysed.Qualitative phytochemical screening carried out on the samples showed that flavonoids, phenol, tannin, phytate and ascorbic acid were present in all the samples. Methanol and water extracts contained over 86-95% phytochemicals found in original samples while chloroform, ethylacetate, acetone contained 20-30% phytochemicals found in the original samples. The phytochemical analysis of the plant samples depicted that phytate had the highest value ranging between 115.79±1.10ppm (banana peel) and 1043.96±3.03ppm (fire weed). Ascorbic acid content was between 19.64±0.42ppm (plantain peel) and 45.87±0.72ppm (wild lettuce). Saponin value ranged between 8.03±0.09ppm (orange peel) and 38.51±0.55ppm (green soko). Plantain peel (9.98±0.26ppm) and red acalypha (35.49±0.57ppm) had the lowest and highest value of total phenol respectively. Flavonoid content ranged between 8.02±0.06ppm (red soko) and 18.49±0.41ppm (plantain peel). Fluted pumpkin (3.86±0.00ppm) and mango peel (9.13±0.06ppm) had the lowest and highest value of total carotenoids respectively. Tannin, oxalate and alkaloid contents ranged between 0.00±0.00ppm and 29.51±0.61ppm.The solvent-extractive values of fruit peel and vegetables showed that methanol (5.39±0.11%-24.52±1.21%) and water (5.91±0.11%-21.33±1.43%) had the highest yield of bioactive extracts. The antioxidant properties of fruit peel and vegetables depicted that reducing power was lowest in red soko (0.29±0.01) and highest in tusil (0.91±0.18). The iron (II) ion chelating activity ranged between 2.77±0.26% (red soko) and 6.29±0.31% (chaya).The lowest and highest values of DPPH radical scavenging activity were 9.40±0.08% (red soko) and 27.95±1.00% (tusil). ABTS+ radical scavenging activity ranged between 8.14±0.37% (Red Soko) and 21.82±1.13% (Tusil). The varying concentrations (200ppm-1000ppm) of methanol and water extracts of orange peel, fireweed, mango peel and red acalypha increased the colour unit by 1-4 for RSBO and RPKO but 0.5-1.0 for CGNO while chaya leaf increased the colour unit by 2-8.5 for RSBO, 0.5-8 for RPKO and 0.5-3.5 for CGNO. The varying concentrations (200ppm-1000ppm) of methanol and water extracts of orange peel, chaya leaf, fireweed, mango peel and red acalypha had a slight difference in refractive index by 0.001-0.003 for RSBO, 0.001-0.002 for RPKO and 0.001-0.006 for CGNO. All the methanol and water plant extracts except mango peel were more effective than 200ppm BHT in combating hydrolytic rancidity of RSBO and RPKO. All the varying concentrations of both methanol and water extracts of orange peel, chaya leaf and red acalypha were more effective than 200ppm BHT in reducing oxidative rancidity of RSBO while 400ppm-1000ppm chaya leaf and 800ppm-1000ppm fireweed extracts had better antioxidant activities than 200ppm BHT in reducing oxidative rancidity of RPKO. All the varying concentrations of methanol and water orange peel, water mango peel and methanol chaya leaf extracts were superior to 200ppm BHT in reducing oxidative rancidity of CGNO. Methanol and water orange peel extracts were the best among other plant extracts in overcoming both hydrolytic and oxidative rancidity of the three edible oils considered in this study. Generally, there are high antioxidant potentials in water and methanol extracts of fruit peels and vegetables which can effectively replace the synthetic antioxidants which are costly and toxic.