8342937113

K. ENDINKEAU AND T. W. WOODWARD

1985, Diehl 1977) will causechanges in theChemical and phy-sical properties of the oil to sonie extent. This paper reports the effects of gamma-irradiation on peroxide formation in palm olein.

MATERIALS AND METHOD

Malm ais

Palm olein was prepared from crude palm oil as previouslydescribed (Endinkeauand Woodward. 1989). Potassium iodide (AR), glacial acetic acid (AR) and sodium thiosulphate were obtained from

BDH Ltd. Sodium dodecyl sulphate (SOS), 2-thiobarbituric acid (TBA), 1,1,3,3-tetramet-hoxypropane (TMP) and an antioxidant 2, 6-di-t-butyl-4-methoxyphenol (BUT) were purchased from Sigma Chem. ('.o. An aqueoussolution ofTBA reagent (0.8%) was prepared according to WaravdekarandSaslow, (1959). A standard solution of malonaldehyde (MA) forcalibration ofTBAtest wasobtained by hydrolysisof TMP according to the method of Kwon and Watts, (1963). Ali other solvents were of reagent grad e and were used without furtherpurification.

Method

Samples of palm olein ( 2 x 40 g) were irradiated with y-rays in glass tubes (5.5 x 3.0 cm) at a dose ratę 65 Gy/min using a ^ł>nCo-gamma source at the University of Salford, England. The dose ratę was determined by Fricke Dosimetry using ferrous sulphate solution (Jayson et ni. 1975). Samples were irradiated at room temperaturę (21 ± 3°C) in the presenceof oxvgen. Portionsof irradiated sample were withdrawn at various limę intervals to give total doses between 2 to 12 kGy. The extent of peroxidation in irradiated samples was then determined and analysed immediately after irradiation. The peroxide value (PV) was determined according to 1UPAC (IUPAC 1979), whilst the thiobarbituricacid (TBA) test wascarried out based on the method described by Pikul et ni (1983). Essentially, the method involved the reaction ofTBA reagent with theMAformed in the irradiated sample. Afterheatinginawaterbath (60 min), the aqueous layer was separated and the absorbance of the pink pigment was measured at 532 nm. The method was calibrated using pure malonaldehyde and the results were expressed as Mgm malonaldehyde/goil. The conjugated diene formation was measured at 232 nm in isooctane solvent. The concentration of dienes was then calculated usinga molarextincdon coefficient (z)

of3x 10^} M"^lcm^_l (Mead 1952) andreported$ molarity per gram oil (M/g).    J

To investigate the post-irradiation effect, th* irradiated samples were kept in the dark at roon temperaturę and at 0°C, and periodically mons tored for TBA values after 2, 4, 7,9 and 1 1 dayso storage. The tubes were loosely stoppered so thai the air could diffuse into the samples during stoi age. Unirradiated samplesservingascontrol wen treated in the same way as the irradiated ones.

Samples were also irradiated in the presena of an antioxidant, BUT. The recjuired amounto¹ the BUT (prepared in acetone) was injected inu the oil sample using a microsyringe. The sample were then stirred with a magnetiestirrer for 10 mir under nitrogen atmosphere to dissoh eand to dis perse the antioxidant. The concentration ofann oxidant added ranged from 50 to 300 ppm (w/w and theextent of peroxidation was determined b PV and TBA methods.

RESULTS .AND DISCUSSION

Gamm a-i rr ad i at i on of u n sa tu r a ted fa ts o r oi I s (R H produces free radicals (R⁵) which reacts witl oxvgen and subsequent abstraction ofa hydroger atom forming a hydroperoxide (ROOH). Tht compound is unstable anddecomposesintovariouf un-desirableoxidation products, partie ularlyshori chain aldehydesand ketones that are responsibk forthechangeof flavoursandodoursof deteriorate oilsandfats. Furthermore,oxidisedoilsaredifficul to bleach and products madę from them art potentially capable of degrading further sina oxidation isaselfpropagating, irreversible reaction

Palm olein is morę prone to oxidativt deterioration compared to palm oil. The most labile component is the unsaturated fatty acid which comprise about 42% oleić (G 18:1), I \^ci linoleic (Cl8:2) and0.3% linolenic (Cl8:3) acids Gamina-irradiation oftheseacids in the presena ofoxygen would lead to the formation of hydropet oxides. The extent of peroxidation as measured b the oxidation indices isshown in Figs I, 2and 7.1 wasobserved that all indices (conjugated diene, P' and TBA values) showed the same generał trend i.e. at aconstant dose ratę (65Gy/min), the valuc inereased with inereasing irradiation doses. The* results suggest that as the dose inereases, mon radicals are formed, inereasing the ratę of peroxi dation and thus, morę hydroperoxides are pro duced. However, the conjugated diene (M/g) atV peroxidevalues (mEg/kg) inereasedslightlyslowe at Iow irradiation doses (<4 kGy) but slightly mon

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PKRTANIKA VOL 14NO.L 1991