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EVALUATION OF ANTIOXIDANT POTENTIAL OF MONODORA MYRISTICA (AFRICAN NUTMEG)

ABSTRACT

This work evaluates the antioxidant potential of Monodora myristica (African nutmeg). Monodora myristica extract was obtained by solvent extraction using n-hexane and used as treatment on freshly prepared crude palm kernel oil and palm oil. Equal volume of oil samples were subjected to different concentration of extract treatment (0.2ml,0.4ml, 0.6ml, 0.8ml, 1.0ml using syringe. These oil samples were equally divided into two groups SS and SR. Group SS was stored under the sun and group SR  was stored in the room for three weeks. These treated oil samples were analyzed on weekly basis at two different parameters: Acid value (AV) of free fatty acid and thiobarbituric acid (TBA) value, using standard methods. The main effect of extract was determined using ANOVA. For the two varieties of oil, the acid value of free fatty acid increased significantly (P<0.05) as the period extends for group SS without extract while those for group SR showed no significant increase. But AV of oil samples treated with higher extract concentration decreased significantly (P<0.05) for both groups SS and SR. TBA value also showed the same trend of AV. Hence, monodora myristica extract yielded reducing effect in the oxidative level of the oil varieties.  

CHAPTER ONE

INTRODUCTION

Lipid oxidation is one of the major reasons that food deteriorate and is caused by the reaction of fat and oil with molecular oxygen, leading to off-flavours that are generally called rancidity(Basturk et al., 2007). Exposure to light, pro-oxidants and elevated temperature will accelerate the reaction (Kubow, 2009). Lipid oxidation and resultant flavour impairment has seriously limited the storage potential of most fat containing foods (Ihekoronye and Ngoddy, 1985). Rancidity covers a wide range of biological activities where the effect is to “make things worse” and thus adversely affect man’s economy. Free radicals and microorganisms are known to cause chemical characteristics that lead to oxidation and deterioration in quality of vegetable oils derived from the seeds or fruits pulps of plants (Basturk et al, 2007). The keeping quality of the oils is basically dependent on their chemical compositions, for instance, the percentages of the degree of unsaturation. Rancidity is associated with off-flavour and odour of the oil. There are two causes of rancidity. One occurs when oil reacts with oxygen and is called oxidative rancidity. The other cause of rancidity is by the combination of enzymes and moisture. Enzymes such as lipase liberate fatty acids from the triglyceride to form di and/or monoglycerides and free fatty acids and such liberation of fatty acid is called hydrolysis, hence hydrolytic rancidity.

The oxidation of fats is an important deteriorative reaction with significant commercial implications in term of product value. The initial oxidation products that accumulate are hydroperoxides, which may subsequently break down to form lower-molecular weight compounds such as alcohols, aldehydes, free fatty acids and ketones, leading to autoxidative rancidity. The peroxide content present in alimentary fats attests to its state of primary oxidation and thus its tendency to go rancid. Unsaturated fatty acids, in fact, react with oxygen forming peroxides, which determine a series of chain reactions whose end result is volatile substances having the characteristic smell of rancidness. These reactions are accelerated by high temperatures and by exposure to light and oxygen (Yildiz et al., 2002). The lower the peroxide and acid values, the better the quality of the alimentary fats and their state of preservation.

Although simple, procedures of  acid value (AV) or peroxide value (PV) determination are cumbersome, destructive to the sample, costly, require potentially hazardous solvents, substantial personnel time, glassware and accurate preparation of reagents and are dependent on a visual endpoint (Ismail et al., 1993; Van de Voort et al., 1994). Oxidation is concerned mainly with unsaturated fatty acids. Oxidative rancidity is of special interest as it leads to the development of off-flavour that can be detected early on in the development of rancidity (Basturk et al., 2007)

Some slight deterioration at least is to expected in any commercial oil-bearing material and is, in fact, inherent in the process by which fat is formed (Morel,1997). In the living plants and animals, fats, carbohydrates and proteins are synthesized in a complicated series of steps with the aid of certain enzymes. These enzymes are capable of assisting the reverse as well as the forward reactions and hence under proper conditions may promote the oxidation and degradation of the very substances that, they have previously been instrumental in synthesizing (Basturk et al., 2007)

Oils in general are known to be susceptible to oxidation and microbial attack. The composition of the various oils determines the extent of oxidation and type of organisms likely to thrive in them (Chow et al., 2000). Several studies have demonstrated that environment factors affect not only the fatty acid composition of vegetable oil, but also, although apparently indirectly, the spatial arrangement of those acids on the triacylglycerol molecule (Tay et al., 2002). Triacylglycerol composition and structure are important in the areas of nutrition, oil stability and possible physiological effects. Palm oil is extracted from the mesocarp of the fruit of the oil palm, Elaeis guineensis. crude palm oil (CPO) has a deep orange-red colour due to the high content of carotenoids and is a rich source of vitamin E consisting of tocopherols and tocotrienols (Nesaretnam and Muhammad, 1999). Both beta carotenes and vitamin E are well known nutritional antioxidants.

Palm oil is known to support the growth of fungi and bacteria especially when it contains moisture (Cornellus, 2001).. Their lipolytic enzymes are so active that even under unfavorable conditions palm oil is seldom produced with a free fatty acid content (FFA) of less than 2% and under favorable conditions of processing, the free fatty acid content of this oil reaches 20%and higher. When the fruit is bruised, lipolytic action occurs and a near maximum FFA (8-10%) is reached within 40 minutes. The FFA of unbruised fruits may increase only 0.2% or less in the course of 4 days (Cornellus, 2001). The exposure in the sun is made under radiations of weak temperatures, varying daily, creating an environment favourable to the chemical and enzymatic reactions of hydrolysis and oxidation (Tan et al., 2002). This study is aimed at examining the oxidative and biodeteriogenic effects of free radicals contaminating the oils from the varieties of the oil palm (Elaeis guineensis) and palm kernel oil and the chemical components of the oils and the effect of solvent extract of ehuru (African nutmeg).

Oil palm is indigenous to the Nigerian coastal area. It was discovered by European explorers in the early 1400’s and was distributed throughout tropical Africa by humans who practiced shifting agriculture about 5000 years ago. The palm plant originated from the jungle forest of East Africa and about 5000 years ago, palm oil was used by the pharaohs for cooking and lighting. The cultivation of oil palm is restricted to the eastern sub zones where its growth is favoured environmentally and climatically. Besides, it is a major cash crop in this region. The first oil palm plantation was established at Sumatra in 1911 and at Malaysia in 1917. About this time it was simultaneously established in West Africa and tropical America.

Over the years, a little attention was paid to the industrial use of palm kernel oil. Nevertheless, recent studies have indicated that apart from their domestic uses that they can be used as engine lubricants, as replacement for biodiesel if their properties are enhanced. Although high in saturated fats, it is a different oil to extract from the nut or kernel of palms which has a yellowish white colour and a pleasantly mild flavor similar to coconut oil in fatty oil acid composition and properties.  Crude palm kernel oil (CPKO) is extracted from palm kernels with palm kernel cake as a by-product. The physical and chemical properties of the various palm oil products have been reviewed by Nesaretnam and Muhammad, (1999).

Monodora myristica is a widespread and attractive small tree with very decorative flowers appearing just before the leaves. The fruit is suspended on a long green stalk with numerous seeds embedded in whitish sweet smelling pulp. The seed is oblong and pale brown when fresh with a thin seed coat and hard kernel (Nesaretnam and Muhammad, 1999). The seed production is seasonal occurring between April to June. The fruits are globular and ovoid; 3-4 inch long and about 3-5 inch diameter. The wood is hard. The seeds are contained in a hard shell and have a very strong aroma . This plant is commonly called Orchid flower tree in English, Ehuru Ofia in lgbo (Okafor, 2003). Monodora myristica is a specie of calabash nutmeg, the edible seeds yield a nutmeg-flavoured oil which is used in West Africa for cooking (Eggeling, 2002). Plants that belong to Annonaceae family are rich in flavonoids and bioflavonoids and are known to have antioxidant activity (Shahidi et al., 2009). Monodora myristica seed extract contains important pharmacological compounds, alkaloids, flavonoids, and vitamins A and E as well as many important lipids; arhinolipids, free fatty acids, glycolipids, phospholipids and sterols. The plant is widely used in ethnomedicine, especially to relieve toothache as well as in the treatment of dysentery. When roasted and ground, the seeds are rubbed on the skin for (unspecified) skin diseases (Irvine, 2000). This suggests that the seeds of Monodora myristica plant could be germicidal or antiseptic. The roasted ground seeds are chewed, then spat into the hand and then rubbed across the forehead to relieve headache. The seeds are also crushed and used as insecticide, while the root relieves toothache when crushed (Ogtinein unet al., 1999). Monodora myristica seeds are also used for the treatment of constipation and as a stimulant (Irvine, 2000). The essential oil from Monodora myristica seed is used in pharmaceutical and dental preparation (Talalaji, 1999).

In this study, we have monitored characteristic parameter, namely acid value and thiobarbituric acid value during storage of palm kernel oil and palm oil at different environmental conditions treated with different concentration of seed extract of Monodora myristica. Whereby, the acid value and thiobarbituric acid value, were assessed by the conventional method and the UV-spectra were registered for each sample. Although simple, procedures of acid value (AV) or peroxide value (PV) determination are cumbersome, destructive to the sample, costly, require potentially hazardous solvents, substantial personnel time, glassware and accurate preparation of reagents and are dependent on a visual endpoint (Ismail et al., 1993; Van de Voort et al., 1994).

1.1   SIGNIFICANCE  OF RESEARCH

The aim and objective of this research is to:

1.  To carryout solvent extraction of Monodora myristica

2. To  investigate the antioxidant effect of Monodora myristica extract on palm kernel oil and palm oil at different environmental conditions.