Physicochemical properties and fatty acid composition of freshly prepared palm oil

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INTRODUCTION
Elaeis guineensis (oil palm) is a monocotyledon belonging to the genus Elaeis.The palm fruit has an outer edible layer known as the mesocarp, an inedible layer of shell and an edible kernel inside the shell (Naiyana, 2003;Edo et al., 2022).It is mostly found in Southeast Asia, especially Malaysia and Indonesia which constitutes about 85% of the total world production (Sime et al., 2014).It is native to many West African countries where local population has used its oil for culinary purposes (Naiyana, 2003) and for production of soaps, detergents, pharmaceutical products and cosmetics (Yeong et al., 2012).It is medicinally used in approximately 35 out of Corresponding author.E-mail: ndididence@gmail.com.
Author(s) agree that this article remain permanently open access under the terms of the Creative Commons Attribution License 4.0 International License Africa's 48 countries and it is used in the treatment of infection and digestive system disorder (Gruca et al., 2014).Palm oil thrives well in many African countries including Nigeria, Ghana, Benin, Liberia, Angola and Congo.Rafiu et al. (2022) noted that crude oil contains the highest concentration of agriculturally derived carotenoids when compared with other vegetable oils.It is known to be among the richest plant source of vitamin A. Sumathi et al. (2008) researched on the utilization of oil palm as a source of renewable energy, revealed that huge quantities of E. guineensis by-products are developed to produce value added products such as methane gas, bio-plastics, organic acids, activated carbon and bio-compost.They added that the biodiesel obtained from E. guineensis is degradable, non-toxic and has significantly fewer emissions than petroleum-based diesel (petrol diesel) when burned.Palm oil is now the most widely used oil in the world.
Physicochemical properties are referred to as the physical and chemical properties of a substance and these include refractive index, acid value, saponification, specific gravity, free fatty acid, viscosity, peroxide value, iodine value, cloud point, flash point, melting point and boiling point.They are qualitative properties of palm oil and do not indicate the position of bonds or the amount of olefinic carbon, but rather it provides an overall status of unsaturation level of oil.These parameters are very useful in evaluating the quality of the palm oil (Tulan et al., 2009).Peroxide value is used as a measure of the extent to which rancidity (incomplete oxidation or hydrolysis of fat) has occurred during storage.It can be used as an indication of the quality and stability of fats and oil (Tulan et al., 2009;Mac-Arthur et al., 2021).It increases with storage time and temperature.
Due to the ever-increasing utilization and demand for palm oil for domestic and industrial purposes like production of paint, soap, pharmaceuticals for local and for export, coupled with a decline in the production of commonly used source of oil in Nigeria, palm oils are produced on large scale for immediate and future use (Edo et al., 2022).Since palm oil processing is a major source of income and employment to a wide area of population in Africa especially in Nigeria, it has been observed that the red colour of the harvested palm fruit begin to change to dark brown as the fruit deteriorated (Mac-Arthur et al., 2021).Also edible vegetable oils are prone to quality deterioration through oxidation and microbial degradation resulting in nutritional loss and offflavors (Mac-Arthur et al., 2021).The quality of the oil depends on the deterioration degree of harvested fruit.The degree of deterioration contributes to the formation of oxidation products that are reactive and toxic, which ultimately pose health risks inflammation (Tulan et al., 2009;Ruswanto et al., 2020), thus the need to investigate the effect of deterioration of the harvested palm fruit on the physicochemical properties and fatty acid composition of freshly prepared palm oil.

Sample collection
The palm fruit sample was collected from Elele Community in Ikwerre Local Government Area of Rivers State, Nigeria.The coordinates of Elele community is 5.6663°N and 6.4948°E.The plant was identified by Dr. Green of Plant Science and Biotechnology Department of Rivers State University, Port Harcourt.

Sample preparation
The harvested palm fruit was divided into four and labeled (1 [fresh], 7, 14 and 21) based on the number of days the fruits were allowed to stay before processing to obtain the oil.At the end of each period of deterioration, the fruits were boiled and pound traditionally.The pound fruit mash was then transferred to an open bowl and washed carefully with warm water and squeezed manually to obtain a red viscous fluid which was heated (temperature) for 30 min for traces of water to evaporate and finally sieved using metal basket to obtain a clear red palm oil.The processed palm oil was stored in an airtight plastic container at 25°C.

Determination of quality indices
The acid values, free fatty acid, iodine value, peroxide value, saponification value, refractive index, specific gravity, viscosity and free fatty acid profile were determined using the AOAC (2012) as summarized subsequently.

Determination of physical properties
Melting, cloud, flash, and boiling points were determined according to the method described by Pike (2003).

Determination of acid value
Twenty-five milliliters of diethyl ether was mixed with 25ml alcohol and 1 ml phenolphthalein (1%) and neutralized with 0.1 M NaOH. 10 g of oil was dissolved in the solvent and titrated with aqueous 0.1 M NaOH, shaken until pink color which persisted was obtained.

Determination of iodine value
Palm oil was poured in a beaker and weighed; the weight was calculated by dividing 20 by the highest expected iodine value.10 ml of carbon tetrachloride was added to the oil and dissolved, 20 ml of Wiji's solution was further added and kept in the dark for 30 min.A 15 ml solution of potassium iodine was then added with water (100 ml) and mixed.The solution is then titrated (liberating the iodine) with 0.1 M sodium thiosulphate using starch as an indicator.A blank sample was also prepared with 10 ml of carbon tetrachloride, the sample result (V 1 ) and that of blank (V 2 ) was recorded.
Iodine Value = (v2 -v1) × 1.269 / Weight of the sample used If (V 2 -V 1 ) is greater than, V 1 / 2 will be repeated using a smaller amount of sample.

Determination of peroxide value (PV)
One gram of oil was weighed into a clean dry boiling tube, 1 g of powdered potassium iodide and 20 ml of solvent mixture (2 volume of glacial acetic acid + 1 volume of chloroform) was added.The resulting solution was then placed in a water bath for 30 s and allowed to boil vigorously; when this is done it was poured quickly into a flask containing 20 ml of potassium iodide solution (5%).The sample solution was then titrated with 0.002 N sodium thiosulphate solution using starch as an indicator.

PV = 2 × V
where V= volume of sodium thiosulphate used, 2 = (N × 1000) / W, N=normality of sodium thiosulphate used, and W = weight of sample used.

Determination of saponification value
The sample (2 g) was weighed into a conical flask and 25 ml of alcoholic potassium hydroxide solution was added, the mixture was then heated in boiling water for 1 h.
Potassium hydroxide (KOH) was then titrated with 0.5 M HCl using phenolphthalein as an indicator.A blank sample was also prepared and back titrated accordingly.The sample and blank titers (V 1 and V 2 ) were recorded.

Determination of specific gravity
Fifty milliliters pycnometer bottle (manufacturer, model) was washed thoroughly with detergent, water, petroleum ether and dried and weighed, the bottle was then filled with water and weighed.After that it was filled with the oil sample and weighed.

Determination of refractive index
Abbe refractometer was reset with a light compensator; the oil sample was then seared on the lower prism of the Abbe refractometer.Light was passed by means of bangle mirror which reflected in the form of a dark background.The telescope tubes were adjusted until the black shadow appeared central in the crosswire indicator; it is then read.

Determination of viscosity
The viscometer (manufacturer, model) was first calibrated using water as the sample at 26 to 28°C.For each sample 200 ml of the oil was measured into a 250 ml beaker and the rotating spindle of the viscometer was immersed in it.The viscosities of the samples were then read from the monitor of the viscometer.

Fatty acid profile
Five hundred milligrams of the sample was weighed in a 25 ml Nwachoko et al. 11 centrifuge; 2 ml of water was added and mixed to dissolve.It was then kept at room temperature for 15 min.5 ml of internal standard (C11:0 FAME + C13:0 TAG) was added at 2 mg/ml in methyl tertbutyl ether and 5 ml of 5% (w/v) methanoic sodium methoxide solution was added also.The tube was then closed and vortexed for 10 s.
After 10 s (time starts when sodium methoxide is added), 2 ml of hexane was added and at 210 s 10 ml of neutralization solution (10% disodium hydrogen citrate / 15% sodium chloride in water).It was shaken gently using vortex mixer and centrifuged at 1,750 rpm for 5 min.200 µl of supernatant was transferred into 10 ml flask and diluted to mark with hexane.

Smoke, flash and fire point
Ten milliliters volume of oil was poured into an evaporating dish, a thermometer was suspended at the center of the dish without it touching the bottom of the dish, and the temperature was gradually raised using a hot plate.
The temperature at which the sample gives off a thin blush smoke continuously is noted as the smoke point; similarly the temperature at which the oil started flashing without supporting combustion is noted as the flash point, the temperature at which it starts to support combustion is known as the fire point.

Statistical analysis
Data obtained were subjected to statistical analyses, mean, standard deviation and one-way analysis of variance to determine the significant difference among the days and compared at 95% confidence limit using SPSS program version 17.

RESULTS AND DISCUSSION
Table 1 shows the effect of days (day 1 [fresh], 7, 14 and 21) of palm fruit deterioration on the physicochemical properties of freshly prepared palm oil.The result shows that the acid value, peroxide value, flash point and boiling point of the sample were above National Agency for Food and Drug Administration Control (NAFDAC) permissible limit while the free fatty acid, saponification value, iodine value and melting point were below NAFDAC permissible limit.The values for days 14 and 21 were significantly (p<0.05)higher when compared with the value for day 1.Acid value is a measure of the free fatty in palm oil and it is used to estimate the amount of palm oil that is lost during refining process (Enyoh et al., 2017).From the table, it was observed that the acid value of the oil sample increased as the number of days increased and Free fatty acid is one of the most important quality parameters in edible palm oil.The amount of free fatty acids in palm oil is an indicator of the quality of the palm oil and high level of free fatty acids is an indication of lipid oxidation (Akinola et al., 2010).The free fatty acid value is half the acid value and for this study, it ranged from 1.96±0.07 to 3.09±0.14mg KOH/g.and it increased as the days increase.The value for day 21 was significantly higher as compared to the values for days 1 and 7.However, the values obtained were below NAFDAC limit, which indicates that the level of oxidation of the palm fruits at the time of processing was low (Akinola et al., 2010).The increase in the free fatty acid (FFA) values as the day increases are in tandem with the findings of Taage et al. (2012) who reported that FFA values of processed palm oil could be influenced by the degree of deterioration.
Peroxide value (PV) is a useful indicator of the early stages of rancidity during storage of palm oil.It can be used to check the presence of unsaturation and to determine autoxidation (oxidative rancidity) as peroxides are intermediates of the autoxidation reaction (Onyeka et al., 2005).The degree of deterioration of the fruit before processing the oil affects its taste, odor and colour due to oxidative rancidity.The result showed that the palm oil from the palm fruit processed immediately after harvest (day 1) had a significant low PV when compared with other days (7, 14 and 21).The values obtained were all above the NAFDAC permissible limit (10) mEq/kg.High peroxide value is a definite indication of high level of unsaturation and susceptibility to oxidative rancidity (Enyoh et al., 2017).The Iodine value is a measure of the degree of unsaturation (C=C) in relation to the amount of fat or oil (Hasan et al., 2016).The result showed that iodine value of the oil processed immediately (day 1) was significantly high when compared with other days (7 and 14).Iodine value of day 1 oil (50.13±2.21) is within the NAFDAC recommended range of 50 to 55, while the values for other days were below the range.The value for day 21 is similar to that reported by Akinola et al. (2010) and Rafiu et al. (2022).These low values suggest that the oil has low level of unsaturation and might not be susceptible to oxidation (Hasan et al., 2016).
Refractive Index (RI) measures how much light bends when travelling through oil and it also assess the purity of the palm oil.RI is mostly used as a quality control measure (Sarkar et al., 2015).It helps to indicate the possible chances of spoilage and higher RI indicates rancidity in palm oil (Sarkar et al., 2015).The values obtained showed no significant difference for all the days and the values for days 1, 7 and 14 were within the limit.The significant decrease observed during the early stages of processing (days 1 and 7) could be as a result of the degree free fatty acids and oxidation (Akinola et al., 2010).
Viscosity is a measure of internal friction of the oil molecules.Relative high viscosity of palm oil may be due to intermolecular attraction between the long chain structures of the triacylglycerol molecules.The result revealed that the viscosity of the oil sample immediately after harvest was significantly higher as compared to the values obtained after 7, 14 and 21 days of deterioration.
Specific gravity also known as Relative Density is one of the most basic bio-diesel feed stock properties because of its correlation with heating value (Kenechi et al., 2017).The result of this findings showed that the specific gravity of the first two days of processing (days 1and 7) were within the NAFDAC acceptable limit of 0.90 -0.92 kg/m 3 while the later (14 and 21 days) values were higher than the standard.High specific gravity of oil is an indication of the level of spoilage and adulteration of the palm oil (Kenechi et al., 2017).
Cloud point is the temperature at which one component of a mixture of liquid begins to solidify on cooling, resulting to invisible cloudiness.The cloud point is closely related to degree of unsaturation as increase in the degree of unsaturation of a sample results to a decrease in the cloud point of the given sample (Khalid et al., 2011).The degree of the cloud point of the sample for all the days except day 7 was above NAFDAC limit of 10°C The Flash Point is the temperature at which the decomposition products produced from frying oils can be ignited.The flash points of the sample in all the days were greater than the NAFDAC range (250°C) for flash point of palm oil (Khalid et al., 2011).
Table 2 shows the effect of deterioration period on the fatty acid component of freshly prepared palm oil.Fatty acid composition is peculiar to each oil sample because it affects the physicochemical properties of the oil sample (Aremu et al., 2015).Lauric acid also known as Dodecanoic is a medium chain saturated fatty acid with 12-carbon atom chain.It can be metabolized into ketone bodies, which are important energy source for extrahepatic organs in the body, such as the brain, heart and muscle (Sodamade et al., 2013).The result showed that oil for day 7 had significant (p<0.05)increase in the composition of lauric and Myristricacid when compared with oils for other days.Myristic acid is a long chain saturated fatty acid with 14 carbons and it accumulates fat in the body.Moderate consumption of myristic acid improves long-chain omega-3 fatty acids level in plasma phospholipid, which could exert improvement of cardiovascular health parameters in human (Sodamade et al., 2013).
Palmitic acid (C16:0) is a saturated fatty acid and it is a principal constituent of refined palm oil.It is the most common saturated fatty acids in the human body and it represents 20 to 30% of total fatty acids in membrane phospholipids and adipose triacylglycerol.It helps in formation of cell membranes, lung secretion and signaling molecules, while also storing and utilizing energy within cells and modifying proteins (Arzoo and Bakeet, 2014).The result revealed that the composition of palmitic acid for day 14 oil was significantly higher when compared with other days.
Stearic acid (C18) also called Octadecanoic is a saturated straight chain fatty acid consisting of 18 carbon atoms without double bonds.Stearic acid has been shown to have a neutral effect on blood total and lowdensity lipoprotein (LDL) cholesterol levels (Arzoo and Bakeet, 2014).The result revealed that the value for day one oil was significantly (p<0.05)higher as compared to the values for other days.Oleic acid (C18:1) is a monounsaturated omega-9 fatty acids (Sodamade et al., 2013).Excess of oleic acid blocks the body from using linoleic acid.From the Table 2, the value for day 21 oil was significantly different when compared with other days.
Linoleic acid also known as omega-6 is a polyunsaturated omega-6 fatty acid.It possesses two more double bonds and lacks several hydrogen atom that are found in saturated fatty acids and it is an important constituent of neuronal membrane phospholipids (Arzoo and Bakeet, 2014).Linolenic acid is an omega-3 polyunsaturated acids which helps to lower blood pressure and also reduce serum triglyceride (Srilakshmi, 2008).The result showed that the values of C18:2 (linoleic acid) and C18:3 (Linolenic acid) for day 1 oil was significantly different than the oil of other days, the values showed that C18:3 decreased as the fruit deteriorate.

Conclusion
The result reveals that the acid value, peroxide value, flash point and boiling point of the sample were above NAFDAC permissible limit while the free fatty acid, saponification value, iodine value and melting point were below NAFDAC permissible limit.The result also shows that most physiochemical properties (such as acid value, free fatty acid, refractive index, specific gravity, cloud point and boiling points) increased in days 14 and 21 oil, thus it could be concluded that allowing the palm fruit to deteriorate after harvest beyond seven days could reduce the quality of oil.

Table 1 .
The effect of palm fruit deterioration on the physicochemical properties of freshly prepared palm oil.
Rafiu et al. (2022)0)ere above NAFDAC permissible limits.High acid value could imply poor refining of the oil, thus lowering the palm oil quality.Also, the values reported in this study were higher than that reported byAkinola et al. (2010)andRafiu et al. (2022).

Table 2 .
The effect of storage duration on the fatty acid component of freshly prepared palm oil.