Evaluation of the Effects of processing on Physicochemical Properties of the Freshly Extracted Crude Groundnut Oil

doi:10.21203/rs.3.rs-2635063/v1

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Evaluation of the Effects of processing on Physicochemical Properties of the Freshly Extracted Crude Groundnut Oil

https://doi.org/10.21203/rs.3.rs-2635063/v1

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The effect of processing on physicochemical properties of freshly extracted crude groundnut oil was determined. Crude groundnut oil was extracted following the traditional method and half of it was processed as done industrially. Chemical properties [acid value, free fatty acid, peroxide value, saponification value, ester value, iodine value, total phenol (TP), p-Anisidine value, total tocopherol (TT), and total antioxidant capacity (TAC) and thiobarbituric acid reactive species (TBARS)] and physical properties (refractive index, surface tension, smoke point, flash point, viscosity and specific gravity) were determined following the AOAC and other standard methods. Analysis of the results indicated that the processing of the crude groundnut oil, although improved the physical appearance and reduced free fatty acid by 56% hence, could improve its stability but it also compromised the levels of antioxidant compounds such as tocopherols in the oil that reduced by 16.4%. TAC, an antioxidant parameter also reduced by 92% while TBARS increased by 15.1%. It can be concluded that processing of groundnut oil could either improve its stability or compromise it, hence, there is need for the stability study of processed groundnut oil.

Groundnut oil

p-anisidine value

Physicochemical

Total tocopherol

Thiobarbituric acid reactive substances

Groundnut, or peanut (Arachis hypogaea Linn), commonly called the poor man's nut is one of the most important cash crops in the world [1]. It is an important food crop, the third most important source of vegetable protein and the sixth most important oilseed crop [2]. It contributes significantly to the protein in the diets of people in many developing countries [1]. It contains 48–50% of oil and 26–28% of protein and is a rich source of dietary fibre, minerals, and vitamins which is evident in its high caloric value [1]. Worldwide, groundnut production is principally dedicated to the production of vegetable oil (49%) and the cake obtained as well as whole nuts (41%) are used as human food and also as feed for animals [1].

Groundnut oil is widely consumed domestically and all over the world. Studies have shown that groundnut oil contains more potassium than sodium and is a good source for calcium, phosphorus and magnesium [3]. It also contains thiamine, vitamin E, selenium, zinc and arginine [3, 4]. Research has shown that diets high in groundnut oil are as effective as olive oil in preventing cardiovascular diseases [4]. The report of the US Department of Agriculture released in 2015 showed that the world vegetable oil consumption has increased from 151.68 to 177.16 million metric tons from 2011/2012 to 2015/2016, respectively [5]. It also projected that it will keep on increasing because of increase in world population. The prices released by the World Bank in 2020 for groundnut, coconut, palm kernel, palm oil and soybean oil are $1493, $993, $955, $835 and $874 per metric ton, respectively [6]. Hence, adequate protection must be given to vegetable oils reported to be barely enough for the world population that is growing astronomically [7].

Vegetable oils are known to be susceptible to lipid auto-oxidation reaction. This reaction is a degradative free radical chain reaction that causes quality loss in food industries, oxidative stress and its attendant diseases such as diabetes, atherosclerosis and cancer [8]. The free radicals generated in this reaction have also be reported to damage macromolecules such as DNA and the cellular membrane [9].

The processing of crude vegetable oil is important due to the presence of some undesirable components of the oil such as lecithin, gum, protein, gossypol and so on. These components are known to cause sedimentation in vegetable oils, making the oil to be cloudy or outright formation of precipitate, hence, making them unattractive and unsuitable for some uses such as in salad dressing. Some of these unwanted constituents such as gossypol could also be toxic [10]. Processing although makes vegetable oils attractive and improves their usage, it has been reported that it could compromise the shelf-life of the processed oil in some vegetable oils [11] and improved it in others [10].

The processing of vegetable oil involves conditions such as heat, steaming, neutralization, decolourization and so on. Some of these conditions are well known to enhance lipid auto-oxidation. As a result of the aforementioned, it has become imperative to examine the effects of processing on groundnut oil with the aim of making recommendations on how to prevent or at least reduce this destructive reaction in groundnut oil, hence this study.

Materials

All the reagents used in this study were of analytical grade and were sourced from BDH Chemicals Ltd, Poole, UK and Sigma Aldrich chemicals Co., St Louis, Mo, USA.

Groundnut was purchased from New Market, Ile-Ife, Osun State, Nigeria.

Methods

Extraction of Groundnut Oil

The extraction of the groundnut oil was done following the traditional method. The groundnut seeds (17.6874 kg) were roasted in a big pan with firewood as the source of fire for 30 minutes. The roasted groundnut seeds were immediately bagged in air tight sack bags for 24 hours. The roasted groundnut seeds were then de-hulled (removal of seed coat) manually and handpicked to separate nuts of good quality from the bad ones. The good nuts were pulverized using a locally fabricated mill (Lawood Metals, Osogbo, Osun State, Nigeria). Water (1000 mL) was added to the groundnut paste obtained and the mixture kneaded manually. The oil content oozed out during the process of kneading. The crude groundnut oil (5.2 L) obtained was scooped out and stored in the dark inside an amber bottle prior to further analysis. Half of the crude groundnut oil (2.6 L) was processed according to the methods described by AOAC [12].

Chemical Parameters

The chemical parameters (Acid, iodine, peroxide, saponification and ester values) were determined according to the methods described by Association of Official Analytical Chemists [13] methods (AOAC 936.16, AOAC, 920.158, AOAC, 965.33 and AOAC, 936.15, respectively). The ester value was obtained as the difference between the saponification value and the acid value [13].

Determination of Antioxidant Parameters

The following antioxidant parameters were determined in the oil samples using standard methods:

Determination of total tocopherol

Total tocopherol (TT) content of the oil samples was determined spectophotometrically using the method of Contreras - Guzman and Strong [14].

Determination of total phenol

Total phenol (TP) was extracted from the crude and processed oils and determined as described earlier [3]. The total phenol content was standardized against gallic acid and expressed as mg/kg gallic acid equivalent (GAE). The linearity range of the gallic acid standard was 0–40 mg/L GAE (R² = 0.9795).

Determination of Thiobarbituric acid reactive substance (TBARS)

This parameter was determined by the spectrophotometric method [15]. The TBARS was standardized using 1, 1, 3, 3-tetraethoxy propane (TEP) and expressed as mg malonaldehyde (MDA) kg^− 1 oil. The linear range for this assay was between 0–2.0 µM TEP (R² = 0.998).

Determination of p-anisidine value

The p-Anisidine value (p-AV) was determined spectrophotometrically using the IUPAC method [16].

Physical Parameters

The moisture content of the oil samples was determined using AOAC method [13]. Sample (2.0 g) was weighed in triplicate into a pre-weighed crucible and then dried in an oven at 105 ^oC until a constant weight was obtained after cooling. The difference in weight of the crucible with sample before and after drying was recorded as weight of moisture. The refractive index and specific gravity (SG) of the oil samples were determined by AOAC method [13]. The surface tension was done by counting the number of drops (n) formed by equal volumes of oil sample and water using vertically held pipette [13]. The viscosity measurement on the other hand was performed using a viscometer [13] and it was determined at various temperatures (30, 50, 70, 90 and 100 ºC). The time of flow of the oil sample under gravity through the capillary of the viscometer was also measured at a known temperature and the viscosity calculated as reported by AOAC [13]. Smoke point was determined according to the method reported earlier [17]. The temperature at which smoke started coming out from the oil sample was recorded as the smoke point. The flash point was taken as the temperature when the vapour from the oil was ignited by the naked flame.

Statistical Analysis

The results of this study were expressed as mean and standard deviation of triplicate analysis. Data were subjected to one-way analysis of variance using GraphPad InSat software (GraphPad InSat Inc. 11452El Camino Real, # 215, San Diego 92130, USA) to estimate the levels of significant difference by performing unpaired t-test, and were considered significant at P ≤ 0.05. Pearson correlation coefficient was also used to determine the correlation between peroxide value (PV) and each of the antioxidant parameters (TP, p-AV, TT, TAC and TBARS) and considered significant at P ≤ 0.05.

Chemical Properties of Groundnut Oil

The chemical properties of the oil samples were investigated to know the effects of processing conditions on the shelf life of the oil samples obtained.

Moisture Content

Moisture content of plant food plays an important role in determining whether a sample will have long shelf-life or not because moisture (as water activity, a_w) enhances some chemical reactions such as Millard reaction – a reaction between protein and reducing sugars as well as auto-oxidation in lipids.

The results of the moisture content of the samples are presented in Table 1. The values are 7.31 ± 0.2 and 1.63 ± 0.2% for crude and processed oils, respectively. These values are significantly different (p ≤ 0.01). It has been reported earlier that a very high or low moisture contents may compromise the quality of groundnut oils [18]. Zehra et al. [19] reported a decrease in the oxidation of their flavoured olive oil with decrease in water activity. The same observation was reported by Oyem [18] who observed a progressive decrease in the formation of free fatty acids with decrease in water activity of crude palm oil stored for 21 days. These results showed that the processing of groundnut oil will give it a better shelf life than the crude groundnut oil.

Table 1

Chemical Properties of Crude and Processed Groundnut Oil
Parameter	Crude Oil	Processed Oil
Moisture Content (%)	7.31 ± 0.2^b	1.63 ± 0.2^a
Acid Value	3.36 ± 0.23^b	1.49 ± 0.13^a
Free Fatty Acid (%)	1.69 ± 0.12^b	0.75 ± 0.07^a
Peroxide values of (meq / kg oil)	2.00 ± 0.01^a	10.0 ± 0.01^b
Saponification value (mg KOH/g)	266.94 ± 0.12^b	207.14 ± 0.25^a
Ester value (mg KOH/g)	263.58 ± 0.25^b	205.65 ± 0.12^a
Iodine value (I₂ g/100 g oil)	89.63 ± 2.89^a	103.21 ± 2.93^b
Total Phenol (mg GAE/g)	3.09 ± 0.04^a	3.83 ± 0.05^b
p-Anisidine	1.650 ± 0.025^a	11.290 ± 0.228^b
Total Tocopherol (mg of α-tocopherol/kg)	35.02 ± 1.13^b	29.26 ± 0.92^a
Total Antioxidant Capacity(mg AAE/g)	1.315 ± 0.015^b	0.101 ± 0.012^a
Thiobarbituric acid reactive species (mg / kg)	1.325 ± 0.021 ^a	1.525 ± 0.021^b
Results are means of triplicate determination ± standard deviation.
Data in the same row followed by the same superscript letters are not significantly different at the 5% probability level.
GAE, KOH and AAE are Gallic Acid Equivalent, Potassium Hydroxide and Ascorbic Acid Equivalent respectively.

Acid value (AV) and Free Fatty Acids (FFAs) Content

The AV indicates the amounts of free fatty acid (FFA) that is present in a vegetable oil. It measures the extent to which triacylglycerol in oil has been decomposed by lipase and other actions such as light and heat. The amount of free fatty acids present is a measure of the quality and stability of the oil [20]. It is well known that FFAs are more susceptible to lipid oxidation leading to rancidity and production of off-odour. The determination of AV is used as a general indication of the edibility of the oils and stability to rancidity. The mean acid value (mg KOH/g oil) and %FFA are presented in Table 1. The AV was observed to decrease significantly (p ≤ 0.01) from 3.36 ± 0.23 mg KOH/g oil for crude groundnut oil to 1.49 ± 0.13 mg KOH/g oil after processing which represents 55.7% decrease. The %FFA value of 1.69 ± 0.12% obtained for crude groundnut oil in this study was significantly higher (p ≤ 0.01) than the value 0.98 ± 0.03% reported for crude groundnut oil by Nkafamiya et al. [21], 0.4% reported earlier for groundnut oil [3] and 0.48% obtained also for groundnut [22]. The higher value obtained in this study compared with what was reported earlier in our laboratory could be attributed to the water used for the extraction of the oil (traditional method) as against n-hexane, a non-polar solvent used in Falade et. al., [3]. Water is well known to aid the hydrolysis of lipids leading to the release of their constituent fatty acids. Hence, processing of the oil obtained using traditional method will no doubt improve the shelf life as well as the quality of the groundnut oil.

Peroxide Value

Peroxide value (PV) is a primary oxidation product of lipid oxidation formed from the decomposition of hydro-peroxide. These compounds could react with heme proteins or low molecular weight metals to produce free radicals [23] implicated in the aetiology of some terminal diseases such as cancer and diabetes [24]. Hence, its determination is important.

The results of PV are presented in Table 1. The values are 2.00 ± 0.01 Meq/Kg oil and 10.00 ± 0.01 Meq/Kg oil for crude and processed oils, respectively, which was observed to be significantly higher (p ≤ 0.05) than the crude oil. The value obtained in this study was significantly lower (p ≤ 0.05) than 10.9 ± Meq/Kg oil reported earlier in our Laboratory for groundnut oil (Falade, et. al., 2008) [3] and 10.60 ± 2.27Meq/Kg oil by Jahurul et. al., [25] for noni seed but compared well with 3.82 ± 0.17 Meq/Kg oil reported for desert date kernel oil [26]. The variation in the levels of this parameter across laboratories could be attributed to the extraction conditions, the climatic conditions under which the plants were cultivated as well as the age of the seeds before analysis. The processing of the groundnut oil was also observed to lead to 400% increase in this parameter. For oil to have acceptable storage stability, its PV should be less than 5.0Meq/Kg oil [27]. The processed oil was higher that this bench mark, hence, it may be necessary to use natural antioxidant compound to defend the oil during the processing of the groundnut oil to prevent the increase of this parameter during processing.

Saponification Value (SV)

Saponification Value is an index of average molecular mass of fatty acid present in the oil sample [28]. High saponification values indicate that the fatty acids present in the oils have a high number of carbon atoms. The higher the SV, the higher the fatty acid chain length and the more suitable the oil is for soap making [28]. The results for the SV for both crude and processed groundnut oil are presented in Table 1. The Saponification Value of the oil decreased significantly (p ≤ 0.01) from 266.94 ± 0.12 mg KOH/g for the crude groundnut oil to 207.14 ± 0.25 mg KOH/g after refining. This represents 22.3% reduction in SV due to processing. The reduction in SV is expected because during the neutralisation step of the processing, free fatty acids in the oil were converted to soap using KOH and the soap stock removed, hence, the reduction in this parameter for the processed oil. The values obtained in this study were higher than the range of 188.00–193.25 mg of KOH/g earlier reported for groundnut oils obtained from two varieties of groundnuts [20] and 162.40 ± 0.07 mg KOH/g reported for desert date kernel oil [26] but compared well with 221.50 ± 0.21–220.20 ± 0.20 mg KOH/g reported by Nkafamiya et al. [21] for different species of groundnut oils.

Ester Value (EV)

The ester value is the difference between the saponification value and acid value. The EV is also presented in Table 1. The EV of the oil decreased significantly (p ≤ 0.01) from 263.58 ± 0.25 to 205.65 ± 0.12 mg KOH/g after refining, representing 22% reduction. The value was higher than 184.15 ± 2.02 mg KOH/g and 206.86 ± 0.79 mg KOH/g reported for sunflower and tithonian oils, respectively [29]. The reduction in ester value as a result of the processing could be due to the hydrolysis of some of the intact triacylglycerols by the KOH used during the neutralisation step of the processing leading to the production of soap which was removed from the oil at the neutralisation step.

Iodine value (IV)

This parameter is used for the estimation of the degree of unsaturation of vegetable oils. Oils with high IV are preferred from nutrition and health viewpoints because it indicates that the oils are rich in polyunsaturated fatty acids (PUFA). For example, oils that are rich in PUFA have been reported to reduce heart diseases that are associated with cholesterol [30]. IV is also used to measure the extent of lipid auto-oxidation. The IV is presented in Table 1. The values are 89.63 ± 2.89 I₂g/100g and 103.21 ± 2.93 I₂g/100g for crude and processed oils, respectively. The IV obtained in this study for groundnut was lower than 109.3 ± 8.3 I₂g/100g reported earlier for groundnut [3] but higher than 5.52 ± 0.05 I₂g/100g reported for desert date oil [26]. The 15.1% increase in IV as a result of processing of groundnut oil in this study could be attributed to the removal of some of the constituents of the oil such as lecithin during the degumming stage which is expected to concentrate the remaining constituents of the oil left such as IV. This shows that the processing of groundnut oil will be beneficial to it from health and nutrition viewpoints.

Total Phenol (TP) Content

Phenolic compounds are known to play an important role in stabilizing lipids against peroxidation [31] and inhibiting various types of oxidizing enzymes [24]. The total phenol is a measure of phenolic compounds which may contribute to the antioxidant property. They have been reported to play an important role in human nutrition and health because they protect against several oxidative stress-related diseases such as cardio-vascular disease and some cancers [32]. The total phenol of the oil samples is presented in Table 1. The values are 3.09 ± 0.04 and 3.83 ± 0.05 mg GAE /g for crude and processed oils, respectively. This represents 23.9% increase in the total phenol after processing. The difference in these values between crude oil and the processed oil was significant (p ≤ 0.01). The increase in total phenol after processing could be due to the concentration of the phenolic compounds by the removal of some of the constituents of the vegetable oil such as lecithin, gum, proteins and so on during the processing. The value of this parameter was higher than 0.03 mg GAE /g reported earlier for groundnut [3] but lower than 48.85 ± 0.28 mg GAE/g reported for noni seed oil [25]. This shows that the processed groundnut oil will be better protected against lipid oxidation, hence a longer shelf life compared with the crude groundnut oil, but the value of PV for the processed oil was saying something else because it was significantly higher (p ≤ 0.05) than the crude. A similar observation was reported earlier in our laboratory where the oil of a specie of watermelon seed (Sugar baby) with the highest total phenol recorded the highest PV and the specie with the least total phenol recorded the least value for PV [33]. This shows that other antioxidant compounds such as tocopherol and carotenoid could be contributing significantly to the stabilization of vegetable oil. It was reported earlier that total phenol contributes 30% to the stabilization of vegetable oils [34].

p-Anisidine Value

This parameter measures the content of aldehydes (mainly 2-alkenals and 2, 4 – alkadienals) formed from the decomposition of hydroperoxides, products of lipid auto-oxidation. It actually measures the extent of lipid oxidation.

The values of this parameter are 1.650 ± 0.025 and 11.290 ± 0.228 for crude and processed oils, respectively. This shows that processing of this vegetable oil led to 584% increase in p-anisidine value. The implication of these results is that some of the polyunsaturated fatty acids would have been decomposed, hence compromising the nutritional value and quality of the oil. The value reported here for groundnut oil was higher than 0.86 ± 0.11 reported earlier in our laboratory for crude groundnut oil [31]. The difference in value could be due to different method used in the earlier study. Hexane (a non-polar solvent) was used for the extraction of the groundnut oil as against the traditional method which involves the use of water, a polar solvent used in this study. Water in term of water activity (a_w) is known to enhance lipid oxidation.

Total Tocopherol Value

Tocopherol is the most important lipid-soluble antioxidant; it protects membranes from oxidation by reacting with lipid radicals produced in the lipid peroxidation chain reaction [35]. This prevents the propagation reaction from continuing by removing the free radical intermediate. The total tocopherol values in the oil samples are presented in Table 1. The value for the processed oil (29.26 ± 0.92 mg of α-tocopherol/kg) was significantly lower (p ≤ 0.05) than 35.02 ± 1.13 mg of α-tocopherol/kg obtained for the crude oil. The value of total tocopherol was lower than 43.4 mg/ kg reported earlier for groundnut oil by Falade et al. (2008) [3] and also below the range of 42 mg/kg and 2680 mg/kg reported for coconut oil and wheat germ oil, respectively [36]. The inconsistence in the level of this parameter is expected because tocopherols are not stable; they vary with time in the process of carrying out their function of stabilising vegetable oils. Walker and Slinger [37] had earlier reported a significant loss of total tocopherols of their processed rapeseed oil which is in agreement with our finding. The effect of processing on the total tocopherol content of groundnut oil was opposite of what was observed with TP which was observed to increase after processing. It is well known that tocopherols are natural antioxidants; hence, they are present in vegetable oils to provide vitamin E for human and animal nutrition as well as to defend vegetable oils from lipid oxidation. The role of these compounds in vegetable oil could account for the reduction of total tocopherol of the processed oil. The implication of this is that the processed oil will be more susceptible to lipid oxidation, hence, should be adequately protected with antioxidants.

Total Antioxidant Capacity (TAC)

Total antioxidant capacity is a versatile analytical tool for estimating the antioxidant potential of an oil sample. It is the measure of the amount of free radicals scavenged by a test solution [38]. The total antioxidant capacity of groundnut oil is presented in Table 1. A high TAC value corresponds to high antioxidant capacity. The TAC of the crude groundnut oil 1.315 ± 0.015 mg AAE/g was significantly higher (p ≤ 0.01) than 0.101 ± 0.012 mg AAE/g for the processed oil. The reduction in the TAC for the processed oil could be due to the effect of processing on the oil. Some of the endogenous antioxidant compounds like α-tocopherol present in rapeseed oil have been reported to be lost during the refining of the oil seed [37]. Reduction in total tocopherol was also observed in this study.

Thiobarbituric acid reactive species (TBARS)

TBARS are secondary products of lipid oxidation. These products are very reactive and are believed to be responsible for the initiation of the oxidation of some macromolecules such as proteins, lipids and DNA leading to ageing and carcinogenesis in human [39]. The results of this parameter are 1.325 ± 0.021 and 1.525 ± 0.021 mg/kg for crude and processed oils, respectively. This represents 15% increase in TBARS as a result of the processing of the oil. The implication of these results is that some of the unsaturated fatty acids (PUFA) have be decomposed in to lower chains unsaturated hydrocarbons such as malonaldehyde thereby reducing the nutritional value and health benefits of the vegetable oil.

Physical Properties of Groundnut Oil

Refractive Index

The refractive index of oil has been reported to increase with increase in the number of double bonds [20]. Rudan-Tasic and Klofutan [27], also observed that a high value of refractive index is an indication of high number of carbon atoms. The refractive index of both crude and processed oil samples are presented in Table 2. The results showed a significant decrease (p ≤ 0.01) from 1.687 for the crude oil to 1.682 after processing. This shows that the processing of this oil has caused significant reduction in both carbon chain length and number of double bonds. Falade and Obuseh [33] had earlier reported a linear relationship between refractive index and iodine value. The refractive indices of the samples were higher than 1.464 ± 0.002 earlier reported for groundnut oil [3].

Table 2

Physical Properties of Crude and Processed Groundnut Oil
Parameters	Crude oil	Processed oil
Moisture Content (%)	7.31 ± 0.2^b	1.63 ± 0.2^a
Refractive Index	1.687 ± 0.001^b	1.682 ± 0.001^a
Surface Tension (mNm^− 1)	37.84 ± 0.94^a	39.63 ± 0.02^b
Smoke Point (^oC)	230.00 ± 2.00^b	220.00 ± 1.00^a
Flash Point (^oC)	300.00 ± 1.00^b	280.00 ± 2.00^a
Viscosity (cSt) 30 ^oC	50.010 ± 0.005 ^b	45.779 ± 0.001 ^a
40 ^oC	39.641 ± 0.001 ^b	37.312 ± 0.001 ^a
50 ^oC	31.973 ± 0.004 ^b	25.058 ± 0.004 ^a
60 ^oC	30.083 ± 0.002 ^b	23.707 ± 0.002 ^a
70 ^oC	23.344 ± 0.005 ^b	20.375 ± 0.002 ^a
80 ^oC	18.249 ± 0.008 ^b	13.664 ± 0.005 ^a
Specific Gravity 30 ^oC	0.912 ± 0.012 ^a	0.911 ± 0.011 ^a
40 ^oC	0.910 ± 0.010 ^a	0.909 ± 0.011 ^a
50 ^oC	0.909 ± 0.010 ^a	0.906 ± 0.011 ^a
60 ^oC	0.904 ± 0.001 ^a	0.903 ± 0.011 ^a
70 ^oC	0.903 ± 0.011 ^a	0.903 ± 0.011 ^a
80 ^oC	0.902 ± 0.011 ^a	0.901 ± 0.001 ^a
Results are means of triplicate determination ± standard deviation.
Data in the same row followed by the same superscript letters are not significantly different at the 5% probability level.

Surface Tension

Surface tension is the energy required to increase the surface area of a liquid due to its intermolecular forces. The surface tension values for both oil samples are presented in Table 2. The values are 37.84 ± 0.94 and 39.63 ± 0.02 mNm^− 1for crude and processed oils respectively. These values are significantly different (p ≤ 0.01) from each other. The difference in this parameter between crude and processed oils could be due to the stronger intermolecular forces that might have existed among the molecules of the purified oil over the crude oil which contains impurities that could affect these forces of attraction. It should be noted that low surface tension oil is suitable for use as a releasing agent in baking processes and in making shortenings [28]. This property given above suits crude groundnut oil over processed oil.

Smoke Point and Flash Point

The smoke point is the temperature at which vegetable oil produces smoke. At this point, vegetable oil will give food that is been fried unpleasant taste and odour. The flash point on the other hand is the temperature at which the vapour of the oil ignites. Results of the smoke and flash points of the crude and processed oils are presented in Table 2. The smoke points for the crude and processed oils are 230 ± 2.00 and 220 ± 1.00 ^oC, respectively. The results showed a significant decrease (p ≤ 0.01) as a result of processing of the groundnut oil. The 4.3% reduction in smoke point as a result of processing is expected based on the colligative properties of liquid. Impurity is known to raise the boiling point of a liquid, hence, the removal of the impurities in the oil will account for the reduction in its smoke point. The flash points for crude and processed oils are 300 ± 1.00 and 280 ± 2.00 ^oC, respectively, representing 6.7% reduction. This significant (p ≤ 0.01) reduction can also be explained using colligative properties of liquid given above because; whatever reduces the boiling point of oil will eventually reduce its flash point. Crude vegetable oils generally have a high FFA content with a corresponding high smoke point and consequently, a high flash point [3]. The implication of these results is that processed oil may not be suitable for deep frying; hence, crude groundnut oil will be better for deep frying.

Viscosity

Viscosity of oil is a measure of the oil’s resistance to shear. The viscosity values of the oil samples measured at temperatures between 30 and 80 ^oC are presented in Table 2. The values ranged from 50.010 ± 0.005 to 18.249 ± 0.008 cSt and 45.779 ± 0.001 to 13.664 ± 0.005 cSt for crude and processed oils, respectively. The viscosity of the processed oil was consistently and significantly (p ≤ 0.01) lower than that of the crude oil at all the temperatures used for this study. This can be attributed to the removal of the impurities in the processed oil. Low viscosity has been reported to be crucial for any vegetable oil to be suitable for use in biodiesel because it will allow a high degree of atomization with short ignition delay as well as easy flow through pipes [35]. In order to use this oil for biodiesel, it is important to first process the oil since processing was observed to reduce the viscosity by 8.5% making it to be at par with Olive pomace oil (45.27 cSt) used for making biodiesel [40].

Specific Gravity

The specific gravity of the oil samples at temperatures between 30 and 80 ^oC is presented in Table 2. The values ranged from 0.912 ± 0.012 to 0.902 ± 0.011 and 0.911 ± 0.011 to 0.901 ± 0.001 for crude and processed oils, respectively. Although the values of this parameter reduced consistently with the temperatures used, the reductions were not significant (p ≥ 0.05). The reduction in specific gravity although not significant could be attributed to the removal of some of the impurities (lecithin, gum, proteins, free fatty acids etc.) thereby reducing its weight.

The study reveals that acid value decreased significantly (p ≤ 0.01) [55.7%] due to processing of the crude oil but lead to 400% increase in peroxide value. Saponification value of the oil was also observed to decrease significantly (p ≤ 0.01) after processing, hence, processing of the oil will not be suitable for oil to be used in soap making. However, processing lead to 15.1% increase in the iodine value, and 23.9% increase in total phenol, hence desirable from nutritional and health viewpoints but this can’t be said of total tocopherol which decreased by 16%. On the other hand, processing of the vegetable oil lead to increase in both p-anisidine value and TBARS and decrease in TAC. The processing of this vegetable oil also affected some of the physical parameters such as refractive index, viscosity, smoke and flash points which decreased significantly (p ≤ 0.01). On the other hand, specific gravity decreased marginally but surface tension increased significantly (p ≤ 0.01).

The processing of the vegetable oil seemed to have both advantages and disadvantages, hence, processing of this oil should be based on what the oil is to be used for. In order to protect the antioxidant compounds in this oil such as tocopherol, it is imperative to defend this oil with antioxidant compounds extracted from medicinal plants. This could form the basis for future study.

CONFLICT OF INTEREST

The authors declare no conflict of interest.

ACKNOWLEDGMENTS:

The grant for this study was provided by Tertiary Education Trust Fund (TETFUND) with reference number DVC/37/TETFUND/RP/2014/3. The authors wish to express their sincere appreciation to TETFUND.

Ethical Approval and Consent to Participate

Ethical approval and consent to participate are not applicable in this study because it does not contain experiments using animals or human studies.

Consent for publication

All the Authors have read the manuscript and approved its submission to Plant foods for Human Nutrition for publication. We will also like to declare that no portion of this work has been published elsewhere, either completely, in part, or in another form or under consideration for publication elsewhere.

Authors' contributions

All the authors contributed significantly to the manuscript. Each author’s contributions are as given below:

Conceptualization: [Falade Olumuyiwa Sunday]; Methodology: [Falade Olumuyiwa Sunday]; Formal analysis and investigation: [Afolabi Funmilayo Patricia], [Ogundele Olamide Rachael] and [Raji Rofiat Oyebisi]; Writing - original draft preparation: [Afolabi Funmilayo Patricia and Aderogba Mutalib Adeniran]; Writing - review and editing: [Falade Olumuyiwa Sunday]; Funding acquisition: [Falade Olumuyiwa Sunday]; Resources: [Falade Olumuyiwa Sunday]; Supervision: [Falade Olumuyiwa Sunday and Aderogba Mutalib Adeniran].

Funding

The funding for this study was provided by Tertiary Education Trust Fund (TETFUND) with reference number DVC/37/TETFUND/RP/2014/3

Availability of data and materials

The data and materials used for this manuscript can be accessed from the google drive link provided below. These data were obtained directly from the Laboratory Notes because we have lost the electronic data:

https://drive.google.com/drive/folders/1VV4Bq_ntMocrBQ4GcGuVCZ6KNPqR1iUc

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No competing interests reported.

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Evaluation of the Effects of processing on Physicochemical Properties of the Freshly Extracted Crude Groundnut Oil

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Abstract

Introduction

Materials And Methods

Materials

Methods

Extraction of Groundnut Oil

Chemical Parameters

Determination of Antioxidant Parameters

Determination of total tocopherol

Determination of total phenol

Determination of Thiobarbituric acid reactive substance (TBARS)

Determination of p-anisidine value

Physical Parameters

Statistical Analysis

Results And Discussion

Chemical Properties of Groundnut Oil

Moisture Content

Acid value (AV) and Free Fatty Acids (FFAs) Content

Peroxide Value

Saponification Value (SV)

Ester Value (EV)

Iodine value (IV)

Total Phenol (TP) Content

p-Anisidine Value

Total Tocopherol Value

Total Antioxidant Capacity (TAC)

Thiobarbituric acid reactive species (TBARS)

Physical Properties of Groundnut Oil

Refractive Index

Surface Tension

Smoke Point and Flash Point

Viscosity

Specific Gravity

Conclusion

Declarations

References

Additional Declarations

Status:

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