Effects of Thermal Treatment on The Characteristics Quality of Some Ghanaian Vegetable Oils (Palm, Coconut and Groundnut)

16 Introduction : Vegetable oils contain natural antioxidants and other properties reported to impart 17 anti-diabetic properties when consumed, in animal study. In humans however, these oils are 18 subjected to high temperatures during cooking before consumption. High temperature tends to 19 affect the characteristic quality and potential to impart on health benefits such as antidiabetic 20 properties. The objective of this work was to determine the characteristics quality of vegetable oils 21 after thermal treatment that equates to temperatures oils are subjected to during food 22 processing/cooking. 23 Methodology : Three portions of 200g of each fresh unrefined red palm oil, coconut oil and 24 groundnut oils in three conical flasks T1, T2 and T3 were heated to room temperature 28 o C (T1) 25 to 100 o C in boiling water (T2) and to 200 o C in electric cooker oven (T3) for 10 minutes. Acid, 26 iodine, peroxide, saponification, unsaponification values of the oils were then determined after 27 cooling to room temperature. Results ; Coconut oil heated to 200˚C had the least Acid value of 2.89±0.135 whiles Palm oil 30 heated to 100 ˚C had the highest value of 19.57±0.165. There were no peroxides formed in Coconut 31 and Palm oils at 28 ˚C as well as Palm oil at 100 ˚C. However , peroxides were highest in Coconut 32 oil at 200˚C with value of 15.28±2.315. Saponification value of groundn ut oil at 28 ˚C was the 33 least at 89.52 ± 2.18 and 296.57±1.045 the highest in coconut oil at 200 ˚C. Heating however 34 increased the unsaponifiable matter in all the vegetable oils used. The in terms of acid iodine vale, peroxide value value were retained after one heat treatment. This implies the quality of the oils are during food


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Vegetable oils are triglycerides extracted from plants and termed as plant oils that are mostly liquid 44 at room temperature or fat when solid [1]. Vegetable oils are used for many purposes, mostly for 45 cooking, as fuels, paints, and in skin care product and other pharmaceutical products [2-3-4]. 46 Medical benefits from the consumption of vegetable oils are conflicting. Some of these oils have  oil also contains some stearic acid, arachidic acid, behenic acid, lignoceric acid and other fatty 70 acids [17]. It is believed that the antioxidant carotenes, unsaturation and short chain fatty acids 71 confer the oils their antidiabetic effect [11].  The 10% vegetable oil added to mouse chaw [11] that showed antiglycaemic effect was at room 77 temperature, However, humans normally process these oils during cooking at high temperatures 78 before consumption. This work is aimed at determining quality of the oils after a single heating at 79 different food processing temperatures, this will help determine the mode of application of the oils 80 in human trials.

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Fresh unrefined palm, coconut and groundnut oils were bought from the Ghana Food        Table 3.  The saponification value of oil is the number of mg of potassium hydroxide (KOH) required to 225 saponify 1g of a fat or oil [34]. High saponification value is an indication that oils are normal 226 triglycerides. The saponification value is also an estimation of the molecular weight of the fat or 227 oil, the higher the molecular weight the smaller is its saponification value because larger molecules 228 have relatively fewer number of carboxylic functional groups per unit mass of the fat.

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Saponification value also indicates the carbon chain length of the acid present in the oil or fat, the 230 higher the saponification value, the greater is the percentage of the short chain acids present in the 231 glycerides of the oil or fats . High saponification number is also an indication of high degree 232 of unsaturation in an oil sample.

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The saponification value of coconut oil was significantly higher than that of groundnut and palm 234 oils, (Table 4) this is possibly because of the higher amount of saturation and shorter chain length 235 (lauric acid (C12:0) and or smaller molecular which confers the property of having a higher 236 saponification value [36]. Even though groundnut oil has the higher unsaturation than the other 237 oils but because of its high chain length and or molecular weight (mainly composed of lenoleic 238 (C18:2) and arachidic acid (C20:0)) therefore rather has the lowest saponification value. However, 239 the saponification values of the oils were significantly increased after thermal treatment  Unsaponifiable are components of an oily (oil, fat, wax) mixture that fail to form soaps when 244 treated with sodium hydroxide (lye) or potassium hydroxide. Unsaponifiable constituents are an 245 important consideration when selecting oil mixtures for the manufacture of soaps.

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Unsaponification values were significantly increased across the three thermal treatments and were 247 higher for palm oils as compared to the other oils (Table 5).

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There were no significant changes of acid value in groundnut oil, palm oil and coconut oil at higher 250 temperature compared to the room temperature value, hence there was no significant degree of 251 degradation of the oil quality resulting from hydrolysis of triacylglycerols of the oils as a result of 252 heating. The Iodine value which is a measure of the degree of unsaturation in the oils was not 253 significantly changed, therefore these oils are less susceptible to oxidation at high temperatures.

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Palm oil and coconut oil had low peroxide value at room temperature, no significant change at 255 100 C but a significant increase at 200 C . Groundnut oil on the other had comparatively had 256 significant peroxide at room temperature and much higher values at the higher temperatures. The 257 saponification value of coconut oil was significantly higher than that of groundnut and palm oils 258 possibly due to its comparative higher unsaturation and smaller molecular weight. The overall 259 quality of the oils was not significantly changed in just one heating.