Thermoxidative Degradation of Commonly Used Vegetable Oils: A Comparative Study

Transcription

1 Journal of Emerging Trends in Engineering and Applied Sciences (JETEAS) 3(6): Scholarlink Research Institute Journals, 2012 (ISSN: ) jeteas.scholarlinkresearch.org Journal of Emerging Trends in Engineering and Applied Sciences (JETEAS) 3(6): (ISSN: ) Thermoxidative Degradation of Commonly Used Vegetable Oils: A Comparative Study 1 C. Andrew; 2 Buba A. A.; 1 A.U. Itodo; 1 E.E.Etim 1 Department of Chemical Sciences, Federal University, Wukari. 2 Department of Chemistry, Modibbo Adama University of Technology, Yola. Corresponding Author: E.E.Etim Abstract The objective of this study was to compare the deterioration of commonly used vegetable oils using one of the three scientific method namely, heating of the oil at high temperature (180 o C) normally used for frying alone and without the presence of material to be fried. This is with the view to comparing the result as carried out using the other method of investigation i.e. under frying condition. The deterioration of the oils under study (Cottonseed and groundnut oils) was evaluated by determining the physical and chemical properties. Among the chemical parameters determined include percentage of Free Fatty Acids (FFA) and Peroxide Value (PV) whereas, physical parameters evaluated include Refractive Index (RI) and Density measurement. The %FFA for both the oils under study showed an increase with an increase in heating time. The values of %FFA for cottonseed oil were higher compared to that of groundnut oil. The refractive index for groundnut oil showed a uniform variation with heating time. While that of cottonseed oil followed in the same manner. The peroxide values of all the oils under study increased with heating time. The findings also revealed that cottonseed oil was more susceptible to deterioration as was characterised by development of a dark colouration than groundnut oil. Keywords: deterioration, cottonseed oil, groundnut oil, %FFA, PV, RI, density INTRODUCTION The term oil is used in generic sense to describe all substances that are greasy or oily fluid at room temperature (Buba, 2005). Generally, fat are considered as extracted triglycerides/lipids that are liquid under the same condition. Fats and oils belong to a larger group of naturally occurring substances called lipids. Because lipids serve as a convenient means of rapid heat transfer, they have found increasing use in commercial frying operations. (Anon 1983). Fats and oils are non-volatile substance insoluble in water but soluble in organic solvent. They constitute along with protein and carbohydrates, the major of food stuffs and are widely distributed in nature. From chemical point of view, oils and fats are products of the reaction between a triol (glycerol) and three molecules of fatty acids. This type of reaction is commonly termed as esterification. Oils/fats are obtained basically from two sources namely: animal and vegetable sources (Snell and Snell, 1992). Animal fats and oils are derived both from terrestrial and marine animals. Marine fats include liver oils etc. Terrestrial fats and oils derived from animals include lard (from Swine), suet (from oxen or sheep), ghee (from cow or buffalo milk) etc. Vegetable oil is found in greatest abundance in fruits and seeds. Oils obtained from plant source are termed vegetable oils. These include: palm oil, cotton seed oil, ground oil, sunflower oil etc. Edible oils from plant sources are of interest in various food and application industries. They provide characteristic flavours and textures to foods as integral diet components (Odoemelam, 2005) and can also serve as a source of oleo chemicals (Morrison et al., 1995). Vegetable oils had made an important contribution to the diet in many countries, serving as a good source of protein, lipid and fatty acids for human nutrition including the repair of worn-out tissues, new cells formation as well as a useful source of energy (Gaydon et al., 1983; Grosso and Guzman, 1995; Grosso et al., 1997; 1999). The groundnut is an herbaceous plant of which there are many varieties, some of which are Boro light, Boro red, Ela, Mokwa, Guta and campala. The fatty acids and physicochemical analysis of the oils of the varieties had been investigated (Anyasor et al., 2009). The quality of groundnut oil depends to a large extend on the oil fraction. The oil content of groundnut differs in quantity, the relative proportion of fatty acids, geographical location, seasons and growing conditions (Adeyeye and Ajewole, 1992). Groundnut seed contains 44 to 56% oil and % protein on a dry seed basis and is a rich source of minerals (phosphorus, calcium, magnesium and potassium) and vitamins E, K, and B group (Savage and Keenan, 1994). Heating is one of the most commonly used methods of food preparation in the home and industries and prolong use of oil for this purpose causes change in 924

2 its physical and chemical properties (Morette and Fett, 1998). The deterioration of several vegetable oils during continuous heating (Andrikopoulos et al., 1989) and after 2 years storage (Andrikopoulos & Demopoulos, 1989) has already been studied and briefly reviewed. The stability of or freedom from the development of rancidity in an oil or fat is one of its most important properties and advance information on it susceptibility towards rancidity permit its quality to be defined. This research work intends to investigate the thermo oxidative degradation of commonly used vegetable oils through measurements of some chemical and physical properties of the oils. This is with the view of comparing the result with degradation under frying conditions. MATERIALS Sampling: Groundnut oil was randomly procured from retailers within Zing town of Taraba State. The cottonseed oil was randomly obtained from Jimeta modern market, Yola, Adamawa state. They were used as received except for the treatment during experimentation. Sample pretreatment entails; i. Heating the oils at high temperature normally used for frying alone and without the presence of material to be fried. Alternative methods include; ii. Heating the oil at frying temperature in the presence of an inert material like cotton. iii. Frying food material yams etc in the oil at frying temperatures. (175 o C- 185 o C) Thermoxidative method by Dobarganes & Perez- Camino, (1988) was adopted with slight modification. About 3.5 litres of the oil (Groundnut and cottonseed) was heated on a hot plate to a temperature of C for 36 hours. Two layers of aluminum foil was made over the frying utensil and three layers of poorly conducting material blanket cloth (insulator) were also made over the foil to retain the heat and also maintain the temperature of the container. A small window was made by cutting a circular shape out of the foil before wrapping the foil and the insulator for dipping thermometer to see that the heating temperature does not exceed and also does not fall below the desired temperature. Samples were taken at an interval of six hours for 36 hours (Dobarganes & Perez-Camino,1988). Evaluation of Chemical and Physical Parameters Chemical and physical parameters used in this current research to evaluate the quality of oils were evaluated following AOAC standard methods as described by Buba (2005). RESULTS AND DISCUSSION This section present the results of values obtained for the determinations described under characterization of groundnut oil during heating at 180 o C for a period of thirty-six hours and this is showed on table 1 below Table 1: Physico-chemical parameters of groundnut oil due to heating at C for thirty-six hours Time of heating (Hrs) %FFA PV Refractive index Density g/cm Keys: FFA=Free Fatty Acid, PV=Peroxide Value Table 1 shows the results of the all the characterization carried out on groundnut oil and the changes or variation in this parameter with heating over a period of 36 hours. The variations are further depicted on figure 1. The FFA for the groundnut oil investigated shows an increase for nearly throughout the heating time. This suggests that reactions, oxidation and hydrolysis which led to the formation of free fatty acids occurred during the heating process. The value of the FFA of unheated oil (fresh oil) was 0.67% and the value shows a high value against that as cited in literature of industrial oils chemicals ( %). The reason can be attributed to the fact that the oil under study is locally processed and also as reported by Pearson, (1970) that genotype, geographical location and weather conditions may have effect on the fatty acid composition of the oil. The importance of initial %FFA can be seen in at least two ways: - FFA content is important during refining where it has a major effect on the final yield of refining oil and - In analyzing the deterioration of the oils during use as indicated by the results reported by Frioriti, (1977) The values of %FFA of the oil investigated generally shows lower values compare to that with frying as reported by Buba, (2005). This would suggest that during frying, the food materials come into contact with the oil at about 180 o C and is also exposed to air so there are a lot of potentials to cause chemical changes in the fat/oil i.e. high contents of free fatty acids products. The fact that the %FFA of the oil under study is lower than that due to frying would equally suggest that greater hydrolysis occurred during the frying condition leading to higher FFA content. The variation in %FFA with heating time indicates that there was a modest increase for 925

3 groundnut oil. This feature would suggest onset decomposition of acid formed to yield other products. Fig. 1: Comparative test analysis of Free Fatty Acid (%FFA) in thermoxidated Groundnut and Cottonseed oils Peroxide Value: The initial peroxide value for groundnut oils very low indicating high purity and freshness. Peroxide value is a measure of the peroxide content of oil, and indirectly, a measure of the degree of rancidity. Fresh oils usually have values less than 10 meq. per kg, and when this value is exceeded, it is an indication that rancidity has set in. (Kirk and Sawyer, 1991). There is a general increase in the peroxide and this would suggest rancidity has occurred during the heating period. would saddest that the average molecular weight of the oil, unsaturation levels are generally higher than that from the literature. But it is very difficult on this basis alone to be conclusive since temperature may play role in this behavior. This is because the relationship between variation in RI and the average molecular weight, unsaturation is complex for example; conjugation also has some effect on RI value. However, there is a uniform variation in the RI of the oil investigated nearly throughout the heating period. Nevertheless, the results of the RI indicate linear relationship with heating and therefore can be used as an index of deterioration. A comparison between the RI of the oil under study with that under frying as reported by Buba, 2005 shows a little variation. This suggest that there is difference in the average molecular weight, and unsatuartion level in both the oils used as these properties has a mark effect on the value of RI. Albi et al. (1997) did not notice any variations in refractive indices during heating olive and sunflower oils at 180 C for 2 h, while significant increases in refractive indices of frying oils have been reported after frying eight successive batches of potatoes (Cuesta et al., 1991) and in discarded oils (Al-Harbi & Al-Kahtani, 1993). It seems that under the conditions of domestic frying, oils do not reach high alteration levels. Fig. 2: Comparative test analysis of Peroxide Value (PV in meq/kg) in thermoxidated Groundnut and Cottonseed oils Refractive Index: Generally, there is a close relationship between the RI of fat/oil and the average molecular weight and the degree of unsaturation. Thus it is reported that the RI of fat is dependent upon its average molecular weight and to a minor degree upon its glyceride structure as well as its degree of unsaturation. The RI of the oil under study generally shows a higher value compare to that obtained from the recommended codex standard. This 926 Fig. 3: Comparative test analysis of Refractive index(ri) of thermoxidated Groundnut and Cottonseed oils Table 2 below shows the results of the deterioration described under characterization of cottonseed oil during heating at 180 o C for thirty-six hours. Time of %FFA PV Refractive Density heating Index g/cm 3 (hours)

4 The FFA shows an increase throughout the period of heating time 36 hours as depicted in figure 1. This suggests that reactions (oxidation and hydrolysis) which lead to the formation of fatty acids occurred during the heating process. A change in the chemical integrity of the triglyceride molecule (deterioration or degradation) is also implied during FFA formation. The initial value of the %FFA of cottonseed oil is higher than the value obtained from recommended codex standards. This may arise as a result of difference in fatty acid composition of the oil used in the literature. Probably, initial primary reaction products are present in the oil used for this study. However, a comparison between the values of %FFA under study to that carried out during frying show that the former have higher values than the later. This would suggest that the composition of the fatty acid of the oil under study is higher than that carried out during frying condition. The refractive index of the cottonseed oil generally shows a uniform variation over the heating period. The RI of the unheated oil was within the range as cited from the recommended codex standards. Generally, presence of primary reaction products, average molecular weight and level of unsaturation of the oil is reported to have close relationship or effect on the refractive index (Boekenogen 1964) CONCLUSIONS The degradation of commonly used vegetable oil (groundnut oil and cottonseed oil) was investigated through measurement of its physical and chemical parameters. Among chemical parameters determined include %FFA and peroxide value. While physical parameters characterized are RI and density measurement. Standard methods of characterization as described in AOCS were used in all chemical and physical determinations. The initial value of %FFA for the oils under study show a little higher value when compared to that cited in the literature of industrial oil and chemicals. Physical changes also occurred as shown by increase in viscosity while heating. Physical changes in colour was also noticed with the oils most especially cottonseed oil. The studies revealed that cottonseed oil showed highest tendency to undergo degradation as indicated by its high value of its %FFA. This will suggest that cottonseed oil is more unsaturated than groundnut oil. Also, as obtained from literature, cottonseed oil contains more percentage of unsaturated fatty acid component; linoleic acid (47%) than groundnut oil (26%) and hence more susceptible to oxidative degradation. The peroxide value (PV) for cottonseed oil under study indicates an increase with heating time. PV is a measure of the peroxide content of oil, and indirectly a measure of the degree of rancidity. The studies also revealed that cottonseed oil was more prone to darkening than groundnut oil as was observed by a sudden change in colour from light yellow to dark colouration. Hence, we can conclude that both heating and frying conditions can be used to study the deterioration of fat/oils. REFERENCES Adeyeye, A. and K. Ajewole, Chemical composition and fatty acid profiles of cereals in Nigeria. Food Chem., 44: Albi, T., Lanzo n, A., Guinda, A., PeÂrez-Camino, M.C. & Leo n, M. (1997). Microwave and conventional heating effects on some physical and chemical parameters of edible fats. Journal of Agricultural and Food Chemistry, 45, Al-Harbi, M.M. & Al-Kahtani, H.A. (1993). Chemical and biological evaluation of discarded frying palm oil from commercial restaurants. Food Chemistry, 48, American Oil Chemist s Society (AOCS) (1997) Official and recommended practices of the American Oil Chemists Society, 5th edn., AOCS Press, Champaign Holaday and Pearson, (1974) Anon Ceral Laboratory Methods, 8 th ed. American Ass. Ceral Chemists, St. Paul, MNAndrikopoulos, N.K., Tzamtzis, V.A., Giannopoulos, G.A., Kalantzopoulos, G.K. & Demopoulos, C.A. (1989). Deterioration of some vegetable oils. I. During heating or frying of several foods. Revue Francaise Des Corps Gras, 36, Andrikopoulos, N.K. & Demopoulos, C.A. (1989). Deterioration of some vegetable oils. II. After two years storage of fried and non fried samples. Revue Francaise Des Corps Gras, 36, Anyasor, G.N., K.O. Ogunwenmo, O.A. Oyelana, D. Ajayi and J. Dangana, Chemical analyses of Groundnut (Arachis hypogaea) oil. Pak. J. Nutr., 8: Buba A. A The use of Spectroscopic, Viscometric and other Techniques in the evaluation of Vegetable oils. PhD Thesis, Federal University of Technology Yola, Nigeria. Boekenogen, H. A Analysis and characterization of oils, fats and afts products. Interscience Publishers, New York. Dobarganes, M.C. & Perez-Camino, M.C Systematic evaluation of heated fats based on quantitative analytical methods. Journal of the American Oil Chemists Society, 65,

5 Frioriti, J.A Measuring Flavour Deterioration of fats, oils, dried emulsions and foods. Journal of the American Oil Chemist Society. 54, Gaydon, E.M., J.P. Bianchini and J. Ratovogery, Triter pene alcohols, methyl sterols, and fatty acid five Malagasy legume seed oils. J. Agric Food Chem., 31: Grosso, N.R. and C.A. Guzman, Chemical composition of Aboriginal Peanut (Arachis hypogaea L.) seeds from Peru. J. Agri. Food Chem., 43: Grosso, N.R., E.I. Lucini, A.G. Lopez and C.A. Guzman, Chemical composition of aboriginal peanut (Arachis hypogaea L.) seeds from Uruguay. Grasasy Aceites, 50: Grosso, N.R., J.A. Zygadlo, A.L. Lamarque, D.M. Maestri and C.A. Guzman, Proximate, fatty acid and sterol compositions of aboriginal peanut (Arachis hypogaea L.) seeds from Bolivia. J. Sci. Food Agri., 73: Kirk and Sawyer, R. Pearson s Chemical Analysis of Food. 8thedition. New York, Churchil livingstone. 1991; Pp Morette, E. and R. Fett, Oils and Food Technology in Industrial Station of Purification. Sao Paulo, Varele. Morrison, W.H., R.J. Hamilton and C. Kalu, Sun flower seed oil. In; R.J. Hamilton,(ed.) Developments in oils and fats. Blackie Academic and Professional, Glasgow, pp: Odoemelam, S.A., Proximate Composition and selected physicochemical properties of the seeds of African oil bean (pentaclethra marcrophylla). Pak. J. Nutr. 4: Pearson, D., The chemical Analysis of Food (6 th ed.) J. A. Church hill, London, pp: Savage, G.P. and J.I. Keenan, The composition and nutritive value of groundnut kernels, In: smart J (ed). The Groundnut crop: scientific basis for improvement. London: Chapman and Hall, pp: