Genetic diversity in chestnuts of Kashmir valley Pak. J. Agri. Sci., Vol. 50(3), 421-425; 2013 ISSN (Print) 0552-9034, ISSN (Online) 2076-0906 http://www.pakjas.com.pk THE EFFECT OF REFINING STEP ON THE CHANGES IN VISCOSITY VALUES OF VEGETABLE OILS Pelin Günç Ergonul Food Engineering Department, Engineering Faculty, Celal Bayar University, Muradiye Campus, Manisa, Turkey * Corresponding author s e.mail: pelingunc81@hotmail.com In this work, the viscosity values of chemically refined vegetable oils (sunflower, corn, soybean and rapeseed) and physically refined vegetable oils (olive and palm) were determined during refining processes. At this point of view, fatty acid compositions and viscosity values of oil samples were determined. The edible vegetable oils presented Newtonian behavior in shear rates at ranges 6.28-20.93 s -1. It was observed that palm oil is more viscous than the others. During physical refining, the effect of both oil type and refining steps were significantly important, whereas in chemical refining only the effect of oil type was found statistically important (p<0.01). It was observed that correlation among fatty acid compositions and viscosity values of the samples showed differences according to oil type. Keywords: fatty acid composition, rheological properties, vegetable oils, viscosity INTRODUCTION Viscosity is one of the most important parameter which characterizes rheological properties of liquid foods. From the physicochemical point of view, viscosity means the resistance of one part of the fluid to move relative to another one (Abramovic and Klofutar, 1998). The estimation of viscosity of a vegetable oil is essential in the design of unit processes such as distillation, heat exchangers, piping and reactors (Rodenbush et al., 1999). Viscosity of vegetable oils is affected by some physical and chemical factors such as temperature, oil density, molecular weight, melting point and degree of unsaturation. Viscosity is known to be variable due to the fatty acid composition. Oils or fats containing a greater proportion of fatty acids of relatively low molecular weight are slightly less viscous than ones of an equivalent degree of unsaturation but containing a higher proportion of high-molecular-weight acids (Lawson, 1995). Fresh vegetable oils show Newtonian flow behavior because of their long chain molecules (Maskan, 2003; Santos et al., 2004). In vegetable oils, viscosity increases with chain length of triglyceride fatty acids and decreases with unsaturation (Abramovic and Klofutar, 1998; Santos et al., 2005). For this reason, viscosity is a function of molecules dimension and orientation (Santos et al., 2005). One of the factors that greatly affect the viscosity of oils is temperature. It has been reported that the viscosity of oils and fats decreased linearly with temperature (Igwe, 2004). Generally the flow behaviors of vegetable oils were characterized by steady shear measurements at 25 C (Kim et al., 2010). From that point of view, the aim of this study was to investigate the viscosity of six different vegetable oils (sunflower, corn, soybean, canola, olive and palm oils) from each step of refining processes at 25 C constant temperature and then correlated with their fatty acid composition. Also, the effect of the type of vegetable oils was investigated. MATERIALS AND METHODS Sunflower, corn, soybean and canola oils were obtained from processing lines of factories using chemical refining including degumming-neutralizing, bleaching, winterizing and deodorizing steps. Olive oil and palm oil samples were obtained from processing lines using physical refining including bleaching and deodorizing steps. All samples are taken after extraction and each refining steps in duplicate. Rheological measurement: The flow behaviors of vegetable oils were measured by using a Brookfield viscometer (DV-I Viscometer, Middleboro, USA) with a small sample adapter, spindle 3, which permits the use of 600 ml of oil in each analysis. Measurements were done at room temperature (25 C), in different shear rates at ranges 6.28-20.93 s -1. Temperature was controlled using a water bath with precision of ±2 C. Sample preparation and measurement of fatty acid composition: Fatty acid composition of vegetable oils was investigated by using 6890 N model Agilent Technologies GC (Palo Alto, USA). Fatty acid methyl esters (FAME) were prepared according to Anonymous (1997). 100 mg of oil was placed into a centrifuge tube and 0.5 ml of 2 N methanolic potassium hydroxide and 2.5 ml of n-hexane (Merck, Darmstadt, Germany) were added. The mixture was then mixed vigorously and centrifuged in 6000 rpm for 10 min by using a centrifuge (Hettich EBA 8S, UK). The upper clear supernatant, the FAME, was transferred to a screwcapped vial. After methylation of oil samples, the fatty acid
Ergonul methyl esters were injected into a capillary DB-23 fused silica capillary column (60 m 0.25 mm i.d., 0.25 μm film thickness; Supelco Inc., Bellefonte, PA, USA). The temperature of GC was 180 C. The injector and detector temperatures were 220 C (15). Hydrogen was used as the carrier gas at a flow rate of 1 ml/min. The fatty acids were identified by comparing their retention times with the retention times of fatty acid standards in chromatograms obtained. Statistical analysis: Factorial design was used in statistical analyses. Two replications were done. The analysis of variance was done using the PROC GLM procedure of SAS (version 8.2., SAS Institute, Cary, NC, USA 2001). LSMEANS for treatments were generated and separated when significant (p<0.0001) using the Duncan procedure. Correlation coefficients among viscosity values and fatty acids composition of samples were also calculated using PROC-CORR procedures of SAS (SAS, 2001). RESULTS AND DISCUSSION Results of chemically and physically refined vegetable oils viscosity values determined after each refining steps are presented in Table 1 and in Table 2. The highest viscosity was observed in palm oil which was followed by olive, rapeseed, corn, sunflower and soybean oils. Kim et al. (2010) was observed the highest viscosity in hazelnut oil, followed by olive, canola, corn, soybean, sunflower and grapeseed oils. Generally, liquid food viscosity depends on its composition and temperature. The oil viscosity has a direct relationship with some chemical characteristics of the lipids, such as the degree of unsaturation and the chain length of the fatty acids that constitute the triacylglycerols (Abramovic and Klofutar, 1998). According to the results, it was observed that vegetable oils viscosity increased due to the removing of free fatty acids and with polymerization and decreased with unsaturation. In case of polymerization, the increase in chain size leads to the increase in the electron number in the molecule, increasing London forces and, one more time, intermolecular forces. Besides this, longer chains are more difficult to move, leading to a higher viscosity (Santos et al., 2005). As seen as in Fig. 1, the viscosity of corn and sunflower oils increased after neutralization and bleaching steps then decreased after winterization step. It is thought that this situation can be explained by the removal of compounds having short carbon chain, especially free fatty acids during neutralization and bleaching steps from sunflower and corn oils which are rich in wax content. By removal of these compounds, concentration of waxes having higher viscosity values, increases in these oils. The degree of neutralization of the acid is of critical importance for the efficiency of the degumming process. If the degree of neutralization is too low, the viscosity of the phosphatides is rather high, which often makes the continuous discharge from a centrifugal separator problematical (Greyt and Kellens, 2000). In the case of rapeseed oil, the viscosity declined rather limitedly after neutralization step and after bleaching step showed a little increase and then remained almost similar. But in general, the viscosity value of rapeseed oil has not changed during refining process. Also, there is no statistically significant change in the viscosity value of soybean oil was determined during refining process. Generally the viscosity values of chemically refined of all oils did not change after winterization step and remained stable. It is obvious that, variety of the oil is an important factor affecting the changes in wax content of the samples. It was observed that changes in wax content of the oils having more wax amount were significant. It can be said that the rheological behaviors of oils did not change significantly in constant room temperature. Among the physically refined oils, the viscosity values of palm-olein increased significantly after bleaching Table 1. Viscosity of chemically refined vegetable oils at 25 C in different steps of refining processes Refining Viscosity (Pa.s) Steps Sunflower A Soybean B Corn A Rapeseed A Crude 7.03 6.24 7.52 8.00 Neutralized 7.18 6.16 8.30 7.36 Bleached 8.00 6.20 8.67 7.71 Winterized 6.67 6.20 7.34 7.79 Deodorized 6.58 6.10 7.37 7.72 Table 2. Viscosity of physically refined vegetable oils at 25 C in different steps of refining processes Refining Viscosity (Pa.s) Steps Olive Oil a Palm Oil b Crude A 8.79 12.03 Bleached B 8.12 17.02 Deodorized C 7.96 10.76 422
viscosity (cp) viscosity (cp) Changes in viscosity values of vegetable oils step and decreased significantly after deodorization step (Fig. 2). It is thought that this prominent change is due to the high saturated structure of palm oil. The viscosity values of olive oil has been shown a consistently decrease during refining (Fig.2). Average viscosity value of all oil samples decreased when compared with the wax content of crude oil. 10 8 6 4 2 0 Crude Neutralized Bleached Winterized Deodorized Figure 1. The changes of viscosity values of chemically refined oils 18 16 14 12 10 8 6 4 2 0 Crude Bleached Deodorized olive oil palm oil Figure 2. The changes of viscosity values of physically refined oils The statistical results demonstrated that in chemically refining only the oil types were significantly affected the viscosity values (p<0.01). The differences between oil types in terms of viscosity are indicated by different letters in Table 1. In different refining steps, the viscosity values of oils did not change significantly due to the applied constant temperature. But the oil type was prominently affected the viscosity (Kim et al., 2010). In physically refined oils, both of oil type and refining steps are statistically important (p<0.0001) and they are indicated also by different letters by themselves (Table 2). For all the oils analyzed in this study, the percent amount of total saturated fatty acids (SFA) and unsaturated fatty acids increased from 7.3% to 44.8% and 11.3% to 69.9% (Table 3-4). In general, when the monounsaturated fatty acids (MUFA) contents of oils decreased, polyunsaturated fatty acids (PUFA) contents increased and when the MUFA contents of oils increased, PUFA contents decreased during refining processes. It was determined that refining steps and oil types were not affected statistically the fatty acid composition of oil samples in chemically refining (p>0.05) whereas in physically refining, only the oil types was statistically important effect on the changes in fatty acid compositions of the samples (p<0.0001). The differences between oil types are indicated by different letters in Table 3. This is because, in physical refining no oil degradation occurred as in chemical refining so the viscosity decreased significantly by removing saturated components during physical refining. The viscosity of olive oil was positively correlated with the amounts of SFA (r=+0.54) and negatively correlated with the amounts of MUFA (r=-0.44). For palm-olein, viscosity was positively correlated with the amounts of SFA (r=+0.85) and MUFA (r=+0.95) and negatively correlated with the amounts of PUFA (r=-0.80). Among the chemically refined oils, the viscosity of sunflower oil was positively correlated with the amounts of saturated fatty acid (SFA) (r=+0.97) and monounsaturated fatty acid (MUFA) (r=+0.56) and negatively correlated with the amounts of polyunsaturated fatty acid (PUFA) (r=-0.62). The viscosities of rapeseed and soybean oils were positively correlated with the amounts of SFA (r=+0.85 and r=+0.84) and negatively correlated with the amounts of MUFA (r=- 0.47 and r=-0.45). The correlation the viscosity of corn oil with the amounts of SFA was quite low (r=+0.27). Santos et al. (2005) was determined that viscosity of oils was better related to the concentration of PUFA chains than to MUFA Table 3. Fatty acid composition of physically refined vegetable oils in different steps of refining processes Oil Type Fatty acids (%) Saturated Monounsaturated Polyunsaturated Olive oil A Crude 17.34 69.92 12.76 Bleached 17.21 70.23 12.6 Deodorized 17.11 70.08 12.82 Palm oil B Crude 44.77 44.01 11.25 Bleached 43.98 44.55 11.5 Deodorized 43.82 44.53 11.66 423
Ergonul Table 4. Fatty acid composition of chemically refined vegetable oils in different steps of refining processes Oil Type Fatty acids (%) Saturated Monounsaturated Polyunsaturated Sunflower oil Crude 10.34 49.87 39.78 Neutralized 10.09 50.07 39.93 Bleached 10.00 50.17 39.85 Winterized 10.03 48.70 41.10 Refined 9.95 48.26 41.81 Corn oil Crude 15.82 38.81 45.51 Neutralized 15.37 38.33 46.28 Bleached 15.20 37.95 46.89 Winterized 14.64 37.32 46.48 Refined 14.60 38.83 47.49 Rapeseed oil Crude 8.21 64.22 27.61 Neutralized 7.73 64.34 27.94 Bleached 7.71 64.46 27.84 Winterized 7.33 64.38 29.31 Refined 7.32 63.72 29.49 Soybean oil Crude 16.55 26.93 56.52 Neutralized 17.18 27.84 55.01 Bleached 16.35 28.20 55.47 Winterized 15.44 29.30 55.36 Refined 15.29 29.51 55.37 chains. Kim et al. (2010) was not observed any positive correlation between viscosity and total saturated or unsaturated fatty acids, in their study. But they indicated positive correlation between the viscosity and both of oleic and linoleic acids. Conclusion: Changes in viscosity values of chemically refined oils during refining process are similar then the physically refined oils. From that point of view, it is thought that average viscosity values of the oil samples obtained from different steps of refining process are similar at ambient temperature 25 C. But, for all oils samples it was determined that viscosity value decreased at the end of the deodorization step when compared to viscosity value of crude oil. The results demonstrated that there was a high positive correlation between oil viscosities and SFA, and low negative correlation between oil viscosities and MUFA or PUFA. Since the value of viscosity did not change significantly, there were no important correlation was determined among viscosity and MUFA and PUFA values at 25 C. REFERENCES Abramovic, H. and C. Klofutar. 1998. The temperature dependence of dynamic viscosity for some vegetable oils. Acta Chim Slov 45:69-77. Anonymous. 1997. AOCS, Official Methods and recommended practices of the American Oil Chemists Society, Preparation of Methyl Esters of Fatty Acids, method Ce 2-66. Greyt, W.D. and M. Kellens. 2000. Refining Practices. In: W. Hamm and R.J. Hamilton (eds.), Edible Oil Processing. Sheffield: Sheffield Academic Press Ltd., U.K. Igwe, I.O. 2004. The effects of temperature on the viscosity of vegetable oils in solution. Ind. Crop Prod. 19:185-190. Kim, J., D.N. Kim, S.H. Lee, S.H. Yoo and S. Lee. 2010. Correlation of fatty acid composition of vegetable oils with reological behaviour and oil uptake. Food Chem. 118:398-402. Lawson, H. 1995. Food oils and Fats: Technology, Utilization and Nutrition. Chapman & Hall, New York. 424
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