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Study of physical property like viscosity of vegetable oils is an important engineering parameter for designing processing equipment which represents a subject of intensive research effort to be carried out in academic institutions and industry. The rheological and thermal degradation in unheated and heated rice bran, palm oil, sunflower oil and groundnut oil is studied as a function of temperature (30°C to 90°C) using low cost temperature controlled Redwood Viscometer. The viscosity decreased with increasing temperature at different rates depending on the saturated and unsaturated fatty acid composition of the oils. Some experimental relations that describe the temperature dependence of viscosity are fitted to experimental data and correlation constants are studied. A novel methodology in gas chromatography (GC-MS) is investigated to quantize the amount of polyunsaturated, monounsaturated and saturated fatty acids before and after heating the oils. The amount of polyunsaturated chains is better correlated (R2= 0.896) with viscosity than monounsaturated chains. The content of unsaturated fatty acids has good correlation (R2=0.867) with viscosity than saturated fatty acids (R2=0.747) for unheated oils and heated oils. The viscosity of heated (to smoke point 210 - 250°C) oil sample is greater than unheated oil due to increase in the content of saturated composite in the oil on heating.
Keywords: Vegetable oils, Viscosity, Thermal degradation, GC-MS.
In food industry and processing applications, the physical properties of foods play an important role in the analysis of quality, process, design and fabrication of processing equipment. Vegetable oils undergo extensive oxidative deterioration due during deep frying, storage, marketing and hydrogenation1. The main factors influencing the entity of these transformations are represented by the temperature of the treatment, the nature of foods being fried, the presence of metals that catalyze the oxidation phenomena, and the composition of the frying oil2. It is known that fats and oils, when subjected to prolonged heating for frying, are subjected to a series of chemical-physical modifications, the effects of which can be observed in the variation of the sensory and nutritional characteristics 3. Hydro-peroxide, which is the major oxidation product, decomposes to secondary products, such as esters, aldehydes, alcohols, ketones, lactones and hydrocarbons 4. These secondary products adversely affect flavour, aroma, taste, nutritional value and overall quality of foods 3. These heated oils get biologically degraded before disposal.
Physical parameter of vegetable oils like viscosity is essential for designing processing system in food industry 5. In vegetable oils, viscosity increases with chain lengths of triglyceride fatty acids and decreases with unsaturation (monounsaturated or polyunsaturated fatty acids) composition in the oil6. Change in the fatty acid composition on heating changes the physical property which is necessary to predict processing variation, predict processing equipment and thermal degradation in oils7. Hence viscosity is a function of molecular dimension and orientation used to estimate the quality deterioration in the oil that causes great economic losses to the food industry 8, 9. Viscosity which is the marker of thermal degradation is also an index for thermal stability of antioxidants in edible oils10.
In present study the viscosity of unheated and heated rice bran oil, palm oil, sunflower oil and groundnut oil is measured using microcontroller based instrument in the temperature range from 30°C to 90°C 11. From the variation of viscosity with temperature the thermal degradation in the heated oil with respect to unheated is studied. Empirical relations that relates kinematic viscosity and temperature is used to find the coefficient R2, variance and standard error. The relationship between the viscosity and fatty acids for heated and unheated oils using GC-MS results are also premeditated.
Commonly available and popular branded rice bran oil, sunflower oil, groundnut oil and palm oil has been collected from a local grocery shop located in Thanjavur district of Tamilnadu, India to assess the thermal effect and possibility of usage of repeatedly heated oil by the society. To get sample for heated oils hundred milliliters of sample has been placed in a copper beaker and heated on an electric device, stirring manually with glass rod. A microcontroller based temperature controller has been designed and has been used to monitor the sample temperature. To mimic the oil oxidation process during frying, the sample has been heated up to 250°C for five times. In order to ensure that the sample has been heated to the temperature greater than its smoke point, it has been exposed to successive heating.
Microcontroller based the Redwood viscometer is used to measure the viscosity in laminar flow method. The microcontroller based instrument is used to measure temperature and time taken for the collection of 50cc of sample (ref- sensor). The temperature is maintained by a temperature controller which has a very good accuracy of ± 1%. The kinematic viscosity is calculated from the following relation:
(Î½) = (A* t - B/t) x 10-6 m2/s --------------------- (1)
Where A and B are constants that are calculated from the diameter and height of the orifice.
t = redwood time which measure the rate of flow in seconds. When t > 34, A = 0.26 and B = 172
The redwood seconds were ramped from 50 to 250 sec at temperatures of 30°C to 90°C. The copper cup in the viscometer is washed with CCl4 after each observation. Each reading is taken from the average of three trials.
For the fatty acid compositional variation determination, an Agilent gas chromatograph from Hewlett-Packard (Palo Alto, CA, USA) equipped with a HP 5971 MS detector was used. Separations were carried out on an Agilent - Hewlet Packard fused silica capillary column HP-5 (30 m x 0.25 mm I.D. column coated with a 0.25 Î¼m film thickness) (Folsom, CA, USA). The GC-MS interface temperature was maintained at 250°C. 1µl of both heated and unheated oil samples were injected manually in splitless mode with injector port temperature at 250 °C. The helium carrier gas flow rate was 1 ml /min. The column temperature program was as follows: 90 °C, held for 1 min, 12 °C min-1 to 150 °C, held for 1 min, 2 °C min-1 to 230 °C, held for 3 min, 10 °C min-1 to 250 °C, held for 25 min. The selective ion mode was used in the analysis. Retention time and abundance of the confirmation ions relative to that of quantification ion were used as identification criteria. Mass Charge range was between 50- 500 amu. Oven temperature programmed to 50° -250°C.
Results and discussion
Figures 1, 2, 3 and 4 show the variation of viscosity of unheated and heated rice bran oil palm oil, groundnut oil and sunflower oil with temperature. It is observed that the viscosity decreases with increase in temperature. This is due to the high thermal movements among molecules reducing intermolecular forces, making flow among them easier and reducing viscosity. The presence of double bonds in fatty acid that exist in cis configurational form, produces "kinks" in the geometry of the molecules 7, 12. This prevents the chains coming close together to form intermolecular contacts, resulting in an increased capability of the oil to flow13.
Figure 1 shows the variation of viscosity of unheated oil and heated rice bran oil in the temperatures range of 30°C to 90 °C. At 30°C the viscosity of heated is 45% greater than unheated, whereas at 90°C the viscosity varies by only 6% that exemplify the thermal degradation in the oil is low and antioxidant stability is appreciable. Figure 2 shows the change in viscosity of heated palm oil is 39% greater than unheated at 30°C, the variation at 90°C is nearly 42% which is very much greater compared to rice bran oil. Figure 3 illustrates the difference between the viscosity of unheated and heated groundnut oil at 30°C is found to be 25% and it increases to 35% at 90°C. Figure 4 confirms the variation in viscosity of unheated and heated sunflower oil. The modification at 30°C is found to be 24% whereas at 90°C it is 9%.
On comparing the viscosity change at 90°C palm and groundnut oil shows more difference between unheated and heated oil than sunflower oil and rice bran oil. This exemplifies that the percentage of thermal degradation is more in the oils. The viscosities at 30°C are about to the maximum of 6-fold greater than those at 90°C. This disparity produce significant effect on the power required to pump the oil at desired temperatures as well as in choosing pipe of definite radius.
The changing of viscosity with temperature relation is modeled using equation 2-4. Equation 2 is the Arrhenius model where viscosity varies exponentially with temperature. This is used by the reachers to characterize Newtonian and non-Newtonian liquids14. Equation 3 shows viscosity relationship in power law model. The logarithmic variation of viscosity with temperature is given in Williams-Landel-Ferry model equation 4.
Where Ea in the equation is the activation energy with unit kJ/kg, R is universal gas constant (8.314 kJ/kg mol K), T is absolute temperature °C and A is a pre exponential constant.
Power law model:
C and n are constants. is the value of reference temperature 273 K.
Modified Williams-Landel-Ferry model:
Where p and q are the constants. Constants A, C, n, p and q in equations (2 - 4) were calculated using nonlinear regression procedure the values of the constants are illustrated in table 1-3.
The quantity of saturated, monounsaturated and polyunsaturated fatty acids are calculated from the results obtained using GC-MS analysis 15. Figure 5 shows the variation of viscosity with unsaturated fatty acids of unheated oils. The correlation between monounsaturated fatty acids and viscosity is less than that of polyunsaturated fatty acid (PUFA) for the correlation is R2 =0.896 a more extended chain makes viscosity smaller and flow easier16. Figure 6 shows the variation of viscosity with saturated and unsaturated fatty acids and it is observed unsaturated fatty acids are highly correlated with viscosity (R2>0.87) than saturated fatty acids which characterizes the chain length of fatty acids. The viscosities of oils are better related to the concentration of polyunsaturated (more than one double bond) chain length than to monounsaturated fatty acids (has only one double bond) like oleic. This is due to the increase in the amount of Ð¿ bonds that makes bonding more rigid by decreasing rotation between C - C bonds 6, 17. The viscosity is negatively correlated with PUFA shows decrease in viscosity due to the presence of unsaturated fatty acids like Linoleic of this type as the unsaturation increases. Figure 7 indicates the disparity of viscosity of heated oils with fatty acid composition calculated from the analysis. It is found from the result of correlation analysis (R2<<1) between viscosity of heated oil with the amount of saturated fatty acids which is very poor. Thus the thermal degradation in the oil is substantiated.
The variations of viscosity with temperature of unheated and heated oils are found and thermal degradation is found to be occurred more in palm oil and groundnut oil compared to rice bran and sunflower oil. The increase in viscosity of oil submitted to frying condition may be due formation of undesirable compounds caused by polymerisation reaction. The study of variation in viscosity with temperature is found to fit more in Arrhenius model which can help to predict viscosity at any temperature within the range. The GC-MS analysis confirms the change in viscosity is due to the composition of more percentage of polyunsaturated fatty acids and increase in viscosity due to saturated fatty acids. From the correlation analysis unsaturated fatty acid decreases with heating shows the increase in the saturation of compounds in the oil on heating.