511 Acta Chim. Slov. 1999, 46(4), pp. 511-521 CHARACTERISTICS OF VEGETABLE OILS OF SOME SLOVENE MANUFACTURERS† D. Rudan-Tasič and C. Klofutar Department of Food Technology, Biotechnical Faculty, University of Ljubljana SI-1000 Ljubljana, Jamnikarjeva 101, Slovenia (Received 6.7.1999) ABSTRACT Eleven samples of vegetable oils were examined and the following indices determined: peroxide value, acid value, iodine value, saponification number, specific gravity, refractive index at 298.15 K and electric permittivity in the temperature range from 298.15 to 313.15 K. Some empirical relations between physical and chemical constants were fitted to the experimental data and the correlation constants for the best fit are presented. In addition, the correlation constants for calculating the electric permittivity of oils in the range of temperature from 298.15 to 313.15 K were obtained and the effective dipole moment was estimated via Debye’s equation. INTRODUCTION Edible oils extracted from plant sources are important in foods and in various other industries (e. g. cosmetics, pharmaceuticals, lubricants). They are key components of the diet and also provide characteristic flavours and textures to foods. During extraction, purification and usage, oils undergo a variety of processing operations, including heating, distillation and chemical modification which may alter their properties. Dedicated to Professor Drago Leskovšek on his 80 birthday 512 Several semi-empirical equations have been developed that relate the property of interest (e. g. time for fat to drain from a fried potato chip) to independently measurable bulk properties (e.g., density, viscosity, surface tension, etc). With these equations, it is possible to predict how changes in the properties of an oil alter the efficacy of a process without resorting to time-consuming trial-and-error experiments. The chemical and physical properties of edible oils depend primarly on composition (and hence on biological origin) and temperature. In this work, we have compiled some bulk parameters for different vegetable oils of some Slovene manufacturers with special focus on dielectric properties of oils because of the lack of corresponding data. EXPERIMENTAL Samples for examination were obtained from a commercial centre and represent three Slovene manufacturers, i. e. GEA Oil Factory, Slovenska Bistrica (G), Oil Factory Domžale (D) and Mercator-Oljarica Kranj (K). The fatty acid composition and source or manufacturer of each vegetable oil are shown in Table 1. Chemical constants of oils The following chemical determinations were carried out according to the methods described in the A. O. A. C. [1]: peroxide value, acid value, iodine number, and saponification number. All tests were performed in triplicate. Physical characteristics of oils Density measurements were carried out using a pycnometer at a temperature of 298.15± 0.05 K; the pycnometer of capacity about 25 cm3 being calibrated with water [2]. The relative error in the density, —, was estimated by the expression P 513 dr dm dmo dmp =+ + r m m m (1) where m, mo and mp are the mass of the pycnometer when filled with oil, water and air, respectively and dm = dmo = dmp = 0.0001 g are their uncertainities. The relative error in the density of water, the others. dro ro was considered to be a negligible quantity in comparison with Table 1. Fatty acid distributions and sources of vegetable oils. Sample Specification and source or manufacturer Composition (%)a Saturated Monounsaturated NAc Polyunsaturated 1 Refined sunflower oil (99%) < 20 NAc and wheat germ oil (1%); G 2 Nonrefined sunflower oil (cold pressed); G 10 25 65 3 Refined corn germ oil (99%) and wheat germ oil (1%); G < 20 NAc NAc 4 Refined extra sunflower oil (high level of oleic acid); G 10b 80b 10b 5 olive oil 20 55-83 3.5-21 (natural + refined); G (palmitic acid) (oleic acid) (linoleic acid) 6 Refined sunflower oil (100%); G 10b 20b 70b 7 mixed salad oil; D 10-12 17-25 >60 8 Unrefined olive oil (cold pressed); D 8-27 55-83 4-20 9 Refined sunflower oil; D NAc NAc >55 (linoleic acid) 10 Refined sunflower oil (100%); K 9-17 17-40 48-70 11 Refined corn germ oil; K 10-15 20-35 50-60 Provided by the manufacturers. b Approximate values. c Not available. 514 The refractive index of the samples was measured at 298.15 ± 0.05 K with a Carl Zeiss Abbé refractometer (32-G 110e) with a precision of 1·10-4 at a wavelength of 589 nm. Electric permittivity (dielectric constant) measurements were carried out at temperatures from 298.15 ± 0.05 K to 313.15 K in 5 K intervals using a WTW dipolemeter (model DM 01) with a DFL 1 cell at a constant frequency of 2 MHz. The calibration of the cell (25 cm3) was performed as in reference [3], with benzene, carbon tetrachloride and cyclohexane as standards. The standard error of the estimate of electric permitivitty, L, was about 1.3-10-3. RESULTS AND DISCUSSION The results of chemical analysis in Table 2 indicate that the characteristics of eleven selected edible oil samples are in agreement with current published values for these indices. Values are typical rather than average and variations may occur , depending on a number of variables such as source, treatment, and age of the oil. Thus, the iodine values, IV ( g I / 100 g sample) for olive oils (samples 8 and 4) are noticeably lower than for the other kinds of oil but within the limits of the literature data (77-95, [4]). The saponification values, SV (mg KOH /g sample), varying slightly from 191.4 for corn germ oil (sample 3) to 193.9 for olive oil (sample 5), show that the fatty acids present in these oils have a high number of carbon atoms. Quality evaluation through acid value, AV (mg Koh /g sample) and the peroxide value, PV (mmol peroxide / kg sample) confirmed that the quality of these oils was satisfactory. Freshly deodorized oil should have zero PV, but in most cases, for the product to have acceptable storage stability the PV of oils used should be less than 5. Olive oil contains components that interfere with conventional PV determination; even freshly pressed olive oils have PV values of about 5, and under certain climatic conditions (dry weather) the PV value can be higher than 5 [5]. 515 The density data are presented in Table 3. On average, the relative error of the density, calculated via relation (1), amounts to 1.0 -10"5 meaning that the density values were accurate to five decimal places. The relative density or specific gravity of an oil at any given temperature compared to water at a specified temperature is known to increase as the mean molecular weight diminishes (i. e. with higher SV), and also as the degree of unsaturation increases (i. e. with higher IV) [4]. For the samples investigated the following expression for the approximate specific gravity, d^ was developed dll = 0.83088 + 0.000306 -SV + 0.000271 -IV (2) on the basis of multiple linear regression analysis; the standard error of the estimate s was found to be 1.4-10"4 and the determination coefficient greater than 0.999. Table 2. Chemical characteristics of the edible oils studied. Sample Characteristics Peroxide value Iodine value Acid value Saponification (mmol peroxide / kg sample) g ( I2 / 100 g sample) (mg KOH / g sample) value (mg KOH /g sample) 1 2.026±0.002 112.444±0.018 0.2120±0.0001 191.65±0.04 2 1.788±0.002 111.043±0.009 1.3442±0.0004 192.07±0.02 3 1.780±0.001 108.593±0.003 0.1294±0.0001 191.44±0.02 4 2.178+0.001 87.738+0.006 0.2785+0.0003 191.46+0.02 5 3.287+0.002 84.638+0.006 0.8293+0.0003 193.95+0.04 6 1.962+0.001 109.860+0.006 0.1954+0.0003 192.55+0.05 7 1.980+0.001 108.658+0.007 0.2286+0.0002 191.72+0.05 8 7.493+0.002 82.553+0.008 2.5987+0.0010 191.62+0.02 9 2.227+0.001 109.380+0.008 0.1784+0.0001 192.26+0.03 10 2.146+0.001 109.231+0.007 0.1144+0.0001 191.85+0.04 11 1.707±0. 001 110.116=1=0.002 0.1137=1=0.0001 191.87±0.02 516 Table 3. Density, relative density, refractive index and specific refraction of edible oil samples at 298.15 K. Sample r / g cm-3 d25 nD r / cm3 g-1 1 0.91710 0.91981 1.4736 0.30622 2 0.91688 0.91960 1.4734 0.30618 3 0.91655 0.91926 1.4727 0.30557 4 0.91047 0.91316 1.4681 0.30537 5 0.91037 0.91307 1.4674 0.30502 6 0.91685 0.91956 1.4731 0.30603 7 0.91674 0.91946 1.4728 0.30590 8 0.90934 0.91203 1.4669 0.30508 9 0.91683 0.91955 1.4731 0.30601 10 0.91678 0.91949 1.4729 0.30594 11 0.91680 0.91951 1.4730 0.30599 The refractive indices of the oils investigated, nD, at 298.15 K are given in Table 3, and tend to increase with the number of double bonds, i. e. with mean unsaturation or iodine value, IV. In general, the refractive indices of natural fats and oils are related to their average degree of unsaturation in an approximately linear way. The relationship between refractive index and the iodine, acid, and saponification values is somewhat more complex. A number of equations have been proposed which are of limited application and fair accuracy, e. g. [4]. The relationship between refractive index and iodine value of the oils investigated has the form nD = 1.4484 + 0.0002247 -IV (3) and a more general relationship was found to be nD = 1.4446 + 0.000019-SV+0.0042-------I-0.000226-IV (4) SV The standard error of the estimate, s was found to be 6.6-10-5 and 7.2-10-5 for rels. (3) and (4) respectively, while the determination coefficient was 0.9997 in both cases. 517 Related to the refractive index is the specific refractivity; the dependence of the refractive index upon specific volume has been related by the equation r = (nD -1) / p, where r is the specific refraction. A more useful relation is the Lorenz-Lorentz equation [6] n 2 D+2 p which was used for calculating the values of r for the oil samples in Table 3. It seems that specific refraction is not a particulary distinguishing characteristic of the investigated edible oils; namely, r = 0.30579 ± 0.00043 cm3 g-1. The dielectric data obtained for eleven edible oils are recorded in Table 4; the values of electric permittivity, e, lie in the range of about 3.0-3.2 (at 298.15 K) as is usually the case for most oils. It is evident that electric permittivity increases somewhat with increase in the unsaturation of the oil, i. e. with IV (see Table 2) and decreases with increasing temperature. The variation of electric permittivity with temperature for the oils investigated is given by s = so+a1(T-To) + a2(T-Tj (6) where T is the absolute temperature, To is 298.15 K while eo, a 1 and a2 are empirical constants which were obtained by the method of least squares [7] on the basis of the data in Table 4. The relevant standard error of the estimate, s is given in Table 5. From Table 5 it is evident that the values of the constant eo, within experimental error, are equal to the electric permittivity of an oil at 298.15 K, while the values of a 1 are negative; in contrast, all the a2 values are positive. The coefficient of temperature dependence of electric permittivity, y, was calculated via the relation dT 7 = -e (7) P de From equation (7) it is apparent that the relative change in electric permittivity, —, is s proportional to the variation of temperature, and the coefficient of proportionality is 518 Table 4. Change in electric permittivity of edible oils with temperature. Sample e 298.15 K 303.15 K 308.15 K 313.15 K 1 3.161 3.110 3.080 3.071 2 3.160 3.123 3.091 3.061 3 3.150 3.106 3.075 3.059 4 3.105 3.067 3.038 3.016 5 3.098 3.067 3.036 3.007 6 3.159 3.121 3.092 3.075 7 3.153 3.116 3.087 3.067 8 3.093 3.070 3.048 3.026 9 3.158 3.115 3.085 3.068 10 3.157 3.116 3.085 3.066 11 3.156 3.116 3.085 3.062 Table 5. Values of regression coefficients of equation (6) for the oil samples studied, together with the standard error of the estimate, s. Sample eo - a1 ¦ 103 a2 ¦ 104 s ¦ 104 1 3.1610 ± 0.0000 12.3 ± 0.00 4.20 ± 0.00 ± 0.00 2 3.1598 ± 0.0007 7.63 ± 0.21 7.00 ± 0.13 ± 6.71 3 3.1501 ± 0.0004 10.28 ± 0.14 2.80 ± 0.09 ± 4.47 4 3.1049 ± 0.0004 8.32 ± 0.14 1.60 ± 0.09 ± 4.47 5 3.0981 ± 0.0004 6.38 ± 0.14 0.20 ± 0.09 ± 4.47 6 3.1591 ± 0.0003 8.83 ± 0.10 2.15 ±0.07 ± 3.35 7 3.1531 ± 0.0002 8.29 ± 0.07 1.70 ± 0.04 ± 2.24 8 3.0929 ± 0.0002 4.61 ± 0.07 0.10 ± 0.04 ± 2.24 9 3.1580 ± 0.0000 9.90 ± 0.00 2.60 ± 0.00 ± 0.00 10 3.1571 ± 0.0004 9.38 ± 0.14 2.20 ± 0.09 4.47 11 3.1559 ± 0.0002 8.81 ± 0.01 1.70 ± 0.04 ± 2.24 519 equal to the coefficient of temperature dependence of electric permittivity. Through relations (6) and (7) the coefficient j of an oil is given by 7 = -1[a1+2a2(T-To)] At the temperature of 298.15 K equation (8) simplifies to a, 7o = 1- The error in the coefficient, ôo was obtained as = 5a1o+a1ôyo /o 2 (8) (9) (10) where ô1 and ôso are the errors in the relevant parameters. The values of the thermal coefficient o for the oil samples are given in Table 6 and, ranging from 1.49-10-3 K-1 (sample 8) to 3.89-10-3 K-1 (sample 1), seem to follow the variations in unsaturation of the oils. Table 6. Values of the coefficient of the temperature dependence of the electric permittivity, specific polarization, specific orientation polarization and effective dipole moment for the oils investigated at 298.15 K. Sample (7o±ô7o)103 / K-1 p / cm3 g-1 po / cm3 g-1 ju/ D 1 3.89 ± 0.00 0.4566 0.1503 2.54 2 2.41 ± 0.07 0.4565 0.1504 2.53 3 3.26 ± 0.04 0.4555 0.1499 2.54 4 2.68 ± 0.05 0.4529 0.1475 2.52 5 2.06 ± 0.05 0.4520 0.1470 2.50 6 2.80 ± 0.03 0.4564 0.1504 2.54 7 2.63 ± 0.02 0.4558 0.1499 2.54 8 1.49 ± 0.02 0.4519 0.1468 2.51 9 3.13 ± 0.00 0.4563 0.1503 2.54 10 2.97 ± 0.04 0.4562 0.1503 2.54 11 2.79 ± 0.02 0.4561 0.1501 2.54 520 From the values of electric permittivity of the oils investigated at 298.15 K the specific polarizations were calculated using the Debye equation p=~- (11) e + 2 p Assuming that the difference between the specific polarization and refraction is a measure of the orientation polarization, i. e. po = p-r, then the effective dipole moment, /i, may be calculated from the orientation polarization as follows [8] V2 =——-po (12) 4nN where k is the Boltzmann constant, T the absolute temperature, M the mean molecular weight and N the Avogadro number. Thus, in the case of edible vegetable oils, the effective dipole moment calculated in this manner represents the effective dipole moment of the large oil molecules or aggregates of molecules, regarded as rigid structures and rotating as such. The corresponding specific polarizations, specific orientation polarizations and effective dipole moments are listed in Table 6; the mean molecular weights of the samples studied, calculated via the relationship M ¦ SV = 168300 [9], are rather close to each other, e. g. from 867.8 (sample 5) to 879.1 (sample 3). So it is evident that the effective dipole moment cannot be a satisfactory index either for identifying e. g. an olive oil, or for differentiating between sunflower and corn germ oils. Likewise the dielectric constant requires at least density and refractive index determinations for purposes of identification. However, food oils in general exhibit significant variations in their composition; consequently, it is impossible to define unique values for chemical and physical constants for any oil and it is usually necessary to combine several empirical constants to predict the chemical and physical properties of edible oils. None of the data reported here represent a survey of the range of parameters for all varieties of a particular oil; so the degree of interspecies variability remains undefined. 521 ACKNOWLEDGEMENTS This work was supported by the Ministry of Science and Technology of the Republic of Slovenia. REFERENCES 1. Official Methods of Analysis, 15 th Ed., AOAC, Arlington, VA , 1990, pp 951- 986. 2. R. C. West, CRC Handbook of Chemistry and Physics, 65 th Ed., CRC Press, Boca Raton, 1985, p. 89. 3. C. Klofutar, D. Rudan-Tasič and I. Turk, Physiol. Chem. Phys. & Med. NMR 1996, 28, 111-122. 4. D. Swern, Industrijski proizvodi ulja i masti po Baileyu, Nakladni zavod znanje, Zagreb, 1972, pp 100, 156 5. Y. H. Hui, Bailey’s Industrial Oil and Fat Products, Vol. 2, 5 th Ed., Wiley, New York, 1996, pp 241-269. 6. K. S. Markley, Fatty Acids, Part 1, Interscience Publishers, New York, 1960, pp 499-607. 7. J. Topping, Errors of Observation and Their Treatment, Chapman and Hall, London, 1972, pp 86-112. 8. B. P. Caldwell, H. F. Payne, Ind. Engng Chem. 1941, 33, 954-960. 9. M. Tels, A. J. Kruidenier, C. Boelhouwer, and H. I. Waterman, J. Am. Oil Chem. Soc. 1958, 35, 163-166. POVZETEK Enajstim vzorcem jedilnega olja smo določili peroksidno število, kislinsko število, jodovo število, saponifikacijsko število, specifično težo, lomni količnik pri 298.15 K ter električno permitivnost v temperaturnem območju od 298.15 do 313.15 K. Preizkusili smo nekaj empiričnih relacij med fizikalnimi in kemijskimi konstantami, d oločili korelacijske konstante za izračun električne permitivnosti olj v temperaturnem intervalu od 298.15K do 313.15 K ter izračunali povprečni dipolni moment olj preko Debyeve relacije.