State of Dissolved Water in Triglycerides as Determined by. Fourier Transform Infrared and Near Infrared Spectroscopy

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1 ORIGINAL State of Dissolved Water in Triglycerides as Determined by Fourier Transform Infrared and Near Infrared Spectroscopy Jun KURASHIGE**, Kyo TAKAOKA*, Masahisa TAKASAGO*, Yasunori TARU*, and Koichi KOBAYASHI* **Oils and Fats Research Laboratories, Central Research Laboratories, Ajinomoto Co. Inc. (7-41 Daikoku-cho Tsurumi-ku, Yokohama-shi, 230) *Department of Chemistry, Musashi Institute of Technology ( Tamazutsumi, Setagaya-ku, Tokyo, 158) The state of dissolved water in triglycerides (TG) such as tristearin, triolein, trilinolein, and trilinolenin, was analyzed by Fourier transform infrared and near infrared spectroscopy at 20 Ž, and compared with that of water. Water was revealed to be mainly composed of polymers larger than dimer clusters at 20 Ž, and of monomers and dimer clusters at 120 Ž. In TG, the state showed considerable variation from monomer to polymer clusters. The distribution ratios of the water clusters observed in TG depended on the kinds of fatty acids of TG. The water state was noted to change due to interactions between unsaturated bonds and dissolved water. With increase in the number of unsaturated bonds of TG, the ratio of monomer water decreased, and clusters larger than those of the original water increased. 1 Introduction Usually 300 to 600 ppm of water dissolves in edible oils opened to the air. The more the humidity in the air is, the higher becomes the water content of oils in the range of 800 to 1000 ppm. Actually, the water concentration in those oils can reach by the level of 1500 ppm, in case of oils being contacted with water. And even a small amount of water can affect hydrolysis, and autoxidation of oils under storage, and even enzymatic reactions as mentioned below; 1) In case vegetable oil containing water is stored in a metal vessel, metal is dissolved out into the water, and those dissolved metal ions catalyze to promote the autoxidation of the oil1). 2) The dissolved water in oil interacts with the dissolved oxygen and, as a result, affects oxidative deterioration of the oil2). 3) As for the autoxidation of unsaturated oil in oxygen gas, changes of POV decreased with decreasing amount of the water (200 `20 ppm) dissolved in the oil. But those of POV were almost constant in the range from about 200 to about 900 ppm of water. And POV increased with increasing water content ( about 900 to 1800 ppm) in the oil3). 4) The enzyme lipase catalyzes either esterification or hydrolysis of oil depending on the water content in the oil. However, most of lipases lose it's activity at such very low water concentration as below 300 ppm in the oil, unless being immobilized together with some of special activators4). As mentioned above, the dissolved water in oils is playing some important roles in the oil chemistry. The states of this dissolved water in the oil are proposed to be free water, structual water, and hydrogen-bonded water, and usually the main state of the water in oils, to be structual water, i. e. clusters'. However, it was not clarified yet in detailes how the states of water clusters specifically in the oils are. The states of dissolved water in oils have been investigated by means of the Fourier transform near infrared spectroscopy ( FT -NIR ), referring to a typical combination vibration (v 2+ v 3) of water appearing at around 5200 cm-1 5),6). On the other hand, the state analysis of the water itself has been developped by means of low temperature matrix method7), or molecular beam method at the range of IR (3000 `4000 cm-1 )8). The characteristic absorptions of each water cluster were determined so as to make it possible

2 J. Jpn. Oil Chem. Soc. (YUKAGAKU) to analyze monomer, dimer, -.polymer of water cluster respectively7) `9). In this paper, FT-IR (especially at 3000 `4000 cm-1) in comparison with FT-NIR (5000 `6400 cm-1) is applied to analyze the states of dissolved water in triglycerides such as tristearin TSt), triolein ( TO), trilinolein TLe) and trilinolenin TLn). 2 Experimental 2.1 Materials All of triglycerides were G. R. reagents from Sigma Corp.. All of these TG were saturated with water before measuring. 2.2 Methods A Fourier transform infrared spectorophotometer FT-IR3 of Nihon Bunko Corporation was reformed to be used for measuring absorption in the range from 8000 to 3000 cm-1 at 20 Ž. Measuring cells used were made from silica for spectoroscopy. The pass length (1 or 5 mm) of cells were selected with water contents in samples. Dissolved water in the samples was measured with a coulombmetry moisturemeter (CA-05) of Mitubishi Kasei Corporation. 3 Results and Discussion 3.1 Absorption bands of water Fig.-1 shows the IR spectra of water at 20 Ž and 120 Ž. At 20 Ž, absorption peaks are distributed in the range from 3000 to 3600 cm-1, and the main peak is at 3400 cm-1 with 80% of total absorption area. On the other hand, at 120 Ž, absorption peaks are rather widely distributed in the range from 3400 to 4000 cm-1. The common wave numbers of each water cluster are summarized in Table-1 from those data reported7) `9). Referring to Table-1 the states of structural water are estimated to be mainly bigger polymer clusters than dimer clusters at 20 Ž and to be mostly monomer water or dimer clusters at 120 C. 3.2 The states of dissolved water in triglycerides Figs.-2, 3, 4, and 5 show the spectra of IR and NIR spectra of tristearin TSt), triolein ( TO ), trilinolein TLe), and trilinolenin TLn) at 20 Ž respectively. Those % figures shown at the lower part of Figs.-2 to 5 are calculated from a share of each absorption area in total absorption area of spectra, assuming that molecular absorption coefficients are 1. 0, and that, as for IR spectra, peaks in the range from 3785 to 3925 cm-1 are grouped as the monomer water, peaks in the range from 3715 to 3538 cm-1 as the dimer clusters, and peaks in the range from 3476 to 3240 cm-1 as the polymer clusters. And as for NIR-spectra peaks are similarily grouped as shown in Figs.-2 to 5. For instance, there observed were those absorption peaks at ƒë 2 ( cm-1) + ƒë 3 in the range of NIR ( cm-1) corresponding to the absorption peaks at ƒë 3 in the range of IR (3000- Fig.-1 IR spectra of water at 20 Ž and 120 Ž in nitrogen.

3 Table-1 Absorption bands of water. All frequencies are in cm-1 Fig.-3 IR and NIR spectra of dissolved water in triolein. Fig.-2 IR and NIR spectra of dissolved water in tristearin. Fig.-4 IR and NIR spectra of dissolved water in trilinolein.

4 J. Jpn. Oil Chem. Soc. (YUKAGAKU) Fig.-5 IR and NIR spectra of dissolved water in trilinolenin. Fig.-6 IR spectra of dissolved water in unsaturated triglyceride. Table-2 Distribution rates of cluster water in triglycerides (calculated from absorption area of IR-spectroscopy.) Table-3 Distribution rates of cluster water in triglycerides (calculated from absorption area of NIR-spectroscopy) (All rates are in %) 4000 cm-1) respectively. As the peaks commonly observed at around 3465 cm-1 among all of TG are the overtone of absorption of the ester, 7% is subtracted uniformly on the calculation of absorption area rates of the peak at around 3465 cm-1. Calculated figures are summarized in Table-2, and Table-3. Those IR-spectra of water, TO, TLe, and TLn are compared with in Fig.-6. As shown in Fig.-6 and Tables-2, 3 the states of dissolved water in triglycerides are widely distributed from monomer water to polymer clus-

5 ters. The distribution of respective water clusters in TG changed depending on the kinds of those fatty acids composing TG. There observed were the sihfts of the water states caused by the interaction between unsaturated bonds and dissolved water, since all of those TG investigated consist of normal chain of fatty acids with the same carbon number numbers of unsaturated bonds. of C18, but with the different As the number of unsaturated bonds of TG increased, the rate of monomer water decreased, and the bigger clusters than those of original water increased. 4 Conclusion 1) The states of the dissolved water in triglycerides (TG) were investigated at 20 Ž by the Fourier transform infrared and near infrared spectroscopy. 2) The states of water itself were mainly bigger polymer clusters than dimer ones at 20 Ž, and mostly monomer water or the small clusters such as dimer ones at 120 Ž. 3) The states of dissolved water in TG varied widely from monomer water to polymer clusters. 4) There observed were shifts of the water states in TG at 20 Ž caused by the interaction between unsaturated bonds and dissolved water. As the number of unsuturated bonds of TG increased, the rate of monomer water decreased, and the bigger cluster water than the original water increased. References (Received July 25, 1990) 1) M. Takasago and K. Takaoka, J. Jpn. Oil Chem. Soc. (Yukagaku), 29, 162 (1980) ; 30, 558 (1981) ; 31, 167 (1982) ; 31, 438 (1982) ; 32, 315 (1983) ; 35, 1010 (1986). 2) M. Takasago and K. Takaoka, J. Jpn. Oil Chem. Soc. (Yukagaku), 31, 91 (1982). 3) M. Takasago and K. Takaoka, J. Jpn. Oil Chem. Soc. (Yukagaku), 29, 162 (1980). 4) J. Kurashige, N. Matuzaki, and K. Makabe, J. Am. Oils. Chem. Soc., 64, 1252 (1987); J. Dispersion Science and Technology, 10, 531 (1989). 5) M. Takasago and K. Takaoka, J. Jpn. Oil Chem. Soc. (Yukagaku), 33, 772 (1984); 34, 102 (1985). 6) O. Bonner and Y. Chol, J. Phys. Chem., 78, 1727 (1974). 7) D. Strommen, D. Gruen, and R. Mcbeth, J. Chem. Phys., 58, 4028 (1973) ; L. Fredin, B. Nelander, and G. Ribbegard, J. Chem.,Phys., 66, 4065 (1977) ; R. Bentwood, A. Barnes, and W. Orville-Thomas, J. Mol. Spectrosc., 84, 391 (1980); A. Pine and W. Laferty, J. Chem. Phys., 78, 2154 (1983). 8) R. Page, J. Frey, Y. Shen, and Y. Lee, Chem. Phys. Lett., 106, 373 (1984); D. Coker, R. Miller, and R. Watts, J. Chem. Phys., 82, 3554 (1985); T. Tso and E. Lee, J. Phys. Chem., 89, 1612 (1985). 9) J. Reimers and R. Watts, Chem. Phys., 85, 83 (1984).