Thermal Stability and Kinetic Study on Thermal Decomposition of Commercial Edible Oils by Thermogravimetry (original) (raw)

Abstract

Thermal stability and kinetic parameters of commercial edible oils were investigated by non-isothermal thermogravimetry/derivative thermogravimetry (TG/DTG). Kinetic parameters were calculated by integral and approximation methods. Results obtained indicated that these parameters were dependent on composition of fatty acids, being influenced by the presence of natural and artificial antioxidants. According to the thermogravimetric curves, the following thermal stability sequence was suggested: corn > sunflower > soybean > rice > soybean + olive > sunflower + olive > canola > olive; while the activation energy indicated the following stability order: sunflower > corn > soybean > rice > soybean + olive > canola > sunflower + olive > olive.

Figures (9)

Table 1—Fatty acid composition (%) of some commercial edible oils A usual characteristic in most vegetable oils is the high amount of unsaturated fatty acids present in the triglyceride molecules (Table 1). In the triglyceride, the main route of de- terioration and possible loss of stability is the oxidative ran- cidity.

Table 1—Fatty acid composition (%) of some commercial edible oils A usual characteristic in most vegetable oils is the high amount of unsaturated fatty acids present in the triglyceride molecules (Table 1). In the triglyceride, the main route of de- terioration and possible loss of stability is the oxidative ran- cidity.

Figure 1—TG curves of the commercial edible oils in ait atmosphere Among dynamic methods, differential, integral, and ap- proximation methods can be cited, where studies were car- ried out in previous works (Souza and others 1998). In this work we have selected the following methods, representative of different categories, and applied several equations to the thermal data: Coats and Redfern (1964), Madhusudanan (Madhusudanan and others 1993), Horowitz-Metzger (Horowitz and Metzger 1963), and Van Krevelen (Van Krev- elen and others 1951) methods. The kinetic parameters cal- culated were: reaction order (n), activation energy (E,), and frequency factor (A).

Figure 1—TG curves of the commercial edible oils in ait atmosphere Among dynamic methods, differential, integral, and ap- proximation methods can be cited, where studies were car- ried out in previous works (Souza and others 1998). In this work we have selected the following methods, representative of different categories, and applied several equations to the thermal data: Coats and Redfern (1964), Madhusudanan (Madhusudanan and others 1993), Horowitz-Metzger (Horowitz and Metzger 1963), and Van Krevelen (Van Krev- elen and others 1951) methods. The kinetic parameters cal- culated were: reaction order (n), activation energy (E,), and frequency factor (A).

Figure 2—DTG curves of the commercial edible oils

Figure 2—DTG curves of the commercial edible oils

Coats and Redfern (1964) developed an integral method which can be applied to TG/DTG data, assuming the differ- ent reaction orders. The order related to the most appropri- ate mechanism is presumed to lead to the best linear plot, from which the activation energy is determined. The equa- tion used for analysis is: Horowitz-Metzger method An other integral method to evaluate a kinetic model was developed by Madhusudanan and others (1993), for estimates the kinetic parameters from TG/DTG curves. The activation energy can be calculated from the following expression:

Coats and Redfern (1964) developed an integral method which can be applied to TG/DTG data, assuming the differ- ent reaction orders. The order related to the most appropri- ate mechanism is presumed to lead to the best linear plot, from which the activation energy is determined. The equa- tion used for analysis is: Horowitz-Metzger method An other integral method to evaluate a kinetic model was developed by Madhusudanan and others (1993), for estimates the kinetic parameters from TG/DTG curves. The activation energy can be calculated from the following expression:

As in the previous derivations, the Van Krevelen method (Van Krevelen and others 1951) is based on approximate in- tegration of the rate equation, resulting in a linear plot of data, from which activation energy and pre-exponential fac- tor can be easily calculated using the following expression: where: T = absolute temperature, R = gas constant, ¢ = heat- ing rate, T, = peak temperature of DTG curve.

As in the previous derivations, the Van Krevelen method (Van Krevelen and others 1951) is based on approximate in- tegration of the rate equation, resulting in a linear plot of data, from which activation energy and pre-exponential fac- tor can be easily calculated using the following expression: where: T = absolute temperature, R = gas constant, ¢ = heat- ing rate, T, = peak temperature of DTG curve.

Table 3—Kinetic parameters obtained for the first step of thermal decomposition of commercial edible oils

Table 3—Kinetic parameters obtained for the first step of thermal decomposition of commercial edible oils

“Dm is the weight loss under heating and T is the peak temperature. Table 2—Thermal decomposition data of commercial edible oils obtained from TG/DTG curves

“Dm is the weight loss under heating and T is the peak temperature. Table 2—Thermal decomposition data of commercial edible oils obtained from TG/DTG curves

Table 4—Kinetic parameters obtained for the second step of thermal decomposition of commercial edible oils

Table 4—Kinetic parameters obtained for the second step of thermal decomposition of commercial edible oils

Table 5—Kinetic parameters obtained for the third step of thermal decomposition of commercial edible oils

Table 5—Kinetic parameters obtained for the third step of thermal decomposition of commercial edible oils

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  20. MS 20010100 Submitted 3/1/01, Accepted 4/6/01, Received 5/11/01
  21. The authors acknowledge CAPES and CNPq for scholarship and financial support of this work.