Introduction: The impurities of the oil and its pigments are basically removed from the oil by physical adsorption using an adsorbent during the bleaching process. The bleaching process involves the removal of pigments, impurities, metals and oxidation products. Removal of these substances is essential in oil refining because it improves the stability, appearance and sensory quality of the oil. Activated bleaching earth is the most commonly used adsorbent for purifying and improving the color of fats and oils. The bleaching process of edible oils is important for producing light colored oils with acceptable quality. The aim of this study was to evaluate the physical and chemical properties of soybean oil bleached with bleaching earth containing increased amounts of aluminum and magnesium oxides.
Material and Methods: Bleaching earth was purchased from Kanisaz Jam Company. Degummed and neutralized soybean oil was obtained from Behshahr Vegetable Oil Company. Different amounts of aluminum oxide and magnesium oxide were added to commercial bleaching earth. Activation of the adsorbents was performed with hydrochloric acid and oil bleached at 110°C for 30 min under vacuum by adding 2% of adsorbent containing different percentages of silica, aluminum and magnesium oxides. A series of physical and chemical tests such as peroxide value, acid value, chlorophyll content, carotenoid content, yellow and red colors and amounts of copper and iron were then carried out on the neutralized and bleached oils according to the standard methods. All the experiments and/or measurements were carried out in triplicate. Data were statistically analyzed using the Statistical Analysis System software package on replicated test data. Analysis of variance was performed by application of an ANOVA procedure. Significant differences between the means were determined using the Duncan multiple range test.
Result and Discussion: The results of this study showed that the examined adsorbents reduced the peroxide value to 98.9-96.3%. Application of the adsorbents containing 95% commercial bleaching earth - 5% aluminum oxide and 95% commercial bleaching earth - 5% acidic aluminum oxide reduced the acid value by 33.33% and 26.66%, respectively. The amount of chlorophyll in the control sample was 7.58 mg Pheophytin A/kg oil, which reduced 65.66% by using adsorbent containing 90% commercial bleaching earth and 10% magnesium oxide and reached to 1.90 mg Pheophytin A/kg. The amount of carotenoids in the control sample was 7.88 mg/kg. Using the adsorbent containing 90% commercial bleaching earth and 10% magnesium oxide decreased carotenoids up to 93.40%. Adsorbents containing 95% commercial bleaching earth and 5% aluminum oxide, 95% commercial bleaching earth and 5% magnesium oxide, 95% commercial bleaching earth and 5% acidic magnesium oxide and commercial bleaching earth had the same effect on red color reduction. Yellow color in the oil samples treated with commercial bleaching earth, adsorbent consisting of 90% commercial bleaching earth - 10% aluminum oxide, and adsorbent containing 95% commercial bleaching earth - 5% magnesium oxide was reduced and reached to 38, 50 and 50 Lovibond, respectively as compared to the control sample with yellow color of 70.00 Lovibond. Copper and iron decreased 100% by using adsorbents containing 50% commercial bleaching earth and 50% aluminum oxide or 50% commercial bleaching earth and 50% magnesium oxide.
According to our findings, the addition of aluminum and magnesium oxides to commercial bleaching earth was effective in reduction of peroxide value, acid value, chlorophyll, carotenoid, red and yellow color, copper and iron. Also, the results showed that the best adsorbent contain about 50% aluminum and magnesium oxides. Aluminum and magnesium oxides can improve the performance of bleaching earths.