ADSORPTIVE BLEACHING OF SOYBEAN OIL WITH NON-MONTMORILLONITE ZAMBIAN CLAYS

As an alternative to montmorillonite clay, three local Zambian clays have been used to bleach soybean oil. The bleaching action of the natural clays was poor when compared with commercial acid-activated montmorillonite (M-C) and activated charcoal (A-C) adsorbents. However, acid-activation of the Zambian clays profoundly increased their adsorptive activity. Reduction of 88% in soybean oil color (Lovibond Red) by each of the three activated Zambian clay samples represented an efficiency close to that of montmorillonite (94%) and better than activated charcoal (63%). Peroxide value (PV) of the oil was reduced by 85% (M-C) and 78% (A-C) while 68% was the highest reduction for the activated Zambian clays. After 12 wk of storage at ambient temperature, the bleached soybean oil samples showed some oxidation. Consideration of the totox values indicated that the Zambian clay-bleached oil was more stable over this length of storage when compared with the M-C bleached oil. The bleaching action shown by aluminum-exchanged clays was closely related to their acid-activated counterparts. These results demonstrate a dependency of adsorptive bleaching with Zambian clays on proton availability. Comparative powder x-ray diffraction analysis of the clays showed that quartz was the major mineral present, followed by kaolinite. No montmorillonite was detected. It was concluded that by appropriate treatment to generate Bronsted acidity (protons), Zambian clays can be converted into potent adsorbents for soybean oil impurities.