Contact angle and surface energy analysis of soy materials subjected to potassium permanganate and autoclave treatment

Abstract

Initial water contact angle, apparent rate of water absorption and dispersive and polar surface energy were investigated for compacts of soy flour, two reference materials, soy protein isolate (protein reference material) and soy hulls (carbohydrate reference material), and the residue obtained after alkaline treatment of soy flour (insoluble soy). Untreated soy flour compacts had an initial water contact angle (57°) similar to soy protein isolate compacts (64°) but significantly lower than insoluble soy compacts (74°) and soy hulls compacts (85°). Potassium permanganate oxidation treatment increased the initial water contact angle of all soy materials compacts except for soy hulls compacts. The apparent rate of water absorption after potassium permanganate treatment, decreased for all types of soy compacts except for insoluble soy compacts. Autoclave treatment did not affect the initial water contact angle of soy flour and soy hulls compacts but increased significantly for soy protein isolates and insoluble soy compacts. The effect of autoclave treatment was reflected by the apparent rate of water absorption where significant increase was measured according to the type of soy material. The total surface energy and the dispersive and polar components, estimated from water and diiodomethane contact angle, indicated similar dispersive surface energy for soy flour, soy protein isolate and insoluble soy compacts (30mN/m) but higher for soy hulls (36mN/m). Significant differences of the polar surface energy component of treated materials compared to compacts of the untreated materials were measured, 18.4mN/m (soy flour), 13.6mN/m (soy protein isolate), 8.2mN/m (insoluble soy) and 2.7mN/m (soy hulls). The potassium permanganate oxidation treatment reduced significantly the polar surface energy component of soy materials compacts except soy hulls which increased. In contrast, autoclave treatment did not affect the polar surface component of soy flour but increased for all other soy materials compacts. The most appropriate soy material, based on the polar surface energy characteristics, for compatibility with hydrophobic polymeric materials, appear to be soy hulls, insoluble soy and soy protein isolate subjected to potassium permanganate treatment.