Suitable inorganic salt and its content inhibiting tension of oil-water interface with wheat germ lipase

Abstract: The stabilization of lipase is an important means to control the rancidity of highly oil-containing agricultural products, such as wheat germ. In this paper, the effects of different concentrations of Na+, K+, Ca2+ and Mg2+ on the oil-water interface characteristics of wheat germ lipase A (LA) were examined, based on the interfacial enzymatic analysis methods using interfacial tension meter and rotary rheometer. Firstly, the influence of the different concentrations of wheat germ LA on the interfacial tension of oil-water was studied to determine the concentration of LA system for the subsequent investigation about the effects of inorganic salts on the oil-water interface. Then, the interface tension and rheological properties of the oil-water after adding monovalent and divalent metal salt were studied to explore the effects of species and concentration on the oil-water interface. Next, the effects of different kinds of inorganic salts on the surface tension and rheological properties of oil-water interface, where there were lipase-acting substrate and product, namely, glycerol trioleate and oleic acid, were studied. Finally, the potential mechanism of inhibiting the catalytic activity of metal ions of inorganic salt in inhibiting wheat germ LA activity was discussed. The results showed that, when the concentration of LA was 1.70×10-6 mol/L, the monovalent metal ions Na+ and K+ were more favorable to decrease the interfacial activity of oil-water and the interfacial activity of wheat germ LA, than other divalent metal ions, i.e. Mg2+ and Ca2+. And the effect of Na+ was higher than that of K+. In the divalent metal ions, Ca2+ could decrease the interfacial tension at a low concentration and increase interfacial tension at a high concentration, while Mg2+ was opposite to Ca2+. At the same time, the decrease effect of Ca2+ on the interfacial tension was more obvious than Mg2+. Besides, the effects of glycerol trioleate and oleic acid on the interfacial properties of wheat germ LA were studied; the effect extent was glycerol trioleate > oleic acid, and the concentration of these 2 compounds was 1.0×10-5 and 5.0×10-6 mol/L, respectively. Under the 2 concentrations of glycerol trioleate and oleic acid, adding Na+ (10-6 mol/L), K+ (10-6 mol/L), Ca2+(10-4 mol/L), and Mg2+(10-9 mol/L)to the oil-water system, could reduce the interfacial tension of oil and water, and then reduce the effect of wheat germ lipase. In summary, inorganic salts play an important role in the inhibition of interfacial activity of wheat germ LA mainly by reducing the catalytic effect through affecting the aggregation behavior of wheat germ LA in oil-water interface and the structure of substrate, and play a role of passivating wheat germ LA. The results of this paper can provide theoretical support for the regulation of lipase activity and wheat germ stabilization in wheat germ industry.