关键词: 酪蛋白酸钠；触变性；黏度；扩散光谱；红外光谱

Abstract: Sodium caseinate is commonly used in the food industry. In its many applications as a food ingredient, significant quantities of salts are simultaneously used. Previous studies have shown that co-existing salts can change physiochemical properties of sodium caseinate, especially rheological properties. NaCl has been shown to be able to dramatically increase viscosity of sodium caseinate solution; however, the mechanism is still debatable. In order to further probe the influence of NaCl on rheological properties of sodium caseinate solution and clarify the mechanism. A rheometer, diffusing wave spectroscopy (DWS) and FTIR were combined to investigate the effect of NaCl on properties of sodium caseinate solution. The results showed that the shape and peak positions of FTIR spectra did not change after addition of NaCl, but the intensity changed. Preliminary analysis suggested that NaCl molecules could compete with sodium caseinate for water and indirectly increased protein concentration, while decreasing diffusion properties of the system, as indicated by the increased characteristic decay time τ. It also indicated that NaCl did not influence the secondary structure of sodium caseinate molecules. Rheological characterization indicated that 3% and 7.5% sodium caseinate solutions were Newtonian fluid while 15% sodium caseinate solution was non-newtonian fluid. Addition of NaCl significantly increased protein solution viscosity, and this phenomenon was more obvious at higher protein concentration. To further investigate the rheological properties of 15% sodium caseinate solutions, time dependence and thixotropy measurements were performed. The results showed that viscosity of all these samples increased in a linear manner with increasing time as thixotropic fluid. Collectively, we concluded that high concentrations of sodium caseinate solutions are negative rheopecticity fluid. All these results could help us better understand interactions of NaCl with sodium caseinate and provide theoretical guidance for using sodium caseinate in the food industry.

Key words: sodium caseinate；thixotropy；viscosity；diffusing wave spectroscopy；infrared spectroscopy