The Interaction Between Dairy Proteins And Insoluble Calcium Salts And Its Effect On Suspension Stability

Calcium-fortified dairy products have attracted more and more attention commercially as a health food contributing various essential nutrients and significant levels of calcium. Calcium supplements could be sorted out as soluble ones and insoluble ones. Protein flocculation would be induced by the strong interaction between calcium ions and dairy proteins. Thus soluble calcium salts tend to cause problem in heat stability which insoluble calcium salts avoid when used in heat-treated calcium-fortified formulations. But insoluble calcium salts have poor suspension stability. In food industry, insoluble calcium salts, often considered to be inert in dairy product, are common used calcium supplements in instant milk powder products. In China, calcium carbonate with high calcium content and low cost is frequently used, while calcium phosphate is the first choice in international market which could be easily absorbed as publicized by advertisements.This project, based on the study of interaction between calcium and protein in biomaterials fields, explored the calcium-fortified dairy products from a brand-new perspective. The experimental data revealed a different fact from the traditional consideration. In this study, the interaction between dairy proteins(i.e., whey protein isolate, sodium caseinate and milk protein concentration) and the insoluble calcium salts(i.e., calcium carbonate, hydroxyapatite and tricalcium phosphate) was confirmed. The protein adsorption subsequently and consequently affected the suspension stability when insoluble calcium salts dispersed in aqueous solution. The research has broken the limitation of the traditional view considering insoluble calcium salt as an inert component in calcium-fortified dairy products.Firstly, the interaction between protein and calcium particles was confirmed by zeta-potential characterization and laser scanning confocal microscope images. It found that dairy protein could adsorb on the surface of calcium particle and led to an increase in surface charge of calcium particle. The adsorption mechanism was discussed deeply. The adsorption between protein and calcium particles was driven by the electrostatic interactions between counter-ions, including Ca2+ sites on the calcium particles and the phosphorylated Ser and carboxyl groups of proteins, as well as the negatively charged sites on the calcium particles and the amino groups of proteins, and the former one was more strongly. In addition, the adsorption varied among different calcium salts, proteins and preparing conditions was described using different types of adsorption isotherm.Secondly, the interaction of individual protein components with calcium particles was investigated by the polyacrylamide gel electrophoresis. The adsorption preference order for WPI was β-lactoglobulin ? α-lactoalbumin; for SC, the preference of α-casein and β-casein depended on calcium salts; the order in MPC was β-casein ? α-casein.At last, the stability of calcium-protein complexes was studied. The heat instability of dairy proteins was well preserved with calcium-protein complexes added. The suspension stability of calcium-protein complexes in both water and protein solution was effectively improved. The stability improvement of calcium-protein complexes suspended in water was quite significant especially coated with WPI. While for the adsorption of SC or MPC, the suspension stability increased similarly with an unusual decrease in stability if small amount of protein was added. The Ca-protein complex made under different solution conditions exhibited similar suspension stability once the adsorption saturation was achieved. Furthermore, the suspension stability of calcium-protein complexes was not sensitive to the suspension p H.The interaction between proteins and insoluble calcium salts was studied in this project. The findings were quite different from the traditional view. Dairy proteins could be adsorbed on the calcium particles with adsorption capacity varied from calcium or protein types and solution conditions. We further investigated the suspension stability which was proved to be affected by protein adsorption. The better improvement of suspension stability was observed for calcium phosphates rather than calcium carbonate when the coated protein reached the adsorption saturation. Meanwhile, the particles coated with WPI were more stable in suspension than that coated with SC or MPC. These experimental facts may provide a better understanding of calcium-fortified dairy products with insoluble calcium salts as supplements. This research also provides important theoretical basis for improving the stability of calcium-fortified dairy products.