Study Of The Interaction Of Hydroxypropyl Distarch Phosphate And Casein In Yoghourt

This paper researches the interaction of casein and hydroxypropyl starch phosphate(HPDSP) in yoghurt system in order to find the mechanism of stability. Casein andHPDSP was placed in simulation yoghurt environment in order to facilitate theobservation and research. This study inchuding four aspeacts: the extraction of casein&sample preparation; rheological measuiement of yoghurt samples; measurement of Zetapotential, partical size&conductivity; microscopic observation.Isoelectric precipitation was used to extract casein. After isoelectic precipitation,dionization and derosination casein flour was obtained. We used kjeldahl determinationand SDS-PAGE gel electrophoresis to test the purity of casein. In order to simulate theactual yoghurt system, the sodium acetate-acetic acid buffe was applied to pH ofsamples. The yoghurt samples which adding hydroxypropyl starch phosphate (HPDSP)showed similar rheological behavior with without samples. The yoghurt samples belongto pseudoplastic fluid. The addition of Hydroxypropyl starch phosphate causedincreasing of elasticity modulus, viscous modulus and colloidal viscosity. We think it ismainly because the molecular modification of hydroxypropyl starch phosphate.Phosphate crosslinking and the introduction of hydroxypropyl increases the swellingproperty of starch, to further increase the apparent viscosity of yogurt system, degree ofcrosslinking increased the molecular weight of the polymer in the yogurt system, thusincreasing the viscoelastic modulus of the system.Dynamic Light Scattering (DLS), Zeta Potential Analyzer and conductivity meterare also be used for measurement. Through zeta potential measurement, we think thatcasein particles absorb on the surface of the HPDSP via electrostatic forces whichweicould be demonstated by AFM observation. DLS measurement and microscopicobservation results show that the sample with HPDSP has lower aggregate degree thanwithout. Light microscope, Scanning Electron Microscope (SEM) and Atomic ForceMicroscope (AFM) was applied to observe the microstructure of the samples. Andthrough AFM further proved that some protein particle adsorption on the surface ofstarch granule at pH3.8.Ultimately we believe that in the yogurt system negatively charged HPDSPparticles and some positive charged asein micelles atttract each other by the electrostaticattraction. Then the casein micelles which have smaller size adsorpt on the surface ofHPDSP particle, formed larger positive charged aggregates. Due to the larger aggregates and casein with the same charge would form electrostatic repulsion to maintain systemstability. At the same time because of the addition of hydroxypropyl starch phosphatecasein aggregates particle size get smaller and smaller that exacerbating the Brownianmotion. And the addition of HPDSP increased the pparent viscosity of yogurt systemwhich is beneficial to maintain system stability.