Browsing by Author "Tang, Qingnong"
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- ItemRheology of whey protein solutions and gels : thesis submitted for the degree of Doctor of Philosophy in Food Technology at Massey University, New Zealand(Massey University, 1993) Tang, QingnongThe use of whey protein products in foods is governed by their nutritional and functional properties. Whey protein products have increasingly been applied in a variety of food systems as functional ingredients. In order to boost applications of whey protein products and to improve, predict and control their functional attributes in food products knowledge is required about how they behave functionally under different conditions, e.g. when product composition, processing history, protein concentration, pH, salt concentration and temperature vary. The flow properties of whey protein concentrate solutions were studied in a Bohlin rheometer. The effects of protein concentration, temperature, pH and salts on the gelation and gel properties of whey protein concentrates and whey protein isolate were also investigated in the same rheometer. Differences in gelation between whey protein concentrates, whey protein isolate, egg white and B-lactoglobulin were studied. Differences between dynamic shear properties determined in a Bohlin rheometer and fracture properties determined in an Instron universal testing machine were also studied. The flow properties of whey protein concentrate solutions changed from Newtonian to pseudoplastic or even thixotropic behaviour, owing to structure formation in the solutions, i.e. to increases in protein intermolecular interactions. Such structure formation resulted from increases in protein concentration, temperature or CaC12 concentration, and from shifting the pH to extreme values. Gelation of whey protein was dependent on protein concentration, gelation temperature, pH, salt content and lactose content. Salt content was the most important factor in determining the gelling properties of various whey protein concentrate products and whey protein isolate. Consistent gelling properties could only be achieved when salt content was carefully controlled. The degree of protein denaturation and lactose content also led to differences in gelling behaviour of whey protein concentrates. Whey protein products, when compared with ·egg white, had a higher gelation temperature, a higher minimum protein concentration for gelation, lower initial gelation rate and lower gel stiffness. The differences in initial gelation rate and gel stiffness could be compensated by adjustment of the salt content of whey protein products. Dynamic viscoelastic measurements on whey protein isolate gels in the region of the sol-gel transition exhibited simple power law relationships between the storage (G' ) and loss (G") moduli and frequency as G' oc ro0·54±0.o2 and G" oc ro051±0.o2 , indicating that the gel in the region of the sol-gel transition could have the geometry of a fractal. The critical exponents calculated from the protein concentration dependence of gelation time and from the site percolation model indicated that the gelation of whey protein is a realization of a percolation process. Compression rigidity modulus (Ec), penetration rigidity (EP), tension rigidity (EJ and storage modulus G' all exhibited a similar pattern of variation with pH. G' , Ec, EP and Ev which were not closely related to the fracture properties and hardness of whey protein concentrate gels, were controlled by electrostatic interactions. The fracture forces and hardness were determined by both disulphide bonds and electrostatic interactions, while fracture strains were mainly controlled by disulphide bonds.
- ItemSteady shear and oscillatory rheological measurements on solutions of commercially available whey concentrates : a thesis presented in partial fulfilment of the requirements for the degree of Master of Technology in Food Technology at Massey University(Massey University, 1990) Tang, QingnongThree commercially available whey protein concentrates (WPC), Alacen 312, Alacen 392, and Alacen 475 were studied by steady shear and oscillatory rheological methods using a Bohlin rheometer at concentrations of 5-40% and temperatures of 5-9o0 c. The WPC solutions showed Newtonian behaviour up to a concentration of 15%, were slightly shear thinning at 20 and 25%, and exhibited time dependent or thixotropic behaviour at concentrations of 30% and above. The apparent viscosity of Alacen 475 solutions of concentration less than 10 percent by weight could be calculated by ns = w(1+28C) where ns, nw and Care the viscosity of the solution, the viscosity of water and the fractional weight concentration. For Alacen 475 solutions of 40% concentration the structure broken down by shearing at a high shear rate of 734 s-1 recovered slowly when the shear rate was suddenly dropped to 147 s-1 or zero. The apparent viscosity of WPC solutions was temperature dependent. It decreased at first as temperature increased until a minimum viscosity was attained and then increased rapidly with further increase in temperature. Temperature also had a marked effect on the time dependency of 20% and 30% WPC solutions - causing time dependent shear thinning at 40 and 50°c and time dependent thickening at 60 and 70°c. The continuous changes in structure of WPC solutions during heat-induced gelation were followed using oscillatory rheological measurements. The effects of temperature, concentration and heating time on the formation and dynamic rheological properties of WPC gels were determined and are discussed in terms of current theories on the rheology of protein solutions and gels. A gelling model for the gelation of globular protein solutions was proposed to interpret the development in protein gel structure during the gelling process as reflected by the continuous changes in dynamic rheological properties.