Rheology of whey protein solutions and gels : thesis submitted for the degree of Doctor of Philosophy in Food Technology at Massey University, New Zealand
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Date
1993
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Massey University
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Abstract
The 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.
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Keywords
Whey proteins, Whey protein products