Globular protein gels have been extensively investigated over many years, although the great majority of studies have used rather crude and/or mixed samples of protein. Moreover, most previous work has concentrated on examining structural and rheological properties of fully cured gels. In the present paper we will discuss heat-induced gelation of the typical globular protein, bovine serum albumin (BSA), not far from critical gel conditions together with aspects of kinetic gelation theory. We will concentrate on a description of the parameters that alter the gelation or gel time, tc. We will develop a new model to describe both the effect of temperature and polymer concentration on tc. Gel formation is achieved by isothermal heating. The change in rheological properties during the gelation process is monitored by dynamic shear rheometry as a function of time. The measurements are carried out at different temperatures and protein concentrations to clarify their effects on the gelation. The results are displayed in various diagrams, including a new form of sol/gel state diagram, and discussed in terms of both phase behavior and gelation kinetics.
In the first paper in this series, we investigated heat-induced
gelation of single component
systems of the globular protein, bovine serum albumin. A series of
rheological experiments revealed
that we can describe gelation time in terms of two physical parameters,
concentration and temperature.
In this paper we examine more complex systems, including the
effect of pH and ionic strength, on heat-induced gelation of bovine serum albumin (BSA), and another globular
protein, β-lactoglobulin (β-Lg).
These factors significantly influenced protein gelation, a result
which has not been reported in detail
before. However, no detailed analysis was possible, because the
behavior is very complex. We also consider
the extension of both model and experiments to (quasi-) binary mixed
systems, viz. the gelation of binary
systems chosen from three globular proteins: BSA, β-Lg, and
α-lactalbumin (α-La). We have obtained
reliable results only for the BSA and β-Lg system because of
experimental difficulties. From the data of
the BSA/β-Lg system, it was found that gelation behavior changed
significantly with the ratio of the two
proteins. The change in the gelation time and temperatures was not
expressed as a linear equation but
required a cross-term. This strongly suggests that current models
for the concentration dependence of
modulus for mixed biopolymer gels will need to be
modified.
Heat-induced gelation of milk was studied using both rheological and structural techniques. The sample was a conventional skim milk, concentrated with an ultrafiltration membrane, which formed gels when heated at appropriate pH. We investigated some factors that are considered to affect the gelation, such as concentration, pH and rennet treatment. The gelation process was monitored with a high precision oscillatory shear rheometer and the structure of gels was evaluated with quasi-elastic laser light scattering. From these results the gelation and phase separation behaviour were determined. By combining the results for different concentrations a phase diagram was obtained, which indicated that skim milk had a two-phase region on the higher temperature side. The effects of pH and rennet treatment were also evaluated with the aid of this phase diagram. The results were discussed on the basis of concepts of the phase behaviour of polymers, which were successfully developed in polymer physics.Milk gels such as yogurt and cheese are amongst the most popular food materials. Although there are numerous studies on milk gels and the gelation process, systematic studies are quite few. This is due mainly to the complexity of milk components and their structure, and also because milk gelation is affected by various factors, such as pH, temperature, metal ions and the presence of rennet. However, there have been some attempts to analyse this complex system theoretically since the late 70s as reviewed by Clark & Ross-Murphy (1987). Payens (1976) analysed the rennet-induced gelation of milk by measuring the turbidity. He introduced two models to explain the process; the Michaelis-Menten model for the enzymic reaction step and the von Smoluchowski model for the casein aggregation step. Parker & Dalgleish (1977) adopted another approach to the milk gelation induced by Ca 2+ or heating. They used a classical branching theory, the socalled Flory-Stockmayer theory (Flory, 1953; Gordon & Ross-Murphy, 1975) to estimate the functionality of casein. Tokita et al. (1984) introduced the three dimensional lattice percolation theory to explain the rheological results of enzymeinduced gelation of casein, and a computer simulation based on this theory was performed by Steventon et al. (1991) for whey protein gels. The percolation theory, which also includes the Flory-Stockmayer theory as a particular case, has been
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