On the structure of particulate gels—the case of salt-induced cold gelation of heat-denatured whey protein isolate

• Published in: Food hydrocolloids Vol. 14; no. 1; pp. 61 - 74
• Main Authors: Marangoni, A.G., Barbut, S., McGauley, S.E., Marcone, M., Narine, S.S.
• Format: Journal Article
• Language: English
• Published: Oxford Elsevier Ltd 2000; Elsevier
• Subjects: Aggregation; Applied sciences; Biological and medical sciences; Calcium; Colloidal; Denaturation; Disperse state. Micelles; Exact sciences and technology; Food additives; Food industries; Fractal; Fundamental and applied biological sciences. Psychology; Gel; General aspects; Kinetics; Molecular biophysics; Natural polymers; Physico-chemical properties of biomolecules; Physicochemistry of polymers; Proteins; Sodium; Weak-link; Aggregation; Fractal; Sodium; Calcium; Denaturation; Weak-link; Kinetics; Colloidal; Gel; Elastic constant; Heat treatment; Texture analysis; Gelation; Textural agent; Thermal denaturation; Colloidal gel; Particle; Image analysis; Fractal dimension; Ionic strength; Microstructure; Whey protein; Mineral salt; Rheological properties
• ISSN: 0268-005X; 1873-7137
• DOI: 10.1016/S0268-005X(99)00046-6

In this work we attempted to define the structure of particulate colloidal protein gels using salt-induced cold gelation of heat-denatured whey protein isolate (WPI) as a model. WPI loses its tertiary structure and forms soluble protein aggregates during heat denaturation as evidenced by near-UV circular dichroism and fluorescence spectroscopy, as well as size-exclusion HPLC. Sodium chloride and calcium chloride induced the aggregation of these particles by dispersing charge, and in the case of calcium, also by crosslinking. Light microscopy revealed that these gels were composed of flocs of aggregated primary particles. Flocs are termed microstructures, while primary particles are termed microstructural elements. The size of the microstructures ranges from 10 to 20 μm, while the size of the microstructural elements ranges from 0.5 to 1.0 μm. These gels were structured and behaved rheologically as stochastic mass fractals, where the elastic constant of the gels ( K) was related to the volume fraction ( φ) of protein in a power law fashion, namely K= γφ m . The gel network was found to be in a weak-link regime [Shih, Shih, Kim, Liu & Aksay, 1990. Physical Review A, 42, 4772–4779]. For this case, m=1/(3− D), where D is the fractal dimension for the packing of primary particles within fractal flocs. The constant γ contains information about the particles that make up the network. The mass fractal dimension increased as a function of increasing calcium chloride concentration in the range 5–150 mM, while it remained unchanged as a function of increasing sodium chloride concentration in the range 225–400 mM. The mass fractal dimension determined rheologically in the weak-link regime agreed very well with the spatial mass fractal dimension determined from image analysis of TEM micrographs of the gels. The constant γ decreased in both cases as a function of increasing salt concentration. Internal- L-value measurements suggested that particle size decreased as a function of increasing protein concentration and increased as a function of increasing salt concentration, as expected from decreases in the constant γ. A mechanical and structural model for particulate gels in the weak-link regime was also developed in this work, providing insight into the nature of the constant γ, where γ is proportional to the force constant of the springs, or bonds, between primary particles ( k p), proportional to the interfloc distance ( d 0) and the number of particle–particle interactions at the interface between two fractal flocs ( m), inversely proportional to the square of the fractal floc diameter ( ξ), and inversely proportional to the diameter of primary particles ( a), namely γ∼( mk p d 0/ aξ 2).