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

In this work we attempted to define the structure of particulate colloidal protein gels using salt-induced cold gelation of heat-denatured whey protei n isolate (WPI) as a model. WPI loses its tertiary structure and forms solu ble protein aggregates during heat denaturation as evidenced by near-UV cir cular dichroism and fluorescence spectroscopy, as well as size-exclusion HP LC. Sodium chloride and calcium chloride induced the aggregation of these p articles by dispersing charge, and in the case of calcium, also by crosslin king. Light microscopy revealed that these gels were composed of flocs of a ggregated primary particles. Flocs are termed microstructures, while primar y particles are termed microstructural elements. The size of the microstruc tures ranges from 10 to 20 mu m, while the size of the microstructural elem ents ranges from 0.5 to 1.0 mu m These gels were structured and behaved the ologically as stochastic mass fractals, where the elastic constant of the g els (K) was related to the volume fraction (phi) of protein in a power law fashion, namely K = gamma phi(m). The gel network was found to be in a weak -link regime [Shih, Shih, Kim, Liu & Aksay, 1990. Physical Review A, 42, 47 72-4779]. For this case, In = 1/(3 - D), where D is the fractal dimension f or the packing of primary particles within fractal Aocs. The constant gamma contains information about the particles that make up the network. The mas s fractal dimension increased as a function of increasing calcium chloride concentration in the range 5-150 mM, while it remained unchanged as a funct ion of increasing sodium chloride concentration in the range 225-400 mM. Th e mass fractal dimension determined theologically in the weak-link regime a greed very well with the spatial mass fractal dimension determined from ima ge analysis of TEM micrographs of the gels. The constant gamma decreased in both cases as a function of increasing salt concentration. Internal-L-valu e measurements suggested that particle size decreased as a function of incr easing protein concentration and increased as a function of increasing salt concentration, as expected from decreases in the constant gamma. A mechani cal and structural model for particulate gels in the weak-link regime was a lso developed in this work, providing insight into the nature of the consta nt gamma, where gamma 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 int erface between two fractal flocs (m), inversely proportional to the square of the fractal flee diameter (xi), and inversely proportional to the diamet er of primary particles (a), namely gamma similar to (mk(p)d(0)/a xi(2)). ( C) 2000 Elsevier Science Ltd. All rights reserved.