Gelation of Milk Protein Concentrates Induced by Moderate Hydrostatic Pressures

Abstract

Gelation of aqueous milk protein concentrates made up of spray dried milk was monitored both at atmospheric pressure and at moderate pressures (up to 1000 bar) using rheological methods. A Paar Physica UDS 200 rheometer was used to measure the gelation time at atmospheric pressure as a function of the temperature for a protein concentration of 16% [w=w]. The temperature dependence of the gelation time is well approximated by the Ross-Murphy model, which was originally developed to describe the concentration dependence of the gelation time. Using a purpose built Haake High Pressure High Temperature Rheometer, the effects of moderate pressures upon the gelation of milk protein concentrates (13% [w=w] protein content) were investigated. It was found that the gelation time reduces significantly (by up to almost an order of magnitude), when pressures of up to 1000 bar were applied. These findings, combined with the work of others indicate that these pressures bring about two effects: (1) casein micelle dissociation and as a result facilitate reassociation of those smaller micelle fragments, and (2) denaturation of beta-lactoglobulin. However, the main contribution to enhanced gelation was attributed to the caseins. It is not clear if association of whey proteins with caseins occur at moderate pressures around 50 C. We have modelled the dependence of the rate of gelation on pressure by an Eyring reaction rate process type equation. The major contribution to enhanced gelation upon pressurisation was attributed to a disruption of hydrophobic groups and ionisation of charged groups caused by an increase in electrostrictionhowever, disruption of hydrogen bonds was suggested to contribute to a lesser but still important extent.

High pressure-induced changes in milk proteins and possible applications in dairy technology

“…Changes detected in viscosity, viscoelasticity and texture of gels made from concentrated milk were a function of the pressure level and, as it was the case of rennet or acid gels, were mainly attributed to the concomitant disintegration of the protein structure into fragments that reaggregate on the release of pressure, giving rise to new structures inaccessible via conventional methods of gelling caseins (Briscoe, Luckham, & Staeritz, 2002;Ve´lez-Ruiz, Swanson, & Barbosa-Ca´novas, 1998). According to Keenan et al (2001), micellar dissociation is a requisite for gel formation, while whey protein aggregation (whether self-aggregation or formation of complexes with caseins) plays a subordinate role.…”