Properties of recombined milk protein composite gels : effects of protein source, protein concentration and processing time : a thesis presented in partial fulfilment of the requirements for the degree of Master of Technology in Food Technology at Massey University, Palmerston North, New Zealand
Increased knowledge of the interactions involved in the manufacture of Milk Protein Composite Gels (MPCGs) is essential for the further development of dairy-based analogue and recombined products and the advancement of novel product development. This study investigated MPCG manufacture using four protein sources (Rennet Casein, skim milk cheese (SMC), milk protein concentrate (MPC 85), calcium-depleted milk protein concentrate (IX MPC 85)), three protein to water (P/W) ratios (0.4, 0.5, 0.6) and four processing times (0, 4, 8, 16 minutes). The properties of the products were investigated using confocal and transmission electron microscopy, as well as rheological and functional tests. Protein source was found to have the greatest impact on product characteristics, followed by P/W ratio with processing time having little, and often inconsistent, effects. Increased protein concentration resulted in a higher viscosity during manufacture, a decrease in fat droplet size, an increase in gel firmness, and a decrease in meltability. Increased processing time resulted in a decrease in fat droplet size, few significant changes in firmness (both small- and large-strain), and an increase in meltability Fracture property analysis showed that SMC produced softer, more elastic gels than Rennet Casein. The whey-containing samples produced softer, more brittle gels with little difference between them Small-strain analysis showed that all samples were weak gels but the results did not follow the same trend as the fracture properties. The samples increased in firmness in the following order: SMC < Rennet Casein < IX MPC 85 < MPC 85. Microstructure analysis showed the presence of whey protein aggregates in the MPC 85 and IX MPC 85 samples. These samples also demonstrated aggregation of the lipid droplets, which was attributed to the presence of whey proteins. Reduced levels of calcium resulted in lower levels of emulsification (larger lipid droplets) due to lower in-process viscosities. Correlations between large- and small-strain testing showed that the correlation coefficient was dependent on the protein source being used and that although the level of correlation was not high, there was a general positive trend The small-strain and UW Meltmeter tests did not agree on the order of increasing meltability except for the SMC samples, which were significantly more meltable than the other protein sources. The two tests were poorly correlated (R² = 0.446).