Diffusing Wave and Ultrasonic Spectroscopy of Rennet-Induced Gelation of Milk in the Presence of High-Methoxyl Pectin

Rennet-induced aggregation was studied in milk systems containing high-methoxyl pectin (HMP) using ultrasonic and diffusing wave spectroscopy. These two techniques allow for in situ measurements of sol–gel transitions without the need for dilution. At low HMP concentrations, the casein micelles aggregation behavior was similar to that of skim milk, although changes could be noted in the microstructure of the renneted gels. At HMP concentrations between 0.1 and 0.15%, phase-separation kinetics were slower than the rennet-induced aggregation, and different microstructures formed caused by different dynamics of interactions between casein micelles present in HMP-depleted flocs. Higher amounts of HMP failed to create a continuous gel, as phase separation occurred at a faster rate than rennet aggregation. These results highlight the importance of non-invasive techniques in the study of concentration-dependent phase separating and aggregating systems, as only with observations in situ is it possible to determine new ways to control the structuring of protein–polysaccharide mixed systems.