The renneting properties of skim milk solutions supplemented with milk protein concentrate : the effect of hydration and storage of the milk protein concentrate : a thesis presented in partial fulfillment of the requirements for the degree of Master of Science in Physics at Massey University, Palmerston North, New Zealand
The purpose of this study was to examine the effect of storage and hydration of milk protein concentrate with 85% protein (MPC85) on the renneting properties of skim milk solutions supplemented with MPC85. The following techniques were used in this investigation: solubility testing, rheology, polyacrylamide gel electrophoresis (PAGE), and mass spectrometry. The solubility of MPC85 samples which had been stored for different periods at temperatures ranging from 30°C to 50°C was found to decrease as storage time increased. In addition, as the storage temperature increased, so did the rate at which solubility decreased. This decrease in solubility dropped to approximately 22% of its original amount. Similar experiments were also performed on samples stored at 20°C (ie approximate room temperature), but showed no change over the time frame of the experiment. Rheology experiments were performed on 10% (w/w) skim milk supplemented with 2.5% MPC85 (w/w). The experimental conditions, such as temperature (30°C), reconstitution time frame, and rennet concentration (100 μL per 50 g sample), remained constant throughout testing. The rheological properties of the samples showed a large decrease in the formed gel strength ot the renneted samples, and an increase in gelation time, as storage time and or temperature were increased. Additionally, the viscoelastic moduli (G' and G") and fracture stress also decreased as storage time and/or temperature were increased. This again excludes samples prepared from MPC85 stored at 20°C which showed no change. The rheological properties of skim milk solutions supplemented with MPC85, with respect to hydration time was studied using three MPC85 powders of low, medium, and high solubility. Hydration time was varied between 1 and 24 hours, and results showed that samples made from high and medium solubility MPC85 increased gel strength with hydration, with high solubility MPC85 producing the stronger gels. Samples prepared with low solubility MPC85, produced very weak gels which only changed minimally with hydration time. Aggregation and gelation times for each set of samples were different, but did not change with hydration time. PAGE was used to analyse the composition of MPC85 to attempt to gain an understanding of what caused the change in properties with storage time and temperature. Experiments revealed that a large decrease in the solubility of caseins, and whey proteins decreased in solubility only minimally. Mass spectrometry was used to analyse samples stored at 50°C, and it was found that the casein proteins suffered glycation. Whey proteins were also analysed, but data proved too noisy for any conclusions. In general this thesis aims to provide useful information on the effects of storage and hydration of MPC85. especially with regard to the production of skim milk solutions supplemented with MPC85.