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The effect of ultra high pressure homogenization (UHPH) on structural and rennet gelation properties of casein micelles

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Title: The effect of ultra high pressure homogenization (UHPH) on structural and rennet gelation properties of casein micelles
Author: Sandra
Department: Department of Food Science
Advisor: Dalgleish, D.G.
Abstract: This thesis focuses on the effect of Ultra High Pressure Homogenization (UHPH) on casein micelle structural and rennet gelation properties. More reduction in casein micelle size was found with more passes and higher pressure through homogenization. Different combinations of heat and pressure treatments were employed: control (C), homogenization at 186 MPa (H), heat (T) at 85°C for 10 min, homogenization then heat (HT), and heat then homogenization (TH). The effect of homogenization on decrease in micellar size was found to be dependent on treatment order and combination, as well as whether reconstituted skim milk powder (RSMP) or fresh skimmed milk was used. Homogenized milks (H) showed different protein size exclusion profiles of the serum phase compared to other treatments. UHPH was found to increase the amount of non-sedimentable caseins (kappa, alpha-s1, and alpha-s2) in the serum. Electron microscopy showed formation of smaller particles with H, TH, and HT when compared to C. Transmission DWS was found to be a sensitive method to monitor the changes in casein micelles' apparent size and interactions during renneting of milk. Different rennet concentration did not produce gels with different structures as indicated by final 1/1* values, even though the gel firmness was found to be dependent on rennet concentration as indicated by storage modulus. Approximately 70-75% and about 95% [kappa]-casein, respectively, needed to be cleaved before a change in interactions could be detected by DWS and network formation be detected by rheology. The rennet gelation mechanism was found to be a complex process where both enzymatic and aggregation stages had to be taken into account, and could probably be thought of as a four steps process: the enzymatic step, the decrease in diffusion coefficient, the constricted movement of casein micelles (aggregation) and network formation. UHPH did not affect the primary phase of rennet gelation. No gelation was observed in T and TH samples. It was concluded that UHPH was capable of modifying the structural and rennet coagulation properties of casein micelles and that the sequence of treatments was an important factor in determining the nature of the modifications.
Date: 2006
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