| Milk coagulation, especially the cutting time was evaluated by means of ultrasonic sine waves, dynamic rheology and continuous shear viscometry.; Ultrasonic sine waves were employed to measure the attenuation of rennet-induced milk coagulation. For this, small voltage amplitudes were given to an ultrasonic transmitter which was directly coupled to the milk system. Resonant frequencies of milk system were found to be 8.3, 5 and 1 MHz. The attenuation plots of milk coagulation at 8.3 MHz were different from those at 5 and 1 MHz for nonstandardized milk (unhydrated casein micelles). The ultrasonic attenuation of gels formed with nonstandardized milk increased at 5 and 1 MHz as the coagulation proceeded. The behavior did not change in response to changes in milk pH. The attenuation of gels formed with nonstandardized milk at 8.3 MHz varied with pH. At pH 6.2, the attenuation increased, and at pH 6.4 and 6.6 the attenuation decreased as gelation proceeded. The plots of gels formed from nonstandardized skim milk showed the same attenuation behavior for all the resonant frequencies studied with attenuation decreasing initially, until perhaps the clotting time, then increasing again. The overall difference in attenuation was negligible during one hour of coagulation. On the contrary, the attenuation plots of gels from standardized milk (32°C for two hours, fully hydrated casein micelles), declined between pH levels of 6.2--6.5. These results were consistent with the gels made from store-bought whole milk. Gels made from standardized skim milk showed the same behavior.; Dynamic rheological measurements of rennet-induced milk gels were carried out to define their viscoelastic characteristics. The experiments were conducted at a constant stress of 0.1 Pa and a frequency of 0.1 Hz for 60 min. Experimental parameters were: temperatures of 30, 32, 34°C; pH of 6.2, 6.35, 6.5; rennet of 30, 35, 45 mL/454 kg of milk. The moduli had a local maxima at (32°C Rennet concentrations mostly affected the primary phase indicated by varying clotting times, and only slightly changed the moduli. Increasing the rennet concentration did not increase the moduli. The pH was known to influence both phases of coagulation as it was the case in our experiments and lowering the pH increased the values of the plateau moduli and clotting times greatly.; Continuous steady simple shear tests (gamma = 0) were conducted to measure the apparent viscosity of rennetted milk gel. In this method, named the "Nazik" method, a double gap geometry was used with a constant shear stress of 0.2 Pa. The measurements were made for 60 min. The experimental parameters were three levels of fat, protein, rennet, calcium chloride, temperature and pH. The semi-log scale plots of viscosity vs. time showed an initial lag period followed by a steadily increasing part until the viscosity reached a plateau value around the cutting time. The plateau values measured in the Nazik Method were about 40 times larger than those obtained from strictly rennet induced coagulation. The apparent viscosity plots in linear scale, showed large viscosity peaks starting around the cutting time. Most importantly, the cutting time was found to coincide with the very first big peak in the ∼ 40 kPa.s range. The results were substantiated by the manual cuttime determinations in the Dairy Plant of Food Science Department at UW-Madison. |
