Abstract--1. Milk proteins of the Galapagos fur seal (Arctocephalus galapagoensis) were separated adequately into whey and casein fractions using bovine milk analysis methods.2. In samples from days 5-30 of lactation 40% of the total proteins were whey and 60% caseins; in mid-lactation, day 150, 25% were whey and 75% casein proteins.3. Electrophoretic and isoelectric focusing patterns of fur seal whey protein differed widely from bovine patterns, whereas those of caseins were similar.4. Polymorphisms of fur seal whey and casein proteins were noted and did not seem related to different stages of lactation.5. C-16 and C-18 fatty acids contributed about 70% of fatty acids; 63% of the total acids in milk fat were unsaturated.
Potentiometric acid-base titration is a simple method that can be used successfully in the investigation of the protein system of milk and other proteincontaining products. Its applications include the examination of conventional heating procedures, cheese ripening, the investigation of processed cheese and the effect of melting salts. The principle depends on the performance of pH-dependent phase transitions which are recorded by a single titration cycle and evaluated quantitatively by means of hysteresis loops.An unusual feature of some proteins and nucleic acids is the existence of hysteresis phenomena in their pH titrations in the acid range. The proton-binding vs. pH data lie on one curve when the pH is lowered by addition of acid, and on a different curve when the pH is raised by addition of base. One thus obtains a closed loop. The loop exists between definite pH limits, while above and below these limits only a single reversible curve is observed. Hysteresis loops for milk reflect non-equilibrium transitions within the system.
RESULTSThe succession of buffering capacities (Kirchmeier, 1976(Kirchmeier, , 1977a shown by milk and by acid casein during acidification from pH 8-6 to 3-6 differed from those exhibited during subsequent back-titration with alkali, the latter being characterized by the occurrence of marked hysteresis between pH 5-0 and 6-5. The degree of hysteresis was greater in heated than in unheated milk and increased with increasing heat treatment. It is apparently due to conformational changes in the casein complex induced by acidification and heat treatment. Hysteresis in a milk system can thus furnish information about its thermal history. Fig. 1 shows acid titration curves of raw milk and milk after different heating procedures in a sealed tube. The maxima of the titration curves are shifted towards lower pH values for longer heating times. This means that higher energy barriers have to be overcome for the protonation process. At higher temperatures the surface of the micelles are covered progressively with serum proteins. Due to this coating additional diffusion barriers are constructed and the diffusion rate of the protons into the interior of the micelles is measurably retarded. The resulting expansion of the hysteresis loop can be determined quantitatively.
Processed cheese was manufactured with different types of phosphates (P1 monophosphate to P4 tetrapolyphosphate) and by various techniques. The limiting viscosity numbers of sodium casein and the casein of processed cheese were determined and the axial ratio calculated. The axial ratio of processed cheese protein was a/b=20,0 and that of sodium casein 10,0 resp. We deduce that the apparent increase of the axial ratio is caused by the depolymerization of casein caused by emulsifying salts. The flow curves of processed cheese were analysed. There is good agreement with the power law of Ostwald tau=k.Dn. Therefore processed cheeses are characterized as pseudoplasts. The influence of melting salts on the flow properties can be described by a differing emulsifying effect (P1 less than or equal P4). Temperature, concentration and salt effects are discussed for the viewpoint of correlation between aggregation-desaggregation processes and dehydration and hydration of casein.
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