Reconstituted whole milk was heated using pilot-scale heating equipment. Kinetic and thermodynamic parameters for the irreversible denaturation of β-lactoglobulins A and B and R-lactalbumin were determined. R-Lactalbumin denaturation was first order, whereas both β-lactoglobulin variants had a reaction order of 1.5. Arrhenius plots for all three proteins showed an abrupt change in temperature dependence. In the low-temperature range, the thermodynamic parameters were ascribed to typical denaturation processes in which the unfolding of the protein tertiary structure is the rate-determining step. At higher temperatures, these parameters were in the range expected for typical condensation reactions, suggesting that aggregation processes may be rate-determining in this temperature range. The rate constants for β-lactoglobulin denaturation were independent of the initial protein concentration at all temperatures. For R-lactalbumin at temperatures below 85 °C the rate constants may have been dependent on the initial R-lactalbumin concentration as higher rate constants were observed with decreasing protein concentrations.

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Two experiments were conducted to establish responses in milk Se concentrations in grazing dairy cows to different amounts of dietary Se yeast, and to determine the effects of the Se concentration of the basal diet. The hypothesis tested was that the response in milk, blood, and tissue Se concentrations to supplemental Se would not be affected by whether the Se was from the basal diet or from Se yeast. In addition, by conducting a similar experiment in either early (spring; experiment 1) or late (autumn; experiment 2) lactation, we hypothesized that different Se input-output relationships would result. Both 6-wk experiments involved 60 multiparous Holstein-Friesian cows, all of which had calved in spring. They were allocated to 1 of 10 dietary Se treatments that included 2 types of crushed triticale grain (low Se, approximately 165 microg of Se/kg of DM; or high Se, approximately 580 microg/kg of DM) fed at 4 kg of DM/d, and 1 kg of DM/d of pellets formulated to carry 5 quantities of Se yeast (0, 4, 8, 12, or 16 mg of Se). Daily total Se intakes ranged from <2 to >18 mg/cow in both experiments. Milk Se concentrations plateaued after 15 and 7 d of supplementation in experiments 1 and 2, respectively, and then remained at plateau concentrations. Average milk Se concentrations for the plateau period increased as the amount of Se yeast increased, and low- and high-Se grain treatments were different at all quantities of Se yeast, although there was a tendency for this difference to diminish at the greatest concentrations of yeast. There were significant positive, linear relationships between Se intake and the concentrations of Se in milk, which were not affected by the source of Se, and the relationships were similar for both experiments. Therefore, the output of Se in milk in experiment 1 was greater than that in experiment 2 because the milk yield of the cows in early lactation was greater. The estimated proportions of Se partitioned to destinations other than milk and feces increased with the amount of Se in the diet and were greater in experiment 2 than in experiment 1, a result that was supported by Se concentrations in whole blood and plasma and in semitendinosus muscle tissue. If high-Se products are to be produced for human nutrition, it is important to be able to develop feeding systems that produce milk with consistent and predictable Se concentrations so that products can consistently meet specifications. The results indicate that this objective is achievable.

Summary:The functional properties of whole milk powder (performance during reconstitution in water and coffee) are routinely measured to determine powder quality. An examination of the microstructure of milk powder and of the insoluble material collected after applying a series of functional tests provided insight into why the insoluble material formed during reconstitution.The microstructures (transmission electron microscopy and confocal microscopy) of four commercial whole milk powder samples and of the insoluble material produced during functional testing were assessed. The microstructure of the whole milk powder could be related to features observed in the microstructure of some of the insolubles collected after powder functional testing. It is likely that heating conditions throughout the powder manufacturing process resulted in the denaturation of β-lactoglobulin and its interaction with other proteins and fat globule membrane components, thus influencing the solubility of the milk powder. The application of shear during processing resulted in the greatest change to powder solubility in this study. Increasing homogenisation pressure during processing resulted in fat globule size reduction, adsorption of casein micelles to the fat globule membrane, and the formation of clusters of fat globules caused by the sharing of adsorbed casein micelles. The presence of large numbers of these clusters in the milk powder increased the formation of insoluble material during powder reconstitution in water and coffee.