Factors associated with the milking cluster and the long milk tube which cause a diminished or a fluctuating vacuum within the liner of the teatcup assembly during simulated milking with an artificial udder fell into 3 groups. 1. Lower milk flow rates and admission of air at the clawpiece greatly decreased vacuum fluctuation within a pulsation cycle and increased the mean level of vacuum in the liner during that part of the cycle in which milk was flowing from the teat. 2. Larger bore of the short milk tube connecting the liner to the clawpiece, pulsation of the teatcup liners in pairs instead of all 4 together, and larger volume of the clawpiece bowl in the main decreased fluctuation in vacuum within the liner. 3. Larger bore and shorter length of the long milk tube connected to the clawpiece and decreased height above the outlet of the clawpiece to which the milk was raised mainly increased the mean vacuum in the liner during that part of the cycle in which milk was flowing from the teat.

In a milking experiment with 75 cows mean overall fluctuations in milking vacuum within the liner in a pulsation cycle at peak flow were 11·5 and 4·8 inHg and corresponding mean levels of vacuum during that part of the cycle when milk was flowing from the teat were 11·75 and 13·0 inHg. There were no differences in milking performance of practical consequence.

The detergents sodium dodecyl sulphate and octyl phenoxy polyethoxyethanol interact with casein and cause dissociation of the high-molecular-weight casein aggregates. It is presumed that the detergent binds with hydrophobic regions in the casein molecule. The size of the complexes formed between detergents and αs1-casein, β-casein and κ-casein, as estimated by gel filtration and sedimentation velocity experiments, suggests that the caseins were complexed as monomers.

During gel filtration under non-reducing conditions, detergent-κ-casein complexes were separated from other major components because of their conversion through formation of disulphide bonds into high-molecular-weight aggregates. This reaction, which did not occur in sedimentation velocity experiments, was presumably facilitated by the changes in the equilibrium between the individual caseins during gel filtration.

Sedimentation velocity experiments showed that a ratio of about 40 detergent molecules to 1 casein molecule was required to give the smallest casein-detergent complex.

Previous work had shown that when milk is boiled in a climbing film evaporator a reaction occurs between the casein micelles and the fat globule membrane, probably at the liquid–gas interface. It now appears that the micelle–globule complex is formed by combination of the κ-casein components of the micelles and the membrane of the fat globule. The reaction is not significantly affected by increases in concentration of individual constituents, or by changes in temperature or pH.

The history of the milk before its concentration is an important factor in its subsequent behaviour. Thus, cooling and stirring of the milk during storage, as normally occurs in a bulk collection tank, promotes the reaction in the evaporator and, for some milks, is essential for starting the reaction.

A high retention of whey by curds made from the concentrated milks, diluted with water to their original volume before renneting, indicates that curd properties other than fat retention are also altered during concentration.