Two means of monitoring water availability in casein model systems were compared. Adsorption isotherms of rennet casein and freezedried casein-sodium chloride (NaCI) (95:5 ratio) were obtained over the a, range 0.43 to 0.85. Over the entire a,,. range the casein-NaC1 samples s&bed more water than the casein only samples. Deuterium (7H) Nuclear Majrnetic Resonance (NMR1 R, and R, relaxation na-;a&eters were measured in the samd casein model systems. A lin'ear relation between a, and both II1 and R2 was observed. However, at equal h,, both R1 and R, values were found to be lower for the caseinNaCl system than for the casein only system. This indicated that the water in the NaCl containing system was more rotationally mobile than the water in the system without NaCl at the same a,.

A collaborative study was conducted to determine the reliability of a Bacillus stearothermophilus disc assay method for differentiating various concentrations of penicillin in raw milk. Participating laboratories tested 10 different samples (including one negative) in blind duplicate. Triplicate standards were alternated with triplicate unknowns around the periphery of each of 5 different plates. Zone diameters were measured and the difference in zone size of pairs of adjacent standard and unknown samples were analyzed by a paired t-test. Penicillin concentrations 0.003 IU/mL different from the reference concentrations were consistently distinguishable at a 95% confidence level. Such discriminatory power was determined to be possible with as few as 3 plates (9 replicates) per unknown. The method has been adopted official first action.