1. Certain starter cultures have been found to be inhibited in the presence of milk peroxidase.2. A high correlation has been established between the inactivation of milk peroxidase and of the inhibitory action by use of critical values of any of the following: temperature, pH, concentration of either sodium azide or hydrogen peroxide.3. The inhibitory action can be overcome by the addition of certain reducing substances such as cysteine and sodium hydrosulphite.4. The inhibitory action is exhibited by the corresponding separated milk and whey.5. It is suggested that this inhibition may be due to the formation of a specific inhibitory oxidation product having a quinonoid structure.6. The addition of 1–2% of pasteurized milk or whey to autoclaved milk caused total inhibition, whereas such additions to milk heated to 190° F. showed no such effect. This is attributed to the presence of SH groups in the latter milk capable of reducing the oxidation product.7. It is suggested that the inhibition due to peroxidase is identical with that ascribed to lactenin 2, and that the inhibition described in our previous paper as due to ‘agglutinin’ is identical with that ascribed to lactenin 1.

The absorption of light in the ligand-to-metal charge-transfer band of PtCl,,-causes both photoaquation to PtC1,0H2-and photoreduction to a PtIII complex, PtC1,-.

1. Commercial samples of milk of Swiss origin, sterilized by rapid heating to high temperatures, developed a positive phosphatase reaction on storage.2. The optimum temperatures of storage for the development of this positive phosphatase reaction was 30° C.3. These commercial samples were sterile when received and remained sterile during storage.4. The developed phosphatase is apparently identical with the normal alkaline phosphatase of raw milk as judged by (a) heat stability, (b) pH optimum and rate of hydrolysis at different pHs, (c) comparison of results by Kay-Graham and Aschaffenburg & Mullen tests.

In our previous paper on this subject(i), it was shown that when raw milk was rapidly heated to high temperatures, cooled and then stored at 30° C, a positive phosphatase reaction developed, and it was further shown that this phosphatase was identical with the alkaline phosphatase of raw milk, and was not of bacterial origin. When tested immediately after the heat treatment, such milks always gave a negative phosphatase reaction.In the course of this work it was found that considerable variation occurred in the degree of reactivation 'with different milks. It was shown that this was not due to variations associated with technique, and from examination of a number of milks it was concluded that this variation was related to the milk itself. Examination of possible factors affecting reactivationIn an attempt to elucidate the cause of this variation in reactivation a number of possible factors was examined, using the capillary technique previously described. Unless otherwise stated, shock treatment of raw milk means immersion of capillaries in a glycerine bath at 135° C. for 20 sec.; the subsequent storage period means 2 days at 30° C. Reactivation is expressed as parts per million chlorine, since the permanent colour standards used in the micro-phosphatase test (1) are calibrated in these units. (a) Fat contentNo correlation was found between the fat content and the level of reactivation within the range of fat content found in bulk raw milk from rail tanks. It had also been noted that marked reactivation had been obtained with both separated milk and milk rich in fat, though separated milk gave less reactivation than the corresponding whole milk. (b) Ascorbic acidThe effect of adding ascorbic acid to the raw milk prior to shock treatment was examined, since in our previous experiments with a bacterial phosphatase-236-it appeared that reducing conditions might play a part. No effect upon the level of reactivation occurred on addition of ascorbic acid at the rate of 2 mg./lOO ml. (c) Metallic ions and amino-acidsSince it has been found (2) that either metallic ions such as Ca 2+ , Mg 2+ Zn 2+ , Mn 2+ , or amino-acids such as alanine, or both, play a role in the reactivation of alkaline phosphatase of intestinal origin, experiments were made to see whether Ca 2+ or Mg 2+ or alanine affected the reactivation of milk phosphatase, but no effect was observed using concentrations likely to occur as contaminants.

1. Reactivation of alkaline milk phosphatase is affected by the presence of ions, Mg2+, Zn2+ and Mn2+ being activating and Cu2+, Ni2+ and Co2+ being inhibitory.2. Cu2+ will inhibit reactivation induced by Mg2+, but has no effect upon Zn2+-induced reactivation.3. Using washed cream, either alone or in admixture with boiled whey treated with cationic resin, reactivation occurs in the presence of added Mg2+, but not when Zn2+ is added.4. Experiments with EDTA. confirm that metallic ions play a part both in alkaline phosphatase activity and in reactivation.5. It is suggested that either Mg2+ or Zn2+ or both play a part in reactivation.