443. Keeping quality and raw-milk grading: III. Comparison of keeping quality tests and dye tests in milk control (results of a field trial)

Abstract

1. The results discussed in this part of the report were obtained by examination of milk samples taken at approximately fortnightly intervals throughout a period of one year from 298 farms in eight widely separated areas of England and Wales. The samples were examined by the routine resazurin test, the methylene-blue (4½/5½ hr.) test, the methylene-blue (½ hr.) test and the temperature-compensated keeping quality test using clot-on-boiling (c.o.b.) to determine the end-point. For the first three tests, the treatment of the samples and the standards applied conformed to those prescribed officially for these tests in England and Wales. Details are given of the treatment of samples and testing procedure for the temperature-compensated c.o.b. test. With this test two standards, equivalent to a residual keeping quality at 22° C. of 21 and 24 hr. from 9 a.m. on the day of collection of the milk from the farms, for both evening and morning milk were used.2. The results are analysed on the basis of the proportion of farms and of samples passing or failing the different tests, the effect of shade atmospheric temperature on the incidence of failures and the relationship between the standards imposed by the c.o.b. tests and the official methylene-blue (4½/5½ hr.) test.The proportion of farms and of samples passing each test was much higher in winter than in summer and was directly related to the prevailing shade atmospheric temperature. However, a proportion of farms was consistently able to comply with the standards. No more than 5% of the total failures by any test could be ascribed to the 60 (20%) best farms, whereas the 60 (20%) worst farms were responsible for as many as 37–60% of the total failures from the 298 farms.