EXPERIMENTS WITH THE METHYLENE BLUE REDUCTION TEST FOR MILK

SUMMARY: The effects of shaking milk samples before the addition of methylene blue, delaying the time of addition of the dye, adding a second quantity of dye with a hypodermic syringe after the initial charge had been decolourized and of increasing the storage time or varying the testing time have been studied in five separate experiments. Vigorous shaking at the time of dye addition lengthened the reduction time. This effect was more pronounced when the milk was stored in a tube than in a bottle. Delayed addition of the dye resulted in a shorter reduction time than expected for inverted samples and longer than expected for samples which were not inverted between the time they were put into the 37° bath and the addition of the dye. Second additons of dye did not usually have reduction times exceeding 1 hr. Methylene blue reduction times did not necessarily decrease with successive tests carried out over a period of time on one sample: a time of 30 min. might persist over a series of tests for 4 hr. or more and small increase and decrease in reduction time with successive tests were also noted.     

THERMODURIC ORGANISMS IN MILK. PART V. A SIMPLIFIED TESTING TECHNIQUE

SUMMARY: Thermoduric colony counts at 30° of laboratory pasteurized milk determined by the roll tube or agar strip methods were lower than those obtained by the standard Petri plate method. The differences in colony count were not of such magnitude, however, as to be likely to result in many errors in grading if a thermoduric bacterial content of greater than 10⁴/ml is accepted as an index of unsatisfactory cleansing of dairy equipment.
The three methods examined were simpler and more economical than the Petri plate technique, but the agar strip method, as described, using the standard loop, was the simplest and gave a sufficiently reliable estimate of the thermoduric colony count for advisory purposes.     

TEMPERATURE VARIABILITY WITHIN BACTERIOLOGICAL INCUBATORS

SUMMARY: Temperature variability within four bacteriological incubators regulated for 30° has been studied using readings taken from the normal thermometers and from others placed on the incubator shelves.
During 5-day incubation periods, differences between readings from individual thermometers and between thermometers on different shelves in the same incubator rarely exceeded 0·5° and 1° respectively. The mean temperature recorded on the top shelf was significantly higher ( P <0·05) than that on the bottom shelf for all incubators, and than that indicated by the normal thermometers for two incubators.
Colony counts for pasteurized milk were not materially affected by within incubator temperature differences in this experiment nor, at three of the four centres, by removal from the incubators for brief periods for counting after intermediate incubation times.