THE ASSESSMENT OF THE BACTERIOLOGICAL CONDITION OF MILK BOTTLES

SUMMARY: A study of the relative values of a number of bacteriological tests for assessing the condition of milk bottles indicated that the colony count of the bottle rinse solution on yeastrel milk agar incubated for 4 days at 30°, combined with a clot-on-boiling test applied to 1 ml. of rinse in 9 ml. of sterile milk after incubation for 72 hr. at 19–20°, gave the most useful results.
The mean of the ratios of colony counts at 30° to those at 37° was 15·1, while it was as high as 22·9 for rinses with 37° of over 600 for an unsatisfactory bottle should be retained when the test is done at 30°. The thermoduric colony count of rinses of milk bottles, even when laboratory pasteurized in milk, did not provide any additional information to that given by the colony count at 30° made without pasteurization. A high proportion of the organisms in bottle rinses survived laboratory pasteurization in milk, the survival rate being highest in efficiently treated bottles.
The clot-on-boiling test gave results in general agreement with colony counts and served to indicate the potential influence of badly contaminated bottles on the keeping quality of milk placed in them. A substantial proportion of rinses with satisfactory colony counts reduced methylene blue within 48 hr. at 19–20°.
Colony counts at 37° were on the average much lower for bottles treated with steam than for bottles submitted to detergent treatment in various types of bottle washing machines. Treatment of bottles by steam or hypochlorite was more efficiently done on the farms than at the dairies.