The Influence of Sterility of Farm Dairy Equipment on the Bacteriological Quality of Raw and Laboratory Pasteurized Milk

Summary. A series of experiments was carried out to ascertain which test or combination of tesrs in common use would provide the best indication of thc keeping quality, methods of production, and adequacy of cooling of milk. Monthly tests were made of milk as produced by two groups of farms (“good” and “poor”) in two widely separated areas. The good farms practised twice daily. sterilisation and efficient cooling; thc poor farms practised neither. The tests examined were: (1) colony count (4 days at 30°C.) on fresh (4 hr.) and aged (22 hr. at 18°C.) raw milk, and (in laboratory pasteurized fresh and aged milk; (b) clot‐on‐boiling (storage a t 18°C.); (c) resazurin to complete reduction at 37°C. on fresh raw and aged raw milk: (d) methylene blue reduction time at 18°C. on laboratory pasteurized fresh and aged milk; (e) methylene blue reduction time at 37°C. on laboratory pasteurized fresh and aged milk. No single test gave complete differentiation between good and poor production mcthods; the colony count of laboratory pasteurized aged milk was the nearest approach. In general the thermoduric colony count increased very little in raw milk aged at 18°C. for 24 hr.: the inean increases after ageing mere only 8.4% and 10.O% in the two areas. In all, 36 combinations of tests were considered, althouglh it has not been possible to present in either graphical or tabular form those combinations which showed promise; this is done for what is considered the most suitable combination‐the tiiermoduric count of aged milk and the clot‐on‐boiling time. Visual evidence of poor production methods was frequently associated with poor keeping quality, but lack of evidence of poor production gave no guarantee of good keeping quality. The keeping quality test alone was not entirely reliable as an index of production methods.