SummaryThe results of five experiments designed to show the magnitude and rate of build‐up of thermoduric organisms on dairy equipment indicate that:—(1) Milk rinses of both hand milking utensils and milking machine clusters may contain over 10,000 thermoduric organisms per ml. within 2 to 4 days of absolute sterilisation, when the equipment is simply washed after milking in hot water or hot detergent and where the conditions of milking allow the introduction of a few thermoduric organisms into the milk at each milking.(2) The build‐up of thermoduric organisms on utensils is mainly due to multiplication of the organisms in the milk‐scale or milky‐film and is hardly affected by cumulative additions during milking.(3) The thermoduric flora increased more rapidly and attained greater numbers on the rough surfaces of a galvanized pail than on a smooth, seamless pail.(4) There was no appreciable build‐up of thermoduric bacteria within a month on hand milking utensils consistently sterilised by steam or hypochlorite.(5) Washing milking pails in warm water at 120°F. led to a rapid build‐up of non‐heat‐resisting organisms, but not to such a rapid build‐up of thermoduric organisms.(6) Washing milking pails in hot detergent solution at 160°‐170°F. did not result in anything like as marked a build‐up of heat‐labile organisms, but the build‐up of thermoduric organisms was appreciable by the third week.(7) Under the conditions of our experiments there was no more rapid or greater build‐up of thermoduric organisms on pails held between milking at summer temperatures than at winter temperatures. This conclusion is tentative and further information should be obtained under varied farm conditions.Our experiments were in the main confined to hand milking utensils and we have no direct evidence to decide whether the thermoduric build‐up is more rapid or of a greater magnitude on inefficiently s
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