ISOLATION AND ENUMERATION OF AEROBIC MESOPHILIC BACILLUS SPP. IN MILKBY HEATING IN THE PRESENCE OF ETHANOL

Colonies obtained from the current standard test for the enumeration o f aerobic spore-forming bacteria (Bacillus) in milk (including a heat treatment of 80 degrees C for 10 min) were examined microscopically an d identified as spore-forming or non-spore-forming, It was found that only 0-4% of the colonies obtained by this method were produced by spo re-forming bacteria. The remaining colonies were produced by heat-resi stant coryneform organisms. Therefore, the use of this test to enumera te spore-forming organisms in milk overestimates the actual numbers of spores present. The optimal growth temperatures of these two types of organisms were investigated. It was found that simply incubating spor e-selective plates at 37 degrees C rather than 30 degrees C after a he at treatment of 80 degrees C for 10 min resulted in the growth of a mu ch lower number of non-spore-forming organisms and a slightly higher n umber of spores. An alternative method for the accurate enumeration of aerobic spore-forming bacteria was developed, in which ethanol was us ed to kill the vegetative coryneform bacteria. Heating milk which has been diluted in a pre-heated ethanol solution to give a final ethanol concentration of 20%, followed by a heat treatment of 80 degrees C for 10 min, completely eliminates non-spore-forming organisms from the sp ore-selective test.