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Concerning the Accuracy of the Methylene Blue Reduction Test¹

Central Experimental Farm, Ottawa, Canada

ABSTRACT

1. The methylene blue reduction test is based upon the rate of oxygen consumption in the milk by the bacteria present during incubation. This rate cannot be measured with reasonable accuracy where varying proportions of the bacteria are removed from the main body of the milk during the creaming process.
   
2. The accuracy of the test is greatly increased by periodical inversion of the tubes during incubation. In addition, reduction time is generally shortened and decolorization is more uniform, especially with the better grades of milk.
   
3. It is believed that these advantages more than compensate for the slight inconvenience entailed by this modification.
   
4. Milks showing similar reduction times by the modified technic sometimes show wide differences in standard reduction times. These appear to be associated with differences in the degree and rate of creaming.
   
5. Reduction time is prolonged by approximately 20 per cent with the proposed stronger dye concentration. This is more than compensated for in all but the poorer grades of milk by the shortening of reduction time when inversion is practised.
   

¹ Contribution No. 46 (Journal Series) from the Division of Bacteriology, Dominion Experimental Farms, Ottawa, Canada.