ESTIMATION OF MICROBIAL COUNT IN RAW-MILK BASED ON A COLORIMETRIC REACTION WITH VITAMIN-K-3 AND TETRAZOLIUM SALT

The microbial count is an index of suitability of raw milk handling, a nd also one of the most essential items to judge milk quality. The mea surement of the microbial count is usually carried out by direct count ing with a microscope or by colony counting on an agar plate. However, these methods involve complicated procedures and need special techniq ues. In order to overcome these drawbacks, a convenient and rapid esti mation method for the microbial count in raw milks was developed based on a colorimetric reaction using vitamin K-3 and nitro blue tetrazoli um (NET) available as a substrate of the electron transport system of microorganism. NET was dissolved at a concentration of 1 mM in 0.2 M p otassium phosphate buffer (pH 7.0) saturated with vitamin K-3 followed by mixing with an equivalent volume of a raw milk for 20 or 40 min. T he change of b value determined with a colorimeter during the reaction was linearly related to the microbial counting by the agar plate meth od on a log-log plot (n = 41, r = 0.942). The lower detection limit of the microbial count by the present method was about 1 x 10(5)/ml. Red ucing substances such as uric acid, ascorbic acid and lactose did not interfere with the color reaction at the usual level in raw milk. As t he color change by the present method can be easily recognized with th e naked eye without a special colorimeter, the method would be practic ally applicable not only in the laboratory but also in field work.