Real-time determination of microbial activity of pasteurized fluid milk using a novel microrespirometer method

The effectiveness of the rapid CO2 evolution rate (CER) method was evaluated by using a novel noninstrumental microrespirometer to determine the microbial activity of pasteurized milk and comparing it with traditional culturing methods in homogeneous milk samples. Three different kinds of milk (skim, 1% fat, and whole) stored at 2 temperatures (4 degrees and 7 degrees C) were measured daily for CER, aerobic plate count (APC), and psychrotrophic bacterial count. The mean initial rates of CO2 evolution for all 3 samples stored at the 2 temperatures ranged from 3.42 to 3.71 microL/h/mL and increased to 29 microL/h/mL and above on the final day of the experiment. Regression analysis showed a high correlation (R = 0.98-0.99) between the APC and CER results in combined milk samples. A cut-off value of CER (25 microL/h/mL) for milk spoilage at refrigeration temperatures was identified. The real-time CER method shows promise as a potential alternative to the traditional culture method.