88D-24 |
Determination of microbial quality in pasteurized milk using a non-instrumental microrespirometer |
Y. H. P. HSIEH, Department of Nutrition and Food Science, Auburn University, 328 Spidle Hall, Auburn, AL 36849 and Z. Ren.
Rapid determination of microbial contamination of a highly perishable food product such as milk is crucial and often required for quality assurance. The most widely used aerobic plate count (APC) method requires at least 48 hours to obtain results. The objective of this study was to validate the performance of a microrespirometer for rapid determination of spoilage bacteria in pasteurized milk by comparing the results with the APC cultural method. Skim and whole milk samples were purchased from a local store and stored at 4oC. An aliquot of sample was removed from each milk container daily for the determination of CO2 evolution rate (L/h/mL) using a microrespirometer and total aerobic count (CFU/mL) using Petrifilms. For the CO2 method, five-mL of each sample was treated with CO2-free air, then placed in the microrespirometer pre-equilibrated for 30 min on a shaker and shaken at 240 rpm. The CO2 evolution rate was determined by the time required for the sample to consume a known amount of alkaline solution as indicated by the color change. The carbon dioxide evolution rate of both skim milk and whole milk samples increased from ~ 2 L/h/mL to > 40 L/h/mL as the storage time increased. The corresponding total aerobic bacterial count also increased from 10 CFU/mL at day 1 to > 107 CFU/mL. pH values decreased from 6.78 to 6.16 in skim milk and 6.67 in whole milk. The skim milk sample was spoiled after 10 d of storage while the whole milk sample was spoiled after 19 d of storage. Results from the CO2 method, Petrifilm, and sensory examination were highly correlated. Under the specified experimental conditions, the cut-off value for the CO2 evolution rate for milk spoilage which corresponds to a value of ~107 CFU/mL was 20 L/h/mL. The microrespirometer can sensitively measure very low level CO2 evolution rates, and efficiently determine microbial activity in pasteurized milk samples within 1 h.
Session 88D, Food Microbiology: General
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