Non-invasive and non-destructive ultrasonic technique for the detection of microbial contamination in packed UHT milk

An eight-channel ultrasonic device was developed to detect microorganism growth in UHT milk contained in carton-like packages without opening the packs. The system analyses automatically the amplitude and the delay of an ultrasonic pulse passing through packed UHT processed milk, being the coupling between the transducers and the packs accomplished in dry conditions. Changes in these parameters produced by different microbes are detected even when other physical-chemical parameters still remain within the sterility margins. Three different strains (Bacillus cereus, Proteus vulgaris and Bacillus pumilus) were inoculated at different concentrations in UHT milk packs. Variations in the velocity and amplitude of the ultrasonic wave show the growth signature of these microorganisms. Growth detection was achieved between 7 and 48 h depending on the number and type of bacteria inoculated. The experiments show that conventional analysis like pH or acidity measurements could be substituted by this non-invasive technique.