Influence of sonication on milk sanitization and sensorial traits

Among new technologies used to improve milk sanitization and shelf life, the treatment with ultrasounds showed positive effects on bacterial inactivation. The aim of this study was to evaluate the influence of different sonication parameters on E. coli inactivation and on milk sensory properties. An ultrasonic processor was used at different combinations of amplitude of ultrasonic wave (A: 70 vs. 100%), cycle (C: 0.5 vs. 1) and treatment time (T: 100 vs. 240 s) on UHT milk inoculated with E. coli (ATCC 11775) cells (105 CFU/ml). Before and after each treatment samples were plated in Plate Count Agar (PCA) and in Violet Red Bile Agar (VRB). All treatments were conducted in triplicate. To evaluate the effect of ultrasounds at different values of A (0-100%), C (0.5-1) and T (50-100s) on whole milk sensory properties, a triangle sensory test was carried out. Data were submitted to ANOVA according to a linear model. The increments of A, C and T, resulted in a higher E. coli inactivation in both PCA and VRB. The values of CFU/ml log reduction were (A: 0.50 vs. 0.67, P<0.01), (C: 0.30 vs. 0.89, P<0.001) and (T: 1.01 vs. 1.92, P<0.001). Depending on intensity and duration of sonication, the taste led to discriminate between treatments at 75 and 100% whereas the smell allowed to identify treated and untreated milk at 0, 66 and 100% with increasing intensity and duration. Treated milk resulted unpleasant if compared to control milk. The rating values were significantly different between treatments (P<0.01) and ranged between 4.5 of control milk to 2.25 of milk sonicated at amplitude 100%, cycle 1 for 100s, where a scale between 1 (unfit for consumption) and 5 (very good flavour) was used. Off-flavors in sonicated milk probably resulted from creation of organic sulfur compounds that arise during the decomposition of reactive protein sulfhydryl groups associated with the amino acids, methionine and cysteine. Sonication resulted effective against bacteria but some corrections have to be found to limit sensory degradation.