Ultrasonic High-Resolution Longitudinal and Shear Wave Measurements in Food Colloids: Monitoring of Gelation Processes and Detection of Pathogens

Abstract

In the present paper we describe the applications of the ultrasonic high-resolution longitudinal and shear wave measurements for food and bio-colloids. In the first example, both ultrasonic methods were used for the monitoring of the acidified milk gelation induced by glucono-δ-lactone (formation of gel network in yoghurt). Ultrasonic measurements demonstrated a high sensitivity to pre-gelation and gelation processes during the formation of acid milk gels. The hydration of colloidal calcium phosphate released into serum and the swelling of casein in micelles at pH 5.6–5.0 are suggested as the main contributors to the ultrasonic velocity and attenuation changes during the pre-gelation. The increase in shear loss modulus of acidified milk at pH 5.0-4.85 can be explained by the aggregation of the casein micelles into clusters. Subsequent reformation of these clusters into a gel network at pH 4.85–4.6 is observed as a sharp rise in the storage moduli of acid milk gels and an increase in the ultrasonic velocity. The second example is the application of the ultrasonic shear wave measurements for the detection of Salmonella in liquids. The antigen-antibody binding monitored by impedance measurements of a quartz crystal at 5, 15 and 25 MHz results in both the decrease in resonant frequency and an increase in the imaginary part of the quartz impedance. The analysis of the data indicates that the bacteria cells on the sensor surface do not exhibit pure mass-load behaviour, and the viscoelastic properties of the interfacial layer must be taken into account for quantitative analysis. Overall, our ultrasonic measurements demonstrate their high potential as non-destructive methods of analysis of complex foods and bio-colloids.