Impact of sol-gel transition on the ultrasonic properties of complex model foods: Application to agar/gelatin gels and emulsion filled gels

Quantitative ultrasound (US) techniques have an attractive potential for the non-invasive and real-time characterization of the structural and mechanical properties of food. The objective of the present study is to unravel the determinants of the variations of US properties during sol-gel transition of emulsion filled gels of agar and gelatin, due to the relative contributions (i) of their individual components and (ii) of their combined interactions. Nine model foods were designed, accounting gradually for the complexity of an emulsion filled gel composed of water, sucrose, agar, gelatin and sunflower oil. A quantitative US device was designed to monitor the sol-gel transition of the model foods. US velocity, reflectivity and attenuation were measured at 50 degrees C (sol-state) and 20 degrees C (gel-state) with a 1 MHz transducer in pulse-echo mode. US data were then compared to visco-elastic shear moduli and density measurements. Bulk elastic modulus was shown to prevail over density for explaining the variations of US velocity and reflectivity during sol-gel transition, while high gelatin concentration and interactions between agar and gelatin had an impact on US attenuation. In addition to the fact demonstrated here that US reflectivity is easier to measure compared to velocity and attenuation, the literature reports that this parameter also highly depends on the mechanical coupling of interfaces. Therefore, this study is a baseline for future investigations on the potential of US reflectivity for the characterization of physical interactions between food and more complex surfaces, so that to mimic tongue-food interaction during oral processing.