Effects of nano-bubbles and constant/variable-frequency ultrasound-assisted freezing on freezing behaviour of viscous food model systems

Abstract

Abstract A technique of introducing nano-bubbles (NBs) followed by constant/variable-frequency ultrasound-assisted freezing (CFUF/VFUF) was developed. Hydrolyzed gelatin-based viscous food model systems in different concentrations (G10%, G15%, G20%, G25%, and G30%) containing NBs were prepared, and their characteristics including bubble size distribution, dissolved oxygen concentration and viscosity were investigated. Freezing behaviours of the food model systems by immersion freezing (IF), CFUF and VFUF were monitored, and the elevation of freezing efficiency was analyzed using multivariate analysis of variance (MANOVA). Results indicated that NBs in G20% and G30% possessed the best size stability, showing migration distances of mean diameters less than 50 nm. Besides, MANOVA showed that the two main factors (“hydrolyzed gelatin concentration” and “freezing method”) and their interaction (“hydrolyzed gelatin concentration*freezing method”) significantly affected the freezing rate elevation and phase transition rate elevation. A tendency of VFUF > CFUF > IF in freezing promotion was observed, and the freezing process was accelerated more for G20% when performing CFUF and VFUF. VFUF could promote the freezing process better through alleviating the decline of ultrasonic efficiency induced by increases in bubble size and system viscosity. This study proved that introducing stable NBs followed by performing VFUF could remarkably improve the freezing efficiency of viscous food systems, showing great potentials for future applications in the food industry.