Effects Of Pre-existing Bubbles On Ice Nucleation And Crystallization During Ultrasound-assisted Freezing Process Of Liquid Samples

Power ultrasound has been proved to be a potential method to monitor and controlcrystallization phenomena, although it is a stochastic and probabilistic process. Cavitationplays a vital role during ultrasound assisting freezing processes since the collapse ofcavitation bubbles can initiate nucleation and enhance heat transfer between cooling mediumand frozen products. In the current study, water and sucrose solutions with and without airbubbles as liquid food models were frozen with ultrasound application to study the influenceof pre-existing bubbles on freezing process assisted with ultrasound.Statistical analysis of the results showed that nucleation temperature of liquid sampleswith bubbles exhibited a better fit to linear equation. The nucleation of samples containingbubbles occurred at temperatures close to the irradiation temperatures with a short delay lessthan1s, which was shorter than that without bubbles. The results indicated that ultrasoundapplied in this study could trigger nucleation of liquid samples with bubbles. The pre-existingbubbles in the liquid may become cavitation bubbles with ultrasound application, whichwould improve the efficiency of cavitation effect. Therefore the method of injecting bubblesinto liquids before freezing could be used to initiate and control the nucleation process forliquid samples during ultrasound assisting freezing.Liquids with and without bubbles were frozen with various ultrasound conditionincluding duration time and intensity, to study the impact of the movements of bubbles onultrasound assisting freezing process. Sucrose solution with bubbles irradiated by ultrasoundat0.38W/cm2nucleated with a low supercooling degree（-0.25oC）. The difference ofnucleation temperature was small with different ultrasound intensities，which implied that partof bubbles in the liquid might turn into cavitation bubbles to initiate nucleation even with aweak ultrasound intensity. And bubbles with smaller sizes would become cavitation bubbleswhen enhance the ultrasound intensity. Under the same ultrasound intensity and duration time,the energy of ultrasound with low frequency is stronger than with high frequency, result ofmore cavitation bubbles and larger bubbles vibration amplitude. This could not only improvethe rate of heat transfer, but also speed up the formation of crystal nucleus. Maltodextrin with and without bubbles was applied as complex liquid food system, andThe influence of bubbles existing in food system with high viscosity on ultrasound assistingfreezing was studied. With increasing viscosity of solution, the difference of the lagging timebetween solution with and without bubbles was growing. Big sizes bubbles could be stable inliquid with large viscosity, which would vibrate and absorb the small bubbles around in thefield, thus reducing the number of cavitation bubbles. But the previous bubbles would becomecavitation bubbles to boost cavitation effect. Therefore considering both of the above effects,this method is suitable for the solution with larger viscosity in the ultrasound assisted freezingprocess.On base of the above results, choosing an appropriate ultrasound condition, the freezingtime of solution with and without bubbles was recorded. And two points ultrasonic frozen wasoperated and compared with one point ultrasound frozen. When the temperature of solutionfell to-1oC again, the ultrasound irradiation was carried for5s, which could reduce the timeof freezing process. Hence two points ultrasonic frozen applied to solution with bubbles couldcut down the whole freezing time by arousing nucleation with shorter time and wiping out theadverse effect of poor heat transfer rate of bubbles in the characteristic freezing stage.