Innovative applications of power ultrasound during food freezing processes—a review

Although the application of power ultrasound to food freezing is a relatively new subject, recent research advances show its potential is promising. The beneficial use of the sound energy is realised through the various effects that ultrasound generates upon the medium where it transmits. Among them, cavitation is perhaps the most significant one, which can not only lead to the production of gas bubbles but also the occurrence of microstreaming. The former can promote ice nucleation while the latter is able to accelerate the heat and mass transfer process accompanying the freezing process. Similar to other dense and incompressible materials, ice crystals will fracture when subjecting to alternating acoustic stress, consequently leading to products of smaller crystal size distribution, which is indeed one of the most important aspects that many freezing processes target at. Resulting from these acoustic effects, the application of power ultrasound is beneficial to many food-freezing processes. If ultrasound is applied to the process of freeze preservation of fresh foodstuffs, it can shorten the freezing process, and lead to product of better quality. If it is applied to freeze concentration and freeze drying processes, it can be used to control crystal size distribution in the frozen product. Furthermore, power ultrasound can also bring several benefits to the process of partial freezing of ice cream inside a scraped surface freezer, e.g. reducing crystal size, preventing incrustation on freezing surface, etc. Therefore, ultrasonic freezing process could have promising applications in freezing of high value food (ingredients) and pharmaceutical products. However, for the future development of this technology, several problems still remain to be explored. More fundamental research is still needed in order to identify factors that affect the ability of power ultrasound in performing the above functions. Considerable research effort is also required with regards to the development of adequate industrial equipment.