3.3. Improving the freezing rate
The rate of freezing is an important parameter for impacting the quality of frozen foods because it determines the size of ice crystals, degree of cell dehydration and damage to the tissue structure. Generally, rapid freezing can better maintain the quality of frozen foods compared with slow freezing, although the mechanism for this is not as well understood as is sometimes stated. Power ultrasound has demonstrated its ability to accelerate the freezing process of food materials [6], [28] and [44]. Hu et al. [27] investigated the effect of ultrasound on the freezing rate during immersion freezing of dough cylinders. The results showed that application of power ultrasound during phase-transition stage of freezing process significantly increased the freezing rate and the freezing time was shortened by more than 11% when ultrasound power levels of 288 W and 360 W were applied. The freezing curves of dough samples obtained from ultrasound assisted freezing at 360 W power level and without ultrasound treatment are displayed in Fig. 7. Similar results were also reported by Li and Sun [28], Delgado et al. [6] and Xin et al. [44]. These studies reported that power ultrasound could accelerate the freezing rates of fruits and vegetables including potato [28], apple [6] and broccoli [44].