The effects of ultrasound-assisted freezing on the freezing time and quality of broccoli (Brassica oleracea L. var. botrytis L.) during immersion freezing

• The freezing time and quality of ultrasound-assisted frozen broccoli were studied.
• Ultrasound-assisted freezing with judicious parameters decreased the freezing time.
• Microstructure of broccoli was better preserved by ultrasound-assisted freezing.
• The firmness of broccoli was better preserved by ultrasound-assisted freezing.
• The drip loss was reduced by the application of ultrasound-assisted freezing.

The application of ultrasound during immersion freezing of broccoli was studied and particular attention was given to the effects on freezing time, microstructure, firmness and drip loss of broccoli. Broccoli florets were immersion frozen in an ultrasound-assisted freezer at two frequencies and four different power levels. The results showed that the total freezing time and times required for pre-cooling, phase change and sub-cooling stages of broccoli were significantly reduced by the application of ultrasound-assisted freezing (UAF) at 150 (30 kHz) or 175 W (20 kHz) power level and with judicious combination of process parameters (exposure time, ultrasound irradiation temperature and pulse mode). The microstructure and the firmness of broccoli tissue were better preserved and the drip loss was significantly reduced by the application of optimized UAF compared to the normal immersion freezing. These findings indicate that there is a great potential of UAF in immersion freezing of food.