Ultrasound-assisted immersion freezing method as a new strategy to improve the quality and microstructure of Chinese beef stewed dishes.

Prepared dishes, as a technological extension of pre-conditioned foods and representative products of the central kitchen, are becoming a major part of people's daily diets. However, the problem of dish quality deterioration owing to freezing in the frozen supply chain has not yet been addressed, and freezing quality enhancement techniques are lacking. Herein, special equipment was used to investigate effects of ultrasound-assisted immersion freezing (UIF) on the quality of the prepared dish (braised beef with potato) and compare it with immersion freezing (IF) and conventional air freezing (AF). Results indicated that UIF effectively shortened the freezing time and improved the freezing rate of the dish. A suitable intensity of UIF treatment (150 W) improved dish quality, and high-intensity UIF treatments (300 and 450 W) caused tissue structure destruction and decreased quality. The hardness of beef and potatoes treated with UIF-150 was significantly higher than that of other treatments (P < 0.05). Scanning electron microscopy demonstrated that UIF-150 reduced the size of ice crystals and maintained the structural integrity of beef and potatoes. Porosity values of beef were 6.91 %, 2.66 %, 2.09 %, 4.30 % and 6.94 % in the AF, IF, UIF-150, UIF-300 and UIF-450 groups, respectively, and the trend of porosity in potatoes was similar to that of beef. Meanwhile, the contents of volatile flavour components in the dishes were remarkably decreased after freezing treatments, and UIF treatment could promote the retention of volatile flavour components. Results of this study provided a reference for the application of UIF technology in the frozen quality enhancement of prepared dishes and their industrialisation.