Infusion of CO2 in a solid food: A novel method to enhance the low-frequency ultrasound effect on immersion freezing process

• Gelatin–sucrose–water model system frozen under different freezing conditions was investigated.
• Infused CO2 (ICO) and low-frequency ultrasound (LFU) decreased freezing time and ice crystal size.
• Better preservation of original physico-chemical properties was achieved by ICO and LFU.
• ICO and LFU treatment will have promising applications in real solid food freezing.

In this study, influence of infusion of CO2 (ICO) in a model solid food system (gelatin gel) was investigated during low-frequency ultrasound (LFU) assisted immersion freezing process. Low-frequency ultrasound will cause a rapid gas bubble formation in the CO2 infused food matrix enhancing the ice crystal nucleation. The freezing time, ice crystal size distribution and physico-chemical properties in the treated sample were investigated by Confocal Laser Scanning microscopy (CLSM), low-field NMR (LF-NMR), SEM, DSC, FTIR and TPA. The results indicated that infused CO2 can significantly (p < 0.05) decrease the freezing time and ice crystal size. The freeze-thawed samples subjected to combined ICO and LFU treatment had significantly (p < 0.05) lower water loss and higher gel strength and texture properties than the sample with LFU treatment only. LF-NMR study demonstrated that the combined treatment also influenced the water mobility and distribution of freeze-thawed samples. Compared to control sample, secondary structure of gelatin proteins of all the frozen samples changed significantly, α-helix decreased, while β-sheet and random coil increased.Industrial relevanceThe existence of CO2 in the samples promoted the crystallization of ice in solid food model during ultrasound assisted freezing process. Thus, the infusion of CO2 will have promising application in freezing of real foods (such as fruits and vegetables, dough, and fish).