Effect of ultrasound on the diffusion properties of casein entrapped in alginate–chitosan gel

• Different kinds of ultrasonic treatments had obvious distinctions on the casein diffusion.
• In the ultrasound-assisted process, increasing frequency made the value of De decreased while ultrasound power played a promoting role on De.
• De get the minimum value at 50 kHz in the pretreatment ultrasound process.
• The maximum De was obtained at 0.25 W/cm2 during the pretreatment ultrasound process.
• The changes (area ratio) of microstructure gel beads were in accordance with De at pre-ultrasound process.

The effects of ultrasound-assisted and pre-ultrasound treatment on the diffusion properties of casein imbedded by alginate–chitosan gel were investigated. The fluorescence spectrophotometry for determining the fluorescence intensity of casein was established to calculate the diffusion coefficient (De). Scanning electron microscope (SEM) was used to observe the microstructure of gel beads. The results showed that two different kinds of ultrasonic treatments had obvious distinctions on the casein diffusion. As the frequency increased, the value of De decreased from 28.56 × 10−4 m2 s−1 (28 kHz) to 2.57 × 10−4 m2 s−1 (135 kHz) during the ultrasound-assisted process. While, the minimum De of 8.6 × 10−4 m2 s−1 was achieved at the frequency of 50 kHz for the pre-ultrasound treatment. The impact of power on the diffusion showed that De increased with the increase of ultrasound power until it reached the highest value 28.56 × 10−4 m2 s−1 (0.45 W/cm2) in the ultrasound-assisted process. It would reach the maximum value (16 × 10−4 m2 s−1) when the power was 0.25 W/cm2 in the pretreatment ultrasound process. SEM analysis exhibited that the gel structural changes (area ratio) were in accordance with De through different ultrasonic treatment. This was mainly due to the mechanical action and cavitation of the ultrasonic treatment. This study is important to explain the diffusion properties of large molecules and explore the mechanism of enzyme immobilization treated by ultrasound.