Positive Effects and Mechanism of Ultrasound on Chitin Preparation from Shrimp Shells by Co-Fermentation

Abstract

The objective of this study is to explore the effect and mechanism of ultrasound on chitin extraction from shrimp shells (SSP) by the co-fermentation of Bacillus subtilis and Acetobacter pasteurianus. After pre-treating the SSP with high-intensity ultrasound (HIU, 800 W), the protease activity in the fermentation solution reached 96.9 U/mL on Day 3, which was significantly higher than for SSP that had not been pre-treated with ultrasound (81.8 U/mL). The fermentation time of the chitin extraction process was 5.0 days without ultrasound pre-treatment, while it was shortened to 4.5 days when using 800 W ultrasound to treat SSP. However, there were no obvious differences when we applied low-intensity ultrasound (200 W, 400 W) treatment. Furthermore, chitin purified from shrimp shells pre-treated with HIU exhibited lower molecular weight (11.2 kDa), higher chitin purity (89.8%), and a higher degree of deacetylation (21.1%) compared to SSP with no ultrasound pre-treatment (13.5 kDa, 86.6%, 18.5%). Results indicate that HIU peels off the protein-CaCO3 matrix that covers the SSP surface. About 9.1% of protein and 4.7% of Ca2+ were released from SSP pre-treated with 800 W ultrasound. These figures are both higher than with no ultrasound pre-treatment (4.5%, 3.2%). Additionally, the amount of soluble protein extracted from SSP through HIU was 50% higher than for the control sample. SDS-PAGE analysis indicated that the soluble protein was degraded to the micromolecule. It also revealed that HIU induced the secondary and tertiary structure destruction of protein extracted from SSP. In conclusion, HIU-induced degradation and structural damage of protein enhances the protein-CaCO3 matrix to be peeled off from SSP. Also, in the co-fermentation process, an increase of protease activity further accelerates deproteinization.