Fabrication of pH-responsive chitosan-based hydrogel beads via electrostatic layer-by-layer assembly for visual monitoring of pork freshness

Anthocyanins possess outstanding bioactivity and pH sensitivity, but their stability is poor. The purpose of this study was to use chitosan hydrogel beads (CH) containing purple sweet potato anthocyanins as the core, with sodium carboxymethyl cellulose (CMC) and cationic soybean protein isolate (NSPI) as the outer layers, to prepare multilayer hydrogel beads via electrostatic Layer-by-Layer (LBL) assembly, to increase the stability of anthocyanins for monitoring pork freshness. The porosity (Pr) of CH is 20.51 %, and the hardness is 0.69 N. Compared with those of CH, the Pr of CH/CMC/NSPI and CH/CMC are reduced by 8.54 % and 1.32 %, respectively, and the hardness is increased by 49.28 % and 18.84 %, respectively. This phenomenon was confirmed by scanning electron microscopy, and the structure of the hydrogel beads became denser with an increasing number of outer layers. Thermogravimetric analysis also revealed that an increase in the number of outers layer improved the thermal stability of the hydrogel beads. X-ray diffraction results revealed that the crystallinities of CH, CH/CMC and CH/CMC/NSPI were 28.82 %, 25.05 % and 17.92 %, respectively, indicating that electrostatic interactions and hydrogen bonds between materials affect the crystallinity of hydrogel beads. The interaction between materials and the generation of hydrogen bonds have also been proven by Fourier-transform infrared spectroscopy. During the release process, the release rate of CH/CMC/NSPI was lower than that of CH and CH/CMC, and the release rate was less than 10 % at 360 min. Compared with pure CH containing PE, the hydrogel beads with CMC and NSPI shells still showed better antibacterial and antioxidant activities after 10 days of storage, and the free radical clearance rates of DPPH and ABTS still remained at 53.86 % and 90.68 %, respectively. Owing to the multilayer structure of CH/CMC/NSPI, it still has excellent antibacterial activity after 10 days of storage. Owing to their superior biological activity, hydrogel beads have the potential to be used as preservatives to prolong the freshness of pork by modulating the microenvironment, whereas CH/CMC/NSPI can more effectively inhibit weight loss, pH and TVB-N increase in pork. The hydrogel beads have good pH sensitivity and can be used to monitor the freshness of pork during storage. Therefore, the multilayer hydrogel beads developed in this study have great potential for monitoring food freshness and extending shelf life.