Understanding the effects of carboxymethyl cellulose on the bioactivity of lysozyme at different mass ratios and thermal treatments

Abstract Lysozyme (Ly) is an excellent natural preservative to prolong the storage and shelf life of food products. However, its bioactivity is often compromised when it coexists with negatively charged polysaccharides due to their unfavorable electrostatic interactions. In this study, we investigated the influence of carboxymethyl cellulose (CMC) on Ly structure and bioactivity under the different CMC and Ly mass ratios and thermal treatments. The CMC-Ly complexes were characterized by turbidity, protein intrinsic and extrinsic (isothiocyanate labeled) fluorescence, Fourier transform infrared (FTIR) and circular dichroism (CD) spectroscopies. The bioactivities of Ly were determined via enzymatic activity and inhibition zone methods. The results showed that their interactions could be modulated by the mass ratios of CMC to Ly. The strongest interactions were found at the mass ratio of 1:5, leading to the significant structural changes and thus the least heating stability of Ly. As a result, its enzymatic and antibacterial activities were restrained at the maximum level at this mass ratio owing to the strongest screening effects on the active sites, aggregation of Ly molecules, altering the spatial structure and neutralizing the surface charges. Notably, further increasing CMC concentration weakened their interactions and thus the complexes dissociated until at the mass ratio of 1:1, which, however, was helpful to recapture the bioactivities of Ly. When the CMC: Ly ratio was higher than 1:1, the bioactivities of Ly were diminished again, possibly due to the steric-hindrance effect of excessive CMC. Thermal denaturation further restrained the enzymatic activities of Ly in CMC-Ly complex. Whereas CMC remained the antibacterial activity of Ly at high temperature heating. In a nut shell, our findings from this work shed some light on the possible mechanistic explanation of negative effects from polysaccharides on the bioactivity of Ly during food processing.