Formation and optimization of chitosan-nisin microcapsules and its characterization for antibacterial activity

• RSM was used to optimize the process of preparing CS–nisin microcapsules.
• CS-nisin showed the maximum antibacterial activity at pH 5.0–6.0
• No significant change in the thermal stability of CS-nisin.
• CS-Nisin showed an enhanced pH stability against Bacillus subtilis CICC10275.

Nisin, a natural antimicrobial agent, was selected as the core material with chitosan (CS) as the wall material for micro-capsulation. Response surface methodology was used to optimize the process of preparing CS–nisin microcapsules. The predicted optimal condition was found to be CS concentration of 2.4 mg/ml, salt addition rate of 8 ml/min, CS: nisin by 3.8:1 (W/W) ratio, Na2SO4 as the precipitant salt. In this condition, the embedding, release rate, Z-average size of the microcapsules were 68.7%, 54.6%, 1441.6 nm, respectively. According to the predicted model, the corresponding values were 66.9%, 50.8%, 1419 nm, respectively. Scanning electron microscope (SEM) revealed that CS-nisin microcapsules were spherical, and the size apparently increased compared with CS. Fourier transform infrared spectroscopy (FTIR) indicated that there was no significant chemical interaction between CS and nisin. Antimicrobial activity assay exhibited CS-nisin microcapsules with the concentration of 0.25% (W/V) showed confirmed inhibitory effect against Gram-positive bacteria Bacillus subtilis. Besides, 1% CS-nisin showed the maximum antibacterial activity at pH 5.0–6.0, when the value of pH was 7.4, it also exert antibacterial ability, indicating the enhanced antimicrobial activity of CS-nisin.