Effect of CaCl₂ on the stability and antimicrobial activity of nisin

Nisin is a natural and safe biological preservative which has great potential in cosmetics and food industry. However, the low thermostability of nisin at high pH limits its commercial applications. In this study, the effect of CaCl₂ on the stability and antimicrobial activity of nisin was studied. The results showed that CaCl₂ has a strong protective effect on nisin at weak acid environment (pH of 4.0-6.0) and heat treatment. The results of X-ray diffraction (XRD) and scanning electron microscopy (SEM) showed that the crystalline particles of nisin increased size and decreased peak and a stable structure was formed by CaCl₂ with nisin. The thermogravimetric analysis (TGA) presented that CaCl₂ led to the temperature of initial (T5) and complete decomposition (T95) of nisin increased by 8 °C and 13 °C, respectively, and enhanced the thermostability of nisin. The result of Fourier-transform infrared spectroscopy (FT-IR) confirmed that CaCl₂ could promote the formation of hydrogen bonds and increase the stability of proteins. The changes of secondary structure of nisin with CaCl₂ was revealed by circular dichroism (CD) spectroscopy. At pH 4.0, the proportion of α-helix, β-sheet and β-turn were same as to that of pH 2.0 without CaCl₂. At pH6.0, the proportion of secondary structure changed a little by heat treatment. This indicated that the addition of CaCl₂ protected the secondary structure of nisin.