Thermal stabilization effects of κ-Carrageenan on water-soluble protein extracted from Pinctada martensii meat

This study aimed to explore the effect of κ-carrageenan on the stability of WSP from the muscle of Pinctada martensii and provide data reference for the use of P. martensii meat and improvement of the functional properties and application expansion of different types of aquatic proteins. The effect of κ-carrageenan (2 mg/mL) on WSP (1 mg/mL) stability before and after heat treatment (50~100 °C) in the range of pH 5.0~8.0 was explored using differential scanning (DSC), Fourier transform infrared spectroscopy (FT-IR) and circular dichroism (CD). Turbidity, ζ-potential, particle size, hydrophobicity, and presence of free sulfhydryl groups and total sulfhydryl groups of WSP and κ-WSP were measured. Compared with WSP, light transmittance of κ-WSP decreased after heat treatments at various temperatures (50~100 °C) under the same pH conditions, and the difference of transmittance reached maximum at pH 5.0 and 80 °C. DSC showed that the denaturation temperature of κ-WSP was slightly higher (an average increase of 4.4 ± 1.2 °C) than that of WSP except at pH 4.0 and pH 5.5. At pH 5.0, the ζ-potential value of κ-WSP was higher than (an average increase value of 2.7 ± 0.9) that of WSP with lower particle size distribution, and the hydrophobic content of WSP and κ-WSP showed different trends with increasing temperature. The total thiol groups of both WSP and κ-WSP increased with increasing temperature, except the decrease at 100 °C, while the content of free thiol groups of either WSP or κ-WSP did not change significantly. The FT-IR and CD results showed that the electrostatic combination of WSP and κ-carrageenan did not produce new functional groups, and κ-carrageenan significantly affected the secondary structure of WSP under heat treatment in the range of 80~100°C. These results suggest that κ-carrageenan has a protective effect on the thermal denaturation of WSP of pearl shellfish, and this protective effect is most obvious at isoelectric point (pH 5.0).