Effect of dextran molecular weight on structure and emulsifying property of zein conjugates

Abstract: In order to investigate the effect of conjugation by dextran on the structure and emulsifying property of zein and the relationship between protein structure and functional properties, zein-dextran conjugates were prepared by conjugating zein and dextran with different molecular weights (6, 20, 40 and 70 kDa) under wet-heated Maillard reaction at 85 ℃ for 2 h. The free amino content, degree of graft, colour and amino acid content for zein-dextran conjugates were measured by UV-vis (ultra-violet-visible) spectrophotometer, colorimeter and automatic amino acid analyzer. For the zein-dextran conjugates, the free amino content was reduced, and a* and b* increased, while L* decreased in comparison with native zein, indicating the covalent graft reaction between protein and polysaccharide. As the decrease of the molecular weight of dextran, the conjugate prepared by zein and dextran with low molecular weight (6 kDa) had lower free amino content, higher degree of graft and darker colour, which suggested that the dextran with low molecular weight (6 kDa) was more reactive. The amino acid analysis indicated that lysine and arginine were the main amino acids involved in conjugation reaction between zein and dextran. The structures of conjugates were researched using Fourier transform infrared spectroscopy (FTIR), intrinsic fluorescence spectroscopy and differential scanning calorimetry (DSC). FTIR demonstrated that zein and dextran with different molecular weights could form the covalent conjugates. The covalent conjugation of dextran could result in the fluorescence quenching of zein and reduce the thermal stability of zein. The bathochromic shift of fluorescence emission maximum wavelength for the conjugate prepared by zein and dextran with low molecular weight (6 kDa) occurred, leading to looser tertiary structure. Moreover, zein conjugating with the dextran with 6 kDa molecular weight had lower thermal stability, revealing that zein-dextran conjugate with higher degree of graft had a looser tertiary structure, and the higher the degree of graft, the lower the thermal denaturation peak temperature and enthalpy, the lower the thermal stability of zein-dextran conjugates, which might be an important reason for improving the emulsifying activity index (EAI) and emulsifying stability index (ESI) of zein-dextran conjugates. The emulsifying properties of conjugates were researched using microplate reader, laser particle sizer and dynamic rheometer. The EAI and ESI of native zein were (11.57±0.54) m2/g and (12.23±0.68) min, respectively. The EAI and ESI of zein-dextran conjugates were higher than that of native zein, indicating the significant improvement of emulsifying properties of zein by the covalent conjugation of dextran. The conjugate prepared by zein and dextran with low molecular weight (6 kDa) had higher EAI of (23.28±0.71) m2/g, which could be related to higher degree of graft, suggesting that the EAI of zein-dextran conjugates improved with the increase of degree of graft. However, the conjugate prepared by zein and dextran with high molecular weight (70 kDa) had higher ESI of (26.44±0.47) min. It was possible that the dextran with high molecular weight (70 kDa) had a large steric hindrance, inhibiting the aggregation of droplets in emulsion and forming more droplets with smaller size. Meanwhile, the emulsion was more viscous, which also resulted in the improvement of emulsion stability. These could be confirmed by particle size and rheological analysis of emulsion. It could be seen that the emulsion stability had little relation to degree of graft. However, the steric hindrance effect of dextran with high molecular weight (70 kDa) was the main factor to improve the stability of zein-dextran conjugate emulsions. The results of this study can provide theoretical basis for improving the functional properties of corn protein and understanding its modification mechanism.