The effect of limited hydrolysis was investigated on the physico-chemical properties of soy protein isolate–maltodextrin (SPI-Md) conjugate. The hydrolysates at a degree of hydrolysis (DH) of 1.8% showed much higher surface hydrophobicity (H0; 71.39 ± 3.60) than that of the SPI control (42.09 ± 2.17) and SPI-Md conjugates (53.46 ± 2.74). Intrinsic fluorescence analysis demonstrated the unfolding of protein molecule and exposure of hydrophobic groups of SPI-Md conjugate hydrolysates. As evidenced by far-UV circular dichroism (CD) spectroscopy, the limited hydrolysis increased the unordered secondary structures of SPI-Md conjugates. The denaturation temperature (Td) of SPI-Md conjugate was significantly increased by subsequent limited hydrolysis from 102.53 ± 0.60 °C to 108.11 ± 0.61 °C at DH 1.8%. In particular, the emulsifying activity index (EAI) was improved notably after limited hydrolysis of DH 1.8% (147.76 ± 4.39 m2 g−1) compared with that of native SPI (88.90 ± 1.44 m2 g−1) and SPI-Md conjugate (108.97 ± 1.45 m2 g−1).Summary
In this study, Neutrase-assisted controlled hydrolysis of SPI-Md conjugates showed significant beneficial changes to their functional properties. It was demonstrated that the emulsifying capability, heat stability and solubility around the isoelectric point, exhibited great improvement after hydrolysis. As confirmed by the results of H0, fluorescence emission spectra and far-UV CD spectroscopy, controlled hydrolysis (DH, 1.8–2.9%) of SPI-Md conjugates led to an increase of its molecular flexibility, by unfolding of peptide molecules which could accelerate the adsorption at the interface of oil droplets and keep the oil-water equilibrium to form a stable emulsion.