The Study On Emulsifying Properties Of Sweet Potato Protein

The waste solution produced in sweet potato processing contains large amounts of sweet potato protein. Recycling sweet potato protein from waste solutions can reduce the waste of resources and the pollution to the environment. Furthermore, sweet potato protein has significant bioactive functionality and it is a high-quality plant protein with good processing characteristics. Therefore, there are important theoretical and practical value to study the emulsifying properties and its mechanism of sweet potato protein.Sweet potato protein (SPP) was prepared through isoelectric precipitation. The effect of physicochemical conditions (protein concentrations, oil volume fractions, pH, NaCl and CaCl2) on properties of stabilized emulsions of SPP were investigated by use of emulsifying activity index (EAI), emulsifying stability index (ESI), droplet size, rheological properties and interfacial properties in different physicochemical conditions, and initially explored the mechanism of the effect of different physicochemical factors on emulsifying properties of SPP.Increasing of protein concentration greatly decreased droplet size, EAI and apparent viscosity of SPP emulsions; however, there was a pronounced increase in ESI and interfacial protein concentration (P < 0.05). In contrast, increasing of oil volume fraction greatly increased droplet size, EAI and emulsion apparent viscosity of SPP emulsions, but decreased ESI and interfacial protein concentration significantly (P < 0.05). The main components of adsorbed SPP at the oil–water interface were Sporamin A, Sporamin B and some high-molecular-weight aggregates formed by disulfide linkage.At higher pH (7, 8), the EAI and ESI is high while with the decrease of pH (8-4) the EAI and ESI decreased. However, when pH is below pI, the EAI and ESI increased again. Compared with emulsion at pH 3, the EAI and ESI of SPP emulsions at pH 2 decreased due to the intense hydrophobic interaction. The structure of SPP changes significantly, which leads to the increase of interfacial protein concentration and the formation of dimmer of Sporamin A. Compared with SPP emulsions without NaCl, the EAI and ESI of SPP emulsions with addition of 0.2 M NaCl decreased significantly, and the initial apparent viscosity increased markedly as well, which can be attributed to reducing electrostatic repulsion between droplets through electrostatic screening. SDS-PAGE show that NaCl has little effect on the composition of interfacial adsorbed protein.The EAI of SPP emulsions decreased significantly from 30.3 m2·g-1 to 27.6 m2·g-1 after the addition of 0.05 mol·L-1 calcium chloride, while the d4,3 increased from 4.2μm to 4.42μm (P<0.05). Nevertheless, with the further increase in concentrations of calcium chloride (0.10-0.25 mol·L-1), the EAI had no obvious difference (P>0.05), but the d4,3 had a marked increase (P<0.05). In addition, the CI, interfacial adsorbed protein concentration and initial apparent viscosity were increased markedly by adding higher concentrations of calcium chloride (P<0.05). Furthermore, SDS-PAGE showed that with addition of calcium chloride Sporamin A was not easy to be adsorbed at oil/water interface, and that the high molecular polymers (>66kDa) formed by S-S bonding existed both in oil/water interface and continuous phase of emulsions. The study found that these physicochemical conditions can significantly affect the emulsifying properties of SPP.