Emulsifying Capacity and Stability
The above study showed that different heating methods and time affected the conformation of protein in samples. It was trusted that the structure changes could influence the efficacy of evolving oil droplets due to the different of morphology and surface groups, thus, the functional properties of protein might be affected [54]. The emulsion capacity (EC) reflected the adsorption capacity of protein at the water-oil interface, and the emulsion stability (ES) indicated the retention ability of protein at the oil-water interface after emulsion [54]. Table 4 shows the emulsion capacity (EC) and emulsion stability (ES) of untreated and treated walnut kernel. Untreated sample had the EC of 44.32%. EC of roasting and steaming treated walnut kernel samples ranged from 55.42 to 56.82% and from 52.37 to 56.38%, respectively. EC of walnut kernel heated for 15, 20 and 30 min increased significantly (P < 0.05). However, EC of untreated walnut kernel exhibited a significant (P < 0.05) difference compared to the heated samples. It was attributed that the heating treatment could cause the changes of native structure or surface properties of proteins in walnut kernel, which promoted more dissolution and the adsorption at the interfacial area [22]. Furthermore, no differences were found between the R15, R20, R30, S15 and S30. In contrast, there was a significant but minor decrease in EA (52.37%) for S30 compared to other samples. In general, the emulsifying properties of samples are due to depending on the strength of the formed interfacial films of protein at the oil-water interface [55]. The increase of protein adsorption and decrease of interfacial energy would hinder the separation of two phases, which resulted in the high EC [56].
Table 4 also shows the variation of the ES at different treatment methods and time. The ES of samples with heating treatment was significantly (P < 0.05) higher compared to untreated sample. It was further proved that the samples by heating treatment had better ES due to the better surface characteristics that could be beneficial to hinder the aggregation of droplet [57]. Untreated sample represented ES of 74.21% (Table 4). The ES of R15-R30 and S15-S30 increased by 92.46 - 96.39% compared to untreated samples, which indicated a greater capability of the protein polymer in heat-treated samples to adsorb on the surface of the oil droplet. It was concluded that the heating treatment had the ability to enhance the ES of protein in walnut kernel. It was farther confirmed that the unfolding of proteins would be helpful to improving the surface assimilation and maintenance of processing proteins at the oil-water interface, which would lead to the better diffusion and ES [57]. On the whole, the protein from heated walnut kernel might more reliably accumulate liquids on the surface, which lead to a relatively stable emulsion. Therefore, the samples obtained by heat treatment of walnut kernel would improve the stability in applications of food products [57].