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].