# High-Pressure Effects on Selected Properties of Pea and Soy Protein Isolates
## 4 Discussion
### Emulsifying Properties  
The general trend observed in previous studies regarding the effects of HPP on emulsifying properties of plant-based proteins was the increase of EAI with pressure [8; 30]. These effects were explained mainly by protein unfolding caused by the HPP and consequent exposure of hydrophobic groups, thus improving emulsifying properties [31], although, the contrary was also already reported [28]. It is expected that a complex set of factors interacting with each other influence the interfacial activity of these proteins, including protein charge, solubility, surface hydrophobicity and the overall distribution of hydrophilic and hydrophobic regions on the protein chains. In fact, and contrarily to what was previously reported [32], in the present study, no significant correlations were observed between the changes of H\({}_{0}\) and EAI with pressure, both in the total and soluble fractions at the different pHs. However, in general, for both proteins under the three pHs analyzed, a strong negative correlation was observed between the effect of HPP on solubility and EAI: for example, under pH 7, r = -0.702 (_p_ < 0.001) for PPI and r = -0.953 (_p_ < 0.001) for SPI. For these samples and under the conditions analyzed, a possible positive effect of HPP on protein solubility and in obtaining more open and extended protein conformations, which, at first, would be advantageous to obtain an increase in the emulsifying capacity of these proteins, may be compromised by an increase in intermolecular protein interactions, even though mostly forming soluble aggregates that will have lower diffusion rates and molecular flexibility, thus compromising their interfacial activity and emulsifying activity. Regarding the effects of HPP on the proteins' ability to stabilize emulsions, the treatment by HPP has been shown to have varied and complex effects, not only dependent on HPP conditions but also on the pH, making difficult a reasoned interpretation of these effects. However, also in this case, for many conditions of HPP and for certain pH values, the observed effects were not statistically significant (the whole set of results is shown in Figures S6 and S7 as Supplementary Data). In general, the effect of HPP on the surface tension of PP and SP solutions was small and, in most cases, not statistically significant, either in relation to the control sample or in relation to the different processing conditions. For PP, the most significant influence of HPP that is worth highlighting is the decrease in surface tension with increasing pressure for samples processed for a reduced holding time (5 min, \(p\) < 0.05) (Figure S3A). The decrease of \(\gamma\) with pressure was also observed for 15 min holding time, but, apparently, higher values of \(\gamma\) were obtained by increasing the holding time. For soy proteins (Figure S3B), the treatment by HPP seems to originate a more pronounced increase of \(\gamma\) in relation to the control. For these samples, it is noteworthy to mention the significant decrease (_p_ < 0.05) of \(\gamma\) with the increase of the holding time for those samples treated at 600 MPa. Thus, the influence of HPP upon the ability of the proteins to decrease the air-water surface tension is dependent on the processing conditions (pressure and holding time), but also on the type of protein, which can help to better understand the diversity of results regarding the emulsifying properties of the pea and soy proteins, as affected by HPP. Previous studies have already shown different effects of HPP on ESI for emulsions prepared with different vegetable proteins, with both increases and decreases in the stability of emulsions being reported [16; 33; 34]. Since pressure affects different proteins in different ways and affects the exposure of hydrophobic groups and/or changes in the molecular flexibility [6], it can either increase or decrease the interactions between the proteins within the interfacial viscoelastic film on the oil droplets, altering the ESI in different ways.