Effects of pH on techno-functionality and interfacial activity of potato protein extract (PPE)

The discharged wastewater from potato starch factories known as potato fruit juice (PFJ) contains 1-1.5% protein with potential techno-functionalities if recovered by non-destructive techniques. In this study, the intact potato protein extract (PPE) in aqueous solution (0.5% w/v) at different pH values (2, 4, 6, 8 and 10) was characterized with respect to solubility, emulsifying activity (EAI) and stability (ESI), droplet size, zeta potential and interfacial activity of stabilized interface. In a protein solubility graph, a valley shape was observed indicating the lowest solubility around pH 6, while the highest solubility was recorded at pH 2 and pH 8 as 53.4% and 42.0%, respectively. A pH-dependent trend was observed as both EAI and ESI were increased by shifting the pH from acidic region to the alkaline with highest values at pH 10. Apparently, the negatively charged protein at the alkaline region have higher emulsifying activity compared to the positively charged protein at acidic pH. Similarly, zeta potential data was dominated by the applied pH, i.e., a remarkable shift was recorded from pH 2 (-6.1), toward pH 8 (-30.3) and 10 (-31.5), corresponding to the droplet size (D_3,2) of 2810 nm, 1013 nm and 947 nm, respectively. In terms of interfacial activity, the aqueous PPE solution actively reduced the interfacial tension (IFT) of stabilized interface in water-in-oil system at different pH values. However, apart from a stronger and much faster decrease of the IFT at the alkaline region, the formed droplet detached from the needle after nearly two min and less than one min at pH 8 and 10, respectively. This phenomenon can possibly be attributed to the synergistic effect of high protein concentration of aqueous phase in our study and pH-dependency of interfacial activity of PPE with very high mobility and flexibility at alkaline pH, yet to be clarified.