Mechanism underlying the enhancement of emulsifying properties of millet bran prolamine via ultrasound-assisted gallic acid covalent-binding or carboxymethylation under pH-shifting.

Natural proteins with high safety and excellent emulsifying properties have a promising prospect. In order to enhance the emulsifying properties of millet bran prolamine (MBP), pH-shift and ultrasound-assisted gallic acid-covalent binding or carboxymethylation were implemented. The results evidenced that ultrasound-assisted gallic acid-binding or carboxymethylation both increased the crystallinity and contents of protein, polyphenols, and β-sheet of MBP. Ultrasound-assisted gallic acid-covalent binding significantly enhanced the hydrophobicity (4922.83), emulsifiability, and emulsion stability of MBP, while ultrasound-assisted carboxymethylation modified MBP exhibited the highest proportion of low-molecular-weight subunits (9.92 kDa), emulsifiability (132.59 m2/g), emulsion stability (62.63%), and centrifugal stability (82.55%). Mechanistic analysis revealed that ultrasound-assisted carboxymethylation or gallic acid-binding improved the emulsion properties via enhancing the interfacial adsorption capacity, solubility, or hydrophobicity of MBP, increasing zeta potential, reducing droplet size and the ratio of storage to loss modulus in the emulsion system. Alkaline pH-shift (from 6 to 10) improved the emulsifiability and emulsion stability of MBP by increasing its solubility, hydrophobicity, interfacial adsorption capacity and zeta potential. In summary, alkaline pH-shift and ultrasound-assisted carboxymethylation was the most effective way for improving its emulsion properties. However, more underlying mechanisms of these modifications need further works.