Fabrication and characterization of whey protein isolate-tryptophan nanoparticles by pH-shifting combined with heat treatment

Abstract

L-Tryptophan (Trp) is an essential amino acid with numerous health benefits. However, incorporating Trp into food products is limited due to its pronounced bitter taste. Encapsulating Trp in nanoparticles by using other natural biopolymers is a potential strategy to mask the bitter taste of Trp in the final products. Whey protein isolate (WPI), composed of alpha-lactalbumin (α-LA), bovine serum albumin (BSA), and beta-lactoglobulin (β-LG), has played a crucial role in delivering bioactive compounds. In order to incorporate Trp within WPI, the present study used a combination of pH-shifting andthermal treatment to fabricatewhey protein isolate-tryptophan nanoparticles (WPI-Trp-NPs). During the pH-shifting technique, WPI unfolds at high pH, such as pH 11, and the dissociated WPI molecules are refolded when pH is shifted back to neutral, creating particles with uniform dispersion and encapsulating smaller particles surrounding them in solution. Further, the well-distributed nanoparticles formed by pH-shifting might encourage the formation of more uniform nanoparticles during subsequent thermal treatment. TheWPI-Trp particles have an average particle size of 110.1 nm and a low average PDI of 0.20. Fluorescence spectroscopy confirmed the encapsulation of Trp by WPI, which shows higher fluorescence when the Trp is encapsulated by the WPI. Surface hydrophobicity, circular dichroism, particle size, free sulfhydryl, and antioxidant activity were used to characterize the WPI-Trp-NPs. WPI-Trp-NPs formed by pH-shifting combined with heating showed a higher surface hydrophobicity and free sulfhydryl content than the untreated WPI-Trp mixture. The conversion of α-helix into random coil in the WPI secondary structure indicated a more disordered structure of the modified whey protein. Molecular docking results indicate the interactions between Trp and WPI, including alpha-lactalbumin (α-LA), bovine serum albumin, and beta-lactoglobulin (β-LG), were mainly driven by hydrophobic interactions and hydrogen bonding. The binding affinity between Trp and these proteins was ranked as α-LA>BSA>β-LG. The combination of pH-shifting and heating improved the functionalityof WPI and was an effective way to fabricate WPI-Trp nanoparticles.

Keywords: Nanoparticles; Thermal treatment; Tryptophan; Whey protein isolate; pH-shifting.