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This work systematically researched the binding behavior mechanism and foaming properties of bovine α-lactalbumin (α-La) and glycyrrhizic acid (GA) or ginsenosides (GR) in bulk at pH 7.0 by using the methods of multi-spectroscopic, molecular docking and molecular dynamics (MD) simulation. The intrinsic fluorescence results showed that binding behavior mechanism of GA and GR bounded onto α-La was static quenching. Moreover, the Ka of α-La/GA (3.44 × 104) was larger than that of α-La/GR complex (9.57 × 102) at 298 K. The main driven forces of α-La/GA were hydrogen bonds, van der Waals and hydrophobic interaction forces, while only hydrophobic interaction force was involved in α-La/GR complex. Meanwhile, molecular docking result showed that the docking site of α-La/GA and α-La/GR was mainly located in sheet with S1, S2 or S3 and helix with H4 or h1. There were six hydrogen bonds in α-La/GA, whereas no hydrogen bond was present in α-La/GR. The MD results presented that the electrostatic interaction energy, the Van der Waals interaction energy and the nonpolar solvent energy were favorable for binding between α-La and GA or GR. Among them, the main contributed amino acid residues of α-La/GA complex were Thr33, Gln54, Tyr103, Trp104 and Leu110, while the main interacted residues of α-La/GR system were Trp60, Val99, Tyr103 and Trp104. Furthermore, GA and GR bonded onto α-La had insignificant impact on the secondary structure of α-La. Finally, the addition of GA and GR (1.0 mg/mL) induced a 155.57% and 111.11% increase in the foaming property of α-La. The obtained results provide some new insights to the interaction mechanism of α-La bounded with GA and GR. Additionally, it also indicates that α-La/saponin complex has the ability to apply in the protein-based foaming foods.

Influence of microwave irradiation heating (MH) and water bath heating (WH) pretreatments (70, 80 and 90 °C) on the structure, emulsifying properties and cell proliferation of laccase-crosslinked α-La was investigated. Compared with WH pretreatment, MH pretreatment can further promote the formation of polymers in laccase-crosslinked α-La. Also, MH treatment further unfolded the secondary structure, increased the surface hydrophobicity, and induced more formation of dityrosines in laccase-crosslinked α-La solution than that of WH pretreatment. Compared with untreated laccase-crosslinked α-La, its emulsifying activity index (EAI) was increased by 28.87% and 25.01% after MH and WH pretreatment at 90 °C. Gel hardness and water holding capacity of laccase-crosslinked α-La was increased by 30.20% and 18.37% after MH at 90 °C compared to untreated laccase-crosslinked α-La. Cell viability of laccase-crosslinked α-La (2 mg/mL) was increased by 18.12% at 90 °C with MH pretreatment. These findings indicated that MH pretreatment could ameliorate the crosslinking degree of α-La and laccase, thus improving its emulsifying, gel properties, and cell viability.

This study aims to explore the interaction mechanisms between alpha-lactalbumin (ALa) and three fat-soluble vitamins (VA, VD3 and VE) at pH 7.0 and their impacts on structure and functionality of ALa. Analysis of fluorescence spectroscopy showed that vitamins quenched intrinsic fluorescence of ALa in a static mode and cholecalciferol (VD3) had a higher binding affinity for ALa than Retinol (VA) and alpha-tocopherol (VE). Synchronous fluorescence and three-dimensional fluorescence spectra demonstrated the conformation changes of ALa. Meanwhile, fourier transform infrared spectroscopy manifested that three vitamins induced a transition from α-helix to β-sheet in ALa, and VD3 notably increased its random coil content. Furthermore, a combination of VE with ALa generated macromolecular aggregates, based on particle size results. The addition of VA and VD3 increased zeta potential and surface hydrophobicity of ALa, while VE triggered that of ALa initially increased and then decreased with vitamins ranging from 0 to 0.35 mmol/L. Additionally, molecular simulation suggested that VA and VE had a same binding site in the cleft of ALa, but VD3 bound in a hydrophobic pocket in protein, and their binding driving forces were hydrophobic interaction and hydrogen bond. This study reveals that ALa might be a potentially suitable carrier for delivering vitamins, providing a theoretical basis of interaction between proteins and small molecules.

The gel strength, microstructure and water-holding capacities (WHC) of emulsion gels, modified by combined ultrasound (US, 20 kHz, 600 W) pretreatment (0, 20 and 40 min) and laccase cross-linking treatment of α-lactalbumin (α-LA)-stabilized emulsions (at protein concentration 12%, w/v; oil volume fraction, 10%, w/v), were studied and compared. Significant differences in gel strength, WHC, and morphological changes have scrutinized among various emulsion gels prepared with laccase cross-linking US-α-LA (α-LA treated with US pretreatment) and US-α-LA. The gel strength of laccase cross-linking US-α-LA and US-α-LA significantly increased from 201.4 to 274.1 g and 22.8–49.4 g, respectively, with US duration ranged from 0 to 20 min. Furthermore, gel WHC of laccase cross-linking US-α-LA and US-α-LA also remarkably increased from 91.1 to 95.9 and 61.7–70.8%, respectively. The laccase cross-linking US-α-LA and US-α-LA had more uniform and denser microstructure than control. Moreover, the emulsion with laccase cross-linking US-α-LA gave a self-standing elastic gel, which showed stability for more than one month without showing any separation. In conclusion, the most appropriate combined processing technique to make a stable gel was the moderate US duration (20 min). It had a remarkable impact on physicochemical and microstructural attributes of emulsion gel, producing better substrates for laccase. Our obtained results suggested that for functionality modification of the globular protein, ultrasound pretreatment combined with laccase is an effective method.