High internal phase pickering emulsions stabilized by pea protein isolate-high methoxyl pectin-EGCG complex: Interfacial properties and microstructure

Highlights

• PPI-HMP-EGCG complex was used as the stabilizer for the PEs and HIPPEs.

• PPI-HMP-EGCG complex produced a thick and ordered viscoelastic interface layer.

• Formation of interface layer and gel-like network structure promoted PEs stability.

• Stability mechanism of PEs was clarified by interfacial behavior and microstructure.

Abstract

The pea protein isolate-high methoxyl pectin-epigallocatechin gallate (PPI-HMP-EGCG) complex was used to stabilize Pickering emulsions (PEs) and high internal phase PEs (HIPPEs), and the effect of interfacial rheology on the microstructure, bulk rheology and stability of these emulsions was investigated. The PPI-HMP-EGCG complex with PPI to EGCG 30:1 exhibited partial wettability (81.6 ± 0.4°) and optimal viscoelasticity for the formation of stable interfacial layer. The microstructure demonstrated that the PPI-HMP-EGCG complex acted as an interfacial layer and surrounded the oil droplets, and continuous phases were mainly filled with excessive HMP, which enhanced emulsion stability. The formation of a firm gel-like network structure required a dense interfacial layer to provide the PEs (complex concentration of 0.1%) and HIPPEs (oil-phase up to 0.83) with ideal viscoelasticity and stability. The results provide the guidelines for the rational design of EGCG-loaded HIPPEs stabilized by water-soluble protein/polysaccharide complexes.

Introduction

Pickering emulsions (PEs) were stabilized by solid particles that adsorb onto the oilwater interface, and the stability was dependent on the physicochemical properties of the adsorbent particles and the viscoelastic behavior of the interface layer (Gao et al., 2014, Van Hooghten et al., 2017). The applicability of inorganic particles in the food industry was limited. Therefore, it was necessary to develop ecofriendly particles which are of great value to food system. Some edible particles have been found to stabilize emulsions, such as cellulose nanocrystals (Chen et al., 2018), modified starch (Li, Zhang, Li, Fu, & Huang, 2020) and zein (Wang et al., 2016). The development of protein-based PEs has been promising for the establishment and wide application of emulsion-based ingredients. Compared with individual protein particles, protein-polysaccharide complexes have shown improved surface wettability, enhanced interfacial properties (Wang et al., 2016), promotion the interfacial adsorption and accumulation, and the formation of a stable interfacial layer (Zeng et al., 2017).

High internal phase Pickering emulsions (HIPPEs) are characterized by enrichment of the internal dispersed phase to more than 0.74 by volume, and exhibit high loading efficiency for active substances and nutrients (Wei et al., 2020a, Wei et al., 2020b, Wei et al., 2020c). HIPPEs stabilized by alcohol-soluble proteins, including zein (Zhou et al., 2018) and gliadin (Zeng et al., 2017), were usually prepared via organic solvent-mediated precipitation, which limited their application in the food industry. Therefore, there is considerable interest in developing effective PE/HIPPE stabilizers with water-soluble plant proteins or protein-polysaccharide-polyphenol complexes.

PPI is a plant protein with a high nutritive value. At present, the utilization of PPI is limited, mainly due to low water-solubility and aggregation near the isoelectric point (Guo et al., 2020). The addition of the polysaccharide (HMP) enhanced the viscosity of aqueous phase and prevented protein aggregation. Meanwhile, many studies have reported on the effect of HMP on stabilizing plant protein dispersion (Wei et al., 2020a, Wei et al., 2020b, Wei et al., 2020c) and the preparation of the alcohol-soluble protein-pectin complex (Zhou et al., 2018). Some researchers pointed out that the addition of EGCG can improve the wettability of the protein-based complex and further enhance the stability of the PEs (Liu et al., 2019, Zhang et al., 2020). Also, polyphenols can participate in the regulation and manipulation of the interfacial layer to stabilize PEs (Zembyla et al., 2019, Zou et al., 2015). Furthermore, the combination of protein, polysaccharide and EGCG can enhance the hydrophobicity of the protein through the interactions between EGCG and protein (Yang et al., 2018). There are some studies on PEs stabilization using plant-derived protein-pectin/polyphenol complexes, such as zein-pectin complex (Jiang et al., 2020) and zein-EGCG complex (Zhang et al., 2020). However, there have been no reports on the usage of the PPI-HMP-EGCG complex as the stabilizer for PEs and HIPPEs.

The objective of this study was to investigate the impact of interfacial rheological properties on the interfacial microstructure, bulk rheology and stability of PEs and HIPPEs. The relationship between EGCG content and interfacial properties were illustrated using surface wettability, interfacial shear rheology and interfacial intension. The PEs and HIPPEs were prepared with PPI-HMP-EGCG complex as the stabilizer, and the formation and stability mechanism of emulsions were explored through the interfacial behaviors.