Understanding the air-water interfacial behavior of suspensions of wheat gliadin nanoparticles

Highlights

• Wheat gliadin nanoparticles (WGNPs) display excellent foam stability at pH 6.0 but not at pH 4.0

• WGNP surface properties and morphology in suspension did not vary much with pH.

• Interfacial films with high viscoelasticity were formed at pH 6.0 but not at pH 4.0

• Disulfide bond formation occurred extensively upon interfacial adsorption at pH 6.0

• High foam stability at pH 6.0 is due to intermolecular cross-linking at the interface.

Abstract

The low solubility of many plant proteins (such as those of cereals) is a main obstacle preventing their use for stabilizing food foams and emulsions. Protein based nanoparticle suspensions hold promise for stabilizing such systems. Here, we shed light on how wheat gliadin based nanoparticles (WGNPs) behave at air-water interfaces, which at present remains largely unknown. At pH 4.0 and pH 6.0, WGNPs display very poor and excellent foam stability and result in interfacial films with low and high viscoelasticity, respectively. Fourier Transform Infra-Red and fluorescence spectroscopy revealed substantial differences neither in structural nor in surface properties of WGNPs, nor in WGNP morphology at varying pH values ranging between 4.0 and 6.0, implying that the differences in interfacial behavior originate at the interface during or after adsorption of WGNPs. Cryo scanning electron microscopy imaging of foams stabilized by WGNPs showed that at pH 4.0 and pH 6.0 NP-like structures and a more coherent film are present at the interface, respectively. This is consistent with the higher viscoelasticity of adsorbed interfacial films at pH 6.0 than that at pH 4.0. Foam fractionation revealed that proteins in foams produced at pH 6.0 contain a substantial amount of intermolecular disulfide bonds. Thus, the excellent foam stability of WGNPs at pH 6.0 may at least to some extent be ascribed to formation of a covalently cross-linked protein network at the air-water interface.

Introduction

Food foams and emulsions are traditionally stabilized by low molecular weight surfactants or proteins. Examples include meringue, mayonnaise, milk foam (on coffee) and cake. Such dispersions often contain animal proteins. Their abundant use is in part due to their favorable nutritional profile and their enjoyable organoleptic properties but mostly to their excellent functional properties (Day, 2013). However, the production of animal protein is more expensive and has a greater environmental impact than that of plant protein (Henchion, Hayes, Mullen, Fenelon, & Tiwari, 2017; Herrero et al., 2011).

Plant-based protein rich side-streams originate from various industrial processes, as is the case in the isolation of starch from maize (Zea mays L.) and wheat (Triticum aestivum L.) (Mulder, 2010; Van Der Borght, Goesaert, Veraverbeke, & Delcour, 2005) or the isolation of oil from soy (Glycine max) beans (Rodrigues, Coelho, & Carvalho, 2012). The low solubility of the proteins in these side-streams, be it due to their innate structure or to their pre-treatment, is often a major obstacle preventing their successful use for stabilizing food dispersions. Strategies for inducing or improving functionalities of plant protein have been researched. A promising strategy is to produce dispersions of nano-sized protein aggregates, hereafter referred to as nanoparticles (NPs), with high colloidal stability in aqueous systems (Joye & McClements, 2013; Wouters & Delcour, 2019). Such NPs may stabilize food foams and emulsions. An important concept in this regard is that of Pickering stabilization whereby interfaces are stabilized by adsorption of hard, rigid NPs (Binks, 2002; Pickering, 1907; Ramsden, 1903). Provided that such rigid NPs have sizes within a 1 nm to 1 μm range and that they have a contact angle at the interface of both dispersed phases in a 30°–150° range, their interfacial adsorption is practically irreversible (Tavernier, Wijaya, Van der Meeren, Dewettinck, & Patel, 2016; Xiao, Li, & Huang, 2016). This then results in a strong steric barrier which protects the system from destabilization (Binks, 2002; Xiao, Li et al., 2016). The concept of interface stabilization by rigid NPs has been investigated for various types of NPs, including some based on silica (AlYousef, Almobarky, & Schechter, 2018; Binks & Lumsdon, 1999), latex (Binks & Lumsdon, 2001) or clays (Abend, Bonnke, Gutschner, & Lagaly, 1998). Such NPs are of course not suitable for many food or pharmaceutical applications. More recently, interest in the production of biopolymer based NPs has risen substantially (Joye & McClements, 2013). Proteins, due to their relatively high hydrophobicity, innate surface-activity and molecular flexibility may well be good raw materials for producing such NPs (Dickinson, 2010).

In many recent studies dealing with stabilization of emulsions by protein based NPs, a Pickering effect is assumed to be a major mechanism (de Folter, van Ruijven, & Velikov, 2012; Xiao, Wang, Perez Gonzalez, & Huang, 2016; Hu et al., 2016; Xiao, Wang, Perez; Zou, van Baalen, Yang, & Scholten, 2018). That NPs outperform the proteins they are made from indeed suggests that their particulate nature contributes to their functionality. However, this does not necessarily mean that other mechanisms are not at play. The notion that protein based NPs are likely to an extent deformable could suggest that they would indeed behave differently as is the case for rigid, inert NPs. Moreover, the behavior of protein based NPs at air-water interfaces has only been reported on in a few instances.

In a recent paper by our group (Wouters, Schaefer, Joye, & Delcour, 2019), the foaming and air-water interfacial properties of NPs based on wheat gliadins (WGs) and maize zeins were studied. One of the main conclusions of this work was that wheat gliadin nanoparticles (WGNPs) efficiently stabilize foams. A similar observation was earlier made elsewhere (Peng et al., 2017, 2018). More specifically, for WGNP suspensions at pH values close to their point-of-zero-charge (pH 6.0), excellent foam stability and an interfacial film with high surface dilatational moduli were observed. At pH 4.5, poor foam stability and films with low surface dilatational moduli were obtained (Wouters et al., 2019). Interestingly, that foam stability of WGNP suspensions seems to be related to formation of a viscoelastic film at the interface to some extent might imply – and only imply – unfolding, reorientation of protein molecules as well as lateral protein-protein interactions at the interface. While such behavior is typical for films made up by protein adsorbed from solution (Murray, 2011), it has not been investigated in the case of NPs. This would to some extent contradict the notion that NPs would merely exert a Pickering type effect. This phenomenon thus requires further in-depth investigation.

In the present paper, we set out to shed light on the behavior of WGNPs at air-water interfaces. The above described pH-dependent air-water interfacial behavior of WGNPs presents an excellent opportunity to investigate this. Here, variations of pH from 4.0 – at which poor foaming properties are expected – to 6.0 – at which good foaming properties are expected – was exploited as a tool.

• to investigate the relationship between the structural and air-water interfacial properties of WGNPs,

• to shed light on how WGNPs behave at air-water interfaces and whether mechanisms other than a Pickering effect can be considered important for interfacial stabilization, and

• to investigate the contribution of molecular changes [disulfide (SS) bond formation] in the protein population to interfacial stabilization

To achieve these goals, we here used a multi-disciplinary approach combining a thorough structural (size, surface charge, protein conformation, surface hydrophobicity, morphology) characterization of NPs, air-water interface dilatational measurements, visualization of NP-stabilized foam structures with cryo-SEM and a foam fractionation experiment to investigate changes in both the NP properties (size, surface charge, surface hydrophobicity) as well as molecular changes in the protein population due to air-water interfacial adsorption.