Impact of pH on molecular structure and surface properties of lentil legumin-like protein and its application as foam stabilizer

Highlights

• High foaming capacity of lentil legumin-like protein and long life foams were obtained at pH 5.0 and 7.0.

• Kinetic suggests that foaming stability is closely related to surface protein conformation.

• Interfacial layer strength is linked to size, conformation and solubility/hydrophobicity balance.

Abstract

The capacity of a protein to form and stabilize foams and emulsions depends on its structural characteristics and its physicochemical properties. The structural properties of lentil legumin-like protein including molecular weight, hydrodynamic size, surface charge and hydrophobicity, and conformation were studied in relation to its air–water interfacial behaviors. Kinetics study suggested that the foaming stability was closely related to the surface conformation of the protein that strongly affected adsorption and re-organization of the protein layer at the air–water interface. Foams prepared at neutral pH showed dense and strong networks at the interface, where combination of the α-helix secondary structure, medium hydrodynamic molecular size, and balance between solubility/hydrophobicity all contributed to the formation of such strong protein network at the interface. At pH 5.0, the protein formed a dense and thick network composed of randomly aggregated protein particles at the air–water interface. Whereas at pH 3.0, the unordered structure increased intra-protein flexibility producing a less compact and relaxed interface that reduces elasticity modulus with time and reduced foam resistance against collapse. This research revealed that lentil legumin-like protein could form long-life foams at mild acidic and neutral pH. The potential for use of lentil protein as a novel foaming plant-based stabilizer is demonstrated in food and non-food applications where stable, long-life foams are required.

Introduction

Increasing cost of dairy-based ingredients, emerging dietary preferences (e.g., gluten-free and vegan) and consumer demand for healthier ingredients are leading the market trends toward lower cost and plant-based alternatives, which are gaining increasing market share as food ingredients and for bio-based material applications [1]. In this context, legume proteins are attracting attention because of potential from both nutrition and health standpoint. In general, legumes are high in protein (20–25%). The majority of storage proteins in legumes are globulins, which can be classified in two groups. Proteins in the first group have sedimentation coefficients between 10.5S and 13.0S and are referred to as ‘legumin-like’ or 11S proteins. The second group has smaller sedimentation coefficients (7.0–9.0S) and are generally isolated from seed extracts as trimmers of glycosylated subunits. This group of proteins is referred to as ‘vicilin-like’ or 7S proteins [2]. Lentil is a leguminous plant high in fiber, low in fat and cholesterol free. Canada is one of the major lentil producers in the world with an annual production of over 1.5 million tons [3]. Lentils contain 20.6–31.4% protein with legumin-like protein (∼50%) as the major globulin fraction, which is comprised of a number of 6–19 polypeptides with a molecular weight (M_w) of 18–43 kDa [4]. Generally, it is accepted that legumin is a hexamer with a M_w of about 320–380 kDa, which consists of six polypeptide pairs that interact non-covalently. Each of these polypeptide pairs is comprised of an acidic subunit of about 40 kDa and a basic subunit of about 20 kDa, linked by a single disulfide bond [4]. Unlike other legumes such as soybean and pea, which have been extensively studied [5], there is limited research on the legumin-like protein of lentil except for some report on its sedimentation speed [6], immunological reactivity, composition [4], and functionalities, including foaming properties [7], [8]. A fundamental understanding of detailed structural features of lentil legumin and its functionalities is important for its potential food applications.

In our previous work [9], extraction process parameters were optimized to obtain lentil protein concentrates, and they demonstrated strong foaming capacity and stability, comparable to whey and egg protein. This superior functionality may provide an opportunity for lentil protein to be used as a foaming agent of plant origin for both food and non-food applications. In spite of its high potential, the underlying mechanism of such foaming properties of a plant protein is still unknown. As the major protein in lentil, legumin-like protein component could play an important role contributing to the foaming properties. Thus, this study aims to isolate and purify the legumin-like protein from lentil, and use it as a plant protein model to understand how protein molecular structure (molecular weight, surface charge, hydrophobicity, and conformation) impacts its surface properties (surface tension, dilatational and shear rheology), and subsequently foaming functionality. The generated knowledge may help develop strategies for modification of plant protein structures to improve their functionality for targeted applications. Most of the previous reports have focused on protein surface properties and foaming capacity at neutral pH. Our previous study revealed that environmental pH significantly influenced protein physicochemical properties and foaming functionality. Thus, the impact of environmental pH on the structure and properties of the lentil legumin-like protein was also investigated.