Barley β-glucan effects on emulsification and in vitro lipolysis of canola oil are modulated by molecular size, mixing method, and emulsifier type

Highlights

• Lipid emulsification depends on mixing order and emulsifier type.

• Barley β-glucan reduces lipid emulsification and digestion independent of viscosity.

• Barley β-glucan promotes depletion flocculation of lipid emulsions.

• Strength of depletion increases with glucan molecular weight.

Abstract

The purpose of this study was to examine the impact of mixing methods, emulsifier types and molecular weights of barley β-glucan (BG) on emulsification and in vitro lipolysis of canola oil-in-water emulsions. Three different emulsifiers, Triton x100 (TTN), lecithin (LCN) and whey protein isolate (WPI), and three orders of mixing high molecular weight barley BG with oil and emulsifier were studied by measuring viscosity, microstructure and particle size distribution of the systems; while extent of lipolysis was quantified by an in vitro digestion method. In addition, lipids initially emulsified with TTN were mixed with barley BG of low, medium and high molecular weight to determine the effect of BG molecular weight on canola oil emulsion structure and lipolysis. In the absence of barley BG, lipid emulsification depended on the emulsifier type with larger lipid droplets found in WPI stabilized emulsions. Incubating emulsions with fibres increased the viscosity and size of the emulsified droplets, but to different extents depending on the emulsifier type and mixing order. Addition of BG reduced lipid digestion to the greatest extent for initially small droplet sizes, but independent of viscosity. Microstructure and particle size analysis suggest that the mechanism involves droplet flocculation restricting the surface area available for lipase action. Changes in droplet size distribution were observed after digestion only for high molecular weight BG, with more large droplets. All BG samples slowed lipolysis of emulsions but to different extents depending on the molecular weight with the largest BG significantly reducing lipolysis (p < 0.05). A rationalization is proposed for the impact of different molecular weights of barley BG on the depletion flocculation of emulsions using two models. These results enhance our understanding of the impact of soluble dietary fibres on the physicochemical and structural changes that may occur to emulsified lipids within the gastrointestinal tract.

Introduction

β-Glucan (BG, also known as mixed-linkage glucan) is a soluble dietary fibre (SDF) readily available from oat and barley grains and has been gaining interest due to its multiple functional and bioactive properties (AbuMweis, Jew, & Ames, 2010; El Khoury, Cuda, Luhovyy, & Anderson, 2012; Rains & Maki, 2013, pp. 239–254). Beneficial effects against cardiovascular disease, dyslipidemia and obesity have been documented (Cook, Rains, & Maki, 2013, pp. 265–279; Wood, 2007; Xin-Zhong et al., 2015), with particular focus on cholesterol reducing properties. These studies have resulted in approval by regulatory agencies of health claims associated with consumption of oat and barley BG and lowering of blood cholesterol. In vivo studies in animal models have shown that non-charged SDFs such as BG (Xin-Zhong et al., 2015) and arabinoxylan (Gunness et al., 2016) can delay triglyceride digestion and absorption from the small intestine into the blood circulation. As the in vivo system is complex, in vitro studies are warranted to better understand how these polymers influence lipolysis.

Cereal BGs are linear homo-polysaccharides of d-glucopyranosyl residues linked by a mixture of β-(1,3) and β-(1,4) linkages, with blocks of consecutive (1,4) linked residues (i.e. oligomeric cellulose segments) separated by single (1,3) linkages (Wood, 2007). They show considerable diversity in their structures, including the ratio of β-(1,3): β-(1,4) linkages which is believed to affect their physical properties, such as water solubility, viscosity, and gelation properties, as well as their physiological action in the gastrointestinal tract (Lazaridou & Biliaderis, 2004, Lazaridou & Biliaderis, 2007). Molecular weight is an important factor that affects viscous and gel-forming properties of polymers. In addition, in the case of food emulsion systems, polysaccharides are often used to improve emulsion stability and textural properties (Paximada, Koutinas, Scholten, & Mandala, 2016; Winuprasith & Suphantharika, 2013; Yusoff & Murray, 2011). However, limited information is available in the literature regarding emulsions with BG, especially in the presence of different commonly used food emulsifiers such as whey protein isolate and lecithin. Previous in vitro studies have suggested that BG influences lipid digestion mostly by its viscous property and gel-forming ability which may hinder the access of lipase to the surface of lipid droplets (Kontogiorgos, Biliaderis, Kiosseoglou, & Doxastakis, 2004; Santipanichwong & Suphantharika, 2009). It has been found that oat BG could increase the droplet sizes of egg yolk stabilized oil-in-water emulsions containing 20% soybean oil, whereas barley and yeast BG showed no such effect (Santipanichwong & Suphantharika, 2009). The common behaviour of these BG polymers is that they can induce flocculation of oil droplets (Santipanichwong & Suphantharika, 2009). Lipid digestion is a complex process that can be affected by other factors including mixing methods, emulsion structure and stability (Golding & Wooster, 2010), initial lipid droplet structure/size (Giang et al., 2015; Grundy, McClements, Balance, & Wilde, 2018; McClements, 2013) and emulsifier type (Mun, Decker, & McClements, 2007; Zeeb, Lopez-Pena, Weiss, & McClements, 2015). It is believed that viscous BG might affect lipolysis by influencing one or more of these factors (Zhang, Zhang, Zhang, Decker, & McClements, 2015).

The objectives of the current study were to investigate the effects of (1) three different emulsifiers (whey protein isolate, lecithin and the model non-food emulsifier Triton X-100) and three ways of mixing barley BG with canola oil and emulsifiers, and (2) different molecular weights (low, medium and high) of barley BG on emulsification and in vitro lipolysis of canola oil emulsion stabilized by Triton X-100. Viscosity enhancement of the lipid emulsion, droplet flocculation, changes in microstructure and extent of lipolysis were studied.