Rheology of common uncharged exopolysaccharides for food applications

Highlights

• Exopolysaccharides modify rheological properties in food systems.

• Intrinsic viscosity reveals macromolecular conformation of exopolysaccharides.

• High molecular weight levan and dextran form weak gels.

• Curdlan can form low-set and high-set gels, due to highly ordered helical structures.

Uncharged exopolysaccharides (EPS) are a promising source of functional hydrocolloids with unique techno-functional properties and health promoting effects. EPS can be used as additives or formed in situ in food to enhance the rheological properties. This article gives a review of the rheological characterisation of EPS in terms of macromolecular conformation and viscoelastic behaviour. The properties of the uncharged EPS curdlan, dextran and levan are discussed in more detail. In addition, recent research on the use of these EPS to enhance the rheological properties of food is reviewed.

Introduction

Exopolysaccharides (EPS) are carbohydrate polymers produced by various microorganisms, including bacteria, fungi, and yeasts. The full function of these EPS in biological systems has not been fully revealed, but for one they play an important role as a structural material in biofilms protecting the cells and promoting adhesion to surfaces [1]. EPS are divided into homopolysaccharides consisting of a single type of monosaccharide and heteropolysaccharides composed of repeating units of at least two different monosaccharides [2]. While heteropolysaccharides are formed in a complex process involving various enzymes, homopolysaccharides are often formed by one specific type of enzyme, a glycosyltransferase, which uses sucrose as substrate. The microorganism excretes the glycosyltransferase into its environment, where the enzyme cleaves the sucrose and uses the released energy of the glyosidic bond to polymerize either glucose or fructose. Two common homopolysaccharides of this reaction are the α-1,6-linked glucose polymer dextran (Figure 1a) and the β-2,6-linked fructose polymer levan (Figure 1b). In addition, levan may have some β-2,1-linked side chains and dextran may have α-1,4-linked, α-1,3-linked, and α-1,2-linked branches [3]. β-glucans such as the β-1,3-linked glucose homopolysaccharide curdlan (Figure 1c) are formed in a process that involves several enzymes located in the cell and on the cell wall [4]. The concentration and the molecular weight of EPS depends on the microbial strain, the way of synthesis (extracellular or intracellular) and the cultivation conditions. Further information on controlled EPS synthesis, including influencing the molecular weight and polymer concentration can be found elsewhere [4,5].

Dextran and levan can be produced by food grade bacteria such as lactic acid bacteria or acetic acid bacteria. On the one hand, the microorganisms can be used for in situ EPS production in fermented products. This allows the tuning of product properties such as water-binding capacity, viscosity or gel strength, without the use of food additives. In contrast, the use of EPS as an additive is not limited to fermented products and may lead to a better control of the product compared to the one from the in situ production because the concentration of the functional ingredient does not change with time.

The present review article focuses on the rheological characterization of EPS describing the functionality in terms of their macromolecular conformation and their viscoelastic properties. For this purpose, the EPS levan, dextran, and curdlan as well as recent applications in food products are examined in more detail.