High Acyl Gellan Networks Probed by Rheology and Atomic Force Microscopy

Abstract

The network structure of high acyl gellan polysaccharide was investigated using dynamic viscoelasticity and steady flow viscosity measurements, as well as atomic force microscopy (AFM). Time-temperature superposition (TTS) of mechanical spectra of aqueous dispersions having a gellan concentration of 0.1% w/w revealed a gel-like response at the lower end of the frequency range. The TTS master curve of the steady flow data exhibited a power-law relationship between shear viscosity and shear rate at the lower end of the shear rate range, instead of a Newtonian plateau. These rheological characteristics suggest the existence of an effective yield stress arising from the presence of a percolated network. AFM images of high acyl gellan revealed micrometer-sized networks composed of double-stranded helices laterally associated to varying degrees. These associated helices did not dissociate fully on heating at 90℃, suggesting that they are partially preserved native networks secreted by gellan-producing bacteria.Keywords: atomic force microscopy, high acyl gellan, network, time-temperature superposition, viscoelasticity, yield stress *To whom correspondence should be addressed. E-mail: sikeda2@wisc.edu (S.I.), qzhong@utk.edu (Q.Z.) † IntroductionGellan is an extracellular polysaccharide secreted by Sphingomonas elodea in the form of hydrated networks encapsulating the bacterial cell (Morris, 1998). Due to its ability to form clear and heat-resistant hydrogels, gellan is widely utilized in industry (Valli and Miskiel, 2001). Naturally occurring gellan exists in the high acyl form with a tetrasaccharide repeating unit consisting of →3)-β-d-glucose-(1→4)-β-d-glucuronic acid-(1→4)-β-d-glucose-(1→4)-α-lrhamnose-(1→ (Jansson et al., 1983;O'Neil et al., 1983). On the (1→3) linked β-d-glucose residues, each C-2 position is esterified with l-glycerate, while approximately one-half of the C-6 positions are esterified with acetate (Kuo et al., 1986). High acyl gellan is alkali-treated to obtain a deacylated product for expanded commercial applications (Baird et al., 1992;Valli and Miskiel, 2001). Deacylated gellan molecules are fully hydrated in an aqueous solution at sufficiently high temperatures. Upon cooling to a certain temperature range, fully hydrated molecules in the disordered coiled conformation transform into the double-stranded helical conformation (Morris, 1998;Ikeda et al., 2004;Rinaudo, 2004). Double-stranded helices aggregate laterally to form thicker rod-like structures in the presence of specific cations, such as potassium, that shield electrostatic repulsion between double-stranded helices arising from negatively charged carboxyl groups, forming macroscopic gels at sufficiently high polysaccharide concentrations (Morris, 1998;Ikeda et al., 2004;Rinaudo, 2004). The aggregation of double-stranded helices results in a much higher gel-melting temperature than the temperature corresponding to gel formation (Annaka et al., 1999; Matsunaka et al., 1999;Miyoshi and Nishinari, 1999).High acyl gellan recovered directly...

The improved thermal stability of anthocyanins at pH 5.0 by gum arabic

“…The height of GA tested in the present study (~2 nm, Table 4) was in agreement with the study that Table 3 The degradation rate constant (k) and half-life of anthocyanins during heating at 80 and 126 C in the 0.51 mg/mL anthocyanin solution with and without 10 mg/mL gum arabic (GA) at pH 5.0. a reported the height of the monomeric GA in aqueous solutions to be 1e3 nm (Ikeda, Funami, & Zhang, 2005). Additionally, the height of the anthocyanin-GA particles was about 5e6 times of the GA (Table 4), suggesting that the particles are composed of several monomeric GA (Ikeda, Gohtani, Nishinari, & Zhong, 2013;Lv, Yang, Li, Zhang, & Abbas, 2014). The results were in agreement with our recent study about the stable norbixin-GA complexes at pH 5.0 after similar thermal treatments (Guan & Zhong, 2014).…”

Effects of the conjugation of whey proteins with gellan polysaccharides on surfactant-induced competitive displacement from the air-water interface

“…The addition of gellan to the sub-phase did not cause any significant changes to the surface pressure. A topographical AFM image of the interfacial whey protein film transferred onto mica at this stage, before the addition of surfactants (Figure 1a), shows no network structures characteristic of the network-forming polysaccharide (Ikeda et al, 2004a(Ikeda et al, , 2013. Because gellan has a glucuronic acid residue in its repeating unit (Chandrasekaran et al, 1992), it may bind electrostatically to positively charged groups of protein molecules.…”

“…The objective of this study was to investigate the effect of conjugation between whey proteins and network-forming polysaccharides on the surfactant-induced competitive displacement of whey proteins from the air-water interface. Gellan, a food-grade gelling polysaccharide, was conjugated with whey protein because its molecular structure and properties have been well characterized and reported in the literature (Chandrasekaran et al, 1992;Ikeda et al, 2004aIkeda et al, , 2013.…”

The Young's Modulus, Fracture Stress, and Fracture Strain of Gellan Hydrogels Filled with Whey Protein Microparticles

“…Food proteins and polysaccharides play essential roles in determining the quality and shelf‐life of such colloidal foods (Dickinson ). The main functionality of these biopolymers is the rheological control of food matrices that frequently involves the formation of supra‐molecular assemblies induced by the conformational transition of biopolymer triggered by changes in environmental factors such as temperature, pH, and ionic conditions (Ikeda and Morris ; Morris and others ; Ikeda and others ; Mezzenga and Fischer ). Furthermore, proteins can function as emulsifying and foaming agents due to their surface activities (Dickinson ; Damodaran ).…”

“…In the native polysaccharide, two acyl groups exist in the tetrasaccharide repeating unit consisting of →3)‐β‐ d ‐glucose‐(1→4)‐β‐ d ‐glucuronic acid‐(1→4)‐β‐ d ‐glucose‐(1→4)‐α‐ l ‐rhamnose‐(1→, while they are removed by alkali treatment in the most readily available form of gellan (Morris and others ). Deacylated gellan molecules hydrate fully in an aqueous solution at sufficiently high temperatures, transform into the double‐stranded helical conformation upon cooling, and aggregate to form gel networks (Ikeda and others, , ; Morris and others ). Small and large deformation properties of gellan gels have been studied intensively due to their relevance to sensory characteristics of food products (Morris and others ), including effects of filler particles on rheological properties of gellan gels (Jampen and others ; Moritaka and others ).…”