Large amplitudes oscillatory shear (LAOS) behavior of egg white foams with apple pectins and xanthan gum

Highlights

• LAOS and FTR measurement were done for wet foams.

• The foam containing only egg white generates the largest number of harmonics.

• The level of de and the pectin concentration in foam do not influence τ₀.

Abstract

The study presents the results of research on non-linear rheological properties of foams based on egg white protein, xanthan gum and apple pectin of various levels of methylation. The rheological studies were carried out by means of large amplitude oscillatory shear (LAOS) and Fourier transform rheology (FTR) methods. The obtained foams revealed elastoviscoplastic properties. The analysis of the results allowed determination of yield stress, shear exponents and the Fourier spectra, which enabled estimation of the quantity and intensity of individual harmonics. The Lissajous patterns as well as coefficients of energy dissipation were also demonstrated. The foam containing solely egg white revealed the largest number of harmonics in the Fourier spectrum (17th), whereas supplementation with hydrocolloids induced a decrease in the amount of harmonics (9th). The image of the Fourier spectra was reflected by the Lissajous curves, which gradually acquired an ellipsoidal shape as the hydrocolloid concentration increased. The coefficient of energy dissipation in the function of the deformation amplitude demonstrated a sigmoid dependence and tended towards the value of approximately 0.8 for higher amplitudes. This indicated that the investigated foams were in a flow area. The statistical analysis (two/three-way ANOVA) of the abovementioned values showed that neither the level of methylation nor the pectin concentration in foam influences the yield stress.

Introduction

Physicochemically, foams are gas–liquid systems where gas constitutes a disperse phase, while liquid is a continuous phase (Campbell & Mougeot, 1999). Foams are classified within a very wide group of the so-called soft matters. They show structural disorder and metastability (Sollich, 1998), which make them similar to glasses. The mechanisms within the systems are shaped by the temperature fluctuation-induced movements of chains which are not able to provoke a total relaxation of the structure. Due to the very long relaxation (slow relaxation modes), soft glassy material (SGM) theory may be applied for a description of the rheological phenomena which occur during shear of foams (Miyazaki et al., 2006, Sollich, 1998, Sollich et al., 1997). The word “soft” was used here in order to emphasize the difference between the discussed systems and glasses in their ordinary meaning.

The foams based on egg white protein or whey protein derivates constitute an important group of food foams (Campbell & Mougeot, 1999). However, the systems are unstable and they undergo disintegration over time. In order to prevent the disintegration various additives, such as monosaccharides and disaccharides and polysaccharides, are applied. The application of these additives allows for the shaping of the properties of foams in accordance with their technological purpose. The use of a specific polysaccharide is determined by the polysaccharide's molecular structure and by the ability of its interaction with a protein applied as a foam-forming agent. Pectin and xanthan gum are polyanion polysaccharides which are the most frequently used factors shaping mechanical properties of foams.

Pectin (P), a polysaccharide of plant origin (Ovodov, 2009, Yapo, 2011), is one of the constituents of the cell wall (from 2% to 35%). Pectin consists of a frame of (1 → 4) linked α-d-galacturonic acid units interrupted by single (1 → 2) linked α-l-rhamnose residue. The carboxyl groups of the galacturonic acid units are partly esterified by methanol. Pectin does not have a specific structure (Pérez, Mazeau, & du Penhoat, 2000) as it is dependent on the conditions of biosynthesis (Mohnen, 2008). “Smooth” molecules, i.e. without branching, and “hairy” molecules, i.e. branched, are the most frequent. Pectin adopts coil conformation of high elasticity, “worm-like”, in an aqueous solution; the highest elasticity was observed for “hairy” areas. The commercially available pectins are divided into two groups on the basis of their degree of esterification: high methoxyl pectin (HM), with a degree of methyl-esterification of de ≥ 50%, and low methoxyl pectin (LM), with a degree of methyl-esterification of de < 50% (Lopes da Silva & Rao, 2006).

Xanthan gum is an extracellular polysaccharide produced by Xanthomonas campestris (Palaniraj & Jayaraman, 2011). Xanthan gum is an anionic polyelectrolyte with a β-(1 → 4)-d-glucopyranose glucan (as cellulose) backbone with side chains of -(3 → 1)-α-d-mannopyranose-(2 → 1)-β-d-glucuronic acid-(4 → 1)-β-d-mannopyranose on alternating residues. Slightly less than half (~ 40%) of the terminal mannose residues are 4,6-pyruvated and the inner mannose is mostly 6-acetylated (that is, the side chains are mainly β-d-mannopyranosyl-(1 → 4)-(α-d-glucuronopyranosyl)-(1 → 2)-β-d-mannopyranoside-6-acetate-(1 → 3)). Some side chains may be missing. In a solution, xanthan gum adopts the conformation of a double helix (Morris, 2006).

Due to their molecular structure and the presence of electrically loaded function groups, both pectin and XG, when combined with proteins, may form solid complexes (Lopes da Silva and Rao, 2006, Morris, 2006).

Large amplitudes oscillatory shear (LAOS) and Fourier transform rheology (FTR) methods are now more often used as a tool for describing the rheological phenomena occurring in complex structural fluids such as molten polymers, polymer solutions, polymer blends, W/O and O/W emulsions and foam. This is because these systems exhibit a broad spectrum of rheological properties, from Newtonian viscosity, plasticity to viscoelasticity or elastic-viscoplasticity. Application of LAOS and FTR techniques allows one to assess the fundamental rheological properties (e.g., shear modulus or yield stress), as well as those that were previously unattainable (e.g. Fourier spectra analysis, geometric decomposition, Chebyshev coefficients) (Hyun et al., 2011). Also a number of completely new rheological quantities have been introduced, whose physical interpretation allows better understanding of the phenomena occurring during the flow. All of these quantities can be derived as a result of only one experiment. Information about the rheological properties of the tested material obtained in this way is complete and characterizes it in terms of non-linear shear flow.

Due to the possibility of structurally complex material testing, LAOS and FTR techniques can be an excellent tool for the analysis of food rheological properties. These methods are starting to be introduced into the canon of rheological testing of food (Melito, Daubert, & Foegeding, 2012). They can be used at the phase of designing a new food product, where the information about the behavior of the product in the case of large values of deformation (and more generally the movement) is essential. In the case of large (non-linear) deformations it is also possible to make deductions about the behavior of the product in the mouth during chewing. For this reason, LAOS and FTR methods can be used for example for preselection of food additives responsible for the shaping of product structure. Through the interpretation of LAOS experimental results, it is possible to indicate the additives responsible for shaping of the viscous, elastic and plastic properties of finished products, and also to investigate the interaction of a selected structure forming additives. LAOS technique is a convenient and inexpensive tool, because of performing only one type of experiment, providing virtually full information about the rheological properties of the additive. So far in the literature there are only the results of xanthan gum solution tests performed by means of LAOS (Carmona, Ramírez, Calero, & Muñoz, 2014), soy isolates with admixture of flaxseed gum (Bi, Li, Wang, Wang, & Adhikari, 2013), as well as for carrageenan gels (Hilliou, Wilhelm, Yamanoi, & Gonçalves, 2009). Also attempts to describe the rheological properties of dough were undertaken (Lefebvre, 2006, Ng and McKinley, 2008, Ng et al., 2011), systems containing tapioca starch pastes and selected hydrocolloids (Fuongfuchat et al., 2012). All of the abovementioned works focused on the analysis of the non-linear properties of the tested systems during shear flow. The obtained results are consistent with those obtained by classical methods (flow curve, yield point). However, by combining in a single experiment research on the properties in the range of linear and non-linear deformations, LAOS and FTR techniques open a new approach to the cumulated research on the mechanical properties of food.

Application of LAOS and FTR techniques requires a specific mathematical formalism. Such formalism was previously rarely encountered in food science. Unfortunately, it is necessary to determine the appropriate physical quantities, which then are easily given an appropriate physical interpretation. It will be discussed in the later part of this work.

The aim of the study is a comprehensive LAOS and FTR analysis of rheological properties of foams obtained on the base of egg white protein, xanthan gum and apple pectins. The influence of the molecular mass and pectin's esterification level on the properties of the obtained foams will be analyzed.