Elsevier

Food Research International

Volume 54, Issue 1, November 2013, Pages 478-486
Food Research International

The non-linear rheological properties of fresh wet foams based on egg white proteins and selected hydrocolloids

https://doi.org/10.1016/j.foodres.2013.07.028Get rights and content

Highlights

  • Xanthan gum is a major factor, shaping the non-linear image of the studied foams.

  • Conformity between two methods of yield stress τ0 estimation has been obtained.

  • The foams containing xanthan gum can be recognized as a phenomenologically metastable.

Abstract

The study focuses on analysis of non-linear properties of foams consisting of egg white protein, xanthan gum and gum Arabic. The measurements of the G′ and G″ values as the function of the deformation amplitude were carried out. The yield stress values τ0 were determined by two methods, and the flow curves investigation were carried out as well. Distributions of characteristic times were determined. It has been proven that xanthan gum is a major factor forming non-linear image of the studied foams. The rheological behavior of xanthan gum-containing systems can be explained by the theory of soft glass, which implies that the studied foams are in a metastable state. The changes of the deformation amplitudes or the changes of the shear rate values cause the transition among subsequent metastable states. These phenomena are not present in foams, which consist of egg white protein only and which are supplemented with gum Arabic only. Furthermore, it has been noticed that there are no evident differences between the correlations of β/δ (G ~ γ0 β, G ~ γ0 δ) and computer simulation for the hard sphere-type systems, presented in the literature. The systems containing higher concentrations of xanthan gum exhibit very complex behavior during the performance of a qualitative test of the hysteresis loop. This results in the extension of the spectrum of characteristic times. Conformity between two methods of yield stress τ0 estimation has been obtained. Furthermore, it has been shown that xanthan gum causes the increase in τ0. This phenomenon was not observed in the case of gum Arabic.

Introduction

Foam has many important applications in a variety of foods. In the food industry as well as gastronomy egg white is commonly used as a foaming agent. Wet foams are used as ingredients in creams and dressings. Foam obtained from egg white is characterized by a low mechanical resistance. According to that some extra substances are added in this case to stabilize the foam and allowing consolidation of its structure. For this purpose, mono-, di-saccharides and polysaccharides are used. Nowadays there is a growing interest in obtaining products with reduced nutritional value. Commonly used mono-and disaccharides maintain the appropriate moisture content and structure but have a high nutritional value. Exchange of simple sugars by polysaccharides allows avoiding the problem and additionally helping in stabilization of rheological properties of foam. From a technological point of view foam should be a viscoelastoplastic system. This means that the foam should be able to accumulate a certain amount of mechanical energy in its structure, which ensures the maintenance of the molded shape in the presence of small deformation. This property is typical for the elastic systems. Plastic behavior, in turn, is associated with the defeat of the yield stress. The system is not able to accumulate more energy and after reaching the yield stress beyond begins flowing. In turn, these properties are very important at the stage of product formation (giving it a suitable shape). In addition, the above described phenomena have a large impact on the sensory properties of the foam.

Both the formation of the product and its consumption is associated with the occurrence of large deformations (displacements) or shear stresses, during which non-linear rheological properties appear. Therefore, rheological tests should disclose precisely those phenomena.

From physicochemical point of view, the foam can be classified as metastable system.

Such materials are frequently called “soft glassy materials,” where the word “soft” is used to emphasize its difference from glass in a classical meaning (Sollich, 1998, Sollich et al., 1997).

The criterion for assessment, whether a system is metastable, is determined by the three conditions, proposed by Myiazaki (Miyazaki, Wyss, Weitz, & Reichman, 2006), which concerns to G′ and G″ curve course as the function of the deformation amplitude (γ0), They are as follows:

  • 1.

    The peak, which is located on G″ curve, must be within the range of amplitude from 10 2 to 100;

  • 2.

    The peak must be observed within frequencies to which the material exhibits elastic response, G > G″;

  • 3.

    Relation of the β/δ (G ~ γ0 β, G ~ γ0 δ) is around 2 for the highest values of the deformation amplitudes.

The system can be recognized as a phenomenologically metastable if the presented conditions have been matched together.

The problems related to the analysis of the rheological metastable properties of the complex fluids are considered theoretically with the aid of computer simulation or experimentally. The research is mainly carried out for such systems as concentrated emulsions, as well as wet and dry foams. The description of non-linear rheological properties of foams is based, among the others, on the definite determination of the yield stress (Mason, 1999, Saint-Jalmes and Durian, 1999, Weaire et al., 2003, Rouyer et al., 2005, Weaire, 2008, Yang and Foegeding, 2011). The works discuss a possibility of determining the yield stress as well as the correctness of the methods used for the determination. The research on the non-linear foam properties with the use of low frequencies and large deformation amplitudes makes up another very wide group of publications. The dependence, expressed as, G* = G′(γ0) + jG″(γ0) is then analyzed (Mason et al., 1995, Sollich, 1998, Sollich et al., 1997). A phenomenological view of the metastability conditions presented above is a result of these dependences. Attempts to phenomenological description of rheological phenomena in foams with the help of the creation of the Maxwell- or Zener-like rheological models have been undertaken. The models include terms which describe plastic effects. This issue was discussed in the works of Marmottant and Graner, 2007, Cheddadi et al., 2012 and Saramito, 2007, Saramito, 2009, who presented very interesting models for a complex fluid system with particular attention to the analysis of the normal stresses arising during the flow of such a system. Research with the use of Fourier transform rheology (Rouyer, Cohen-Addad, Höhler, Sollich, & Fielding, 2008) has been also carried out. In these studies, not only amplitudes are analyzed, but also whole time range resulting from the experiment.

The aim of the present work is to determine non-linear rheological properties of the wet egg white protein-based foams supplemented with xanthan gum (XG) and gum Arabic (AG) and to carry out an analysis of the obtained results in the scope of the theory of soft glassy materials.

Section snippets

Materials

The commercially available food egg white (Ovopol, Poland) and the following food additives: gum Arabic (AG) (Regis, Poland) and xanthan gum (XG) (Hortimex, Poland) were used in this research. The protein content in egg white determined by the Kjeldahl method was (83.87 ± 0.1)%. Average molecular weights and dispersity of hydrocolloids were determined chromatographically. For xanthan gum the following were obtained: weight average molecular mass Mw = (19.6 ± 0.9)·105 g mol 1, number average molecular

Analysis of the G′ and G″ dependences in the function of the deformation amplitude

The G′ and G″ dependences at 23 °C as the function of the deformation amplitude were shown in Fig. 2. The dependences are characterized by a parallel course within the linear viscoelasticity area. Further, the curves intersect and the G″ value becomes higher than the one of G′. The effect has been observed for all analyzed foams. Supplementation with AG does not cause any visible changes in G′ and G″ courses. Though the module values initially decrease (0.3% AG), the values become similar to

Discussion

The data obtained at 23 °C indicate that the foams supplemented with XG exhibit a typical phenomenological metastable behavior. This is proved by the meeting of three postulates which are described in the introduction. The peak, present on G″ curve (Fig. 2), appears in the area of the elasticity response (G > G″) for the amplitude values within the range of 0.1–1.0 and the ratio of exponents β/δ = 2. The obtained results strictly correspond to the results obtained in a computer simulation for hard

Conclusions

At the temperature of 23 °C Xanthan gum addition is a key factor, shaping the non-linear image of the studied foams. The rheological behavior of the systems containing XG can be explained by the soft colloidal glass theory, which proves that the studied foams appear in a metastable state. Changes of deformation amplitudes or the changes of the shear rate values cause the transition among subsequent metastable states. These phenomena are not observed in the foams, which consist of egg white

Nomenclature

    D

    down curve

    G*

    complex relaxation modulus, Pa

    G

    real part of relaxation modulus (storage modulus), Pa

    G

    imaginary part of relaxation modulus (loss modulus), Pa

    G0

    share modulus, Pa

    max(*)

    max value

    t

    time constants, s

    U

    up curve

    Greek letters

    β

    share exponent

    δ

    share exponent

    γο

    amplitude of deformation

    γ˙

    share rate, s 1

    Φ

    volume fraction of gas phase

    ρ

    density, kg∙m 3

    η

    viscosity, Pas

    τ0

    yield stress, Pa

    τ0osc

    yield stress, Pa

    ω

    frequency, Hz

Acknowledgment

The work is dedicated to the memory of Professor Jerzy Skrzypek (1942–2012), my teacher and an outstanding exemplar of a scientist and to the memory of John Hunter (1968–2012), a creator of the Matplotlib.

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