The role of hydrocolloids in mechanical properties of fresh foams based on egg white proteins
Introduction
Foam is a multi-phase system consisting of a liquid or solid continuous phase in which dispersed gas bubbles are suspended (Adamson and Gast, 1997). Currently, there is a great interest in fluid-gas systems of foam structure being observed. They are used for creation of new products or are applied as raw materials in food industry. One of the most often used foaming agents is egg white protein (ovalbumin).
The quality of the foam mainly depends on the conformation assumed by the emulsifier on the interfacial surface. In order to obtain stable foam, a protein molecule must be opened and its hydrophilic and hydrophobic groups must be activated. Moreover, a good foaming agent should be characterized by a good flexibility of its material molecule. In the case of egg white protein, it is done through the partial unfolding of macromolecules what enhances amphiphilic properties, which, in turn, facilitate the formation of foam. Foam, obtained in such a way, is unstable; hence it is necessary to stabilize it. Stability improvement of such foams is achieved through addition of low molecular weight compounds such as monosaccharides and disaccharides, or by addition of high molecular weight compounds – polysaccharides. Besides influencing stabilizing properties, addition of polysaccharides can also shape the rheological properties of the system (Miquelim et al., 2010). Viscoelastic properties are particularly crucial as they can be used in predicting many important technological parameters. Moreover, the viscoelastic properties are closely correlated with other physicochemical parameters of foam such as density or gas volume fraction.
Studies on the viscoelasticity of the foam produced on the base of egg white protein alone or with addition of polysaccharides are mainly focused on the analysis of the variability of the complex modulus in function of frequency (Mleko et al., 2007). Attempts have been also made to determine the influence of individual hydrocolloids on the viscoelastic properties of foams (Chávez-Montes et al., 2007). In addition, research on the broadly defined rheological properties of foams has been carried out (Lau and Dickinson, 2000). Problems related to the measurement of thixotropic properties of foams should be emphasized (Miquelim and Da Silva Lannes, 2009, Mleko et al., 2007).
The issues related to yield stress and nonlinear rheology frequently appear in the literature on rheological properties of foams (Rouyer et al., 2005, Weaire, 2008, Hutzler and Weaire, 2011). Moreover, two constitutive equations for elastoviscoplastic systems, proposed by Saramito (Saramito, 2007, Saramito, 2009), turned out to be highly efficient in describing of nonlinear rheological properties of foams (Cheddadi et al., 2008, Cheddadi et al., 2012). In the works of Marmottant and Garnier (Marmottant and Graner, 2007) a rheological model of foam as a elastoviscoplastic solid was also presented, which made it possible to describe the changes of G′ and G″ values in the function of the deformation amplitude. Saint-Jalmes and Durian, who have examined the influence of gas volume fraction on rheological properties of wet foam, also obtained interesting results (Saint-Jalmes and Durian, 1999).
However, the professional literature lacks works related to the analysis of rheological and viscoelastic properties based on linear models. There is no application of Maxwell or Burger type viscoelastic continuous models used for description of the characteristic time values (such as relaxation or retardation) in foam systems, either. There is no literature data related to the scaling of the rheological properties of food foams against protein or polysaccharide concentration. The aim of this study was to analyze the viscoelastic properties of egg white protein foam supplemented with xanthan gum and carrageenan. The main focus was the estimation of stress relaxation spectrum based on the continuous Maxwell model. An attempt was also undertaken to associate the characteristic rheological values with other physical properties of obtained foams.
Section snippets
Materials
The commercial food egg white protein (Ovopol, Poland) and the following food additives: mixture of carrageenan gums (CA) (Regis, Poland) and xanthan gum (XG) (Hortimex, Poland) were used in this research.
The protein content in egg white determined by the method of Kjeldahl was (83.87 ± 0.10)%. Molecular weights and polydispersity of hydrocolloids were determined chromatographically, details were described in earlier work (Ptaszek et al., 2009). For xanthan gum there were obtained: weighted
Basic physical properties of foams
Foam obtained from the pure egg white protein is characterized by the largest gas volume fraction (ϕ = 0.967, Table 2) and by the lowest density (ρ = 54.7 kg m−3). It is also interesting to observe how density and gas volume fraction of the obtained foams undergo changes upon supplementation of protein with hydrocolloids. As the data given in Table 1 show, addition of 0.6% xanthan gum increases the density of foam more than two fold (ρ = 115.2 kg m−3 for 0.3% XG system). A similar situation occurs for
Conclusions
The presented considerations reveal, that xanthan gum clearly stabilizes the foam structure and enhances its native viscoelastic properties. Addition of carrageenan causes changes in the viscoelastic properties, which are manifested in the degradation of the relaxation spectrum.
Increase of XG content induces the growth in the density of the system which clearly correlates with its viscoelastic properties. This phenomenon can be explained by the fact that together with the increase of XG
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