Comparison of Interfacial and Foaming Properties of Soy and Whey Protein Isolates

Abstract: A comparative study on the foaming properties and behavior at the air-water interface of soy and whey protein isolates were made. Foams were obtained by the method of gas bubbling. The initial rate of passage of liquid to the foam (vi) and the maximum volume of liquid incorporated to the foam (VLEmax) were determined. The destabilization process of the formed foams was analyzed by a biphasic second order equation. Measurements of equilibrium surface tension (water/air) and surface rheological properties were carried out in a dynamic drop tensiometer. The foaming capacity (vi and VLEmax) and the stability of foams prepared with the whey protein isolates (WPI) were better than those formulated with the soy protein isolates (SPI). WPI foams were more stable showing the lower values of rate constants of gravity drainage and disproportion. There were significant differences (P ≤ 0.05) in the dilatational modulus in the surface rheology measurements， which were higher at the interface with WPI， implying greater resistance of the film formed to collapse and disproportion. In conclusion， WPI formed better and more stable foams than the SPI.

Key words: Soy protein isolates (SPI)， whey protein isolates (WPI)， disproportion， gravitational drainage.

1. Introduction

Many natural or processed foods are dispersions or have been dispersions during a stage of its production. Most of these dispersions are foams. Therefore， the analysis of food dispersions or colloids is of great practical importance [1].

Food dispersions are thermodynamically unstable. From a practical standpoint it is possible to produce a kinetically stable (or metastable) dispersion for a reasonable period of time， which is what the consumer demands for products [1].

Foams are dispersions of air bubbles in a liquid medium containing a surface active agent， also known as foaming agent. The surface active agent tends to be on the surface of the bubbles， protecting them from the collapse. The composition and properties of the adsorbed layer determines the resulting stability and physical properties of the foam [2].

The process of destabilization of foam is the tendency of the discontinuous gas phase to form a continuous phase by approximation and fusion of the bubbles， achieving a minimum surface area (minimum free energy). This process is opposed by the protein surface film， which as a mechanical barrier the more effective it is， the greater their viscoelasticity and rigidity are. The mechanisms of foam destabilization are liquid drainage by the effect of gravity， disproportion or Ostwald ripening， in which large bubbles grow at the expense of small bubbles by gas diffusion through the lamellae， and collapse of the foam by lamellae rupture. All of these mechanisms occur simultaneously and synergistically [3].

There have been several studies of the kinetics of foam destabilization. Previous works studied the foam destabilization by a specific rate constant of drainage that does not differentiate between the various processes of destabilization [4-8]. Panizzolo and collaborators [9] proposed that there are two distinct processes of fluid drainage from the foam， one due to fluid drainage itself and another to the Ostwald ripening. On this basis， a two-phase second-order model which determines the rate constants and maximum volumes drained due to gravity drainage and disproportion was considered [9].

In the present work we study the foaming properties of the whey protein isolate (WPI) and the soy protein isolate (SPI). Both of whey and soy proteins are by-products of the industry.

The functional properties of whey proteins are given by the α-lactalbumin and β-lactoglobulin (main components) properties. Among them are its solubility， emulsifying and foaming properties and the gelling ability [10-12].

Isolated soy proteins are manufactured from defatted soy flakes by separation of the soy proteins from both the soluble and the insoluble carbohydrate fractions of the soybean.

Functionality is determined， in a large part， by the specific processing parameters used for the manufacture of a given isolated soy protein. Gelation， emulsification， viscosity， water binding， dispersebility， and foaming or whipping properties are important functional characteristics associated with isolated soy proteins [13].

There are many studies on the literature of the foaming properties of whey [14-17] and soy protein isolates [8， 18-20]. but there no comparative studies between them.

Therefore， the objective of this work is to conduct a comparative study on the foam ability and foam destabilization by gravitational drainage and disproportion of bovine whey and soy protein isolates and relates them with the behavior at the air-water interface.

5. Conclusions

These results show that WPI has better foaming ability than SPI， as it has greater values for vo and VLEmax. Foams made with WPI also have greater stability to drainage and disproportion (lower kg and kd). WPI forms more cohesive films in the interface as deduced from the values of E， Ed and Ev and this is why they were more resistant to disproportion and collapse. In conclusion， WPI had better foaming properties than SPI.

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