β-Lactoglobulin aggregates in foam films: Effect of the concentration and size of the protein aggregates
Graphical abstract
Top view of foam films (bulk concentration 1 g/L) containing 50% protein aggregates for: (a) Rh = 35 nm (b) Rh = 71 nm; (c) Rh = 117 nm and (d) Rh = 197 nm. A pressure ramp is applied (10 Pa per 10 min) and, for all images, the final pressure is 100 Pa.
Introduction
Aqueous foams, which are dispersions of gas bubbles in a liquid, are stabilized by amphiphilic molecules and especially proteins in the food industry. The foam stability is determined by, among other things, the properties of the foam films, which separate the bubbles. The understanding of the behavior of these films and of their rupture is essential to control the foam stability [1]. Many studies have been devoted to the properties of proteins at the interfaces and in foam films [2], [3], [4], [5]. Proteins stabilize foams by strongly adsorbing to the air/water interfaces, forming viscoelastic adsorbed layers and leading to a protein network with high viscosity.
Processes such as thermo-mechanical treatments lead to protein aggregation into well-defined self-assembled structures. Depending on the experimental procedure, it is possible to obtain aggregates of different shapes, sizes and concentrations [6], [7]. Protein aggregates are complex structures whose size may be as large as a few hundred nanometers. It can be expected that their behavior is different from that of nonaggregated proteins. Because of their size, the ability of aggregates to diffuse to the interface is reduced and this should lead to lower foaming properties. Moreover, protein–protein interactions within aggregates can be of a different nature (electrostatic, hydrophobic, etc.), which can lead to different kinds of cohesiveness. This could affect the mobility of proteins and their ability to spread at the interface and influence their capacity to stabilize air/water interfaces needed to produce a foam. However, it has been shown that the presence of some aggregates increases the foaming stability [8], [9], [10], [11], [12]. More particularly, the effect of β-lactoglobulin aggregates on the foaming properties depends on the aggregate size and on the ratio between aggregated and nonaggregated proteins [8], [9]. β-lactoglobulin aggregates alone are not able to form stable foams. Under certain conditions, in the presence of a sufficient amount of nonaggregated proteins, the foam stability is improved by the aggregates [8]. If the link between the foam film stability and that of foams stabilized by nonaggregated proteins has been well discussed [1], [2], [13], [14], [15], much less is known about the foam films stabilized by protein aggregates. We have shown in a previous article that for solutions containing only protein aggregates a strong correlation exists between the stability of a foam and the properties of the corresponding foam film (stability and resistance to pressure). For large amounts of aggregates, the foam film stability is dependent on the aggregate size: foam films are either fluid or gelled. If the amount of nonaggregated protein is sufficient, foam films made from mixtures of nonaggregated proteins and protein aggregates form a gel-like network, leading to a better foam stability [9].
One may ask the question if this correlation is still valid for mixtures containing small amounts of protein aggregates. In this case, the nonaggregated proteins will be preponderant and govern the interfacial properties, due to their fast diffusion kinetic from the bulk to the interface compared to that of protein aggregates. However, a small amount of protein aggregates can affect the foaming properties even if it seems to have no influence on the interfacial properties [8], [10], [11]. Indeed, foams made with a small amount of aggregates are very stable compared with nonaggregated protein foams [8], [10]. Not only is the foam stability affected by the presence of protein aggregates but probably also the foam structuration and texturation. However, if the protein aggregates concentration is low, it is reasonable to think that in that case, the gel-like network will be difficult to form. So, other mechanisms of foam stabilization will be implicated like for example the confinement of aggregates in the films or a cork effect in the Plateau borders.
The study of these mixtures at the foam film scale could allow us to better understand the role of protein aggregates in foam structuration and stabilization. This will be relevant with a better understanding of what could occur during industrial processes as only a small fraction of proteins is aggregated during thermo-mechanical treatments [11], [12].
The present paper aims at identifying the different types of behavior of foam films made from solutions containing varying amounts of protein aggregates. The objective was to determine the respective role of nonaggregated proteins and aggregates on the structuration and stabilization of foams. The properties of foam films (stability and resistance to pressure) were compared to those of the corresponding 3D-foams. A phase diagram as a function of protein aggregate concentration and size was established.
Section snippets
β -Lactoglobulin and protein aggregates
β-lactoglobulin was purified from whey protein isolate (Prolacta 90 batch n 273 supplied by Lactalis Industrie, Laval, France), prepared and characterized according to the experimental protocol detailed in [8].
β-lactoglobulin aggregates were formed upon heat induced denaturation of protein at different concentrations at a temperature of 80 °C during 24 h following the protocol detailed in [7]. The samples were cooled in a bath at room temperature. In this paper, aggregates were formed at pH 7 and
Results
Mixtures containing various amounts of protein aggregates of different sizes were studied. Their foaming properties were determined in terms of the foam stability by measuring the remaining final foam volume. The behavior of the foam films stabilized by these mixtures was also investigated and its properties are compared with the properties of the corresponding 3D-foams.
Discussion
The appearance, stability and resistance to pressure changes of foam films obtained from mixtures of nonaggregated proteins with protein aggregates are strongly dependent on aggregate size and on the ratio between nonaggregated and aggregated proteins. We determined from our previous works that a critical minimal quantity of nonaggregated proteins is required to produce foams having better properties than that of nonaggregated proteins [8]. For high amounts of aggregates in the mixtures, the
Conclusions
Foam films made of mixtures containing nonaggregated proteins and protein aggregates have been studied using a thin film balance apparatus. In all cases, foam films exhibit a inhomogeneous profile and are either fluid or gelled, depending on the aggregate size and the amount of nonaggregated proteins. We have established a phase diagram of the different features of these foam films. This phase diagram revealed the existence of an critical amount of aggregates above which a gel-like network
Acknowledgments
This work was partially supported by the “Groupe De Recherches Mousses” of the CNRS. Jérôme Delacotte from Laboratoire Physique des Solides, Orsay, FRANCE is gratefully acknowledged for his assistance and for many discussions during the Thin Film Balance experiments.
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