Food emulsions and foams: Stabilization by particles

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Abstract

Recent advances in the stabilization of emulsions and foams by particles of nanoscale and microscopic dimensions are described. Ongoing research in this highly active field is providing insight into (i) the molecular factors controlling particle wettability and adsorption, (ii) the structural and mechanical properties of particle-laden liquid interfaces, and (ii) the stabilization mechanisms of particle-coated droplets and bubbles. There is much potential for exploiting the emerging knowledge in new food product applications. The preparation of cheap and effective colloidal particles based on food-grade ingredients, especially proteins, is the key technological challenge.

Introduction

Food systems commonly contain particulate material that accumulates at oil–water and air–water interfaces and contributes to the colloidal stabilization of emulsions and foams [1]. The range of particle sizes involved in structuring such systems during and after emulsification and foaming can vary widely from just a few nanometres to tens of micrometres. As applied to emulsion droplets coated by a layer of adsorbed solid particles at the oil–water interface, the mechanism is commonly referred to as Pickering stabilization. Oil-in-water (O/W) or water-in-oil (W/O) emulsions can be produced depending on whether the particles are predominantly hydrophilic or hydrophobic. Important examples of Pickering-type food emulsions are homogenized and reconstituted milks (O/W emulsions stabilized by casein micelles) and margarines and fatty spreads (W/O emulsions stabilized by triglyceride crystals). Examples of edible foams stabilized by particles include whipped dairy cream and dessert toppings stabilized by partially aggregated emulsion droplets [1].

During the past decade there has been increasing interest by physical scientists in the interfacial properties of particles, especially nanoparticles. Some of this progress has been well described in previous reviews [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12•]. Attention here is directed towards research published during the past 3–4 years. The purpose of this article is focus on recent developments in (nano)particle stabilization that seem particularly relevant to food emulsions and foams. The perceived relevance may be because the investigators have worked directly with food-grade ingredients, or, alternatively, because the research offers potential strategies or methodologies for developing improved (nano)particle-based systems in food product formulations. This article is mainly concerned with experimental investigations of dispersed systems stabilized by inorganic particles (especially silica nanoparticles), fat crystals, and protein-based nanoparticles. Structures primarily based on supramolecular networks/gels or self-assembled lipids/surfactants are considered to lie outside the scope of the review.

Section snippets

Emulsions

According to the conventional explanation for emulsion stabilization by solid particles [1•], [2], [3], there is an accumulation of particles at the oil–water interface in the form of a densely packed layer. This thick particle layer then prevents droplet flocculation and coalescence by a steric mechanism.

When two particle-coated droplets come close together we can envisage a sort of particle bilayer arrangement as shown schematically in Fig. 1(a). The effectiveness of the physical barrier in

Foams

The stabilization of bubbles and foams by particles has been receiving great attention from researchers in recent years [2], [7], [8], [9], [10]. A strong motivation for this activity is the general recognition that long-term stability is much more difficult to achieve for aerated systems than it is for emulsions. Hence the incorporation of colloidal particles as a possible way of enhancing foam stability is a matter of great technological and commercial significance. This is especially

Conclusions and outlook

The past few years have seen enormous growth in the amount of fundamental and applied research relating to particle-stabilized emulsions and foams. Some of the newly discovered enthusiasm for this subject is undoubtedly attributable to a desire amongst some researchers to join the potentially lucrative bandwagon of nanoparticle research which exists now across the whole breadth of the scientific community. A cynical observer with a long memory could therefore be forgiven for thinking that the

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