Aerated milk protein emulsions — new microstructural aspects

Assessment of partial coalescence in whippable oil-in-water food emulsions

Partial coalescence influences to a great extent the properties of final food products such as ice cream and whipped toppings. In return, the partial coalescence occurrence and development are conditioned, in such systems, by the emulsion's intrinsic properties (e.g. solid fat content, fat crystal shape and size), formulation (e.g. protein content, surfactants presence) and extrinsic factors (e.g. cooling rate, shearing). A set of methods is available for partial coalescence investigation and quantification. These methods are critically reviewed in this paper, balancing the weaknesses of the methods in terms of structure alteration (for turbidity, dye dilution, etc.) and assumptions made for mathematical models (for particle size determination) with their advantages (good repeatability, high sensitivity, etc.). With the methods proposed in literature, the partial coalescence investigations can be conducted quantitatively and/or qualitatively. Good correlation were observed between some of the quantitative methods such as dye dilution, calorimetry, fat particle size; while a poor correlation was found in the case of solvent extraction method with other quantitative methods. The most suitable way for partial coalescence quantification was implied to be the fat particle size method, which would give results with a high degree of confidence if used in combination with a microscopic technique for the confirmation of partial coalescence as the main destabilization mechanism.

Stabilising emulsion-based colloidal structures with mixed food ingredients

The physical scientist views food as a complex form of soft matter. The complexity has its origin in the numerous ingredients that are typically mixed together and the subtle variations in microstructure and texture induced by thermal and mechanical processing. The colloid science approach to food product formulation is based on the assumption that the major product attributes such as appearance, rheology and physical stability are determined by the spatial distribution and interactions of a small number of generic structural entities (biopolymers, particles, droplets, bubbles, crystals) organised in various kinds of structural arrangements (layers, complexes, aggregates, networks). This review describes some recent advances in this field with reference to three discrete classes of dispersed systems: particle-stabilised emulsions, emulsion gels and aerated emulsions. Particular attention is directed towards explaining the crucial role of the macromolecular ingredients (proteins and polysaccharides) in controlling the formation and stabilisation of the colloidal structures. The ultimate objective of this research is to provide the basic physicochemical insight required for the reliable manufacture of novel structured foods with an appealing taste and texture, whilst incorporating a more healthy set of ingredients than those found in many existing traditional products.

Implications of interfacial characteristics of food foaming agents in foam formulations

The manufacture of food dispersions (emulsions and foams) with specific quality attributes depends on the selection of the most appropriate raw materials and processing conditions. These dispersions being thermodynamically unstable require the use of emulsifiers (proteins, lipids, phospholipids, surfactants etc.). Emulsifiers typically coexist in the interfacial layer with specific functions in the processing and properties of the final product. The optimum use of emulsifiers depends on our knowledge of their interfacial physico-chemical characteristics - such as surface activity, amount adsorbed, structure, thickness, topography, ability to desorb (stability), lateral mobility, interactions between adsorbed molecules, ability to change conformation, interfacial rheological properties, etc. -, the kinetics of film formation and other associated physico-chemical properties at fluid interfaces. These monolayers constitute well defined systems for the analysis of food colloids at the micro- and nano-scale level, with several advantages for fundamental studies. In the present review we are concerned with the analysis of physico-chemical properties of emulsifier films at fluid interfaces in relation to foaming. Information about the above properties would be very helpful in the prediction of optimised formulations for food foams. We concluded that at surface pressures lower than that of monolayer saturation the foaming capacity is low, or even zero. A close relationship was observed between foaming capacity and the rate of diffusion of the foaming agent to the air-water interface. However, the foam stability correlates with the properties of the film at long-term adsorption.

Effect of surfactant sucrose ester on physical properties of dairy whipped emulsions in relation to those of O/W interfacial layers

Dairy foams were manufactured on a pilot plant with various sucrose ester contents. Oil-in-water emulsions were produced by high-pressure homogenisation of anhydrous milk fat (20 wt%) with an aqueous phase containing skim milk powder (6.5 wt%), sucrose (15 wt%), hydrocolloids (2 wt%), and sucrose esters. Sucrose ester content was varied from 0 to 0.35 wt%. Firmness and stability of dairy foams were determined. The fraction of protein associated with emulsion fat droplets and the compression isotherms of those droplets were determined as a function of sucrose ester content. With less than 0.1 wt% sucrose ester, no foam could be produced. The most firm and stable foams were obtained with ca. 0.1 wt% sucrose ester. The fraction of protein associated with emulsion droplets suddenly falls from 60% at a sucrose ester content lower than 0.1125% down to ca. 10-20% for higher surfactant content. Compression isotherms of emulsion droplets at the air-water interface show that, in the presence of surfactant, emulsion droplets disrupt and spread at the interface whilst without surfactant they become dispersed. This means that the presence of sucrose ester causes some destabilisation of fat droplet interfacial layers. There is hence an optimal sucrose ester content that allows some destabilisation of the oil-water interface without concomitant protein displacement from that interface. Consequently, with the recipe and manufacturing process used to produce dairy foams, there exists a compromise in sucrose ester content with regards to manufacture and shelf-life of dairy foams.

Composition of Interfacial Layers in Complex Food Emulsions Before and After Aeration: Effect of Egg to Milk Protein Ratio

Whipped emulsions were prepared at pilot scale from fresh milk, whole egg, and other ingredients, for example, sugars and stabilizers (starch, polysaccharides). Egg content was varied: 4 recipes were studied differing in their egg to milk protein ratio (0, 0.25, 0.38, and 0.68). Protein and fat contents were kept constant by adjusting the recipes with skim-milk powder and fresh cream. Emulsions were prepared by high-pressure homogenization and whipped on a pilot plant. Particle-size distribution determined by laser-light scattering showed an extensive aggregation of fat globules in both mix and whipped emulsions, regardless of recipe. Amount of protein adsorbed at the oil-water interface and protein composition of adsorbed layer were determined after isolation of fat globules. Protein load is strongly increased by the presence of egg in formula. Values obtained for the whipped emulsions were dramatically lower than those obtained for the mix by a factor of 2 to 3. Sodium dodecyl sulfate-PAGE indicated a preferential adsorption of egg proteins over milk proteins at the oil-water interface, regardless of recipe. This phenomenon was more marked in aerated than in unaerated emulsions, showing evidence for desorption of some milk proteins during whipping. Egg proteins stabilize mainly the fat globule surface and ensure emulsion stability before whipping. Air bubble size distribution in whipped emulsions was measured after 15 d storage. When the egg to milk protein ratio is decreased to 0.25, large air cells appear in whipped emulsions during storage, indicating mousse destabilization. The present work allows linking the protein composition of adsorbed layers at the fat globule surface to mousse formula and mousse stability.

Stability of whey-protein-stabilized oil-in-water emulsions during chilled storage and temperature cycling

The stability of heat-treated and/or acidified, partly-crystalline-fat-based, whey-protein-stabilized oil-in-water (o/w) emulsions against partial coalescence was investigated during chilled storage (at 5 degrees C) and repeated temperature cycling (three times between 5 and 25 degrees C). Experiments focused on the evolution of firmness and droplet size (using pulsed field gradient NMR and scanning electron microscopy). Besides the effects of denaturation and/or acidification, the influence of the droplet size of the dispersed phase on emulsion stability was investigated also. It was found that heat treatment or acidification before emulsification led to unstable emulsions during temperature cycling, whereas heat treatment after acidification resulted in stable emulsions.