An Overview

Fractures in smoothie lookalike, an alkane in water emulsion

Iron is a well-known solid just as water is a well-known liquid. In between is a wide range of materials, some called soft solids and others called complex fluids, whose properties are somewhere between these two extremes of solid and liquid. Many of the foods we eat, viewed as materials, belong to these in-between classes.

A common factor of these food materials is that they are not made up of a single continuous substance, but typically comprise several components. The form those components take in the final food material, and the inevitable interfaces between them, play an important part in the food’s properties.

We are interested in a group of food-relevant materials where the soft solid or complex fluid aspect is manifest, and where the multi-component aspect is explicit.

A fruit & yoghurt food product that has overstayed its welcome in the family fridge

Our particular focus is on dispersions of particles or droplets in a fluid background, where some degree of attraction causes the dispersed phase to aggregate together and form a gel. The concentration of the dispersed phase and the strength of the aggregating interaction control how strong the gel is, and this in turn dictates whether the material is more a "soft-solid" or a "complex fluid". In food materials, these properties are apparent to the consumer as "mouthfeel", or the way the sauce gloops out of the bottle, or the way some products develop unsightly liquid layers or other signs of aging which, though they may not affect the food safety or nutritional quality, are nevertheless deemed undesirable.

This last property, the physical stability and aging of dispersions, in particular in emulsions, is an on-going interest at the IFR. An emulsion is a dispersion of droplets of one liquid, typically oil, in a background of another liquid, usually water. Butter is an emulsion consisting of water droplets in oil, whereas cream is an emulsion of oil droplets in water: same stuff, but different material properties and also different microbial aging properties (cream goes sour more quickly than butter because bugs can move about better in a water background). Working with emulsions, we are looking at the way weak gels, in other words aggregated dispersions, collapse in time due to the influence of gravity. This is an issue of interest to food manufacturers who don't wish to see products such as dressings separating out during their sojourn on the supermarket shelf, for example. However we like to think that what we are doing is also applicable to other industries, such as cosmetics and agro-chemicals, and in addition will be of interest in terms of the fundamental science of particulate materials.

More information on Emulsions