A recent paper on using atomic force microscopy to study and measure properties of oil droplets, such as those in food emulsions, was featured on the back cover of the journal Soft Matter. Understanding, at the molecular level, more about emulsions, and how fats and oils behave during digestion is an important part of IFR’s research into how food structure affects health, as their properties affect the rate of digestion and satiety.
Patrick Gunning, the first author of this paper, blogs about the technique he and colleagues at IFR have developed.
The swirling coloured patterns caught in this snapshot of the surface of a soap bubble are caused by variations in the thickness of the liquid film, revealing the highly mobile nature of a surfactant covered interface. (Image courtesy of Paul Gunning http://www.flickr.com/photos/59096190@N07/)
The stability of emulsions may seem like an esoteric scientific problem but it has a massive effect on the digestibility of fats present in the foods we eat. Everyone knows that oil and water don’t mix, but by forming an emulsion we can apparently achieve this.
Emulsions usually consist of small droplets of oil suspended, but not dissolved, in water (or the other way round – small droplets of water in oil). If oil and water were the only components then the droplets would quickly coalesce and revert back to a layer of oil sitting atop the water. However, with the addition of special types of additive the coalescence can be prevented and the oil remains suspended within the water as tiny droplets.
These ingredients are known as ‘surfactants’ a short-hand term for molecules which exhibit ‘surface activity’. As the name implies these molecules seek out the region where the two liquids face each other. They do this because they are amphiphilic – they have an affinity for both the water and the oil and so naturally congregate at the interface.
The congregation of surfactants creates a film around all of the droplets forming a barrier that prevents the oil in neighbouring droplets coming into direct contact with each other and so prevents coalescence.
However, the big issue is that this prevention is not permanent.
The challenge which has eluded scientists to date is measuring the detail of how the nature of the surfactant layer can influence the stability of the emulsions against coalescence. Factors which drive coalescence include temperature, time and changing environmental conditions such as salt concentration and pH. All of these are part of the digestive process that controls the uptake of dietary fat.
Figures selected for the back cover of Soft Matter
This paper, published in the journal Soft Matter, shows for the first time a new method capable of quantifying the differences between the two principal classes of stabilising molecules found in food emulsions, detergent-based surfactants and proteins. The former form films which are highly mobile on the droplets, and migrate around the surface to stabilise the droplets against coalescence. Proteins, on the other hand, form films with completely opposite characteristics, in that they form an immobile and gel-like surface on the oil droplets.
The new method of using an atomic force microscope to measure the relaxation that occurs as droplets are pushed and then held together shows how these two different characteristics influence the forces that occur when droplets collide. Of course, the ultimate fate of whether the droplets will coalesce or bounce off each other remaining intact is determined by how big the squeezing force is and how close the oil in each droplet gets. As they approach one another the droplets squeeze out the intervening water layer and the new method can directly measure the speed at which it escapes.
The data obtained show that with a mobile surfactant film on the droplets the intervening water layer escapes quickly, meaning it thins right down as they approach so that the chances of the oil in one droplet coming into contact with the other is increased (and thus also the probably of coalescence is increased).
For protein coated droplets the immobile nature of the coating on the droplets prevents fast escape of the intervening water layer keeping it thicker and thus keeping the oil in each further apart, so they are much less likely to coalesce.
This new insight provides for the first time a means of measuring how the interfacial coating dictates the changes in force between droplets as they collide with each other. This will help us to understand how emulsions are sensed in the mouth, to help improve the quality of low fat foods. It also helps us to understand how emulsions and fats behave during digestion, so that we can control the breakdown of fats in the gut which is thought to be important for controlling appetite.
Probing the role of interfacial rheology in the relaxation behaviour between deformable oil droplets using force spectroscopy. Soft Matter, 2013,9, 11473-11479