Can colloids and emulsions be the answer to engineering healthy foods and keeping the four horsemen away?
A glut of grim statistics chart how our love of saturated fat, sugar and salt is spreading our waistlines and eroding our health. Obesity levels in some areas of the world including the USA, the UK, eastern Europe, and China have grown three-fold since 1980 according to the World Health Organisation. Wheezing and gasping behind (in stretchy tracksuits, no doubt') are the four horsemen of this new apocalypse: type-two diabetes, cardiovascular disease, hypertension/stroke, and certain forms of cancer.
Eating better and exercising regularly is one way to avoid these horrors. And yet, even if the remedy grows on trees, the statistics suggest other factors are winning. One way forward is to engineer novel foods that keep the four horsemen at bay and yet still let us indulge. But how practical is this?
Two years ago, as part of a UK DTI Foresight programme, chemical engineers at the University of Birmingham published a paper suggesting chemistry and materials science could help food manufacturers to encourage a change in lifestyle by making foods that boosted perception of ingredients such as fat or salt, or slowed stomach emptying.
The researchers' main interest was in using a class of materials called colloids, which include emulsions as a sub-category. These are mixtures of tiny particles of one or more material (solid, liquid or gas) suspended in another, stabilised by emulsifiers. Emulsifiers act as bridges between the different particles. Mayonnaise, for example, is an emulsion in which lecithin (found in eggs) holds together oil and water by forming a layer around the oil droplets.
Other colloid foods include mousses, souffl's, set yoghurts and milkshakes. To make such products healthier, the idea is to wholly or partially replace the fat particles with calorie-free substances such as water, air or gels, while retaining the indulgent sensory properties.
Among the most promising novel colloids are WOW (water-in-oil-in-water) emulsions, which reduce fat content by filling the tiny fat droplets with even tinier water droplets. Leatherhead Food Research in the UK, for example, which carries out research for global food and drinks companies, has been developing these for nearly four years and has reported 40 per cent fat reduction in mayonnaise and cream products made in the lab, without a sensory difference.
To date, there are no products on the market that contain double emulsions, because they are difficult to keep stable and also to manufacture in volume. Part of the problem is that when a double emulsion containing water has sugar or salt in one of the other phases, the combination creates osmotic pressure. This means that the water migrates towards the more salty or sugary side of any interface, and so the emulsion breaks down.
The Birmingham team has recently made progress on this front, helped by the industrial know-how of Ian Norton, former chief scientist of Unilever, who is chair of soft solid microstructure engineering at the university. The work has led to two patents: a water-in-oil emulsion technique that can contain up to 60 per cent water, suitable for making low-fat chocolate; and a WOW emulsion that can reduce salt content up to 80 percent.
For the low fat-chocolate, Dr Phil Cox's group found a way to shore up the water droplet interfaces with sintered cocoa butter crystals, so the water is trapped inside little shells, isolating it from the osmotic pressure of the sugar in the surrounding chocolate. Polyglycerol polyricinoleate (PGPR) and soy lecithin, common ingredients in chocolate manufacture, are the emulsifiers and the cocoa butter crystals were found to be in the form that consumers find the most attractive, as it melts between 32 and 34'C, according to findings published in the November 2009 issue of the Journal of Food Engineering. With the water droplets less than 40'm in size, the team believes that the water should not be perceptible to human taste buds and so should go unnoticed by anyone eating the chocolate.
Dr Jennifer Norton, Ian Norton's daughter, did the early work on the chocolate while she was a postgraduate student in 2008. Their discussions led Norton senior to create the stable salt-reducing WOW double emulsio, with a similar technique to the chocolate, using PGPR with sintered fat crystals as a shell around the innermost water droplets. Other emulsifiers, such as lecithin, stabilise the interface between the containing oil droplet and the main water phase. Hiding some of the water content inside the oil droplets means that only the continuous outer water phase needs to have salt in it, to give the impression of saltiness. The emulsions have remained stable for more than six months so far, losing only 4 or 5 per cent of the internal phase.
As well as their propensity to collapse under osmotic pressure, double emulsions are also shear-sensitive. This means they don't fare well under the usual high volume food manufacturing processes, which in the case of emulsions involve pushing materials at high speed through nozzles (droplet sizes down to 100nm) or through rotor/stator devices (minimum droplet sizes around 1000nm).
A decade ago, the Japanese developed a way to make double emulsions by pressing the inner phrase emulsion (made using valve homogenisation) through a membrane to form droplets at the pore openings that can then be detached by the cross-flowing outer phrase. But it's a very slow process. Norton and his colleagues have been experimenting with a rotating membrane device, which would appear to solve the problem to some extent. 'If you rotate a membrane at low speeds, this is more like a normal emulsification process, so you have the advantage of a membrane plus some sheer,' he says.
Hairy gels and self-structuring colloids
Colloids, based on zero-calorie 'hairy' gel particles, are another promising route for combating obesity. Adding turbulence into a gel as it sets means the gel polymers only associate in the eddies, resulting in a sea of tiny particles (sized at 1,000 to 5,000nm) which have un-bonded polymers sticking out around their edges hence the term 'hairy'. The particles flow like a liquid but can be set and joined under a change in temperature or chemical environment.
Such particles can potentially replace fats in spreadable or semi-solid products like mayonnaise. Today, low fat mayonnaise uses swollen starch grains, which can be 50 to 100 times the size of the usual mayonnaise oil drops. 'If you have gel particles with the same dimensions as the oil droplets, you start to get the same properties as the oil,' says Norton. A totally fat-free mayonnaise would taste odd because the oil carries the flavour, however, just 3 per cent fat will apparently retain the flavour properties. The Birmingham group is now in discussion with various companies about using this technique in foods ranging from meat products to desserts.
Full but not fat
An alternative way to curb greed is to engineer food that makes us feel fuller for longer. Dr Luca Marciani from the University of Nottingham and Dr Martin Wickham from the UK Institute of Food Research (IFR) last year reported on an olive oil in water emulsion stabilised using the commonly used food-grade emulsifier polyoxyethylene sorbitan monostearate, that would remain stable (and not break up) under the acid conditions of the stomach.
With their acid-stable emulsion, the average time for half of the stomach to empty was almost three times longer than for an acid-unstable emulsion (185 versus 67 minutes, respectively). This work is at a very early stage and a long way from commercialisation, but it suggests that it is possible to design oil-in-water emulsions with different behaviours in the gut to influence our appetite.
In Birmingham, the researches have been looking at similar ideas. At this stage, they are writing a patent for a biopolymer that can change from a liquid to a 3D gel when it hits the acidic environment of the stomach, with the gelling process controllable over time''scales from minutes to half an hour.
If all else fails, can our noses help? Using olfactometry, a way of testing and measuring the sense of smell, Dr Rianne Ruijschop of the NIZO Food Research in the Netherlands has discovered that the length of release, the complexity of the aroma compositions and the duration of processing in the mouth are all valuable concepts for the development of foods to induce or increase feeling full.
The approach is intriguing because it allows smells to be administered separately from other factors like ingredients, texture and taste. Dr Ruijschop has found that a drink with an aroma release profile similar to a (soft) solid food could increase the feeling of fullness significantly. A 'multi-component' strawberry aroma, perceived as being more complex, yet of similar aroma quality, intensity and pleasantness to a single-component strawberry aroma, also made people feel fuller.
The industry standard metric of whether low-fat food matches the indulgent variety is viscosity. Dr Norton and others argue that if consumers are to accept healthier substitutes, more sophisticated techniques will be needed to characterise how these novel foods behave in the mouth including pressure, velocity fields and molecular diffusion rate conditions and their relationship to sensory measurements.
Diffusion rate is important, for instance, because it gives some guide as to whether materials that stick to the mouth (like alginate and pectin) might be added as flavour enhancers, delivering salt, say, over extended periods of time. As salt impacts flavour, it might allow salt reduction.
Quantifying a strongly hedonistic attribute, like creaminess, requires many factors. Some time ago, the food scientist Josef Kokini tried to link the perception of creaminess (intuitively related to fat content) with other sensory attributes from trained panelists and created the following rather curious equation:
Creaminess = 0.54 log (Thickness) +1.56 log (Smoothness) -0.32 log (Slipperiness)
In measurement terms, thickness is related to the rheological properties i.e. the way a product flows and deforms; smoothness is associated with particle size in the colloidal system; and slipperiness relates to friction.
Modern imaging techniques may help to reveal more about how such foods behave once in the stomach but this is a very new area. The Institute for Food Research has done some preliminary work using high- resolution MRI imaging with the Norfolk & Norwich University Hospital's (N&NUH) Radiology Academy to look at the differences in rates of stomach emptying for different types of meal. It is also developing and designing pulse sequences that will optimise contrast between the various components (such as lipid or starch) of meals, so it is possible to see changes in the food structure and composition.
Whether this kind of engineering will help to combat diet-related diseases is difficult to predict, but the research shows that it is at least becoming practical. If these foods can meet the recommendations of nutritionists and agencies such as the FDA, FSA and WHO, while keeping consumers happy, then perhaps everyone can have their cake and eat it.
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