SATURDAY, 7 MAY 2011
Making a low-fat chocolate bar that tastes and feels right is not easy. “The big question is how chocolatiers can meet the growing demands of the market for a lower-fat chocolate while keeping true to the classic taste and texture,” says Cambridge chemical engineer Joel Taylor.
British consumers are particularly picky. Chocolate crumb, the basis of the chocolate we know today, was developed in Britain during the early twentieth century to increase the shelf-life of chocolate; and is the source of its unique flavour. It is made by combining dry ingredients with water to form a paste. The paste is then dried and milled, lowering water content and preventing moulds from growing. The drying process causes a reaction between proteins and sugars known as the Maillard reaction, which introduces a ‘cooked’ flavour. British expatriates in the US and elsewhere pay top dollar for imported British chocolate so that they can enjoy the tastes of home.
The classic chocolate bar is adept at satisfying our exacting taste buds. Most of the flavour of chocolate comes from sugar, which along with cocoa comprises a particle suspension in a fatty fluid, usually cocoa butter. To keep the mixture smooth, chocolate manufacturers add emulsifiers. Emulsifiers eliminate friction between particles by sticking to non-fat particles and making it easier for fats to coat them.
Manufacturers are already making progress towards a healthier chocolate by adding oil substitutes and varying ingredients in the emulsion to reduce fat content. However, chemical engineer Phil Cox and his team at the University of Birmingham went one step further in 2009, producing chocolate using more water than oil while still retaining the taste of conventional chocolate. Their cocoa butter emulsions, which are suspensions of two liquids that do not mix, contained up to 60% water. Most emulsions rely on the propensity of oil to remain separate from other liquids and, as a result, are fattier. The water-based emulsions remained stable during storage and the cocoa butter melted around 33°C, the temperature which consumers find the most attractive according to Stephen Beckett, a former Nestlé chocolate researcher. Further research at Birmingham has led to the development of a protein structure filled with air that mimics the properties of fatty molecules in cocoa butter, and can replace some of the fattening oils in foods such as mayonnaise and salad dressings.
However, these innovations have not been without their own problems. Less fatty chocolates have a harder texture. In an attempt to counter this, biochemist Bettina Wolf and students at the University of Nottingham tried adding limonene to low-fat chocolate. Limonene is a citrus fruit-derived oil-soluble substance that compensates for the compromised softness and quality in reduced-fat chocolate. It decreases cocoa butter viscosity by mixing within the cocoa butter’s structure and diluting the fat. It also reduces the formation of fat crystals in cocoa butter, decreasing the solid fat content and hardness of the chocolate.
Cost is always a concern, too. Taylor notes that “cocoa butter is expensive, so substituting other fats in chocolate is important economically as well as for health reasons. Hence you get combinations of milk fats and cocoa fats, which affect taste and texture.” Manufacturers make particles in the chocolate mixture as large as possible to reduce costs. Smaller particles have a higher surface area and require more fat to coat. On the other hand, larger particles make for less smooth, grittier chocolate, which does not taste as good. Getting the size of the particles right is yet another challenge for researchers.
During the manufacture of chocolate, mixtures of different particle sizes clump together tightly leading to thicker fluid and rougher textures. To avoid this, manufacturers use energy-intensive techniques including conching, a process of kneading and stirring at high temperature for many hours in a seashell-shaped vessel.
Taylor questions whether the conching process is as efficient as it could be. He studies the flow, or rheology, of molten crumb chocolate using a machine called a rheometer that applies a shear stress or strain over time. The most commonly used rheological model for chocolate was originally developed to model the rheology of printing inks. However, after studying chocolate over a wider range of shear stresses, Taylor and fellow chemical engineer Alex Routh recommended a different model. Their new, improved model better describes the properties of crumb chocolate rheology. Routh argues: “if you can understand chocolate structure over time, maybe you can achieve [the same] structure using less energy, and perhaps eventually bypass the conching process.”
However, as Taylor highlights “it is not just chocolate, all food manufacturers face the same challenge” of producing lower fat products while maintaining their traditional taste. Therefore, the food industry has entered a new period of research into health and nutrition. Nutritionist Alison Lennox of the Human Nutrition Research (HNR) centre in Cambridge notes that recent years have seen food companies’ research teams increase their interest in nutrition quality. Research at the HNR has provided an understanding of specific nutrients and health implications, but Lennox needs to explain to those who support government initiatives “why you cannot make a low-fat biscuit and still have it taste right.” That is where the scientists are needed.
Will food companies have trouble finding talented young minds to do the research? Not likely, if the Cambridge research group is anything to go by. “I have always liked eating chocolate,” explained Taylor, “so researching the stuff seemed like a pretty good idea.”
Rachel Berkowitz is a PhD student at the BP Institute in the Department of Earth Sciences
