Think about a piece of chocolate. Imagine it melting in your mouth. The sensation is delicious. Now think of the same image, but this time the chocolate is covered by a white film on its surface. This white film is produced when chocolate is poorly crystallized or when it is stored under the wrong conditions. We eat also with our eyes, so such bad-looking chocolate seems less nice to the palate. Here is where scientists come into the picture. Researchers from The Netherlands working at the ESRF try to avoid this white layer, called fat bloom, by studying the structure of chocolate. Their aim is to optimize the pleasure of eating it. They publish this week in the Journal of Physical Chemistry B the structure of a component of cocoa butter and also the crystal structure of the most common form of cocoa butter in chocolate, a result that is of great importance for chocolate production. The ESRF synchrotron light was essential for this research.

There is a lot of science in the process of making chocolate. Dark and bitter sweet chocolate contain from 31 to 38% of cocoa-butter, 16 to 32% of cocoa powder and 30 to 50% of sugar. Cocoa butter determines the physical properties of the chocolate. It has a high degree of crystallinity and may crystallize in six different crystalline forms in the course of the production process. This process includes tempering, which consists of repeatedly heating the chocolate to a specific temperature and then cooling it down. It aims to bring the cocoa butter in one of the most stable crystal forms. The different crystalline phases are numbered from phase I to the most stable phase VI. The lower-numbered phases are unstable and do not give a good product, but manufacturers nowadays manage to set the chocolate in phase V. Nevertheless, even this chocolate phase can suffer from phase transition during storage, resulting in fat bloom. This explains the importance of crystallizing the chocolate properly.

A team of scientists from the University of Amsterdam, with help of the ESRF, has made a major step forward by identifying for the first time the crystal structure of one of the three main triglycerides that make up chocolate butter. The triglyceride, called SOS, is a cis-mono-unsaturated type and represents one quarter of the chocolate butter. This breakthrough helps in better understanding the melting behaviour of cocoa butter and better controlling the production process. According to Dr. René Peschar, first author of the paper, "This work is expected to be highly relevant to confectionery research and industry and the first step to a better understanding of the mechanism of the fat bloom phenomenon at the molecular level."

The researchers used the synchrotron light to collect data from which they determined this structure using the X-ray powder diffraction technique. They also stored completely molten cocoa butter at room temperature (around 22°C) for several weeks to get the phase V. Then they studied it at the ESRF with the same technique and managed to construct a crystal structure model of this cocoa butter phase V. "It is impossible to get these results with laboratory data; you really need a synchrotron facility because of its superior data quality", explains Dr. Peschar, from the University of Amsterdam.

The chocolate research based on data measured at the ESRF has also had impact on industry. The Dutch machine manufacturing company Machinefabriek P.M. Duyvis acquired a patent concerning an improved method of making chocolate that is based on the results of experiments carried out by the Dutch researchers at the ESRF over the last few years. The company built a prototype, tested and fine-tuned it together with the University of Amsterdam and a major European chocolate producer. The company is situated in the middle of the "Zaanstreek", a region hallmarked by a huge diversity of foodstuff manufacturers and processing more than 20% of the world's cocoa bean crop.

Source: European Synchrotron Radiation Facility

Is a white bloom enough to put you off your chocolate? Scientists are hard at work to find out exactly how this bloom forms and how to stop it.

Next time you reach out for your favourite chocolate bar you will probably pay little attention to its fat crystals. However, should you be unfortunate enough to peel back the wrapping to reveal a chocolate covered in a mouldy-looking white 'bloom', then perhaps you might spare a thought for its crystal structure. The chocolate industry ploughs a lot of money into investigating chocolate crystals and bloom.

The industry takes bloom seriously – not only because it is unsightly, but also because it can change the texture and the flavour release properties of the chocolate. Manufacturers are keen to invest in research, using expensive techniques such as X-ray scattering and atomic force microscopy (AFM), to help understand exactly how bloom forms and how to stop it forming. With the average person in the UK eating 10kg chocolate each year (according to Cadbury's confectionery review of 1999), it is easy to see why the industry wants to create a perfect chocolate bar that stays temptingly glossy with a good 'snap'.

Chocolate bloom develops naturally with time, but it can be brought on prematurely. How many of us have left a chocolate bar on the car dashboard in the sun and been disappointed to find that it has been spoilt by a bloom? In this case, the bloom develops because the crystals melt and then recrystallise in a different form when the temperature drops again. Chocolate bloom can also form if the manufacturing process doesn't include a tempering step (see Box 1), when the temperature is carefully raised and lowered to ensure that fat crystals grow in the correct form, size, shape and number.

Chocolate crystals

Cocoa butter, perhaps the most important ingredient of chocolate, is composed of a mixture of saturated and unsaturated fats (triglycerides), the relative proportions of which depend on the country of origin. Some of the unsaturated triglycerides in cocoa butter have low melting points, making it partly liquid at room temperature. Adding milk fat to chocolate raises the level of unsaturated triglycerides and increases the proportion of liquid fat, which explains why milk chocolate is so much softer than its dark counterpart.

The fat crystals in cocoa butter pack together in six different formats (polymorphs). The chocolate industry labels these polymorphs forms I to VI (form I being the least stable) and aims to get the cocoa butter to crystallize in a stable form V to give the chocolate a glossy appearance and a good snap.

Surface science

The surface of a good quality chocolate contains lots of tiny fat crystals that can reflect light, giving it a glossy appearance. Any cracks or crevices (or even fingerprints) on the surface of the chocolate can encourage small, spiky fat crystals to grow. When the crystals reach a size that can diffuse the reflection of light from the surface they give it a dull appearance.

Although the exact mechanism of bloom formation remains disputed, most scientists agree that it involves fat crystals transforming from form V to form VI. Because form VI crystals are more stable than form V, chocolate should inevitably form a bloom at some stage, unless preventive measures are taken.

Richard Hartel at the department of food science in the University of Wisconsin, US, believes that although the form V to form VI transformation always accompanies bloom formation, it does not necessarily cause it. With John Bricknell at Mars in New Jersey, US, he has analyzed a 'model' chocolate using X-ray spectroscopy, to identify the types of fat crystals that develop. Their model chocolate contains amorphous sugar particles - created by spray drying a mixture of corn syrup and sucrose and sieving the mixture to ensure that all the particles are the same size. The chocolate is made by blending and tempering a mixture of cocoa butter, lecithin (an emulsifier), sieved cocoa powder, milk fat and the amorphous sugar.

Because the model chocolate contains no crystalline sucrose, the researchers were able to see clearly the changing polymorphic forms of the cocoa butter. They also used a colorimeter to measure the amount of white bloom that developed on the chocolate samples, enabling them to link changes in polymorphic form to the onset of visual bloom.

They discovered that the form V to form VI crystal transformation took place not only in all of the samples that developed a visual bloom but also in some of the samples that remained bloom-free. Hartel says that 'most people thought they understood bloom formation in chocolate to be the polymorphic transition of cocoa butter. What our results show is that the polymorphic transition indeed occurs, but that something else is needed to create visual bloom'.

Hartel's research team has come up with a theory to explain how visual fat bloom develops in well-tempered chocolates. They suggest that, first of all, liquid fat must be able to get to the surface of the chocolate. The 'pumping action' required to do this could be induced by temperature fluctuations, which cause the fat crystals to melt and then to recrystallise. Fat crystals with high melting points 'dissolve' in this liquid fat and are taken along to the surface where they can recrystallise as spiky crystals. Any cracks and crevices can help the liquid fat get to the surface. The way that the spikes grow from the surface of the chocolate, says Hartel, is 'open for debate' although the 'nature of the sites available for growth undoubtedly plays a role in their formation'.

An interesting and unexpected result emerged from Hartel's study: the amorphous sugar used to make the 'model' chocolate seemed to be able to prevent a visual bloom developing. When the researchers looked at the samples through a microscope, they saw that the fat crystals on the surface of the model chocolate were smooth, rounded and flat, causing little more than a slight dulling of the surface. These crystals were markedly different to the spiky, needle-like crystals of 'real' chocolate that can take away its gloss.

Hartel thinks that, because the smooth, spherical sugar particles pack together more tightly than the irregular-shaped sugar crystals in commercial chocolate, this reduces both the rate of liquid fat migration and hence the rate of bloom formation.

Despite the success of the amorphous sugar at inhibiting fat bloom, Hartel says that it could not be used in commercial chocolate because the sugar 'picks up moisture easily and gives a gummy texture in the mouth'.

By adding high melting point milk fat fractions to their chocolate mix, Hartel and his team have been able to delay substantially the transition from form V to form VI. Indeed, milk fat is commonly used to inhibit fat bloom, and skimmed milk powder is better than whole milk at preventing bloom formation.

How milk fat reduces bloom formation remains a mystery, but minor lipids in the milk fat (eg mono- and diglycerides) are generally thought to influence the kinetics of cocoa butter crystallization. The denser crystal structures that form could potentially stop liquid fat from moving to the surface and recrystallising. The minor lipids could also affect the amount and type of high-melting lipids that dissolve in the liquid fat and could even slow down the transformation of crystals from form V to form VI. Another theory is that because milk fat can decrease the rate of fat crystallization, the chocolate contracts less on cooling. Fewer microscopic cracks appear, reducing the likelihood of liquid fat reaching the surface.

Hartel predicts that 'understanding how the chocolate microstructure influences the rate of bloom formation will ultimately allow the chocolate manufacturer to produce high quality chocolates with enhanced resistance to bloom'.

An even temper 

Researchers at the University of Leeds have been working with Cadbury to help make its tempering process more efficient and reduce the amount of money it spends on heating and cooling vast quantities of chocolate during tempering.

Industrial tempering usually involves applying shear forces (stirring) while changing the temperature. The shear rate has to be chosen carefully because if it is too low then not enough crystals will be generated, and if it's too high the crystals could melt.

Scott MacMillan and Kevin Roberts, from Leeds' chemical engineering department, have developed a method that enables them to look at crystal changes during tempering, with the aim of optimizing the process in order to guarantee the growth of form V fat crystals. They have designed a temperature-controlled shear 'cell', similar to the cone and plate system commonly used in rheometers, placing the fat sample on the bottom plate and rotating the top 'cone'. This set-up allows the researchers to heat and cool fat mixtures while at the same time varying the shear rate. Using the small angle X-ray scattering (SAXS) facility at Daresbury, they have been able to monitor changes in crystal structure in the shear cell during tempering.

When no shear stress was applied to cocoa butter samples, the fat crystals transformed slowly from form III to form IV. However, on shearing the samples, the crystals transformed from form III to form V. Macmillan believes that because the results 'give a strong indication of the inherent mechanisms taking place', they should be able to help Cadbury determine the optimum shear rate and temperature to ensure that the chocolate crystallizes in form V.

Soft in the middle

Those of you with sufficient self-restraint to put aside a half-eaten selection box of chocolates may have noticed, on reopening the box, that the pralines are generally the first to develop a bloom. The nut-based filling contains fat that is liquid at room temperature and, as this fat migrates from the filling to the chocolate exterior, some of the cocoa butter in the chocolate moves in the opposite direction. The appealing texture contrast between the inside and the outside of the praline can then be lost as the liquid fat softens the chocolate exterior and the cocoa butter hardens the soft centre. The liquid fat that moves to the surface of the chocolate can also drag some of the cocoa butter with it, which can recrystallise at the surface and form a bloom.

These problems can be solved to a certain extent by adding a layer of a harder fat (more saturated triglycerides) in between the outer chocolate layer and the soft interior, or alternatively to the centre where it can act as a sponge for the liquid fat.

Paul Smith and researchers at the Institute for Surface Technology in Stockholm, Sweden, are working on the problem of fat bloom in soft-centred chocolates and have developed a technique using radiolabelled (14C) triglycerides to study the fat exchange process. They use differential scanning calorimetry (DSC) to determine the polymorphic form of the triglyceride crystals and a 14C radio detector to follow the movement of the radiolabelled compounds. So far, they have worked mainly on model fat systems, adding unlabelled fat crystals to an oil saturated with a 14C labelled triglyceride and gently stirring the mixture. At regular intervals they remove samples and measure how many of the 14C triglycerides in the liquid oil phase crystallize out. Preliminary results suggest that the exchange rate between fat crystals and dissolved fat is relatively fast when the crystals are small but slow when the crystals are large.

Smith is currently using atomic force microscopy (AFM) to study the changes in the structure of the surface of the chocolate that occur when bloom forms. The diamond tip of the AFM probe moves over the surface of the chocolate and deflects as it passes over any undulations. Smith has chosen the technique over the standard methods of scanning electron microscopy or optical microscopy which can generate artifacts, he says. Optical microscopy, explains Smith, is difficult to use with chocolate because of its dark colour. In addition, the limit of resolution means that only the large crystals can be picked up. Smith has yet to release the results of the study but hopes to use them to help understand the methods of bloom formation and to observe the early onset of bloom.

There is clearly more work to be done on bloom but new techniques and R & D investment should lead the chocolate industry to its holy grail: a long-lasting chocolate that doesn't lose its gloss with storage.