Food scientists are keenly aware that microstructure links the molecular composition and macroscopic and functional properties in foods, such as texture. From an industrial perspective, for foodstuffs that rely heavily on fats and oils for texture and sensory properties (tablespreads, chocolate, etc.), a thorough grasp of microstructure is all-too important in new product development, preservation of quality in existing foods and ensuring a long shelf life. Food microstructure studies thus provide a means to understand, and therefore control, the behaviour of food.

Perhaps more so than in many foods, understanding and controlling chocolate microstructure is integral to optimizing sensory quality, namely texture, melting profile and mouthfeel. Though there is no conclusive single theory that relates all the properties of chocolate, cognizance of microstructure is an essential piece of the chocolate structurefunctionality puzzle.

Milk chocolate consists primarily of cocoa mass, milk solids, and sugar particles dispersed within a continuous cocoa butter fat phase, made up of triglyceride crystals. The sought after properties of chocolate are strongly dependent on the size and distribution of these particles, and on the crystal properties of the fat phase. The triglycerides in cocoa butter have a well-defined melting profile that is key to the snap and sharp melting profile of chocolate at body temperature. Below are two pictures of the surface of properly-crystallized chocolate, at a microscopic scale. The first image is taken using atomic force microscopy (AFM) and the one below using scanning electron microscopy (SEM). Both images show that the surface of chocolate is relatively smooth and is laden with small pore-like openings.

Fat bloom is the chief defect that affects chocolate and chocolate products. This physical imperfection makes the chocolate undesirable for consumers who expect chocolate products to have a glossy surface and desired colour. Instead, bloomed chocolate appears old and stale and is identified by a beige-white film on the surface of the chocolate. Fat bloom is known to be the result of improperly-formed fat crystals larger than 5 μm located at the surface of the chocolate. With very small crystals (less than 5 μm) at the surface, chocolate appears glossy. Larger crystals can diffuse the reflection of light from the surface giving chocolate a dull, and hence, bloomed appearance.

Fat bloom formation is a complex phenomenon whose origins remain somewhat murky. An intriguing feature during bloom formation is the appearance of volcano-like cones on the surface of the chocolate. These cones develop with time. Initially, the cones themselves grow, implying that they are likely liquid-like. Once they have formed, they appear to harden and act as nurseries for nascent crystals. It may very well be that such growing crystals are responsible for bloom formation. The images below obtained with AFM and DSC clearly show the presence of numerous cones on the surface of chocolate.

So the next time you bite into your favourite chocolate bar, perhaps you'll be astonished at just how important all those little cocoa butter fat crystals are!                                                                     Top