The Effect of Shear on the Crystallisation of Cocoa Butter
Many materials such as polymers, metals and some foodstuffs exist normally in the crystalline state. The properties of these materials are predominantly influenced by the characteristics of their crystals (i.e. crystal structure, crystal size and crystal morphology), which are in turn controlled by many parameters during the crystallisation process (i.e. temperature, cooling rate). In some cases, shear or flow, which is often present in commercial processing, has been reported to influence crystallis ation kinetics and crystal structure.
This project was concerned with the effect of shear on the crystallisation of cocoa butter, which is the main fat ingredient of chocolate (Sato, 1998). It is known that shear can strongly affect the way cocoa butter crystallises. Feuge et al. (1962) reported a solid-to-solid transformation of cocoa butter to the thermodynamically stable polymorphic form due to shear. Ziegleder (1993) mentioned the effect of shear on the nucleation rate and the crystal size of cocoa butter. MacMillan et at., (1999) has observed the crystallisation of molten cocoa butter into the stable form induced by shear. However, the detailed mechanisms and parameters associated with the way that shear controls the crystallisation of the fat have not yet been understood. Therefore, the effect of shear on crystallisation kinetics, crystal structure and crystal morphology of cocoa butter was the aim of study in this work.
Cocoa butter is an attractive confectionery fat, and is an important commodity in the food industry. Solid cocoa butter can exist in up to six crystal forms with six different types of molecular structures. The ability of the fat to crystallise into more than one crystal structure is called polymorphism. Polymorphism makes cocoa butter a nice and convenient model system for the study of crystallisation. In nature, cocoa butter exhibits monotropic behaviour (Stoimenof, 1996), which is the ability to irreversibly transform from the less stable forms to the more stable ones. The time taken for the phase transition strongly depends on storing temperature.
In order to study the effect of shear on the crystallisation of cocoa butter, a number of techniques for following crystallisation were employed. These techniques include thermal characterisation, rheological measurement, optical observation and a combination of Multipass Rheometer (Mackley et al., 1995) and X-ray diffraction studies. In particular, a then recently developed MPR (MPR III) was used in combination with a specially built X-ray diffraction system. This allowed rheological and quantitative in-situ X-ray (both wide angle X-ray scattering, WAXS, and small angle X-ray scattering, SAXS) characterisation to be performed simultaneously, to determine the shear-induced crystalline structure and the orientation present in the material.
The results from these experiments elucidated that shear applied to cocoa butter during crystallisation process induces the fat to crystallise into form 3 before rapidly transforms to form 5. Without shear, the fat crystallises into form 3, then slowly transforms to the more stable ones.
Feuge, R.O., Landmann, W., Mitcham, D., and Lovegren, N.V., Tempering Triglycerides by Mechanical Working, J. Am. Oil Chem. Soc. 39, 310-313 (1962)
Mackley, M.R., Marshall, R.T.J., and Smeulders, J.B.A.F., The Multipass Rheometer, J. of Rheology, 39(6), 1293-1309 (1995)
Macmillan, S.D., Roberts K.J., Wells, M., Polgreen, M., and Smith, I., An Examination into Improving the Rate of Crystallisation of Single and Mixed Confectionery Fats Using Synchrotron Radiation, Abstracts for the 23rd World Congre ss and Exhibition of the International Society for Fat Research (ISF), October 1999, Brighton, UK.
Sato, K., Physical Properties of Confectionery Fats, In Ziegler, G.R., editor, Proceedings of the 2nd International Symposium on Confectionery science, 155-176 (1998)
Stoimenof, L., A Morphological Study of Chocolate, Ph.D. Thesis, University of Cambridge. 22-40 (1996)
Ziegleder, G., Vorkristallisation von Schokoladen: Einfluesse durch Produkt und Maschine, Susswaren., 54-58 (1993)