High-Resolution and Time-Resolved XRD Studies of Cocoa Butter lead to better Chocolate

Henk Schenk and René Peschar

Laboratory for Crystallography, HIMS, FNWI, University of Amsterdam,

Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands. (h.schenk@uva.nl)

Crystallization of cocoa butter is the critical step in making chocolate and confectioneries. Poorly crystallized chocolate may result in the formation of fat bloom, a greyish-white film at the chocolate surface. Cocoa butter has been subject to research for many years resulting in at least six different polymorphs, indicated by Greek letters g, a, b' and b. In good quality chocolate predominantly the b(V)-polymorph is present, but may be also the b(VI)-phase.

Fig. 1. Diagram of a triacylglycerole. The three fatty acid residues may have different lengths indicated by p, q, and r. Natural fats as cocoa butter are mixtures of different TAG’s in which also unsaturated fatty acid residues may be present.

X-ray powder diffraction (XRD) is a very useful technique to identify the fingerprint region (d = 3.0 -6.0 A) of the cocoa butter polymorphs, which are different for all. In fat research XRD has the advantage over DSC of giving unambiguous phase information. Time-resolved XRD (trXRD) is a very suitable tool for studying the phase transformations of the system as well as for following melting and solidification processes. It is being used extensively in our research and may be presented in movies.

Also full XRD patterns, including peaks at long d-spacing values, are being used to characterize the various cocoa-butter phases. Furthermore, the components of cocoa butter, the triacylglyceroles (TAG, see figure), are subject to structure determination studies using High-Resolution Synchrotron data. This eventually will lead to more suitable molecular models for cocoa butter to understand its crystallisation and phase transformation. Both single crystal and powder methods are being used, but, remarkably, the latter methods are more successful as it is very difficult to obtain suitable single crystals and to handle them. In order to unravel the mechanism of fat bloom, we recently took up the crystal structure solution of the b(V)-polymorph of cocoabutter and we expect to be able to report on this research at the meeting.

Crystallization of cocoa butter in the b phase directly from the melt under static conditions is only possible employing the memory effect of cocoa butter [1]. When b-phase cocoa butter is heated below a maximum temperature and above the melting end point, it re-crystallizes in the b-phase at crystallization temperatures. The influence of the maximum and crystallization temperatures on the re-crystallization behaviour has been investigated for cocoa butter from Bahia (Brazil). Re-crystallization into the b(VI)-phase appears to be faster than into the b(V)-phase. Structural information on the seed material initiating the re-crystallization process has been collected through Small-Angle X-ray Scattering. High-melting SOS-rich seeds in triple chain-length packing initiate the rapid-starting re-crystallization. The seed crystals initiating the slow-starting re-crystallization resulting in the b(V)-phase are different from those resulting in the b(VI)-phase.

As a spin-off of our basic research a new, patented, way of chocolate making [2] has been tested successfully at industrial scale. The manufacturing is based on a new and revolutionary tempering process, using seed crystals in the liquid phase and driven by a feedback system. A research machine has been developed at the mechanic laboratory of Duyvis Machine Factory [3] and produces on demand from 100 to 1000 kg of well-tempered chocolate in a continues process. A 3-ton prototype is currently being designed and first results may be reported.

Acknowledgements.
The investigations have been supported by the Netherlands Foundation for Chemical Research (NWO/CW) with financial aid from the Netherlands Technology Foundation (STW). The authors thank Drs A.J. van Langevelde, K.F. van Malssen, R.A.J. Driessen (FNWI, UvA), P. Visser (Duyvis Machinefabriek), M. Pop, D.J.A. de Ridder and J.B. van Mechelen for collaborating in the research, the STW User Comittee, and the ESRF (Grenoble, France) for support at the beamlines BM1A (SNBL), BM16, ID11, and, in particular, BM26B.

[1] Van Langevelde, A.J., Driessen, R., Molleman, W., Peschar, R. and Schenk, H. (2001). Cocoa butter long spacings and the memory effect. J. Am. Oil Chem. Soc. 78, 911-918. Van Langevelde, A.J., Van Malssen, K., Peschar, R. and Schenk, H. (2001). Effect of temperature on recrystallization behaviour of cocoa butter. J. Am. Oil Chem. Soc. 78, 919-925.

[2] Van Malssen, K. F., Van Langevelde, A. J., Peschar, R. and Schenk, H. (2001). Method for the manufacturing of chocolate. International Patent Application PCT/NL00/00525 (WO800148-Al).

[3] Schenk, H., Visser, P. And Peschar, R. , abstract ISF 25, Bordeaux, 2003.