Identification of the Initial Nucleating Form Involved in the Thermal Processing of Cocoa Butter Fat as Examined Using Wide Angle X-ray Scattering (WAXS) S. D. MacMillan and K. J. Roberts* Institute of Particle Science and Engineering, Department of Chemical Engineering, University of Leeds, Leeds, West Yorkshire, United Kingdom
CRYSTAL GROWTH & DESIGN 2003 VOL. 3, NO. 2 117-119
M. A. Wells and M. C. Polgreen Cadbury Trebor Bassett, P. O. Box 12, Bournville Lane, Birmingham, United Kingdom
I. H. Smith Reading Scientific Services Ltd., Whiteknights, Reading, United Kingdom Received June 1, 2002;
Revised Manuscript ReceivedDecember 19, 2002
ABSTRACT: In situ wide angle X-ray scattering studies have confirmed that the initial nucleating phase involved in the tempering of cocoa butter fat is consistent with the form III polymorphic structure rather than the form II alternative. Introduction. Chocolate production involves the creation of a formulation containing cocoa butter, cocoa solids, vegetable fats (optional), sugar, lecithin, and in the case of milk chocolate, milk fat and milk solids. It is well-known that the optimum manufacture of chocolate products involves a tempering process, in which the polymorphic structure of the fat component is manipulated to result in its final crystal structure having the desired product properties (melting point, rheology, snap, gloss, and fat bloom stability).2-3 While the triglyceride fats such as cocoa butter can exist in six different polymorphic states,9 only one of these forms produces the desired chocolate properties, form V. Through the tempering process, it is possible to facilitate crystallization of this form in chocolate by cooling molten chocolate to induce the nucleation and growth of seed crystals.1 However, as only form V is desirable in the chocolate product, the sample is then reheated to melt out any unwanted crystalline forms. The chocolate can then be cooled again, with the desirable form acting as a seed, to produce final crystallization in the fat phase to form V. Previous work by MacMillan et al.5 showed that in situ small-angle X-ray scattering (SAXS) could be used to study polymorphic changes in cocoa butter samples as a function of temperature. In this study, SAXS data were collected for both sheared and stagnant samples during in situ cooling experiments, where the samples were crash cooled from 50 to 20 °C (time taken ca. 2-3 min) and then held at this temperature for 1 h. While the resultant SAXS data were ambiguous in terms of being able to characterize the initial polymorphic form produced, i.e., form II or form III, it did show that through the early stages of sample cooling no other possibly less stable intermediate polymorphs were formed. For the sheared sample, the II/III polymorph was observed after 4.5 min from the onset of cooling followed by the formation of form V after 12 min. This was in direct comparison with SAXS data taken for the stagnant sample, which showed the formation of form II/III after 6 min and form IV after 31.5 min. These in situ cooling experiments, while showing the importance of shearing to produce the desired form V polymorph, also provided time-dependent information in relation to polymorph formation. Studies by Wille and Lutton9 revealed that through the use of wide angle X-ray scattering (WAXS) data, see Table * To whom correspondence should be addressed. Tel: +44(0)113 233 2400. Fax: +44(0)113 233 2405. E-mail:
[email protected].
Figure 1. Photograph of variable temperature cell for WAXS studies as used on station 9.1 at the Daresbury SRS.
1, forms II and III could easily be distinguished. It is thus the discriminatory power of WAXS data that is utilized in this short communication to determine the initial polymorphic state formed for the more simplistic stagnant cocoa butter sample under temperature programming. Experimental Section. Cocoa butter was obtained from Cadbury Ltd. and was Ghanaian in origin. Palmitic, stearic, and oleic acid side groups dominated its composition with a constituent make up of 27.5, 32.8, and 34.6 wt % (with the rest being made up by small quantities of other fatty acids, e.g., linoleic). To test the effect of formulation on the crystallization process, a sample containing a 70: 30 mixture of cocoa butter and milk fat, see Table 2, was also made up. Fresh samples were used for each WAXS experiment and were typically 0.2 g. In situ variable temperature WAXS measurements on the fat samples were carried out using the two circle goniometer available on station 9.1 (step size 0.01°, counting time 2 s/step) at the Daresbury Synchrotron Radiation Source (SRS), in conjunction with a small X-ray cell; see Figure 1.6 The samples were examined under stagnant conditions in the cell, whose temperature was controlled via a recirculating Haake F3-CH thermostat, and temperature differences within the cell were kept to a minimum through the use of a small sample area. A platinum resistance probe (Pt100), mounted onto the cell, allowed point contact with the sample, thus providing an accurate measurement of sample temperature.
10.1021/cg025533t CCC: $25.00 © 2003 American Chemical Society Published on Web 02/07/2003
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Table 1. X-ray Diffraction Data on Short Spacings (Units in Å) of Cocoa Buttera polymorphic form melting point (°C) X-ray diffraction peak assignment
I
II
III
IV
V
VI
17.3
23.3
25.5
27.5
33.8
36.3
3.70 (s)
4.24 (s) 3.87 (m) 4.17 (vs) 3.68 (w) 3.71 (s)
4.19 (vs)
4.25 (s) 4.35 (vs) 3.78 (w) 4.63 (m) 3.88 (w) 3.99 (w) 4.60 (vs) 5.43 (m)
3.88 (s) 4.04 (m) 4.28 (m) 4.60 (vs) 5.16 (w) 5.47 (m)
a Letter in brackets signifies diffraction peak intensity, i.e., w ) weak, m ) medium, s ) strong, and vs ) very strong (Willie and Lutton, 1966).
Figure 3. WAXS patterns shown over a 2θ range of 8-18° as recorded using station 9.1 at the Daresbury SRS. The main characteristic peak d spacings have been defined (units are in Å) for a pure cocoa butter sample after cooling from 50 to 23.4 °C.
Figure 2. WAXS patterns shown over a 2θ range of 8-18° as recorded using station 9.1 at the Daresbury SRS. The main characteristic peak d spacings have been defined (units are in Å) for a pure cocoa butter sample at ambient temperature (20 °C). Table 2. Triglyceride Compositions for Cocoa Butter and Milk Fat Feedstocks
constituents
myristic
palmitic
stearic
oleic
other (carbon chain length range)
cocoa butter milk fat
0.1 10.2
27.5 28.8
32.8 12.3
34.6 25.7
14-20 4-24
The WAXS results are summarized in Figure 2a-e. Except for the data taken in Figure 2a, all samples were preheated into their molten states at ca. 50 °C before they were crash cooled back to different end temperatures (ca. 2-3 min). As cooling rate can affect the final polymorphic forms produced, quick crash cooling was essential to prevent the formation of higher-melting polymorphs.7,8 Once the desired end temperature was reached, the samples were then held at this point for 1 h, before the collection of WAXS data. Previous SAXS studies, undertaken by MacMillan et al.5 have encapsulated the whole cooling cycle until this point, revealing no evidence of any other polymorphic states. It was therefore felt unnecessary to repeat this process for the WAXS data presented in this short communication. Results and Discussion. Figure 2 shows a benchmark WAXS experiment carried out to provide peak positioning information in relation to the crystallization process. These data were taken for a fresh cocoa butter sample at the
Figure 4. WAXS patterns shown over a 2θ range of 8-18° as recorded using station 9.1 at the Daresbury SRS. The main characteristic peak d spacings have been defined (units are in Å) for a pure cocoa butter sample after cooling from 50 to 20 °C.
ambient temperature of 20 °C and show that a 2θ scan between 8 and 18° will encapsulate the entire range of peak positions. This scan range was used for subsequent experiments. Examination of these data, with respect to Table 1, also reveals a structure consistent with form V, as expected for a tempered cocoa butter fat. Figure 3 shows data taken from a cocoa butter sample cooled from 50 to 23.4°C, i.e., above the melting point of form II. While the WAXS data clearly show evidence for the formulation of form IV (e.g., d spacings ) 4.34 and 4.17 Å), the presence of a strongly amorphous background prevents identification of either form II or form III. To improve the data resolution, the final temperature for fresh cocoa butter samples was lowered to 20 and 14°C, as shown in Figures 4 and 5. Both sets of WAXS data clearly show defined peak positions consistent with the formation of form IV and form III (e.g., d spacings ) 4.64 and 3.88 Å) despite the fact that at 14 °C the sample is below the crystallization temperature for all six polymorphs. Examination of the short spacing detailed in Table 1 confirms the latter. Finally, in Figure 6, the experimental conditions used in recording Figure 5 are reproduced for a sample containing a mixture of cocoa butter and milk fat, i.e., a sample representative of the kind of formulation present in confectionery products. Again, the nucleating phase is found
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Crystal Growth & Design, Vol. 3, No. 2, 2003 119 ally, some minor changes to the short spacings are observed but it is not clear from this work whether these are significant. Conclusion. This short study reveals the nucleating phase of butter fat to be consistent with its form III structure rather than form II, while the addition of milk fat was not found to appreciably alter the above situation. It should also be noted however, that the experiments conducted here only involved static samples and therefore are not representative of shearing experiments, i.e., it may still be possible that under sheared conditions, form II occurs rather than form III.
Figure 5. WAXS patterns shown over a 2θ range of 8-18° as recorded using station 9.1 at the Daresbury SRS. The main characteristic peak d spacings have been defined (units are in Å) for a pure cocoa butter sample after cooling from 50 to 14 °C.
Acknowledgment. This work was carried out via a grant (GR/K/42820) from the EPSRC’s soft solids processing initiative. S.D.M. gratefully acknowledges Cadbury Trebor Bassett for the award of a research studentship (MacMillan 2000). We are also grateful to CCLRC for beamtime on the Daresbury SRS.
References
Figure 6. WAXS patterns shown over a 2θ range of 8-18° as recorded using station 9.1 at the Daresbury SRS. The main characteristic peak d spacings have been defined (units are in Å) for a 30 wt %:70 wt % milk fat:cocoa butter mixed sample after cooling from 50 to 14 °C.
to be consistent with form III albeit with an enhanced amorphous content due to the milk fat component, which has a much wider range of triglyceride species. Addition-
(1) Davis, T. R.; Dimick, P. S. J. Am. Oil Chem. Soc. 1989, 66, 1488. (b) Davis, T. R.; Dimick, P. S. J. Am. Oil Chem. Soc. 1989, 66, 1494. (2) Dimick, P. S. Compositional Effect on Crystallisation of Cocoa Butter. In Physical Properties of Fats, Oils and Emulsifiers; Widlak, N., Ed.; AOCS Press: Champaign, 1999; Chapter 9, pp 140-163. (3) Dimick, P. S.; Manning, D. M. J. Am. Oil Chem. Soc. 1987, 64, 1663. (4) MacMillan, S. D. Studies of the Crystallisation of Mixed Confectionery Fats under Sheared Conditions using On-line Synchrotron Radiation SAXS Techniques, Ph.D. Thesis, Heriot-Watt University, 2000. (5) MacMillan, S. D.; Rossi, A.; Roberts, K. J.; Wells, M. A.; Polgreen, M. C.; Smith, I. H. Cryst. Growth Des. 2002, 2, 221. (6) van Gelder, R. N. M. R. Structuro-Kinetic Studies of the Crystallisation of Straight Chain Surfactants and Homologues Mixtures, Ph.D. Thesis, Strathclyde University, 1998. (7) van Malssen, K.; Pescher, R.; Schenk, H. J. Am. Oil Chem. Soc. 1996, 73, 1209. (b) van Malssen, K.; Pescher, R.; Schenk, H. J. Am. Oil Chem. Soc. 1996, 73, 1217. (8) van Malssen, K.; van Langevelde, A.; Pescher, R.; Schenk, H. J. Am. Oil Chem. Soc. 1999, 76, 669. (9) Wille, R. L.; Lutton, E. S. J. Am. Oil Chem. Soc. 1966, 43, 491.
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