Understanding the structure of chocolate
The following is a summary of the stated research mentioned above. The content summarized here, including the figures and tables, all belong to the researchers (unless otherwise indicated). The summary attempts to stay as close to the original paper as much as possible with some adjustments in regards to jargon, length, or to focus on bean to bar aspects.
Introduction
Fat bloom results when chocolate is not crystallized in the optimum form, or from storing chocolate in the wrong conditions.
Fat bloom appears usually as a grayish-white film on top of the chocolate surface. Bloomed chocolate looks unappealing, and also may have a texture that is less than desirable.
At the time of this publication, the researchers state no uniform nomenclature exists for the different phases of cocoa butter.
In this research, they used both Greek nomenclature (γ, α, β’, and β) and Roman numbering system (I-VI), which will result in phases γ, α, β’, and two β phases: β (V) and β (VI). Each phase has its unique melting range and stability.
All polymorphs of the cocoa butter crystallization have different patterns, and so X-ray powder diffraction (XRPD) was used to identify the fingerprint region of the cocoa-butter phases.
Molecular Models for cocoa-butter and butter from crystal structures of TAGs
Cocoa butter is made up of a mixture of about 40-50 TAGs (triacylglycerols). Three TAGs dominate, including POS (35%), SOS (23%), and POP (15%). P stands for palmitoyl (palmitic), O stands for oleoyl (oleic), and S stands for stearoyl (stearic).
Ideally, chocolate should be in the β (V) and β (VI) forms. By looking at Figure 4b, you can see that the layers of the β’ structure are more loosely packed than the β structure (Fig. 3).
Figure 5 shows the packing along the fatty acid chains are perpendicular to each other, a key feature of the β’ structure. However, Figure 3 (β phase) shows how all the chains are parallel to each other, which will result in closer packing of the crystals.
Therefore, by looking at the physical structures, we can understand why the optimum β forms of chocolate are harder, while the β’ form is softer.
The Isothermal phase-transition scheme of cocoa-butter
The phases of cocoa butter transition from less stable to more stable, are generally irreversible, and depend on temperature and time. The first few phases crystalize from molten (melted) cocoa butter, except for the two β phases.
Figure 6 illustrates a complete isothermal (constant temperature) phase-transition scheme under mechanically static conditions. It appears that phases β(V) and β(VI) were created via a transformation from the β’ phase.
They also state that phase β’ exists as a range, and within this range there is no isothermal phase transitions.
The B(V) and B(VI) Phases
Crystallization of cocoa butter into any of the β phases, directly from melt, is possible via the “memory” of cocoa butter.
Figure 7 displays the melting curve of β (VI) cocoa butter of Bahia, Brazil origin. Melting starts at 26°C and ends at 34°C. The cocoa butter is half-melted at 30.5°C. However, even when the cocoa butter is heated slightly above 34°C, there appears to be structural information about the crystalline state. Therefore, when the chocolate cools, it re-crystallizes into the β phase.
After extensive studying, they defined the β-memory point temperature (β-MPT) of a cocoa butter. This is the temperature in which a cocoa butter in the β phase has to be heated to prevent it from re-crystallizing into the β phase within 45 minutes after cooling to 25°C.
The β-MPT value of cocoa butter was found to be related to their composition, in particular the percentage of SOS (1,3-distearoyl-2-oleoyl-glycerol) and stearic acid.
To obtain structural information on the seed material initiating this re-crystallization, they performed a small-angle X-ray scattering station of the DUBBLE-beamline at the ESRF in Grenoble. Figure 8 illustrates this, where cocoa butter was heated 36°C (Tmax), cooled to 20°C (Tcryst), and then held at that temperature. The β-polymorph is produced within 10 minutes as seen in Fig. 8.
Crystallization of cocoa butter in the β phase directly from melt is only possible using the memory effect of cocoa butter. Both the Tmax and Tcryst impact re-crystallization behaviour. Depending on the maximum temperature before cooling (Tmax), cocoa butter re-crystallizes in the β(V) and/or β(IV) phase.
The diffraction patterns shown in Table 1 for the βphases depend on the composition of the cocoa butter and on the crystallization method used. Therefore, one cannot take any X-Ray powder diffraction pattern as standard for the β(VI) of all cocoa butters. From the re-crystallization experiments it was concluded that the seed crystals that initiate the re-crystallization to β(V) are likely different than the ones giving the β(VI) phase.
Re-crystallization of cocoa butter via its memory effect is similar to crystallization via seeding. In both cases:
crystal-packing information is present in the melt which directs the crystallization
In the seeding method: the crystal-packing information is directed by the milled cocoa butter (or pure TAGs in the desired phase) into the melted cocoa butter
In the memory effect: the crystal-packing information is still present
Both of these are different from a memory-free method, in which the crystal nuclei (from which other crystals will build upon) are formed by primary nucleation during the cooling of totally molten cocoa butter. The way of achieving this is through a structured technique of mechanical and temperature control of the molten cocoa butter or chocolate.
A New Way towards the production of high quality chocolate
The various solid phases of cocoa butter (polymorphism), has a big impact on the quality of the final chocolate product. Understanding the phase behaviour of cocoa butter is important to optimize production and maintain product quality.
Researchers here have designed and patented a new method to manufacture chocolate (Van Malssen et al., 2001). It replaces the traditional tempering step by an alternative pre-crystallization process based on the use of seed crystals in the liquid phase and driving a feedback system.
