Hoofdpagina van Kristallografie. Summary
Chocolate and chocolate products are favourite confectioneries and hence
they are produced at large scales. However, badly processed or stored chocolate
may result in the formation of bloom, a greyish-white appearance at the chocolate
surface, causing the products to appear aged and musty. Therefore, chocolate
manufacturers receive complaints of many consumers, who doubt the quality
of their products.
The main components of chocolate are cocoa butter, cocoa powder and sugar.
The formation of bloom is caused by physical changes of either the fat part
(mainly cocoa butter) or sugar, referred to as fat bloom and sugar bloom respectively.
In contrast to sugar bloom, which arises during storage from changes of sugar
crystals due to moisture, fat bloom may result from badly crystallized cocoa
butter in the course of the production process. Since cocoa butter shows
polymorphic behaviour, it may crystallize in various crystalline phases (γ,
α, β’, β(V) and β(VI)), which each have its own physical properties. Cocoa
butter crystallized in an unstable (γ, α) or metastable phase (β’ and β(V))
may undergo unwanted phase transitions during storage. Therefore, crystallization
of cocoa butter is a key step in the production of quality chocolate.
Cocoa butter is a complex mixture of about thirty different types of triacylglycerols
(TAGs) though also small amounts of some other components are present. The
physical properties of TAG mixtures such as cocoa butter are largely determined
by the physical properties of the individual TAGs constituting the fat. TAGs
are esterifications of glycerol with three long-chain fatty acids and since
each chain may differ in chain length and degree of saturation, many different
types of TAGs exist. These hydrocarbon chains can easily pack in different
ways resulting in the various polymorphic phases. With X-ray powder diffraction
(XRPD) these phases can be identified unambiguously, because the various packing
modes result in characteristic peaks in the 3 – 6 Å region of the XRPD
pattern.
To make quality chocolate which is free of fat-bloom formation, the cocoa
butter of chocolate should crystallize directly in the β(VI) phase. A better
understanding of the irreversible β’ ® β and β(V) ® β(VI) phase transitions
of cocoa butter at the molecular level and of the experimental conditions
at which the phase transitions occur are essential to realise this. Since
the physical behaviour of cocoa butter is determined by the properties of
its individual TAGs, insight in the phase-transition mechanisms of TAGs at
the atomic scale is recommended. Therefore, accurate three-dimensional atomic
models of different types of pure TAGs in various phases are necessary. These
models are very scarce, due to many problems such as the absence of good single
crystals, so a major research effort has been put into crystal-structure determination
of TAGs and, in particular, of TAGs crystallized in the β’ phase. Molecular
models of the various cocoa-butter phases and their transitions can not be
constructed yet. Therefore, the second part of the research focused on the
crystallization conditions of the various phases and, in particular, on the
dependence of time and temperature of the re-crystallization behaviour of
cocoa butter.
In Chapter 2 the crystal structure of β-1,2,3-tri-hexadecanoyl-glycerol
(β-PPP), determined from single-crystal X-ray diffraction data, is described.
This molecule is crystallized in space group P1bar in an asymmetric tuning-fork
conformation. By assuming appropriate unit-cell transformations it is shown
that this structure and the two other known crystal structures of this β-CnCnCn-type
(n = even) TAGs series form a homologous series. Using these three structures
an overlap model has been build from which the structure of another series
member is predicted. This implies that with one known crystal structure of
a homologous series, the structure of the other series members may be determined
from high-resolution XRPD data. In this way no exhausting efforts have to
be undertaken to grow measurable single crystals for all series members. To
show the effectiveness of this approach in Chapter 3 the crystal structures
of β-1,2,3-tri-tetradecanoyl-glycerol (β-MMM) and β-1,2,3-tri-octadecanoyl-glycerol
(β-SSS), determined from high-resolution synchrotron XRPD data, are discussed.
Grid-search techniques and Rietveld refinement have been used to determine
and refine the structure respectively.
In Chapter 4 a second example of this approach is described, the crystal
structure of β-1,2,3-tri-tridecanoyl-glycerol (β-C13C13C13), an odd-numbered
member of the CnCnCn-type TAG series, as determined from high-resolution synchrotron
XRPD data. This compound, the first known crystal structure of an odd-numbered
TAG, crystallized in space group P1bar and in a tuning-fork conformation
as well. The molecular packing within a layer of this odd-numbered member
β-CnCnCn-type series is identical to the packing of the even-numbered ones.
However, the mutual position of two adjacent layers and, consequently, the
methyl-end group region, is different for the odd and even-numbered series
members. On basis of this information melting-point alternation of odd and
even-numbered β-CnCnCn-type series members is explained.
In Chapter 5 unit-cell parameters and space group (Ic2a) are presented of
the β’-stable CnCn+2Cn-type (n = even) TAG series. A packing model is constructed
under the assumption of straight TAG molecules so implying that the acyl chains
of this structure are not tilted with respect to the methyl-end group plane.
As a result, with only one molecule in the asymmetric unit, overall orthogonal
chain packing is obtained while the intramolecular acyl zigzag planes are
parallel. Chapter 6 describes the first detailed crystal-structure analysis
of TAGs in the β’ phase, both members of the β’-CnCn+2Cn-type (n = even)
TAG series. One compound, β’-1,3-di-decanoyl-2-dodecanoyl-glycerol (β’-CLC),
has been established from single-crystal synchrotron data and the other,
β’-1,3-ditetradecanoyl-2-hexadecanoyl-glycerol (β’-MPM), from high-resolution
synchrotron XRPD data. Both are crystallized in space group Iba2 in a chair
conformation having a bend at the glycerol moiety. On basis of these crystal
structures the differences between the β’ and β phase are discussed.
Chapter 7 gives a complete isothermal phase-transition scheme of cocoa butter
under mechanically static conditions, as obtained from real-time XRPD measurements
during crystallization of cocoa butter at various temperatures. Both the β(V)
and β(VI) were obtained directly through transformation of the β’ phase, which
exists as a phase range rather than as separate phases. The observed phase
behaviour of cocoa butter is explained on basis of the concept of individual
crystallite phase behaviour of cocoa butter. During this extensive study no
direct crystallization of cocoa butter from the melt in the most stable β
phase has been observed.
Applying re-crystallization of incomplete molten cocoa butter, where remaining
crystalline material initiates the crystallization, the β phase can be crystallized
directly. This re-crystallization behaviour, influenced by the maximum and
crystallization temperatures, is the subject of Chapter 8. Depending on these
parameters, rapid-starting re-crystallization into the β(VI) phase and slow-starting
re-crystallization into the β(VI) phase have been monitored using real-time
XRPD measurements. It is concluded that rapid-starting re-crystallization
is induced by high-melting SOS-rich crystals.
To characterize the cocoa-butter phases and the seed material initiating
the re-crystallization process in more detail, the long d-spacing region (40
Å < d < 70 Å) of crystallizing cocoa butter has been monitored
with small-angle X-ray scattering stationed at the ESRF. From the re-crystallization
experiments described in Chapter 9 it is concluded that the seeds initiating
rapid-starting re-crystallization have an SOS-dominated triple chain-length
packing. Furthermore, the β(VI) phase of cocoa butter seems to a adopt a
similar packing. The seed crystals that initiate the slow-starting re-crystallization
and results in the β(V) phase are likely to be different from those giving
the β(VI) phase.
The knowledge of the polymorphic behaviour of cocoa butter obtained in this
study has been utilized to develop a new chocolate manufacturing process.
Since this newly developed process is still in a patenting procedure, it is
not described in this thesis.
Overzicht proefschriften kristallografie Amsterdam.
Hoofdpagina van Kristallografie.