Crystallization of cocoabutter
Polymorphism of cocoa butter
In the confectionery
industry, crystallization of cocoa butter (alone or in chocolate) is
carried out in two
steps:
(1) pre -
crystallization, or tempering;
(2) crystallization
by cooling (moulding etc.) and in storage.
Since cocoa butter
has six crystal modifi cations, the purpose of pre - crystallization
is to produce the
necessary amount of crystal seeds of the least unstable modifi
cation, β (V) (see below). Here, ‘
least unstable ’ means that this modifi cation remains unchanged
over several months.
To produce the
stable modifi cation β (VI) directly needs sophisticated technology,
and such technology is
not yet in everyday use, but investigations aimed at solving this problem are in
progress. If the correct technology is used, the proportion of the β
(V) modifi cation
generated by tempering is about 1 – 5%, and the proportion
generated by cooling is about 45
– 60%. Crystallization is finished in storage, when the proportion
of crystals of the β
(V) modifi cation increases to 60 – 80%.
Several authors have
discussed the crystalline forms of cocoa butter polymorphs (Duck 1964 , Wille and
Lutton 1966 , Huyghebeart and Hendrickx 1971, Lovegren et al. 1976, Dimick and Davis
1986, Jovanovic et al. 1995). For the determination of the melting points of cocoa
butter, see IOCCC Analytical Method 4 ( 1961 ). The data on the
melting points are rather different for the various crystal modifi
cations.
In the confectionery
industry, the data provided by Wille and Lutton ( 1966 ) are perhaps the most often used,
although in a ‘ mixed ’ form (Greek letter + numbering):
γ = I, 17.3 ° C;
α 1 = II, 23.3 °
C;
α 2 = III, 25.5 °
C;
β ′ = IV, 27.3 °
C;
β (V), 33.8 ° C;
β (VI), 36.3 ° C.
The idea of
polymorphic crystalline forms of cocoa butter – as well as of other
fats refers not to the
external microscopic or macroscopic geometrical appearance of the fat crystals but to the
internal structure of the crystals at a molecular level, i.e. the
packing of the triglycerides
in the molecular crystal lattice.
Figure 10.17
represents the transition β ′ (IV) → β (V) of the crystal
modifi cations of cocoa butter. The
transition β ′ → β is stimulated by shearing, which is caused
by strong mixing of chocolate
mass. The characteristic feature of the α (II and III) modifi
cations is that the TAGs start
to align along the axis of the fatty acids and a chair - type
arrangement is formed. (The
modifi cation I is designated by γ in the literature.) The β ′
(IV) modification is more
compact, its consistency is harder and two chairs form one bond (DCL arrangement). The
characteristic feature of the β (V) modifi cation is a compact
structure in which three
chairs form one bond (TCL arrangement). In the modifi cation
β (VI), which is the stable one and evolves over weeks or months, the consistency
becomes more compact through the development of a curved tuning -
fork
shape of the parts
of the TAGs where the oleic acid groups are located.
Tempering of cocoa butter and chocolate mass
The crystallization
of cocoa butter or chocolate mass, usually containing about 28 –
38% cocoa butter, means
the solidifi cation of the material in such a way that the cocoabutter is crystallized in
the form of the β (V) modifi cation. The series of operations starts
with tempering, the next
operation is the shaping of the cocoa butter or chocolate mass and, finally, the
operation of cooling finishes this series.
In the following
descriptions, the tempering of cocoabutter and of chocolate mass are presented together.
However, there is an important difference: since the contraction of cocoabutter in a
chocolate mass is proportional to the volume ratio of cocoabutter,
the contraction of a
cocoa butter bar is about three times higher than that of a chocolate
bar, assuming that they
are of the same volume. Therefore, the moulding of cocoa butter bars, which is a
relatively rare task, needs more cautious cooling because the bars
can crack. The risk of such a
phenomenon is less in the case of the moulding of chocolate mass.
Tempering
From the point of
view of the technology, the control of the transitions α (III) → β
′ (IV) → β (V)
plays an essential role. This is the tempering operation. At the end
of tempering, all of
the β ′ (IV) modifi cation has to be melted and, at the same time,
tempering must provide a
seed concentration of the β (V) modifi cation of 0.1 – 1.15% of
the cocoabutter mass
according to Loisel et al. ( 1997 ). Jewell ( 1972 ), however,
reported that larger amounts of seeds, 2
– 5% of the cocoa butter, were needed for good temper. According to Lonchampt and Hartel
( 2004 ), this difference may be due to differences in seed size,
which affects the number
of seed crystals. Von Drachenfels et al. ( 1962 ) specifi ed the
importance of crystal size. The
smaller and more regular the size of the seed crystals, the glossier
the chocolate and the
greater its bloom resistance. On the other hand, if the crystal size
is too large, the crystals
tend to recrystallize during storage. It was mentioned
above that the transitions from modifi cation I to modifi cation VI are increasingly
slow. At the beginning of the cooling of cocoa butter, the γ (I) and
α (II and III) modifi
cations occur but they change rapidly to the β ′ (IV) and β (V)
modifi cations. For details,
see Ziegleder ( 1988 ).
Since the
crystallization of cocoa butter follows monotropic polymorphism, the
direction of the changes
is exclusively γ (I) → α (II) → α (III) → β ′ (IV) → β
(V) → β (VI). Moreover, under the
usual conditions all the modifi cations can be crystallized directly
from molten cocoa butter
except for β (VI), which crystallizes slowly from the β (V) modifi
cation (Fig. 10.18 ).
The stable form β
(VI) cannot be produced directly from melted chocolate except by the addition of β
(VI) cocoa butter seeds and under very well - controlled conditions (Giddey and Clerc
1961 , van Langevelde et al. 2001 ). It should be emphasized that the target of tempering
is to bring about the β (V) modifi cation, which is unstable,
although its transition to
the stable β (VI) modifi cation is very slow: it needs weeks or
months. During these
monotropic changes the Gibbs free enthalpy decreases continuously;
its minimum is reached
in the β (VI) modifi cation.
However, if the
tempering results in a majority of crystals of the β ′ (IV) modifi
cation, the transition β ′
(IV) → β (V) will take place in the chocolate product within hours
or days, and the
consequence of such a transition will be the appearance of fat bloom
on the surface of the
chocolate product. This is a severe quality defect, called blooming . Taking into account
all the considerations above, the principle of the tempering process is to produce the β
′ (IV) and β (V) modifi cations, and then to melt the β ′ (IV)
modifi cation while the β
(V) modifi cation is retained. Although the β (V) modifi cation can
be produced directly
from a molten chocolate mass, such a direct method cannot exclude the development of
crystals of the β ′ (IV) modifi cation. A warming period necessary in the tempering
operation which destroys the crystals of the β ′ (IV) modifi
cation – this is the way to avoid fat
bloom.
Figure 10.19 shows
the temperature profi le of a correct tempering operation for chocolate mass, which
consists of three steps: two steps of cooling and one step of warming between them. The
traditional tempering machine is similarly partitioned in the
direction of advance of the
chocolate mass. It is evident that a simple conical double - jacketed chocolate tank with
a mixer is hardly suitable for performing tempering correctly,
because it is diffi cult to
carry out the warming phase. Strong mixing of the
chocolate mass during tempering promotes the development of crystals of the β
(V) modifi cation by the shearing effect. The measurement of
tempering, for which the ‘ temperimeter ’ is a practical
instrument, provides important
technological parameters. This instrument includes a small vessel, which the tempered
chocolate mass is poured into. The vessel is placed in an ice –
water bath, and the
temperature of the chocolate mass is measured as a function of time.
The resulting
temperature vs time plots are represented in Fig. 10.20 .
The curve for a well
- tempered chocolate mass is characterized by a horizontal line :
the amount of
crystals of the β (V) modifi cation is suffi cient, and in the time
interval represented by this
line the latent heat generated by crystallization (an exothermic
effect) and the cooling
effect of the bath (an endothermic effect) are in balance.
Consequently, the temperature does
not change in this interval.
When the chocolate
mass is undertempered, too many crystals of the β ′ (IV) modifi
cation develop, which
rapidly transform to the β (V) modifi cation. Consequently, the
latent heat dissipated by
their crystallization exceeds the cooling effect of the bath.
Therefore, an increase in
temperature occurs. When the chocolate mass is overtempered, too many crystals of the β
(V) modifi cation develop, which melt too slowly to compensate the cooling effect of
the bath. Consequently, the temperature decreases continuously. In many
publications, bloom in chocolate is often described as a process
involving the migration by
capillary action of a liquid fat to the surface (Kleinert 1962 ).
Loisel et al.
( 1997 ) considered
chocolate as a porous material and were able to determine, by mercury porosimetry, the
porosity volume of well - tempered dark chocolate [ β (V)],
undertempered chocolate [ β (IV)]
and overtempered chocolate [a mixture of β (V) and β (VI)]. The
volume of air bubbles due
to the process was determined by X - ray radiography to be less than 0.1% of the sample
volume. The porosity of normal chocolate was about 1% of the total volume, and
this increased to 2% for the undertempered chocolate and 4% for the overtempered
chocolate. The results did not allow determination of the precise
pore diameter, but
suggested that the chocolate did not have open, interconnected pores
with a mean diameter
larger than 0.1 μ m at the surface. Moreover, it seems that the
pores were fi lled by the
liquid fraction of cocoa butter at room temperature. As a result, it
is better to talk about empty
cavities rather than pores. Khanet al. ( 2003 ) highlighted the
presence of pores at the
surface of milk chocolate by scanning the surface with an atomic
force microscope. These
authors estimated the concentration of pores to be thousands/cm 2 ;
the pores, 1 – 2.5 μ
m in depth, were randomly distributed on the surface. As mentioned
previously, a preferable method of crystallization from melts is to
add crystal seeds of the
stable modifi cation to the molten substance, which start an overall crystallization in
the stable modifi cation. This is the principle of the Seedmaster
tempering machine manufactured
by Bindler, in which crystals of the stable β (VI) modifi cation are produced by
intensive shearing ( ‘ Seedmaster cryst ’ ), and the pre -
tempered chocolate mass is seeded by these
stable crystals in the Seedmaster mix.
Beside cocoa butter,
several types of chocolate may contain milk fat (milk chocolate) and/or oils derived
from added almonds or hazelnuts (dark and milk chocolate) if these
nuts are refi ned
together with the chocolate mass. Since the properties of these
fats/oils are essentially
different from those of cocoa butter, they can exert an important
effect on the crystallization of
cocoa butter in chocolate. As a rule, it can be stated that in the
case of milk fat, almond oil
or /hazelnut oil the end point of cooling will be ∼ 26 ° C instead
of 27 ° C, and the end
point of warming will be 29 – 31 ° C instead of 30 – 32 ° C.
The decrease in temperature that is
to be used is dependent on the amount of these fats/oils. For further details, see Kniel (
2000 ) and McGauley ( 2001 ).
Fonte:
Confectionery and
Chocolate Engineering / Principles and Applications
Professor Ferenc Á.
Mohos, PhD
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