1
Milling, mixing and tempering—an engineering
view of chocolate
S T Beckett
Nestlé Product Technology Centre, PO Box 204, York YO91 1XY, UK
Abstract: The confectionery market in the UK is larger than that of tea, newspapers and bread put
together. Chocolate accounts, to some degree, for nearly two-thirds of this market and, as its manufacture is relatively complex, carefully controlled processing is required. These processes include,
grinding, mixing and crystallization. This review outlines the most common methods of making
chocolate by describing the machines that are used and explaining what the individual processes are
designed to achieve.
Keywords:
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chocolate, roasting, mixing, milling, crystallization, viscosity, particle size
INTRODUCTION
Most people probably think that chocolate is manufactured as in Willy Wonka’s factory [1] and expect
chocolate waterfalls, etc., but it is, in fact, a very complicated procedure compared with many industries. The
making of a bar of chocolate involves many processes.
Each in its turn is critical in producing this unique
product, with its pleasant avour and texture, which is
hard to bite and yet melts easily in the mouth. Unlike
many processed foods, e.g. potato crisps or breakfast
cereals, the cost of ingredients is relatively high. This
means that the process machinery must be designed to
minimize costs and to optimize the use of the more
expensive ingredients.
It is important to realize what chocolate is made of, in
order to understand what makes it so special (Fig. 1). To
be called chocolate, a product must in part be made from
the components of cocoa beans. These beans consist of
about 55 per cent cocoa butter, a fat that is solid at room
temperature and yet melts almost entirely below body
temperature. The remainder is solid material composed
of cellulose, proteins and carbohydrates, which make up
the major proportion of cocoa powder. Dark chocolate is
chiey manufactured from these materials together with
sugar, while milk chocolate also contains dehydrated
cows’ milk. White chocolate is made from the same
ingredients as milk chocolate, but without the brown nonfat cocoa particles (cocoa powder).
In order to obtain the correct avour and texture, solid
particles must be ground to a size smaller than about
30 mm, as larger particles feel gritty in the mouth. In
addition, they must all be coated in fat (which forms the
continuous phase of chocolate) so that the chocolate will
melt smoothly. This fat must also be crystallized
correctly so as to give the product a good gloss and a
sharp snap when it is broken at ambient temperatures.
It is interesting to note that, in terms of their original
ingredients, almost all chocolates are the same. There are,
however, many different textures and avours. These are,
Fig. 1
The MS was received on 21 September 1999 and was accepted after
revision for publication on 15 May 2000.
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A simplied schematic view of a section through a
piece of chocolate showing the solid milk, cocoa and
sugar particles and the continuous fat phase
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to a large extent, due to the variety of processes and
machinery that are used to produce them.
A general overview of chocolate processing is given in
Fig. 2. Fuller details about chocolate manufacture can be
obtained from references [2] to [4].
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PRODUCTION OF COCOA LIQUOR (MASS)
The majority of chocolate in the United Kingdom is
manufactured from cocoa beans grown in West Africa
and, in particular, Côte D’Ivoire, Ghana and Nigeria. The
beans are fermented to form the pre-cursors of the
chocolate avour and then dried before being transported
to cocoa-processing factories.
Although the growing region of the bean, the cocoa
type and the growing conditions all affect the product
avour, especially of dark chocolate, the subsequent
roasting process is critical in determining the correct
avour. The roasting temperature and duration both have
a signicant effect, as does the moisture content of the
bean and the surrounding air.
Traditionally beans are roasted whole, at temperatures
of about 105–145 °C for an hour. The machinery used
may be a rotating drum, which can be heated externally
or can have hot air passing through it. Alternatively,
continuous processes are available, such as that illustrated in Fig. 3. Here the beans are heated on a tray,
before falling on to the tray below when the tilting slats
open. Each of the series of trays can be individually
temperature controlled. This treatment develops many of
the chemicals required to give the chocolate its desired
avour, together with some unpleasant avours, which
are removed by subsequent processes. In addition it
loosens the shell around the outside of the bean, making
the shell easier to remove.
Whole bean roasting has two disadvantages, however.
Firstly, beans have a range of sizes so that, if the roaster
conditions are set for the mean size, the small beans may
be burned and the larger beans under-roasted (Fig. 4).
Secondly, some of the cocoa butter, which is the most
expensive part of the bean, melts and migrates into the
shell during the roasting. This shell is subsequently
removed and thrown away.
In order to overcome this, two alternative processes
have been developed. Both begin by removing the shell.
This is not easy as it is relatively rmly attached to the
central cotyledons, known in the industry as cocoa nibs.
This separation must be carried out relatively carefully as
legislation limits the maximum amount of shell that can
Fig. 3
Fig. 2
Schematic diagram of the processes used to produce
cocoa powder and to manufacture liquid chocolate for
sweet making
Proc Instn Mech Engrs Vol 215 Part E
Diagram of machine to roast cocoa beans or cocoa nibs
continuously at rates of up to several tonnes per hour.
(Lehmann Maschinenfabrik GmbH, Germany): A,
product feed; B, feed rollers; C, exhaust air fan; D,
air heater; E, air lter; F, extraction screw. (Reproduced from reference [2] by permission of Blackwell
Science Limited)
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MILLING, MIXING AND TEMPERING—AN ENGINEERING VIEW OF CHOCOLATE
Fig. 4
Diagram to illustrate that uneven roasting (and hence
avour differences) can arise due to variations in
cocoa bean size
be present in chocolate. Just as importantly the shell itself
is very hard and will damage any milling machines that
are used to grind the cocoa. In order to aid the release of
the shell, the beans are normally heated very rapidly
using steam or infrared radiation. This causes the
moisture in the centre of the bean to evaporate, expanding
the shell away from the nib. The beans are then broken
and passed through winnowing machines. There the
broken beans are size selected on vibrating sieves, before
being subjected to rapidly rising currents of air. The light
platelet-shaped shell particles tend to rise and are ltered
Fig. 5
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off, whereas the denser round pieces of nib fall into
collecting channels.
These pieces of nib can then be roasted in batch or
continuous machines of similar design to those used for
whole beans. Alternatively, they can be ground to form a
material known as cocoa liquor. This is a liquid at temperatures above about 35 °C and can be roasted using
thin-lm heating devices.
The cocoa butter is contained in cells within the nibs.
These cells, which are mainly smaller than 25 mm (Fig.
5), must be broken to release the fat, which will then help
the chocolate to ow once it melts within the mouth.
Because the fat is the most expensive part of the
chocolate, it is important to release as much of the cocoa
butter as possible. This means that mills must be able to
crush and shear particles with diameters of 5 mm or
more, down to diameters of less than 25 mm. Within the
operating capabilities of most commercial mills, the ner
the particles become, the lower will be the viscosity of
the cocoa liquor. This is in total contrast with chocolate,
which becomes thicker upon grinding, largely due to the
breakage of the sugar particles, which creates new surfaces but does not release any fat. This will be explained
in more detail in Section 4.
Because the milling takes place over such a large range
of particle sizes, it is normally carried out in two or more
stages. The majority of the world’s cocoa is ground using
ball mills. These, however, need a liquid feed material
and operate more efciently below about 200 mm. The
nibs are therefore pre-ground using hammer or pin mills,
which are designed for larger solid particles and produce
a liquid containing relatively large cocoa particles. Many
Section of cocoa bean as seen through a microscope. The individual cells give a net-like appearance.
These cells are approximately 25 mm by 10 mm and can be lled either by large droplets of cocoa butter
(stained black) or by small droplets of fat (small dark spots) within a continuous water phase
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different types of ball mill exist, but a typical one found
in the industry is show in Fig. 6. Where a small particle
size is required, two ball mills may be used in series, the
second having much smaller balls, so as to optimize its
performance with the smaller cocoa particles. Many other
types of mill are also used, with disc mills (based on the
original stone mills) and roll mills being relatively
common.
3
COCOA BUTTER PRODUCTION
Dark and milk chocolates are made from cocoa liquor,
but the fat that it contains is insufcient to coat the
remaining milk and/or sugar particles and additional
cocoa butter is required. This is obtained by pressing the
liquid cocoa liquor. The liquor contains about 55 wt %
fat, and about 35–45 per cent is normally pressed out,
leaving a solid ‘cake’ material with about 10–20 per cent
of the remaining fat. (Solvent extraction is used to
produce lower fat power). This ‘cake’ is then ground to a
powder, which is used to make drinks, or chocolateavoured coatings and compounds. In the latter, the
cocoa butter is replaced by fats obtained from other
sources such as palm kernel and/or illipe [2].
4
CHOCOLATE GRINDING
The production of cocoa liquor means that the vast
majority of the cocoa powder particles are small enough
to produce chocolate, but the sugar and milk components
still require grinding. This can be carried out separately
on a hammer-type mill, or more commonly together with
the liquor and some of the cocoa butter on a roll rener,
where the particles are broken by shearing them in the
gap between counter-rotating rolls. It is interesting to
note that the two processes tend to give chocolates with
different avours. This is said to be because, as sugar is
broken into very ne particles, the surface becomes very
hot and changes from a crystalline to a glassy structure.
Although this eventually reverts to crystals, the surface
is, for a short period, very chemically reactive and
absorbs any avours in the neighbourhood. If cocoa and
sugar are being milled together, cocoa avours are
absorbed on to the sugar, changing the avour of the
chocolate, compared with separate milling.
Once again the range of milling is from several
millimetres to less than 30 mm; therefore a double-milling
procedure is desirable. This is particularly important
because the particle size distribution greatly affects the
ow properties of molten chocolate, as well as its
eventual taste and texture in the mouth.
If there is a very high proportion of small particles
present, they have a large surface area and much more
Proc Instn Mech Engrs Vol 215 Part E
cocoa butter is required to coat them with fat, thus
enabling them to ow past one another. This means that
the chocolate often feels pasty in the mouth and harder to
swallow unless extra fat is added. It is also desirable to
have all the particles small enough for the required
product texture. This normally ranges from 20 to 30 mm,
with the ner size often being found for chocolate tablets
and the coarser size for chocolate coatings on larger
components such as biscuits and coconut. The tongue is
very sensitive and differences in maximum particle size
(traditionally the 90th percentile is quoted) of 3 mm can
often be detected. This means that a narrow particle size
distribution is required, centred below the desired
maximum size, and with a minimum proportion of very
ne sugar or milk particles.
Traditionally the sugar was pulverized in a hammer
mill to about 150 mm, before being fed to a ve-roll
rener for comminution to the nal particle size. It has,
however, become more common to use two-roll reners
in series; in particular a single two-roll followed by three
or more ve-roll reners (Fig. 7). The ingredients,
including granulated sugar are pre-mixed before being
fed into the two-roll rener, which turns them into a paste
with a maximum particle size of about 150 mm. This is
fed directly into ve-roll reners. This process gives a
narrower particle size distribution than the traditional
distribution and can also operate at a lower fat content,
which can be an advantage for subsequent processing.
The ve-roll rener has ve barrel-shaped rollers,
which are normally between 800 and 2500 mm wide and
Fig. 6
Schematic diagram of a typical ball mill used to
produce cocoa liquor. The column is packed with
balls, which vibrate against each other when the
central shaft turns
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MILLING, MIXING AND TEMPERING—AN ENGINEERING VIEW OF CHOCOLATE
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and milk particles with fat, many newly broken surfaces
remain uncoated. In addition, some unwanted avours
are still present. The machine used to liquefy the chocolate and to adjust the avour is known in the industry as a
conche, because the rst machines developed by Lindt in
1876 were the shape of a conche shell.
5
Fig. 7
Illustration of a two and ve-roll rener system used to
grind chocolate. The ingredients are put into the mixer,
which discharges into a two-roll rener. This in turn
feeds three ve-roll reners in parallel
are approximately 400 mm in diameter. These rolls
become parallel under the pressure of operation. The
particles are broken by the shearing action between the
two counter-rotating rolls. The gap between them
becomes narrower until the top gap is the same size as
the largest particles within the chocolate, i.e. less than
30 mm. The material, with a maximum particle size of
around 150 mm, is fed in between the two lower rollers
and is transferred up the ‘stack’ due to the increased
speed of the next roller. The lm of material becomes
thinner by a factor related to the speed difference. The
ratio of roll speeds in fact is one of the ways of
controlling the particle size of the machine. This is
normally between 1.5 and 1.7 with the roll speeds varying
from less than 50 r/min to greater than 400 r/min. In some
ve-roll reners the rolls have set speeds and in this case
the maximum particle size is manipulated by adjusting
the feed gap between the rst two rolls. The greater this
feed gap, the larger will be the particles between the top
two rolls. The pressure between the rolls does not control
the particle size but is just used to ensure a uniform
coating of the rollers by the chocolate. The Bühler
company [5] in Switzerland have produced an automated
feedback system, in which a thickness-measuring device
on the nal roll gives a measurement which adjusts the
relative speeds of the rst two rollers.
The viscosity of the chocolate is critical, however, and,
if there is insufcient fat to bind the solid particles into a
lm, the material may be thrown from the rollers. On the
other hand, if the feed material is very thin, because too
much fat is present, some of the sugar and milk particles
will segregate out in the hopper, resulting in uneven
grinding. Temperature has a very large effect upon the
ow properties of this fat and so accurate cooling and
heating of the rolls is required to obtain an optimum
particle size distribution within the chocolate.
A knife edge removes the ground particles from the top
roller. The chocolate is then in a powdery form. Although
the shearing action of the rolls coats some of the sugar
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CONCHING
Conching is essentially a mixing process, which normally
takes place in three phases:
(a) dry phase,
(b) pasty phase and
(c) liquid phase.
Moisture is highly detrimental to the ow of liquid
chocolate, because it causes the sugar particles to stick
together. Very approximately, for every extra 0.3 per cent
of moisture present in the chocolate, it is necessary to add
1 per cent of extra cocoa butter. Because this fat is
relatively expensive, it is desirable to remove as much
water from the chocolate as possible. Moisture is present
in the cocoa and milk ingredients and is most easily
removed before the particles become coated with fat, i.e.
in the initial dry phase. This moisture takes with it many
of the undesirable highly acidic avours, which have
been formed during the fermentation and roasting of the
beans. This means that this early phase of the processing
needs a machine that is well ventilated, but it must also be
highly temperature controlled. The latter is because, if the
chocolate heats too rapidly, the moisture cannot escape
into the air quickly enough and instead sticks some of the
sugar particles together to form agglomerates. This will
cause the nal chocolate to taste gritty in the mouth. In
addition, too high a temperature will introduce a cooked
avour into the product due to Maillard-type reactions
[2]. As the temperature rises, the fat becomes more liquid
and starts to coat the solid particles. As it does so, the
viscosity and power input increase dramatically as the
chocolate changes from a powder into a thick paste (Fig.
8a) but then falls again as more particles become coated
with fat. The actual nal viscosity of the chocolate
depends upon how much work the mixer can put into the
paste. A typical batch conche is shown in Fig. 9. Here the
wedge-shaped cutters on the ends of the motors smear the
paste against the sides during this initial period. As it
becomes thinner, the rotors can be reversed, giving a
higher shear against the at ends of the wedges. In
addition, the speed can also be increased. This accounts
for the different peaks in the power versus time curve
shown in Fig. 8a. For much of the process, however, the
conche is operating well below its maximum work input.
More modern control devices, however, enable conche
speeds to be varied so as to give the maximum input for
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most of the process (Fig. 8b). This has led to lowerviscosity chocolates (at the same fat content) and/or
considerably reduced conching times.
Even after most of the moisture has evaporated from
the chocolate, avour compounds continue to be removed
and/or developed, although at a lower rate. The chocolate
remains as a thick paste until the liquid phase, which is
designed to give the chocolate its correct ow properties
for subsequent processing. This is achieved by making
further cocoa butter and emulsier (usually lecithin)
additions. The chocolate very rapidly becomes much
thinner, as the conche mixes in this extra liquid component, and within a relatively short time it can be easily
pumped into storage tanks.
There are a wide variety of conche designs, some batch
and some continuous. The process can also be divided
into different parts. Some manufacturers pre-treat their
ingredients so as to reduce conching to a liquefying
procedure. Others use an initial batch machine for mixing
and developing avour, but then pass the paste through
high-shear devices to produce efcient liquefaction.
6
Fig. 8
(a) Typical power versus time curves for a traditional
conching process where the mixer shafts have two
speeds and can be driven in either direction. (b) Power
versus time curve for a conche with feedback control,
which enables the speed of the mixing arms to be
varied according to the viscosity of the chocolate
paste. (Reproduced from reference [2] by permission
of Blackwell Science Limited)
Fig. 9
Diagram of a batch chocolate conche as manufactured
by Richard Frisse GmbH. (Reproduced from reference
[2] by permission of Blackwell Science Limited)
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PRE-CRYSTALLIZATION (TEMPERING)
Just as carbon atoms group together in different ways to
produce substances with a wide range of textures and
surface properties (graphite to diamond), cocoa butter
also crystallizes in six different forms (Fig. 10). Forms I
to IV would produce a crumbly product, without any
gloss or snap. The fat would also soon migrate to the
surface, giving a white sheen known as chocolate
‘bloom’. In addition, the chocolate expands initially
upon setting, as the crystals are not tightly packed. This
means that, if a liquid chocolate is in the wrong crystalline form when it is poured into a mould, it is very
difcult to get the solid product to come out again. For
this reason the chocolate must be seeded with form V
Fig. 10
Illustration of the different crystalline states of cocoa
butter. Form V is the one required for chocolate
making
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MILLING, MIXING AND TEMPERING—AN ENGINEERING VIEW OF CHOCOLATE
crystals. Form VI does not normally crystallize directly
from the liquid chocolate but is a solid–solid transformation with time, also giving rise to chocolate bloom as it
does so.
If chocolate is maintained at the appropriate temperature for many hours, form V crystals will form, but these
will probably just be a few large crystals. What is
required is a large number of small well-distributed
crystals. Chocolate containing these crystals must be
produced at the rate in which it is being used to make
confectionery products, which may be more than 1 t/h. It
is known that, the higher the shear rate that is applied to
the cooled chocolate, the faster the cocoa butter crystallizes. There is, however, a limit, as shear produces heat,
which then melts the crystals. Any device for generating
these seed crystals, known in the industry as a temperer,
must take this into account.
In order to obtain the throughputs required, the whole
process is accelerated by cooling under shear to produce
both stable and unstable crystals. The chocolate, still
under shear, is then reheated to a temperature corresponding to form V. This then melts the unstable crystals,
transforming some into the stable state.
This process is normally carried out in a series of
scraped heat exchangers (Fig. 11). The scraper arms are
modied to provide higher shear rates. The retention time
within the machine is frequently of the order of 5 min.
Each of the shearing elements is temperature controlled
to within a fraction of a degree. Often the machines have
three temperature sections; to provide the initial cooling,
to strike many seed reheating crystals and then to reheat
in order to remove the unstable crystals. In some designs,
each element is individually controlled. Other designs
add streams of uncrystallized chocolate to already seeded
chocolate, in order to increase the throughput.
7
7
order to produce the required shell, the mould is rstly
lled with tempered chocolate and then allowed to set
partly. The mould is then inverted to enable the centre
chocolate to run out. The mould is then turned back
again, lled with the centre material and more chocolate
poured over the top. This requires very careful viscosity
control; otherwise the shell may be uneven or of very
variable weight.
The enrobing process involves passing the centre of the
product through a curtain of tempered chocolate—almost
Willy Wonka’s waterfall. The centres are placed on an
intermeshing wire belt, which passes over a tray of
chocolate as well as the curtain so that they are
completely coated (Fig. 12). Excess chocolate is then
removed by blowing, vibration and rotating rollers
touching the belt. The product is then cooled as for the
moulded product.
The excess chocolate that comes off the sweet falls
CHOCOLATE USAGE
Having manufactured and tempered the liquid chocolate
it must then be formed into the solid product. This is
normally carried out on a moulding or an enrobing plant.
In a moulding plant, the chocolate is poured into preheated plastic moulds, vibrated to level it out and to
remove air bubbles and then allowed to set in a cooling
tunnel. The loss of heat is largely by convection; therefore a good air ow is required. Too low a temperature
can result in the chocolate setting in the wrong crystal
type and subsequent blooming. In addition, if the temperature falls below the dew point, moisture can condense on the chocolate surface, which then dissolves
some of the sugar in the chocolate. Upon subsequent
evaporation, this also gives a white sheen on the surface,
called ‘sugar bloom’.
Many products have hollow centres, e.g. Easter eggs,
or contain another material such as toffee or biscuit. In
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Fig. 11
Schematic diagram of an Aasted tempering machine.
When the central shaft rotates, the attached discs
shear the chocolate between themselves and the
stators, which are xed to the wall. The temperature
of the individual stators is carefully controlled.
(Reproduced from reference [2] by permission of
Blackwell Science Limited)
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Fig. 12
Diagram of a typical chocolate enrobing system: 1,
drive wire conveyor belt; 2, stirred reservoir tank; 3,
chocolate pump; 4, riser pipe; 5, top ow pan; 6,
surge roller trough; 7, air nozzle; 8, shaker; 9, licking
rollers; 10, heated trough. (Reproduced from
reference [2] by permission of Blackwell Science
Limited)
back into the bottom of the enrober tank, from where it is
recirculated. Tempered chocolate is, however, unstable
and will eventually set solid. In order to prevent it from
doing so, some of the chocolate is taken from the enrober
and heated to remove all the crystals. This is then
retempered before being fed back to the enrober. The
amount of chocolate to be recirculated depends strongly
upon the amount being removed by the product.
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CONCLUSION
Mechanical engineering has helped to change chocolate
Proc Instn Mech Engrs Vol 215 Part E
manufacture from a high-manpower craft industry to a
mechanized large throughput industry. Chocolate has
also changed from a largely indulgent food for special
occasions, to something that is eaten by many people on
most days. For instance more than 5 million Kit Kat are
produced each day at a single UK factory and its sales
alone are more than twice that of the newspaper market
[6].
Not only is mechanical engineering important for the
manufacture of confectionery products, but also it has
played a vital role in developing wrapping machines that
can package more than 500 items/min. This is at a speed
that is so fast that it is impossible to see the individual
items clearly by eye.
Further developments are continuing to take place, not
only to reduce processing times and costs, but also to
make new shapes, avours and textures to meet the everincreasing demands of the chocoholics in this new
millennium.
REFERENCES
1 Dahl, R. Charlie and the Chocolate Factory, 1964 (Penguin
Books, Harmondsworth, Middlesex).
2 Beckett, S. T. Industrial Chocolate Manufacture and Use,
3rd edition, 1999 (Blackwell Science, Oxford).
3 Cocoa and Chocolate Manual, 1997 (Lobas, Ede, The
Netherlands).
4 Minie, B. W. Chocolate Cocoa and Confectionery, 3rd
edition, 1989 (Van Nostrand Reinhold, New York).
5 Manufacturer’s Catalogue, Bühler, AG, CH-9240 Uzwill,
Switzerland.
6 Sweet Facts Nestle, 1998 (Nestle UK Limited, York).
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