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3.1.3 Cocoa liquor pressing

The liquid cocoa liquor is stored in large storage tanks where it is kept at a temperature of about 70°C to ensure that the liquor remains liquid.
From there the liquor is pumped to the liquor conditioning tanks mounted on each press, where the product is ‘prepared’ to achieve optimum conditions when it is pressed into cocoa butter and cocoa cake.

The liquor is heated to the required temperature in the tank, while high-speed stirring gear ensures quick heat transfer and homogenization of the product as well as reducing the viscosity. This gives the product a relatively thin-fluid consistency, and improves its flow and pressing properties.

Figure 3.4: Cocoa press with auxiliary equipment(Duyvis)

A. cocoa press
B. support structures
C. hydraulic pump unit
D. control panel
E. conditioning tank
F. liquor pump
G. cake conveyor
H. cake breaker
I. butter pump

Figure 3.5: Press positions (Duyvis)

Click on the picture for enlargement.

The press has 3 positions: open, closed and filled, closed and end of press cycle
Pressing is a cyclic process.

  1. The press is closed. As soon as the liquor pump is started, the filling valves open and liquor flows in to fill the pots (position 2).
  2. As soon as the hydraulic pump is started, pressing starts. The butter flows out of the press, while the cake remains inside.
  3. When the required amount of cocoa butter has been pressed out of the liquor (position 3), the press opens by means of the push-out cylinders, allowing the cocoa cakes to fall out of the press (position 1). The press then closes again, ready for the next cycle to begin.

Depending on the type, a press can hold anything between 165 kg (12 pots) and 220 kg (14 pots). The liquor is pumped from the conditioning tank to the press, with the press closed.
The liquor feed pump operates at a filling pressure ranging from 15-20 bar. As soon as the liquor is fed into the press, cocoa butter is pressed out due to this pressure, and discharged through the pipes to the butter chute.

The first butter produced immediately after the hydraulic pump has been started is impure because of the presence of very fine cocoa particles that are not filtered out by the filter screen. However, as soon as coarse cocoa particles start to accumulate on the screen, thereby forming an additional filter, the butter then pressed will be clear. The butter is discharged through several pipes flowing into the butter chute that leads to the butter storage tank.

As the pressure increases the residual fat content of the product decreases: at 100-150 bar the fluid liquor in the pots has been pressed into hard cakes, while at about 350 bar the residual fat content of the cake has been reduced to between 22% and 24%. To obtain a percentage of about 10-12%, the pressure must be increased to over 500 bar and maintained for about ten minutes. Under these conditions about three to four pressing cycles per hour are possible.

The pressing cycle is halted as soon as the cake has the required residual fat content, which can be determined by:

  • weighing the pressed cocoa butter: when a certain weight is attained the press is stopped and the fat content of the cake is analysed, after which the butter-weight/fat-content ratio may be adjusted;
  • adjusting the pressing time: the operator starts a timer when the pressure is 150 bar; operators can set the required pressing time based on experience and fat analysis and obtains with incremental  adjustments the required residual fat content in  the cake;
  • adjusting the distance of ram travel: the ruler mounted on the cylinder body indicates the distance of ram travel,  i.e. the degree that pots and counterpots are pressed together. This, too, is a measure for the residual fat content of the cocoa cake.

Some liquors can be easily processed, whereas others require special attention during pressing. The category they fall into depends on the physical properties of the liquor and is also influenced by the methods that were used for processing and grinding, as well as the degree of alkalization. When pressing an easily processable liquor, the pressure on the ram can be increased soon after pressing has started, while a more careful approach is recommended for liquors that are more complex in terms of processability. Studies have shown that each type of liquor has its own specific pressing curve, i.e. the graph in which the pressure on the ram is plotted against time.

Figure 3.6: Press curve - pressure from hydraulic pump unit

Modern hydraulic pump units use fully programmable pressing curves.

Figure 3.7: Press curve with the relation fat content cake in a press cycle  and the amount of pressed butter and the plunger displacement

At the end of the pressing cycle, the press opens to allow the cakes to drop onto a conveyor belt or into a chute to be transported to the cake breaker, usually a hammer mill where the cylindrical cakes of approximately 34-35 cm in diameter are broken into small pieces not larger than 3 cm.

Figure 3.8: Cake breaker

The hammer mill is equipped with a flywheel mounted on the shaft of the breaker to ensure maximum impact of the hammers. The cake is broken into fragments small enough to pass through the screen, after which the broken cake is transported, mechanically (by means of conveyor screws, elevators or redlers) or pneumatically to the storage silo, sometimes under cooled conditions.

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