Countercurrent extraction of coffee

To reduce insoluble solids contained in a coffee extract obtained from a countercurrent extraction process, the percolation rate of extraction liquid passing through at least one extraction cell, which is positioned intermediately in the countercurrent cell series between extraction cells containing the least extracted coffee material and the most extracted coffee material, is reduced. The flow of extraction liquid may be increased through an extraction cell in the series downstream from the at least one cell in which the percolation rate is reduced.

BACKGROUND OF THE INVENTION 
This invention relates to a process and an apparatus for the production of 
soluble instant coffee in powder form. 
Soluble coffee powder is conventionally produced by freeze-drying or 
spray-drying after evaporation of a coffee extract obtained by the 
percolation of an extraction liquid through cells filled with ground 
roasted coffee (Sivetz, Coffee Processing Technology, Volume 1, pages 262, 
263, AVI, 1963). 
Extraction is carried out in countercurrent fashion, i.e., water under 
pressure at a temperature of 150.degree. to 180.degree. C. is introduced 
into the cell containing the batch of ground roasted coffee which has been 
most intensively extracted (having undergone N extractions) at the bottom 
of that cell, and then liquid extract of this extraction cell is passed 
through the extraction cell containing the batch of coffee which has been 
extracted (N-1) times, and so on, until the liquid extract passes through 
the cell which has just been filled with fresh ground roasted coffee, and 
the final extract leaves the last cell at a temperature on the order of 
100.degree. C. 
In such countercurrent extraction, the most intensively extracted coffee is 
thus subjected to the highest temperature while the fresh coffee is 
subjected to the lowest temperature. 
A distinction is normally drawn between the hot cells, which contain the 
most intensively extracted coffee, and the cold cells, which contain the 
least intensively extracted coffee. 
After each extraction cycle, the cell containing the most intensively 
extracted coffee is emptied, filled with fresh coffee and, after the cells 
have been suitably interconnected, another extraction cycle begins. 
Although the final extract obtained at the exit of the extraction cell 
containing the freshest coffee contains only a small quantity of ground 
coffee particles, fines always being entrained, it is necessary to filter 
the extract. 
Finally, after the filtration phase which eliminates the particles larger 
than about 1 mm in size, solids, such as polysaccharides or proteins, are 
still present in suspension and have to be eliminated to enable a coffee 
powder which dissolves perfectly without any solids appearing in the cup 
to be obtained after concentration and freeze-drying or spray-drying of 
the extract. 
The suspended solids are normally eliminated by centrifugation, the 
sediment obtained then being decanted, the supernatant decantation liquid 
being reintroduced into the final filtered extract while the solid residue 
obtained is eliminated. 
The main disadvantage of this process is that it produces a sediment which 
has to be retreated by decantation and which is not easy to handle. 
In addition, the suspended solids cannot always be satisfactorily 
eliminated by centrifugation. 
Accordingly, the problem addressed by the present invention was to provide 
a process for the production of instant soluble coffee powder in which 
extraction of the coffee in the liquid phase would enable the content of 
insoluble solids in the final extract to be reduced. 
SUMMARY OF THE INVENTION 
Accordingly, the present invention relates in particular to a process for 
the production of soluble instant coffee powder in which an extraction 
liquid is percolated countercurrent fashion through cells filled with 
ground roasted coffee material and operating in series, the final liquid 
extract then being converted into powder form, characterized in that, 
after the hot cells containing the most intensively extracted coffee, the 
rate of percolation of the liquid extract through at least one of the 
extraction cells positioned between the cells containing the least and the 
most extracted coffee material is reduced. 
By virtue of this reduction in the rate of percolation through the 
extraction cells, the cells act as filters which, by retaining the 
suspended solids, enable a final extract having a greatly reduced 
insoluble fraction in relation to the prior art to be obtained. 
The present invention also relates to a process of the type described above 
in which, after reduction of the percolation rate, the percolation flow is 
regulated by increasing the flow rate of the extraction liquid to achieve 
the desired extraction downstream of the cells where the reduction is 
effected. 
The present invention also relates to an apparatus for carrying out the 
process described above in which, after the hot cells containing the most 
intensively extracted coffee, means are provided to reduce the percolation 
rate in at least one cell downstream of the said hot cells. 
The present invention also relates to an apparatus of the type described 
above in which, after at least one cell where the percolation rate has 
been reduced, means are provided to regulate the percolation flow in the 
cells containing the freshest roasted coffee as a function of the desired 
extraction level. 
Other features and advantages of the invention will become apparent from 
the following description in conjunction with the accompanying drawing 
which is provided purely by way of example and which diagrammatically 
illustrates an apparatus for carrying out the process according to the 
invention.

DETAILED DESCRIPTION OF THE DRAWING AND THE INVENTION 
As can be seen from the drawing, an apparatus for the extraction of coffee 
may be made up of several extraction cells operating in series, each of 
which is formed by a column of which the lower part is connected to the 
upper part of the preceding column and of which the upper part is 
connected to the lower part of the following column. 
Generally, an extraction apparatus is made up of four to eight extraction 
cells and, preferably, six extraction cells. 
Cell 1 contains the most intensively extracted coffee while cell 6 contains 
the least intensively extracted coffee, the level of extraction decreasing 
from cell 1 to cell 6. 
The extraction liquid, which may consist of water under pressure at a 
temperature of 150.degree. C. to 180.degree. C., is introduced at the 
bottom of cell 1, passes upwards through that cell, taking up soluble 
product in the process, leaves at the upper end of cell 1 and passes 
successively through each of the cells up to and including cell 6 which is 
the last cell and which contains fresh ground roasted coffee. 
Accordingly, the final extract issues from cell 6 and is subsequently 
filtered and, optionally, centrifuged and then evaporated and finally 
converted into powder form by freeze-drying or spray-drying. 
The crucial parameter to be taken into account for understanding the 
invention lies in the rate of percolation of the liquid extract through an 
extraction cell. 
This percolation rate should be understood as the ratio between the flow of 
liquid extract, expressed for example in liters per minute, and the 
cross-section of the extraction cell, the value obtained being 
dimensionally comparable with a velocity. 
Accordingly, it will be understood that, for the same flow, a percolation 
rate can vary considerably from one extraction cell to another if the 
cross-sections of those cells are different. On the other hand, if the 
cross-sections of all the extraction cells are identical, the percolation 
rate as defined above is unambiguously related to the flow. 
Now, in an installation for the liquid-phase extraction of coffee by 
percolation through extraction cells, all the cells being intended 
successively to occupy all the places of an extraction cycle, they all 
have the same geometry and, hence, particularly the same cross-section. 
Thus, any reduction or increase in flow will reciprocally produce a 
reduction or increase in the percolation rate. 
In the following description of the present invention, the percolation 
rates are expressed in centimeters per minute and the flows in liters per 
minute. Conventionally, the rate of percolation of the liquid extract 
through the extraction cells is between 12 cm/min. and 15 cm/min. 
In the process according to the invention, the percolation rate is reduced 
after the hot cells, the liquid extract issuing at a temperature above 
140.degree. C. By virtue of this reduction in the percolation rate after 
the hot cells, there is a distinct reduction in the insoluble fraction in 
the final extract. Nevertheless, it appears that, if the percolation rate 
is excessively reduced after the hot cells, the flow of the liquid extract 
falls to an inadequate level in the extraction cells, adversely affecting 
the degree of extraction which can thus become inadequate in an industrial 
process where all the cells are identical. 
To overcome this drawback, it can be of advantage to regulate the flow of 
the liquid extract as a function of the desired degree of extraction 
measured in the final extract after the rate of percolation in one or more 
extraction cells has been reduced downstream of the hot cells. 
If all the cells have the same cross-section, there will be an increase in 
the percolation rate in the cells downstream of those cells where the 
percolation rate has been reduced. However, this increase in the 
percolation rate should not be compared in any way with the previous 
reduction in the percolation rate. 
This is because, although it is advisable to reduce the percolation rate 
after the hot cells to obtain a reduction in the insoluble fraction in the 
final extract, the reduction in flow being only one of a number of ways of 
achieving this result, the extraction level in the extraction cells is 
dependent inter alia on the ratio between the volume of percolating liquid 
and the mass of roasted coffee, irrespective of how the coffee is 
arranged, i.e. in a thin bed of wide-cross-section or in a thick column of 
narrow cross-section. 
Thus, since an extraction cycle has a predetermined duration, the volume of 
percolating liquid is determined by the flow rate and not by the 
percolation rate. 
The following EXAMPLES illustrate the process according to the invention 
and demonstrate the importance of the parameters selected. 
In a conventional installation for the liquid-phase extraction of coffee, 
the percolation rate in the extraction cells is of the order of 15 
cm/minute, the insoluble fraction in the final extract being capable of 
reaching 3.3% and more. 
The following Table, which relates to an installation comprising two hot 
cells, two intermediate cells where the percolation rate is reduced and 
two cold cells where the flow rate is regulated, regulation being effected 
in fact by an increase in the percolation rate, all the cells having the 
same-cross-section, demonstrates the development of the insoluble fraction 
in the final extract as a function of the percolation rate in the 
intermediate cells. 
______________________________________ 
Percolation rate in the 
Insoluble fraction in 
intermediate cells 
the final extract 
cm/min. % 
______________________________________ 
15.0 3.3 
13.6 2.9 
12.9 2.8 
10.3 1.4 
9.6 0.8 
9.1 0.4 
______________________________________ 
It can thus be seen that, for a percolation rate of 10 cm/min., the 
insoluble fraction is of the order of 1.2%, which is advantageous, as will 
be seen hereinafter. 
In addition, the following Table illustrates the effect of the number of 
intermediate cells on the insoluble fraction in the final extract, the 
installation comprising two hot cells upstream and two cold cells 
downstream where the percolation rate is increased. 
______________________________________ 
Number of Insoluble fraction in 
intermediate cells 
the final extract % 
______________________________________ 
4 0.8 
3 0.7 
2 1.2 
1 2.1 
______________________________________ 
Finally, the following Table demonstrates the effect of the flow rate in 
the cold cells on the extraction level measured in the final extract. 
______________________________________ 
Test 1 Test 2 
______________________________________ 
Temperature in the 180.degree. C. 
180.degree. C. 
hot cells 
Flow rate in the 24.8 l/min. 
24.7 l/min. 
hot cells 
Flow rate in the 33.0 l/min. 
31.5 l/min. 
cold cells 
Temperature in the 105.degree. C. 
105.degree. C. 
cold cells 
Extraction level measured 
42.7% 40.4% 
in the final extract 
______________________________________ 
It can thus clearly be seen that, all things otherwise being equal, the 
extraction level is a function of the flow rate in the cold cells. 
Without wishing to be limited to any one explanation of the phenomenon 
observed, it appears that the reduction in the insoluble fraction in the 
final extract is attributable to the fact that, as the percolation rate 
decreases at least in the intermediate cells, the intermediate cells act 
as filters which retain the suspended solids entrained from the hot cells. 
The solids retained in these intermediate cells would then either be 
hydrolyzed when the intermediate cells have become the hot cells or simply 
removed when the grounds contained in the most intensively extracted hot 
cell are eliminated. 
This filtration effect can be demonstrated by calculating the extraction 
level achieved in each cell. 
Thus, for the intermediate cells, it has been found that the extraction 
level can be negative which clearly shows that solids present in the 
extract before passage through the intermediate cells are no longer 
present at the exit of those cells and have therefore been retained in the 
bed of coffee. 
The following Table illustrates this phenomenon for an installation 
comprising six cells, including two intermediate cells. 
______________________________________ 
Number of Extraction level achieved 
cells in the cell (%) 
______________________________________ 
6 11.83 
5 5.83 
4 -8.45 
3 -2.06 
2 12.39 
1 18.57 
______________________________________ 
To carry out the process according to the invention in the preferred 
embodiment illustrated in the accompanying drawing, means 7 are provided 
after two hot cells 1, 2 to reduce the percolation rate in at least one 
cell 3 after the last hot cell 2. 
The means 7 may consist of an evaporator 7 which, by reducing the volume of 
the liquid extract issuing from the cell 2 and intended to percolate 
through the cell 3, will produce a reduction in the flow rate and hence in 
the percolation rate in the cell 3. 
To prevent the extraction level achieved in the cells containing the 
freshest coffee from being overly penalized by this reduction in the 
percolation rate and hence in the flow rate, a device 8 for increasing the 
flow rate in the following cells is arranged between two successive 
extraction cells, for example between the cells 4 and 5, downstream of the 
cell 3. 
The device 8 may be formed by an additional inlet for extraction liquid, 
for example hot water, which, added to the liquid extract coming from the 
cell 4, thus increases the volume of liquid percolating through the cells 
5 and 6. The use of a hot water inlet also enables the temperature to be 
regulated. 
By virtue of the process and apparatus according to the invention, it is 
thus possible readily to obtain a final extract at the exit of the cell 
6--containing the freshest roasted coffee--which has an insoluble fraction 
of less than 1.2%. 
Now, various tests have shown that, for a final extract having an insoluble 
fraction of less than 1.2%, a single filtration step with no subsequent 
centrifugation is sufficient to obtain a juice from which an instant 
coffee powder dissolving in hot water without the appearance of any 
suspended solids can be obtained after evaporation followed by 
spray-drying or freeze-drying.