Soluble coffee - manufacturing methods
Extraction. Optimum extraction of
soluble coffee solids depends on the temperature of the extraction water and
its rate of flow through roasted, ground coffee. In practice incoming water can
be approaching 200°C under high pressure. Extraction requires a row of
inter-connecting percolators or cells, using a continuous reverse flow
principle. Each cell is filled in turn with fresh coffee. Incoming hot water is
introduced into the cell containing the least fresh, most extracted coffee,
where it collects those soluble solids that are vulnerable to the high
temperature and carries these to the next cell in the cycle, and so on. In each
cell the coffee liquor collects more soluble solids.
By the time the sixth cell in a cycle has been reached the liquor’s temperature
has been reduced and so inflicts minimum damage on the delicate flavour
constituents of the freshest roast coffee that are essential to the final
quality. The liquor is then drawn off and cooled. It now consists of
approximately 85% water and 15% soluble coffee. Meanwhile the first cell in the
cycle (that underwent extraction with the hottest water), is emptied of the
spent grounds and is recharged with fresh coffee to start the cycle again. Thus
there is always one cell outside the process, which requires seven cells
altogether.
Evaporation
is necessary to reduce the liquor’s water content to 50%. But first the liquor
is centrifuged to remove non-soluble particles. To evaporate liquor at normal
pressure would require very high temperatures that would cause the liquor to
acquire off flavours and lose valuable coffee aromas as well. Consequently
evaporation takes place under low vacuum and low temperature conditions.
Spray drying
requires a large cylindrical tower with a conical base. The concentrated liquor
is introduced into the top under pressure, with a jet of hot air. The falling
droplets dry into a fine powder that cools as it descends. These particles may
then be agglomerated into granules by wetting them in low-pressure steam,
allowing them to stick together. The wet granules are then dried as they
descend through a second tower and are sifted to provide a uniform final
granule size.
Freeze drying
consists of freezing the coffee liquor into a ¼ inch (about 6 mm) thick cake on
a moving conveyor at a temperature of –45°C. The frozen cake is then broken
into small particles and the ice crystals are removed under very high vacuums,
being converted directly to water vapour by a process known as sublimation.
Freeze drying is more energy expensive but is gentler on the product as less
heat is applied to evaporate the water content. Consequently, freeze drying is
used for the finer tasting and more expensive blends of instant coffee.