• back

  • 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.