Spray-drying - The most commonly used industrial method of manufacturing instant coffee is by the spray-dried process. The water is again allowed to evaporate, forming a concentrate. The concentrated coffee is sprayed from a high tower in a large hot-air chamber. As the droplets fall, the remaining water evaporates. Dry crystals of coffee fall to the bottom of the chamber. The high temperatures involved in this method do tend to effect the oils of the coffee and more flavour is lost. More...
Freeze-drying - The freeze-drying method preserves the most ‘coffee flavour’ but its a more involved procedure. First, the coffee is allowed to sit so the water evaporates naturally, leaving a concentrated coffee solution. This concentrate is then frozen to around -40 Celsius. The remaining water freezes into ice crystals. Sublimation (a natural process similar to evaporation) is used to remove the ice. What's left is dry grains of coffee. More...
Spray drying is the most economic method of producing soluble coffee. The feed to the spray dryer is a mixture of concentrated aroma and hydrolysed fractions, with the preserved aroma components added. In order to maximize aroma retension, drying of the extract takes place under conditions that maintain low powder temperatures. Different types of spray dryers can be used for drying of cofee. Bulk density and colour control is possible by means of in-line gas mixers. Inert gas is injected into the feed system just prior to the nozzle atomizer used in the spray drying system.
In cases where spray dried powders require further agglomeration, an additional process stage is used involving powder wetting, afterdrying and cooling. Control of weeting is carried out with water and/or saturated steam in an agglomeration chamber equipped with a rotating impacter. The agglomerates are then dryed and cooled in the attached fluid bed, followed by sieving and packing. Fines and oversize fractions are reprocessed within the agglomeration plant.
Agglomerated wet coffee granules are frozen. For instant coffee this is a very important stage. Freezing too fast leads to large ice crystals and a very porous product and can also affect the colour of the coffee granules.
Frozen coffee is placed in the drying chamber, often on metal trays.
A vacuum is created within the chamber. The strength of the vacuum is critical in the speed of the drying and therefore the quality of the product. Care must be taken to produce a vacuum of suitable strength.
The drying chamber is warmed, most commonly by radiation but conduction is used some plants and convection has been proposed in some small pilot plants. A possible problem with convection is uneven drying rates within the chamber, which would give an inferior product.
Condensation - The previously frozen water in the coffee granules expands to 107 its volume, the removal of this water vapour from the chamber is vitally important, making the condenser the most critical and expensive components in a freeze drying plant.
The freeze dried granules are removed from the chamber and packaged.
Spray-drying - The most commonly used industrial method of manufacturing instant coffee is by the spray-dried process. The water is again allowed to evaporate, forming a concentrate. The concentrated coffee is sprayed from a high tower in a large hot-air chamber. As the droplets fall, the remaining water evaporates. Dry crystals of coffee fall to the bottom of the chamber. The high temperatures involved in this method do tend to effect the oils of the coffee and more flavour is lost.