Pretreatment of coffee
In order to produce a freeze dried instant coffee powder, it is necessary to control the pre-process prior to the actual freeze drying operation.
The pretreatment consists of the following processes which will have an influence on the colour, bulk density, solubility, particle size and mechanical stability of the final powder.
- Cooling, foaming and pre-freezing – Fresh concentrated extract with the aroma fraction is mixed with fines from the granulation (milling) of the frozen coffee flakes. In the mixing tank the temperature is adjusted before it is pumped through a heat exchanger to cool the concentrate to a temperature ideal for the foaming. The foaming is done by injecting CO2 or N2 (they have a slightly different effect on the final colour). After the foaming the concentrate is further cooled and the first ice crystals are formed. This process step has a significant influence on the final colour, which is detected and controlled automatically.
The pre-cooling also has an influence on the viscosity, which is important for the further freezing.
- The actual freezing takes place in either a CAB freezer (Continuous Air Blast) used for coffee concentrate with up to approx. 45% TS or in a ROTA freezer used for concentrates with 35-37% TS. The ROTA freezer is cheaper, it takes up less space, but the freeze drying operation becomes more expensive, since more water needs to be removed in the freeze dryer. The frozen concentrate is granulated (milled) and finally filled into trays that will be stored in a cold store at -40oC, ready for the final freeze drying.