Prior to the drying, the whey is preconcentrated which removes about 90% of the water. This is carried out in a multi-effect falling film evaporator with vapour recompression in order to save steam. Apart from the actual evaporating calandrias the evaporator is equipped with preheaters for heating the whey by means of vapour before it is finally pasteurized by means of live steam, either direct or indirect.
The evaporated whey is pumped either to crystallization tanks or direct to the spray dryer by a pump with variable speed. The spray dryer may be with either pneumatic conveying or fluid beds for after-drying and cooling.
The pretreatment prior to evaporation is as follows: The raw fresh whey should be cooled to 5-10ºC immediately after it is drained off the cheese vat in order to avoid bacterial decomposition with a troublesome drying as a result. Preservation of the cooled raw whey is possible with H2O2 or formaldehyde. Formaldehyde is better, see Fig. 122. Further the presence of H2O2 will result in a significant pitting corrosion of the impellers in the pumps of the evaporator.
Fig. 122 Preservation of whey with formaldehyde and peroxide
It is advisable to clarify the whey in order to remove casein micelles, which will burn and stick to the evaporator tubes and result in discoloured particles in the final powder. The remaining fat in the whey should also be removed by the separation, as fat may tend to give deposits in the drying chamber.
The whey will typically correspond to the following specification:
| - Sediment test: |
max. 0.1 mg/25 ml |
| - pH: |
min. 6.3 |
| -Titratable acidity: |
max. 0.12% |
| - True lactic acid: |
max. 20 mg/100 ml |
| - Fat content: |
max. 0.05% |
| - Lactose content: |
min. 70%, max. 74% |
| - Protein content: |
min. 12% |
| - Non condensable gases: |
max. 0.02% by weight |
| - Calcium content: |
max. 300 ppm |
| - Magnesium content: |
max. 100 ppm |
| - Chloride content: |
max. 1,200 ppm |
When the whey is stored between the cheese factory and the evaporation process at a temperature exceeding 10ºC, then the storage time should not exceed one hour. If the storage temperature is below 10ºC, then the storage time should not exceed ten hours.