Depending upon need and quantities of material to be handled, different extraction techniques are used in food industries. Some popular extraction techniques are
1. Single stage batch extraction
2. Multi stage cross flow extraction
3. Multi stage counter current extraction
1. Single stage batch extraction:
It is a batch extractor in which the solids are brought in contact with adequate quantity of solvent so as to leach out the solute from solid into the solvent. The process can be accompanied with heating or stirring as the process demands. When the extraction process reaches equilibrium, the solution is decanted and excess solvent from the solution is removed to obtain pure solute. Example; brewing tea or coffee, extraction of oleoresins.
Advantage of single stage batch extraction:
1. Simple in operation
2. Easy in maintenance and flexible in utilization
3. Same batch extraction can be used for different matrix / commodities if needed.
Disadvantage of single stage batch extraction:
1. Low throughputs, restricted to one ton batch
2. Solids retain some amount of solute after every extraction as the solids are saturated with solution containing solutes during decantation.
2. Multistage cross flow extraction:
In this type of extraction, solids are contacted every time with batch of solvent as in batch extractor. Another way of achieving this is similar to soxhlet extractor in which solids are extracted with batch of solvent. This is an excellent process for recovery of complete solute from solids. Example; fat extraction in laboratory analysis.
Advantage of multistage cross flow extractor:
1. Almost complete extraction of solute is possible.
2. Appropriate for small batch and analytical application.
Disadvantage of multistage cross flow extractor:
1. Continuous heating may cause thermal degradation of solute.
2. Lot of energy is consumed in vaporizing and condensing the solvents so not applicable for industrial use.
3. Multi-stage counter current extraction: In this extraction process a battery of extractors is used. The solids flow in one direction of the battery while the solvent flow in opposite direction. Fresh solids are brought in contact with solvent which contain major part of solute and is about to leave the extraction batteries.
Similarly, solids containing very little amount of solute before discharge are brought into contact with fresh solvent and hence any last traces of solute adhering to the solids are also extracted almost completely into the solvent. Thus the solids, even though they carry some amount of solution before discharge, will be carrying very little amount of solute with them. Here, the number of extractors can be adjusted (added or reduced) according to the value of solute extracted. More the number of extractors, more the complete extraction.