The objective of extraction is to recover valuable soluble components from raw materials by primarily dissolving them in a liquid solvent, so that the components can be separated and recovered later from the liquid.
Field of application
Extraction is applied to a wide variety of food products. Typical examples are:
- the extraction of sugar from sugar-beets or sugar-cane
- the extraction of oil from oilseeds and from virgin pomace
- the extraction of coffee extract from coffee beans
- the extraction of caffeine from coffee beans
- the extraction of various other compounds such as proteins, pectins, vitamins, pigments, essential oils, aroma compounds, flavour compounds etc. from many different materials.
Description of techniques, methods and equipment
Extraction works according to the principle that soluble components can be separated from insoluble or less soluble components by dissolving them in a suitable solvent. Raw materials that are suitable for extraction may contain either solids only, solids and a solution, or solids and a liquid. Solid/liquid extraction is sometimes called leaching. When the soluble component is incorporated in a liquid, liquid/liquid extraction may be applied to recover the valuable soluble component. Commonly the extract is the product or intermediate product and the residue is a waste or byproduct. It is not always the objective to recover one particular compound in pure form from a raw material, i.e. sometimes extraction is intended to separate all the soluble compounds from the residue; an example of this is the extraction of coffee. The efficiency of the extraction process depends on the selectivity of the solvent. Common solvents include:
- organic solvents, such as hexane, methylene chloride, ethyl acetate
- supercritical CO2.
Raw materials are usually pretreated in order to ensure an efficient extraction of the desired compounds. For example, sugar beets and sugar cane are cut into thin slices, nuts and seeds are ground or flaked, coffee beans are roasted and ground, and tea
leaves are dried and ground.
Methods and equipment
The simplest extraction method is a repeated extraction with fresh solvent (lateral flow
extraction). However, this is rarely used because of the costs of solvent and because it results in an extract of a very low concentration. The most common method used is countercurrent extraction, either in a batch or continuous process. Batch-wise countercurrent extraction is normally only used for the processing of small amounts of material. In continuously operating extractors, the solid material and the liquid (solvent) flow countercurrently. In principle, many different methods of transport are possible, for the material and liquid flows, some examples of transport systems commonly used include:
- perforated trays connected to an unbroken conveyor or loop, moving horizontally or vertically
- screw conveyors, which transport the solid material in a countercurrent flow vertically or upwards under an incline slope. The screws are perforated in order to obtain a uniform flow of liquid
- an unbroken perforated belt; the solvent is circulated under pump action and sprayed on top of the solid material.
One difficulty with extraction is the recovery of the extracted material from the solvent. This can be carried out by evaporation, crystallisation, distillation, or steam stripping, etc. Some typical examples of extraction are given below:
a) Extraction of sugar from sugar beets and sugar cane (sugar diffusion)
The beets, cut into thin slices called cossettes, are passed into a water-based countercurrent extraction apparatus (diffuser) and emerge as impure sugar juice and beet pulp. The fresh water
used in the extraction process is actually condensed water from the subsequent evaporation steps together with recirculated water from the pulp pressing. The temperature
inside the diffuser is around 70˚C (68 to 72˚C).
a1) Horizontal diffusers
These are large revolving drums, separated into “cells” by a helix attached to the interior surface. As the drum and helix revolve, the juice, which stays at the bottom of the vessel, is transported countercurrently to the cossettes, i.e. the exhausted beet pulp leaves the diffuser at the same end where the fresh water enters.
a2) Vertical diffusers
The extractor is composed of two main and distinct parts: the countercurrent mixer and the extraction tower. The tower is a 14 to 20 metres high cylinder. Inside the tower, a tubular shaft rotates slowly. Special helicoidal steel pieces, or flights, are fitted on the shaft and move the cossettes upwards. The juice and the cossettes move countercurrently.
a3) Slope diffusers
Slope diffusers consist essentially of a U-shaped sloping vessel in which two overlapping screws with opposite pitches rotate. Fresh cossettes fall from a conveyor belt into the lower end. The cossettes are transported upwards by the two screws to a paddle wheel, which lifts the exhausted cossettes out of the extractor. Fresh water is introduced at the upper end and the sugar juice leaves the extractor through a screen at the bottom end.
b) Extraction of oil from oilseeds
The production of crude vegetable
oil from oilseeds (e.g. soybeans, sunflower seeds or rapeseed) is a two-step process:
- the first process step involves the cleaning, preparation (i.e. drying), dehulling, flaking, conditioning and pressing of the oilseeds. Pressing takes place in one or two steps resulting in a crude pressed oil and a cake with an oil content of 12-25%. After the first hexane extraction (pressing only) is carried out the cake oil content is reduced to between 6 and 12%.
Beans (with 20% oil or less) are not pressed, because of their low fat content, but are extracted directly after cleaning and preparation.
- the second proces step includes extraction of the oil from pressed cake or flakes beans with hexane. The extracftion takesplace in a countercurrent flow.
The mixture of hexane and oil, called miscella, is further processed in a distillation process to recover hexane from the vegetable oil. The solvent is passed through a hexane/water separator and then re-used in the extraction process.
The remaining hexane in the cake is recovered by a stripping process, using steam. This dezolventising-toasting process also reduces the enzyme an micro-organism activity in the meal.
The hexane/steam vapours are used in the miscella-distillation process for solvent and heat recovery.
The meal is dried and cooled by air, befor being stored in silos or before loading.