Extraction in food industry

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1. OBJECTIVE

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. 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 (BAT in the Food, Drink and Milk Industries, June 2005).

2. FIELD OF APPLICATION

Extraction is applied to a wide variety of food products. Typical examples are (BAT in the Food, Drink and Milk Industries, June 2005):

• the extraction of sugar from sugar-beets or sugar-cane
• the extraction of oil from oil seeds 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.

3. DESCRIPTION OF TECHNIQUES, METHODS AND EQUIPMENT

Principal of function

(BAT in the Food, Drink and Milk Industries, June 2005)

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. Commonly the extract is the product or intermediate product and the residue is a waste or by-product.

Types of extraction

(BAT in the Food, Drink and Milk Industries, June 2005)

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.

Common solvents

(BAT in the Food, Drink and Milk Industries, June 2005)

The efficiency of the extraction process depends on the selectivity of the solvent. Common solvents include:

• Water
• Organic solvents, such as hexane, methylene chloride, ethyl acetate.
• Alcohol
• Supercritical CO2.

Method description

(BAT in the Food, Drink and Milk Industries, June 2005)

Raw materials are usually pretreated in order to ensure on 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.

There are two extraction methods:

• Lateral flow extraction:

This is the simplest extraction method. It 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.

• Countercurrent extraction:

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.

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.

Transport methods for the food to be extracted

(BAT in the Food, Drink and Milk Industries, June 2005)

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 conve 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.

Use of extraction in food processing

(BAT in the Food, Drink and Milk Industries, June 2005)

Some typical examples of extraction are given below:

• Extraction of oil from oilseeds

The production of crude vegetable oil from oil seeds (e.g. soybeans, sunflower seeds or rapeseed) is a two-step process:

(a) the first process step:

It 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 reduced to between 6 and 12 %. Beans (with 20 % oil or less) are not pressed, because of their lower fat content, but are extracted directly after cleaning and preparation.

(b) the second process step:

It includes extraction of the oil from the pressed cake or flaked beans with hexane. The extraction takes place in a countercurrent flow. The mixture of hexane and oil, called miscella, is further processed in a distillation process to recover the 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 desolventising-toasting process also reduces the enzyme and 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, before being stored in silos or before loading.

• Extraction of coffee for the production of soluble coffee:

In this process, water is the extracting solvent. The coarsely ground coffee is extracted in a battery of batch percolating columns. The process is operated semi-continuously with water in a countercurrent flow to the coffee, from the most extracted cell to the one just filled with fresh roast and ground coffee. The series of cells used in the extraction from a train. The extract is recovered from the fresh or least-extracted cell. One consequence of using of high temperatures is that the system must be kept under pressure, so the individual cells and associated piping must be designed accordingly. Once a batch of solids has been extracted, the exhausted cell is isolated from the train and the spent grounds are discharged. At the same time, a cell containing fresh roast and ground coffee is added to the train. The extraction yield is expressed as the amount of recovered water-soluble dry solid content of roasted coffee as a percentage of roasted coffee (dry weight). Yields of 40 to 56% are obtainable.

• Decaffeination of coffee and tea:

Decaffeination is carried out by extraction with decaffeinating agents, such as water or another solvent such as methylene chloride, coffee oils, ethyl acetate or supercritical CO2. Extraction yields of 97% of the caffeine can be obtained. Two main extraction methods for decaffeination can be distinguished:

• Solvent decaffeination (direct method):

In this method solvents, such as methylene chloride, coffee oils, ethyl acetate or supercritical CO2 are used. The process can be described as follows:

1. Green beans are first soaked with water and steam Beans swell by 30 – 40%
2. A decaffeinating agent is added to the wet beans. The agent solubilises the caffeine from the beans.
3. The agent is drained or steamed away. The agent, together with 97% of caffeine, is removed from the beans.
4. The beans are dried by hot air or vacuum drying. The beans are dried and are now ready for roasting.

The main process parameters are temperature and time. These vary depending on the type of coffee and on the type of solvent. For example, when coffee oil is used as the decaffeinating agent the process may take 6–9 hours at 95–105°C, whereas CO2 is used under high pressure at 40–80°C for 5–30 hours.

• Water decaffeination (indirect method)

In this method water is used as the extraction solvent. The process can be described as follows:

1. Green beans are first soaked with water and steam. The water extracts the caffeine, but also some soluble coffee solids.
2. The beans are separated from the aqueous solution. The water extract containing caffeine and coffee solids is isolated.
3. Water extract is passed over an activated charcoal bed. A solvent, e.g. methylene chloride, may be used instead of activated carbon. The caffeine is removed from the solution by activated carbon filtration or methylene chloride. The coffee solids remain in the aqueous solution.
4. The decaffeinated extract is concentrated and added back to the pre-dried decaffeinated beans. Beans and coffee solids, now without the caffeine, are ready for roasting. Environmental issues. If water is used, water consumption and waste water production are issues. If solvents are used, VOC emissions may be an issue.

4. COMPETITIVE TECHNOLOGIES AND ENERGY SAVING POTENTIALS

a) Changes in the process

Extraction with supercritical media: (Ullmann's Encyclopedia of Industrial Chemistry, Vol.A7, 1994)

A competitive technology of the conventional extraction is the extraction with supercritical media. For example, supercritical carbon dioxide, i.e., at temperatures and pressures above the critical point of 31.06°C and 73.8bar can be used to decaffeinate coffee by a process called destraction. The solubility of substances increases with the density of supercritical carbon dioxide, thus increasing with pressure at constant temperature and decreasing with temperature increases at constant pressure. This property can be used in the separation step. The supercritical extraction is less energy consuming but requires high investment costs.

Supersaturated steam or dichloromethane can also be used. Solid particles can be washed with liquid carbon dioxide.

b) Changes in the energy distribution system

No information is available.

c) Changes in the heat supply system

No information is available.

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