All engineering studies of food processes are related to the basic unit operations of mass transfer and/or heat transfer.

These principles are applied to the numerous procedures of transporting, preparing, processing, packaging, and distributing of basic and value-added foods. Secondary unit operations include techniques of separating components, reducing particle size, mixing ingredients, and concentrating desired components. The unit separation operations, distillation, evaporation, dehydration, and filtration all involve the extraction of a component from a liquid, gas, or solid by physical or chemical means. However, each category listed has such different applications of scientific and engineering principles that it is too unwieldy to place them all under a combined unit operation. Thus, due to these complexities, the procedures have been divided into numerous categories related to the mechanisms of separation. This greatly simplifies the scientific and engineering studies of various unit operations and the subsequent engineering design, manufacture, and integration of the process facilities into an overall food process. Various separations of components in a basic harvested food or a partially processed product are the most important sequences of processing a food to a finished product for the market. The unit operation of extraction is considered to be the removal or separation of a component by material that has greater affinity for the component being removed.

Extraction, the separating of a component from a liquid or solid by another liquid is handled as a separate unit operation since the process is based on diffusion of one component from the base material to an extracting liquid. Since the mechanics of these processes are based on diffusion, study of the processes and design of extracting equipment is based on the diffusivities of the solute being removed from the base material. This differs from other separation processes such as distillation that involve a change of phase in one or more components requiring different scientific principles. The original applications of extraction processes began many years ago with application of gas absorption and solvent extraction in the industrial chemical industry. It has been in relatively recent years that the technology has been extensively applied to separating components in food (1).

Extraction as practiced in the food industry is essentially the operation of removing or separating a component from the food to ensure food safety or to alter the properties. A rudimentary form of extraction begins with the basic separation or removal of components from an as-received food. With the growing problems of microbiological contamination in harvested fruits, vegetables, and animals, the extraction operation of washing as-received raw materials has received much emphasis over the past few years. The simple washing of a vegetable or fruit is necessary to ensure that contamination from soil, fertilizers, living organisms, pesticides, and so on is removed or at least reduced to an acceptable level, making the vegetable safe to eat. Proper washing is particularly important in slaughtered animals since extremely dangerous microorganisms such as Escherichia coli and Salmonella contaminate many of them. A much more complex procedure for removing or extracting a component is found in the process for producing vegetable oils. For example, many vegetable and seed oils are extracted from the base material by solvent extraction. A solvent in which the corn oil is soluble contacts a product, such as ground corn. This results in an oil-rich solvent with little of the solvent-insoluble products remaining in the oil fraction. The oil is then removed from the solvent by other separation operations such as distillation. Extraction processes can be carried out on a batch basis or as continuous steady-state operation. As is the case with most unit operations, the continuous operation is the easiest to analyze and design. Continuous operations are particularly important to maximize the operating costs and efficiency of extraction processes used in the food industry (1-3).

The basis for success of extraction processes is the difference in affinity for one component or material over another. For example, water has little affinity for vegetable oil, but the oil is completely soluble in an organic solvent. Hence, when an oil-containing food raw material, such as soybeans or corn, is placed in contact with water, the water will not absorb any oil. Oil will only be released if the food is in hot water, such as during cooking, when the structure of the food is changed so that oil is released. In this case the oil is not extracted by water but the water acts as a vehicle for washing out the oil and floating it to the surface.

Extraction during food processing is involved with the diffusion mass transfer of one component of the food being extracted, or leached, in the solvent phase. Although the extraction is enhanced by thorough contact or mixing, the controlling factor is the diffusion as measured by the dif-fusivity of the component being removed from the base material. There are two primary unit operations to consider under extraction of foods. The first is a liquid-solid extraction in which a soluble component is removed from the solid by a liquid. This process is known as leaching. The second, liquid-liquid extraction involves the removal of a component in a liquid by another liquid.

There are several special extraction processes whereby the extraction liquid changes phase during the process. However, the basic diffusivity of the condensing extracting material is still the basis for the component transfer rate. For example, steam is used for stripping volatile materials from both solids and liquids. Thus, the condensing steam is actually the solvent phase that either remains a vapor or changes from a vapor to a liquid during the extraction process. Some extractions, such as the use of hexane in extracting oil from soybeans, utilize a vapor solvent that condenses during the extraction and is recovered as an oil-rich liquid solvent.

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