Introduction

The separation of components from natural products is becoming an important part of the food, flavour, fragrance and pharmaceutical industries. The trend towards greater consumption of processed foods has created a dynamic food industry that converts raw materials into foods for which chemical and nutritional content, flavour, colour, texture and other properties are carefully defined and controlled.

Food processing industry involves handling of complex raw materials having complex molecules as chemical constituents. The need to process these materials without loss of aroma, flavour and taste is of paramount importance. Extraction is an excellent separation technique for recovery of active constituents from raw materials and for removal of undesirable constituents. The recent progress in extraction technology has been in the use of novel techniques for separation using supercritical fluid, aqueous two-phase and reverse micelle extractions.

This chapter briefly reviews the conventional solvent extraction techniques of leaching and liquid-liquid extraction used in food processing. The supercritical fluid extraction in which the dissolving power of a solvent can be manipulated to produce improved quality food materials is discussed in some detail. The aqueous two-phase extraction technique is presented, with emphasis on the separation of proteins. The other extraction techniques using reverse micelle systems and emulsion liquid membrane systems for extraction of biomaterials are discussed at the end of this chapter.

2. TERMINOLOGIES

Solvent extraction is a separation process which involves the removal of individual constituents from a mixture of solids or liquids upon addition of a solvent in which the original constituents have different solubilities. In leaching or solid extraction, a liquid solvent is used to dissolve soluble material from its mixture with an insoluble solid. In liquid extraction, a liquid solvent, usually an immiscible liquid, is used to separate miscible liquids by preferentially dissolving one of them. The extracted solute is separated from the solvent by distillation or evaporation.

The process of separating a mixture which is in solid state or in liquid state or in solid and liquid state by contacting it with a fluid maintained under conditions of temperature and pressure above its critical point is termed as supercritical fluid extraction. Separation of the dissolved substance can be accomplished either by reducing the pressure at constant temperature or by raising the temperature at constant pressure. Other terms used for this separation process are dense gas extraction and supercritical gas extraction.

Aqueous polymer/salt systems and aqueous polymer/polymer systems have the property of forming two immiscible phases. When high molecular weight, water-miscible polymers are used in these systems, biomaterials have a tendency to distribute with different concentrations between these phases. Aqueous two-phase extraction is the separation process which involves partitioning of solute between the two aqueous phases. The partition coefficient is manipulated by changing the type and molecular weight of polymers, type of phase-forming salt, and type and ionic strength of the added salts.

Reverse micelle extraction is a separation process where reverse micelles are used to extract solute from an aqueous phase to a reversed micellar organic phase. The reverse micelles are monodisperse aggregates of surface active molecules in an organic solvent. The Winsor type II system, a water-in-oil (W/O) microemulsion in equilibrium with an aqueous phase is of particular interest in extraction of biomolecules.

Emulsion liquid membrane extraction is a three phase process where extraction is performed by dispersing water-in-oil (W/O) emulsion in a continuous water phase (W/O/W) or dispersing oil-in-water (O/W) emulsion in a continuous oil phase (O/W/O). Usually the solute is selectively transferred from the continuous (outer) phase through a selective liquid membrane which separates the two miscible outer and inner phases. In contrast to the reverse micelle extraction where drop size may be in nanometers, the encapsulated inner droplets in the liquid emulsion membrane extraction are typically a few micrometers in diameter.

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