FE 480-Food Operations Laboratory
Dr. A. Kaya

SUPERCRITICAL CO 2 EXTRACTION SYSTEM MILK FAT FRACTIONATION

Introduction

Many technologies have been developed for the separation and fractionation of different food compounds in the food industry. Conventional processes such as crystallization, filtration, distillation or precipitation are being substituted by new processes that use membranes or supercritical fluids. Supercritical fluid extraction (SFE) is a separation process where the substances are dissolved in a fluid which is able to modify its dissolving power under specific conditions above their critical temperature and pressure (supercritical region). The properties of a supercritical fluid are used to extract selectively a specific compound or to fractionate mixtures by changing the temperature and pressure without any phase change. A supercritical fluid is a liquid or a gas at atmospheric conditions which is operational when compressed above its critical pressure (50–250 bar) and heated above its critical temperature (20–60 o C). The most important property of these fluids is the dissolving power in their supercritical region. In the phase diagram for a pure compound, it is possible to distinguish the three material states: solid, liquid and vapor. There are also two important points: the triple point and the critical point. The triple point is the point at which the three states coexist. The critical point lies at the end of vaporisation curve, where the gas and liquid phase merge to form a single homogeneous fluid phase, and beyond this point is the supercritical fluid region. A supercritical fluid exhibits physicochemical properties between those of a gas and a liquid, and has the capacity to dissolve compounds that may only dissolve poorly or not at all in the gas or liquid state. The dissolving power of a supercritical fluid varies with density, which can be as high as a liquid or as low as a gas depending on small changes in pressure and/or in temperature. These properties of supercritical fluids provide a good extraction of the compounds due to their high dissolving power at high densities, and consequently a good fractionation and separation of the compound from the fluid (at lower densities) by reducing the pressure or changing the temperature in a separator. Another important factor is the penetrating power based in the high mass transfer rate of the solutes into the fluid. The low viscosity and high diffusivity of the supercritical fluid enhance this property allowing an efficient extraction of the compounds from the raw material. Supercritical extraction is not widely used yet, but as new technologies are coming there are more and more viewpoints that could justify it, as high purity, residual solvent content
and environment protection. The basic principle of SFE is that when the feed material is contacted with a supercritical fluid than the volatile substances will partition into the supercritical phase. After the dissolution of soluble material the supercritical fluid (SCF) containing the dissolved substances is removed from the feed material. The extracted component is then completely separated from the SCF by means of a temperature and/or pressure change. The SCF is then  may be recompressed to the extraction conditions and
recycled. Some of the advantages and disadvantages of SCFs compared to conventional liquid solvents for separations:

Advantages

 Dissolving power of the SCF is controlled by pressure and/or temperature

 SCF is easily recoverable from the extract due to its volatility

 Non-toxic solvents leave no harmful residue

 High boiling are extracted at relatively low temperatures

 Separations not possible by more traditional processes can sometimes be effected

 Thermally labile compounds can be extracted with minimal damage as low temperatures can be employed by the extraction

Disadvantages

 Elevated pressure required

 Compression of solvent requires elaborate recycling measures to reduce energy costs

 High capital investment for equipment

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