T

Desolventising and toasting

Meal

Fig. 8.7 Flow diagram for soybean oil extraction (from Aguilera and Stanley,

viscosity and soften for flaking. Distillation is required to separate the solvent and extracted oil since solvent extraction also results in a carry over of impurities leading to a need for further oil refining. Solvents for extraction include hydrocarbons, alcohols, ketones and water.

The plant cellular structure is disrupted progressively by crushing and flaking. Pressing is an alternative process that can be carried out hydrauli-cally or in a screw press, although while the pressed material is free of solvent contaminants the efficiency of extraction is less. Following defat-ting, the protein in the meal can be removed by water extraction using similar approaches to those used in oil extraction.

In beet extraction for sugar, beet is cut into thin slices or cossettes. Heating above 50-60 °C causes plasmolysis leading to the cell membrane and cytoplasm becoming permeable.

Fruit juice extraction occurs by pressing or squeezing. Primary extraction again involves heat treatment of fruit slices to achieve plasmolysis. Secondary extraction involves treatment of the residue or pomace. Spices, flavour and colours are also extracted using variations of the same approaches.

In terms of animal residues, a process of rendering is employed involving pressure cooking, then centrifugation followed by pressing of the residue to obtain solid fats.

186 Handbook of waste management and co-product recovery Supercritical fluid extraction

The behaviour of a fluid in the supercritical state can be described as that of a very mobile liquid. The solubility behaviour approaches that of the liquid phase while penetration into a solid matrix is facilitated by the gaslike transport properties. As a consequence, the rates of extraction and phase separation can be significantly faster than for conventional extraction processes. Supercritical fluid extraction (SFE) is known to be dependent on the density of the fluid, which in turn can be manipulated through control of the system pressure and temperature. A supercritical fluid can be used to extract a solute from a feed matrix as in conventional liquid extraction. However, unlike conventional extraction, once the conditions are returned to ambient the quantity of residual solvent in the extracted material is negligible.

A principal advantage is that the dissolving power of the supercritical fluid is controlled by pressure and/or temperature and it is easily recoverable from the extract because of its volatility. Carbon dioxide is the most commonly used supercritical fluid, due primarily to its low critical parameters (31.1 °C, 7.38 MPa), low cost and non-toxicity. Separations that are not possible using more traditional processes can sometimes be effected and thermally labile compounds can be extracted with minimal damage, since low temperatures can be employed in the extraction.

The disadvantages of SFE arise from the high capital investment required for equipment, largely as a result of the elevated pressures involved and the need for compression of the solvent, which requires elaborate recycling measures to reduce energy costs. Notwithstanding, the special properties of supercritical fluids bring certain advantages to chemical separation processes (Rozzi and Singh, 2002).

Food and flavouring

The advantage of SFE here is that the residual solvent can be easily removed from the product no matter whether it is the extract or the extracted matrix. The biggest application is the decaffeination of tea and coffee. Other important areas of use are the extraction of essential oils and aroma materials from spices. The brewery industry uses supercritical fluids for the extraction of flavours from hops. The method is also used in extracting some edible oils and producing cholesterol-free egg powder.

Pharmaceutical industry

SFE is used to produce active ingredients from herbal plants while avoiding thermal or chemical degradation. The elimination of residual solvents from the products applies equally to wastes and co-products.

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