Dutch advance boosts success of supercritical extraction process

1st April 2013

A supercritical fluid is one whose temperature and pressure have been raised above a critical point. Above the critical temperature, the gaseous component of a compound cannot be liquefied no matter how much pressure is applied. The critical pressure is the vapour pressure of the gas at this critical temperature.

In this supercritical situation, only one phase exists. Neither a gas nor a liquid, it is termed a fluid. In terms of solubility this fluid behaves like a substance in the liquid phase, but with the transport properties of a gas. It is this mixture of useful properties that give supercritical extraction a substantial edge over conventional extraction processes.

When a material of interest comes into contact with a fluid in the supercritical state, volatile substances partition into the supercritical phase. From here they can be isolated using pressure or temperature changes. The supercritical fluid itself can then be recycled and reused. It's no surprise, therefore, that supercritical extraction holds a number of advantages over conventional processes. For example, the dissolving power is governed by pressure and temperature changes; the supercritical fluid is easily recovered; as the solvents are non-toxic, there are no harmful residues; high boiling point components can be extracted at relatively low temperatures; and, as low temperatures can be used, it is not so difficult to extract heat-sensitive products.

As with conventional extraction techniques, the choice of solvents is crucial. Typical considerations are whether or not the solvent is inert to the product, whether it can be separated easily, and whether or not it is cheap. Because of its low cost and low toxicity, carbon dioxide is the most commonly used supercritical fluid today. However, many other fluids have been investigated with some becoming commercial successes. These include ethane, propane, ammonia, water, cyclohexane, toluene, tricholormethane and chlorotrifluoromethane.

And industry is becoming more interested in supercritical technology. For example, in the petrochemical industry, it is used for the regeneration of used oils and lubricants. In the pharmaceutical industry it used to isolate active ingredients from plants. Here, its ability to prevent both physical and thermo degradation is key.

For the food and drink industry, one of the main benefits of such fluids is that the residual solvent is easily removed from the product, whether it is in the extract or the extracted matrix. Today, its biggest applications are in the decaffeination of tea and coffee and the extraction of essential oils ­ both flavour and aroma ­ from spices. Brewers also use the method to extract flavour from hops. The only real disadvantages with such processes are that they require elevated pressures, can involve high capital expenditure on equipment, and are batch procedures.

Now Dutch researchers in the department of Agrotechnology and Food Innovations at Wageningen University claim to have developed a continuous process as part of the EU'sC-REX project.

In contrast to other conventional processes, they say, this new innovation allows an easily-controlled continuous process that leads to higher extraction yields. The result is reduced energy consumption, reduced carbon dioxide loss and lower processing costs.

An extruder is at the heart of the new, continuous process(Fig. 1). This machine, which can be roughly described as two rotating screws in a metal tube, is normally used for mixing and shaping plastics and food products.

The Dutch researchers have adapted the machine in such a way that it can be employed as a high-pressure vessel in which the continuous extraction can take place.

First, they create two dynamic material plugs (or filled zones), one at the beginning and one at the end of the extruder(Fig. 2). In between the plugs they inject supercritical carbon dioxide. Because the plugs are highly compressed they can withstand the high pressure and stay gas proof. The carbon dioxide mixes with the solids, and automatically flows in the opposite direction of the solids taking up all the target components. Finally, the carbon dioxide and dissolved products are released from the extruder through a filter after which the extract can be easily separated. In doing this, the scientists have created a continuous countercurrent extracto.

"We have performed successful experiments with caraway seeds, where we extract the essential oil carvone, and on hops where we have extracted important acids used in beer production. In the case of the hops, we have already reached an extraction pressure of 150bar in the extruder,“ researcher Frank Giezen told eFood.





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