What is supercritical fluid extraction and why is it gaining popularity?

Supercritical fluid extraction (SCFE) is a technique that uses a fluid in the supercritical state to separate target molecules (required molecules) from plant- or animal- based raw material. A fluid at a pressure above its supercritical pressure and at a temperature above its supercritical temperature is a supercritical fluid. Such a fluid has some properties like a liquid, some like a gas, and some in between.

Most importantly, the solvent power of such a fluid can be manipulated with pressure changes to extract the target molecule in its relatively pure form. That is, supercritical fluid extraction separates the target molecules with minimal or zero inclusion of other molecules in the raw material.

Extracted target molecules are used in the food, pharmaceutical, and nutraceutical industries. The regulations in these industries with regard to safety, toxicity, residues, and quality are getting increasingly stringent. SCFE is gaining popularity because SCFE systems in India and the rest of the world are capable of separating target molecules in their relatively pure form.

Speaking of properties, a supercritical fluid has:

• Higher density like that of liquids that lends it higher solvent power.
• Lower viscosity like that of gases helps it to diffuse into porous solids and have greater mass transfer rates.
• Diffusivity midway between that of liquids and gases.
• Low surface tension as that of gases allowing it to seep more into porous solids.

Generally speaking, solvent power of a supercritical fluid increases with density. And the density of a supercritical fluid changes by a large amount with minor modifications in its pressure and temperature. In short, the solvent power of supercritical fluids can be tuned to extract a specific molecule i.e. these fluids are selective.

Selectivity-based versatility is another reason why producers prefer supercritical extraction in India and the rest of the world. Take for example the case of Euterpe oleracea, commonly known as cabbage palm. The SCFE process is able to isolate phenolic phytochemicals from cabbage palm at a certain pressure while isolating anthocyanin from the same cabbage palm at another pressure. Apart from the purity of extraction, which is absolutely necessary, such versatility of the SCFE process can cut the operational cost and time of extraction.

Carbon dioxide and water are the two most popularly used supercritical fluids. SCFE plants in India and elsewhere operate by first hiking the pressure and temperature of the fluid to above its supercritical pressure and temperature respectively. Raw material is placed in the high pressure extractor. The supercritical fluid passes through the raw material and dissolves the target molecule. Thereafter, the supercritical fluid enters the separator which operates at a pressure lower than that of the extractor. Here, the target molecule separates from the supercritical fluid because its solvent power decreases with falling pressure, and is collected for further processing.

Multiple extractors working at different pressures are used when the objective is to separate more than one target molecule from the raw material - the separation of anthocyanin and phenolic phytochemicals from cabbage palm being an example.

All in all, the precise and targeted nature of the SCFE process is a huge plus point which is making it more and more popular.