Curing
the root
Professor
Sandeep Roy gives a detailed insight into the utilisation of super critical
fluid extraction for reduction of residues of organic chemicals in the
final food product
The
employment of organic solvents for extracting active or key substances
from both synthetic and natural sources has been the traditional and
favoured route for over a century now. It is still, by far, the most
widely adopted technique for separating and purifying products in a
wide variety of industries, including the food industry. Nevertheless,
the reduction of undesirable residues of organic chemicals (suspected
carcinogens) in the final product remains the prime concern of the industry.
Increasing regulatory and environmental pressures in this aspect, particularly
over the last few decades, have propelled the search for optional technologies.
Added to this is the burgeoning global drive to phase out ozonedepleting
chemicals in the atmosphere, mostly chlorofluorocarbons.
Amongst the several innovations in this field, the commercial use of
supercritical fluids (SCFs) as a substitute to hydrocarbon solvents
has emerged as an attractive contender. SCF-based processes have helped
in eliminating the use of hexane and methylene chloride that were popularly
used.
Supercritical fluid extraction
The discovery of SCF dates back to more than a century.
A pure SCF can be defined as any compound at a temperature and pressure
above the critical values or critical point. For example, for carbon
dioxide (CO2), one of the most widely used SCFs. The critical temperature
and critical pressure are 31oC and 74 atm respectively.
Above the critical temperature of a compound the pure, gaseous component
cannot be liquefied regardless of the pressure applied. Around the critical
point the visual distinction between liquid and gas phases and the difference
between liquid and gas densities disappears. In the supercritical environment
only one can phase exist.
In short, a solvent for extraction combines the best of properties of
both gases and liquids.
For any substance as the critical point is approached, many of its important
properties undergo drastic changes like thermal conductivity, surface
tension, heat capacity and viscosity. Supercritical fluid extraction
(SCFE) is based precisely on this feature of rapid property change with
only slight variations of pressure in the vicinity of the critical point.
These drastic changes make SCFs a preferred choice over liquid solvents
with the same density. The basic principle of SCFE is that the solubility
of a given compound (solute) in a solvent varies with both temperature
and pressure. At ambient conditions, (say 25°C and 1 atm) the solubility
of a solute in a gas is generally negligible. In SCFs, however, solute
solubility corresponds to that in liquids that can be almost 10 orders
of magnitude higher than that in gases. In practical terms this means
SCFs can be used to extract a solute from a feed matrix as in conventional
(organic) liquid extraction.
Indeed, 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 gas-like diffusive properties. Thus, when the feed material (for
example, ground spice) is contacted with an SCF, the active substances
(spice oils, as well as oleoresins) in the matrix gradually partitions
into the supercritical phase.The supercritical fluid containing the
dissolved substances is then removed from the feed material contact.
Following which, the extracted component is separated from the SCF by
means of a simple process of pressure reduction. Essentially, this lowers
the fluid's solubilising power and precipitates the extracted substance.
The SCF after then may be recompressed to the extraction conditions
and recycled.
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