Posted on 25 April 2016
From food and drink to dietary supplements and cosmetics, consumers favour “natural” products. But it can be a challenge to find the best processes for extracting key natural substances from plant matter, either to use as ingredients – such as naturally occurring anti-oxidants, flavours or colours – or to remove them, as in the decaffeination of coffee.
The use of chemical solvents is increasingly seen as undesirable. So there has been a trend towards using carbon dioxide, in its special “supercritical” fluid form, as a natural solvent for these extractions. But unfortunately, supercritical CO2 (scCO2) is often a poor solvent for many interesting natural substances, and its solvency has to be boosted with an “entrainer” – a small amount of another solvent such as water or ethanol, which can dramatically increase the solubility.
Until now, the way in which these entrainers boost solubility with scCO2 has only been partially understood, which has made it difficult for food scientists to know how to select the perfect entrainer for any given foodstuff, or the optimal conditions (e.g. temperature, pressure) for the process.
A new paper published online on 11 April by Dr Seishi Shimizu of the York Structural Biology, Department of Chemistry, uses fundamental thermodynamic theory to solve this problem. Shimizu disproves two of the common hypotheses regarding the behaviour of entrainers in scCO2 extraction processes, concluding that it is molecular-level affinity between the entrainer and the solute (e.g. caffeine) which determines efficacy. The more the entrainer likes to associate with the substance being extracted, the stronger the effect.
But these conclusions do not remain in the realm of dry theory. With collaborator Professor Steven Abbott of industrial process consultancy TNCF Ltd, Dr Shimizu has created a free, open-source app that runs on phones, tablets and laptops. The paper, published in the Journal of Physical Chemistry, focuses on the scientific theory, but it also encourages readers to go to the app and play with all the factors involved in the process.
The beauty of this approach is that it does not involve any complicated analytical equipment. It simply uses the data that the scCO2 community regularly gathers – solubilities and densities at various temperatures and pressures. The app can read the data directly and do all the calculations to reveal which elements of the process (such as entrainer-solute interactions) are the most important.
Prof. Steven Abbott, TNCF Ltd
Both guiding theory and app will be welcomed by a wide range of industrial scientists looking to optimise their extraction processes.
Shimizu, S & Abbott, S 2016, 'How Entrainers Enhance Solubility in Supercritical Carbon Dioxide'Journal of Physical Chemistry B., (advance online press http://pubs.acs.org/doi/abs/10.1021/acs.jpcb.6b01380 )