Microwave extraction is oddly newer then its’ synthetic brethren. Not really sure why — digestion and synthesis had the jump — maybe, it was application or research driven. At any rate, this has quickly changed……and at the end of the day, may have a broader spectrum of usefulness. It probably seems obvious to the folks reading this blog that microwave solvent extraction of natural products is an area where work has been done ( a nice review has been published specifically on natural products 2010)…..that is true, although not as much as one would think. I believe this partly since the art and science of natural product extraction itself has been an arduous journey…..picking the right solvent, finding the right temperatures, purification of the concentrates, etc…..that paints a pictures. This picture has changed dramatically over the last ten years……The EPA approved microwave extraction for semivolatiles and non-volatiles in soil and solids…such as PCBs, PAHs, pesticides. Microwave extraction has also found its way into the extraction of API in simple and difficult formulations — more control, multiple samples, easy…..minimal solvent. And some of the more recent developments include — biomass, sedimentary rock and essential oil extraction in the flavor and fragrance industry…….I am sure you see the payoff and also why it makes sense.

Rather than an exhaustive account of a thousand references, I have highlighted a few areas where there has been some advancement in both, microwave-assisted solvent and solventless extraction.

1) EPA 3546 — microwave extraction of contaminants in soils, solids and sediments. This is a fairly benign application validated using hex, hex:acetone, CH2Cl2 and CH2Cl2 variations to extraction known analytes of interest 100-110 for 30-30 min. The acetone is used to drive through the matrix and absorb microwave power so that the mixture can be heated above its’ boiling point. You remember — for every ten degrees it cuts the rate in half. Very powerful application and nearly every environmental contract lab is using this for sample testing.

2) API extraction – There are several reports on the utility of microwave extraction of API in raw and formulated forms. Most of the activity is done on the development end of things so the number of journal articles has been limited. Having said that, a research group at Pfizer, in the middle as a transition to development published a couple of papers evaluating several techniques used in the pharma industry. A more in depth look can be found in a recent book by Beverly Nickerson on sample preparation of pharmaceutical dosage forms. To simplify, microwave heating is simply a heat source so solvent and temperature studies are developed to maximize extraction without degradation. As formulations become more challenging — and they are — this technology is adding promise to helping keep the process efficient. The benefit in the end is that multiple batches can be performed simultaneously – allowing for a reproducible method with a simple process (now hours and days turn into minutes).

3) Solventless (solvent-free) microwave extraction — this is the current state-of-the art technology, both in concept and practice. Championed primarily by Farid Chemat and Giancarlo Cravotto, essential-oil, biomass and natural products have been isolated in a much more efficient way. The primary advantage of this process comes from the advantage of using a plant’s water content as a microwave absorbing source — water is internally heated and the cells within a material burst with a release of product as part of the process. For a microwave configuration to work in this manner, volatile components are condensed into a trap over the microwave as you would set up a distillation, and heavier or higher molecular weight compounds are isolated below the microwave — which can be cooled or isolated under reduced pressure….actually the process is quite cool and extremely effective. A few resources are shown below but there have been a number of books and journal articles to both processes:

Solvent Free Microwave Extraction: An Innovative Tool for Rapid Extraction of Essential Oil from Aromatic Herbs and Spices 2004 The Journal of Microwave Power and Electromagetic Energy.

Microwave Assisted Extraction for Bioactive Compounds 2013. This is a comprehensive book with several chapters dedicated to basic microwave processing and applications.

Green Extraction of Natural Products: Concepts and Principles 2012 (International Journal of Molecular Sciences)

Solvent-free microwave extraction of essential oil from aromatic herbs: From laboratory to pilot and industrial scale: Food Chemistry

Go ahead and give some of these a read — it is an expanding area of research and application. As we start to move into more enabling technologies, the research is starting to stretch into utilizing natural resources to provide more efficient processes.