molecules Communication An Example of a Novel Efficient Plant Extraction Technique: Electromagnetic Induction Heating Francesco Epifano * , Francesca Preziuso, Vito Alessandro Taddeo, Serena Fiorito and Salvatore Genovese Dipartimento di Farmacia, Università “G. d’Annunzio” of Chieti-Pescara, Via dei Vestini 31, 66100 Chieti Scalo (CH), Italy; [email protected] (F.P.); [email protected] (V.A.T.); serena.fi[email protected] (S.F.); [email protected] (S.G.) * Correspondence: [email protected]; Tel.: +39-0871-3554654 Received: 15 October 2018; Accepted: 20 November 2018; Published: 21 November 2018 Abstract: A simple and easy to handle extraction procedure based on the use of electromagnetic induction heating is described. To assess the potential, scopes, and limitations of this novel process, extraction and subsequent HPLC quantification of emodin from an hydroalcoholic extract of rhizome of Rheum palmatum (Chinese rhubarb) was selected as the reference experiment. Maceration at room temperature and by heating, ultrasound-assisted, and microwave-assisted extractions were also carried out for comparison. Results obtained with electromagnetic induction heating showed that this methodology performed largely better both in terms of time process and extraction yields. Keywords: anthraquinone; electromagnetic induction heating; emodin; extraction methodologies; HPLC; Rheum palmatum 1. Introduction Conventional extraction methodologies of plants include maceration, heating, or refluxing in the presence of solvents with different polarities [1]. Drawbacks currently associated with such processes often include the loss of the desired secondary metabolites due to the occurrence of several side reactions like hydrolysis, air oxidation, and chemical degradation in general. These are also due to the long operation time required to accomplish the above-mentioned experimental steps. Thus, quite recently, the attention of academic and industrial research has been focused on the search for alternative and more efficient plant extraction processes and these include ultrasound [2] and microwave-assisted techniques [3], pulsed electric field extraction [4], the use of supercritical fluids [5], and pressurized liquids [6]. However, most of the recent set-up methodologies may have some drawbacks in some cases (e.g., use of expensive equipment and maintenance). Hence, the search for novel, alternative, more economic, and easy to handle procedures to accomplish plant extraction is a field of current and growing interest. In this short communication, we wish to describe a simple, very rapid, and effective method based on the use of electromagnetic induction heating. Extraction and subsequent HPLC quantification of emodin (Figure1) from the rhizome of Chinese rhubarb, Rheum palmatum L. (Polygonaceae) was selected as the reference process. Molecules 2018, 23, 3048; doi:10.3390/molecules23113048 www.mdpi.com/journal/molecules Molecules 2018, 23, 3048 2 of 6 Molecules 2018, 23, x FOR PEER REVIEW 2 of 6 OH O OH OH O Figure 1. Structure of emodin. 2. Results and Discussion Figure 1. Structure of emodin. To test the effectiveness of electromagnetic induction heating as an efficient plant extraction 2. Results and Discussion methodology, four other processes with an equal amount of solvent and vegetable material were carried out:To test “classic” the effectiveness maceration of (forelectromagnetic 15 days), macerating induction heating at reflux as temperaturean efficient plant (85 ◦ Cextraction for 4 h),methodology, ultrasound-assisted four other and processes microwave-assisted with an equal processes amount (80 of ◦solventC both and for 10vegetable min), employing material were the samecarried optimized out: “classic” conditions maceration that we(for have 15 days), adopted macera for theting extraction at reflux temperature of anthraquinones (85 °C fromfor 4 h), plantsultrasound belonging-assisted to the genusand microwaveRhamnus-assisted[7]. The p indicatedrocesses (80 times °C andboth temperatures for 10 min), employing for these latterthe same experimentaloptimized steps conditions correspond that towe the have best adopted extractive for obtainable the extraction yields of in theanthraquinones reference compound from plants emodin.belonging For the to electromagnetic the genus Rhamnus induction [7]. heating-assisted The indicated extractiontimes and of temperatures the rhizome powderfor these of R.latter palmatumexperimental, the following steps correspond parameters to were the best screened: extractive induction obtainable power yields (from in 1 the to 9,reference from 200 compound W to 1800emodin. W) and timeFor the (10 electromagnetic s to 1 min). After induction several attempts, heating- theassisted optimized extraction experimental of the rhizome conditions powder were of R. set atpalmatum an induction, the following power of parameters 9 (corresponding were screened: to 1800 W induction and 83 ◦C power into the (from reaction 1 to 9, vessel) from 2 and00 W 30 to s. 1800 The firstW) and value time allowed (10 s ato rapid 1 min). boiling After of several the extraction attempts, solvent the optimized (<10 s), while experimental extending theconditions process timewere set overat 30 an s. induction It did not resultpower in of better 9 (corresponding yields of emodin, to 1800 and, W and more 83 disadvantageously, °C into the reaction a vessel) deep darkening and 30 s. The of thefirst extraction value allowed solution a rapid (probably boiling indicating of the extraction a chemical solvent degradation (<10 s), ofwhile the chemicalextending components) the process time andover a massive 30 s. It evaporation did not result of in the better solvent yields (thus, of emodin, rendering and, the more subsequent disadvantageously, work-up more a deep difficult) darkening occurred.of the Theextraction content solution of emodin (probably in each indicating of the five extractsa chemical obtained degradation was assessed of the chemical by HPLC components) analysis coupledand toa massive UV/Vis evaporation detection, following of the solvent the well-validated (thus, rendering methodology the subsequent we set-up work in-up 2010 more for difficult) the anthraquinonesoccurred. The screening content inof plantsemodin belonging in each of to the the five genus extractRhamnuss obtained[7]. The was calibration assessed by curve HPLC of pure analysis emodincoupled was drawnto UV/Vis using detection, weighted following (1/x2) linear the well least-squares-validated regressionmethodology analysis we set and-up shown in 2010 to for be the linearanthraquinones along the entire screening concentration in plants range belonging applied, to providing the genus r 2Rhamnusvalues ≥ [7]0.9996.. The c Thealibration retention curve times of pure of emodinemodin detected was drawn in each using extract weighted matched (1/x that2) linear recorded least from-squares chromatographic regression analysis runs of pureand shown emodin. to be Repeatability,linear along precision, the entire and concentration trueness of range the analytical applied, providing methodology r2 values were ≥ assessed 0.9996. The as previouslyretention times reportedof emodin [8]. Results detected of the in quantificationeach extract matched of emodin that in extractsrecorded obtained from chromatographic by the different extractionruns of pure methodologiesemodin. Repeatability, listed herein precision, are reported and in trueness Table1, whileof the Figureanalytical2 illustrates methodology the chromatogram were assessed as of rhubarbpreviously extract reported obtained [8]. Results after application of the quantification of electromagnetic of emodin induction in extracts heating. obtained Figure by the2 alsodifferent accountsextraction for the methodologies choice of emodin liste andd hereinR. palmatum are reportedas the referencein Table compound.1, while Figure The best2 illustrates baseline the separationchromatogram (allowing of arhubarb sharp quantification) extract obtained of after the peak application of this anthraquinoneof electromagnetic and induction no co-elution heating. of otherFigure phytochemicals 2 also accounts were for the achieved choice of among emodin the and several R. palmatum chemical as standardsthe reference and compound. plant extracts The best thatbaseline were screened separation for our(allowing purposes. a sharp Results quantification) from Table of1 clearlythe peak indicate of this howanthraquinone electromagnetic and no co- inductionelution heating of other performed phytochemicals much better were thanachieved all other among extraction the several methods chemical applied. standards The increase and plant in emodinextracts concentration that were screened was from for nearly our 3-fold purposes. to more Results than 5-fold.from ThisTable positive 1 clearly trend indicate may be how explainedelectromagnetic by the most induction favorable heating features performed of electromagnetic much better than induction all other heating extraction in that methods a uniform applied. heatingThe across increase the in whole emodin of the concentration extraction vessel was from can benearly achieved 3-fold in to a fewmore seconds than 5- leadingfold. This to positive a massive trend and rapidmay be release explained of secondary by the metabolitesmost favorable from plantfeatures cells of and electromagnetic preventing their induction chemical heating degradation, in that a pendinguniform a short-lasting heating across process the whole being of applied. the extraction