ARTICLE pubs.acs.org/JAFC Identification and Quantification of Major Steviol Glycosides in Stevia rebaudiana Purified Extracts by 1H NMR Spectroscopy Valerio Pieri,† Andrea Belancic,‡ Susana Morales,‡ and Hermann Stuppner*,† † Institute of Pharmacy/Pharmacognosy, Center for Molecular Biosciences Innsbruck (CMBI), University of Innsbruck, Innrain 52c, 6020 Innsbruck, Austria ‡ Prodalysa Ltda, Lote 13, Parque Industrial Gulmue, Camino Internacional, Concon, Chile bS Supporting Information ABSTRACT: The use of 1H NMR spectroscopy for the characterization of Stevia rebaudiana extracts is presented. The developed method allows qualitative and quantitative determination of the major steviol glycosides in purified extracts and fractions obtained from various stages of the purification process. Moreover, it proved to be a powerful tool to differentiate between glycosides which are naturally occurring in the stevia plant and artifacts formed in the course of the manufacturing process. Identification of steviol glycosides was achieved by the use of 2D NMR techniques, whereas quantification is based on qHNMR using anthracene as internal À standard. The solvent mixture pyridine-d5 DMSO-d6 (6:1) enabled satisfactory separation of the signals to be integrated. Validation of the method was performed in terms of specificity, precision, accuracy, linearity, robustness, and stability. Quantitative results were compared to those obtained with the JECFA HPLCÀUV method and were found to be in reasonable agreement. NMR analysis does not rely on the use of reference compounds and enables significantly faster analysis compared to HPLCÀUV. Thus, NMR represents a feasible alternative to HPLC-based methods for the quality control of Stevia rebaudiana extracts. KEYWORDS: Stevia rebaudiana, steviol glycosides, stevioside, rebaudioside A, qHNMR, quality control ’ INTRODUCTION substitutes” must be substantiated as well. This is of critical fi Steviol glycosides are ent-kaurene diterpene glycosides found importance, because consumers have speci c expectations for in the leaves of Stevia rebaudiana Bertoni (Asteraceae), a plant products claimed as natural, which include the use of materials of native to Paraguay and Brazil. These constituents are responsible natural origin, processed without the aid of methods that could for the sweet taste of the leaves and extracts derived thereof. potentially alter the naturally occurring substances. In this regard, Purified S. rebaudiana extracts are used as natural noncaloric quality control procedures should be able to detect the presence sweeteners and represent an alternative to sucrose and artificial of chemicals such as residual solvents and artifacts formed during the manufacturing process that could modify the native glycosi- sweeteners. Their regulatory status largely varies by country and fi determines the availability of S. rebaudiana purified extracts as dic composition. Thus, the puri cation of steviol glycosides food additive or dietary supplement. It was only very recently that represents an excellent case study which combines the need for a positive opinion on the safety of steviol glycosides was issued by standardization, food safety, and the expectation of customers for the European Food Safety Authority (EFSA),1 which will be of natural products. Quality assessment relies on the use of validated analytical outmost importance in view of a future EU approval. fi Known steviol glycosides include stevioside (St), rebaudioside methods. By far the most popular approach for the quanti cation of the individual steviol glycosides is LC in combination with UV A (RbA), rebaudioside B (RbB), rebaudioside C (RbC), rebau- À or MS detection.3 5 The recently revisited Joint FAO/WHO dioside D (RbD), rebaudioside F (RbF), dulcoside A (DuA), 3 rubusoside (Rub), and steviolbioside (Stb) (Chart 1). The Expert Committee on Food Additives (JECFA) method is fi based on RP-HPLCÀUV and enables separation and quantifica- composition of puri ed Stevia extracts ultimately depends on ff the production approach employed by the manufacturer. Efforts tion of nine steviol glycosides. This approach su ers from the to improve the sweetening properties of the final products intrinsic limitations of chromatography-based methods, namely, 2 the need of standard compounds for analyte identification/ resulted in the development of several manufacturing strategies. fi While some of them have been directed toward the isolation of quanti cation and relatively long analysis times. In addition, as specific constituents, such as RbA, others have focused on the column performance worsens over time, retention time drifts and fi insufficient separation may arise, which result in overall poor puri cation of the total glycosidic fraction, avoiding any mod- 6 ification of the native glycosidic composition while removing reproducibility. specific impurities. Quality control procedures for Stevia sweeteners must be Received: December 23, 2010 employed to ensure that standardization and safety requirements Accepted: February 25, 2011 are met, as set by regulatory agencies. In addition, claims referring Revised: February 25, 2011 to consumer products as “natural extracts” or “natural sugar Published: March 21, 2011 r 2011 American Chemical Society 4378 dx.doi.org/10.1021/jf104922q | J. Agric. Food Chem. 2011, 59, 4378–4384 Journal of Agricultural and Food Chemistry ARTICLE Chart 1. Structures of Stevioside, Rebaudioside A, Rebau- as mobile phase. Detection was performed with H2SO4 (5% v/v dioside B, Rebaudioside C, Rebaudioside D, Rebaudioside F, methanolic solution). Residual water content was determined using a a Dulcoside A, Rubusoside, and Steviolbioside Perkin-Elmer (Wellesley, MA) TGA 7 thermogravimetric analyzer (heating rate 10 K/min). Liquid extracts were freeze-dried using a Thermo Scientific Heto PowerDry PL6000 freeze-dryer. Samples and Reference Compounds. Purified S. rebaudiana extracts (SR-1 to SR-8) and liquid extracts from the different stages of the manufacturing process (SR-9 to SR-15) were obtained from Prodalysa LTDA. St (declared purity g98%), was purchased from Sigma Aldrich. RbA (95.0% purity by NMR against anthracene; residual water content 4.2% by thermogravimetry) was purchased from different suppliers. The amount of RbA used for recovery and linearity experi- ments was corrected using its purity as determined by NMR. Anthracene was purchased from Sigma Aldrich (declared purity >99%). Isolation of RbB from Sample SR-7. Sample SR-7 (507 mg) was fractionated by CC over silica gel (300 g, 90 Â 3.5 cm) using À À CHCl3 MeOH H2O (60:40:8; v/v/v) as mobile phase (ca. 1.5 mL/ min). Fractions were collected every three minutes and monitored by TLC. Fractions 137À150 were combined, and the resulting solution was evaporated under reduced pressure to yield 23 mg of RbB as determined by NMR and HPLCÀMS investigations. NMR. NMR spectra were acquired at 300 K with a Bruker (Bruker Biospin, Rheinstetten, Germany) Avance II 600 spectrometer equipped with a Bruker 5 mm TXI probehead with Z-gradient, using pyridine-d5 (99.50%), DMSO-d6 (99.90%, containing 0.03% TMS), deuterium oxide (99.90%), methanol-d4 (99.8%), and acetonitrile-d3 (99.80%), all purchased from Euriso-Top (Saint-Aubin, France). Data acquisition and processing were done with Bruker Topspin 2.1. 1H NMR spectra were acquired using the Bruker zg0 or zg0pr pulse programs using the ° following settings: relaxation delay (d1) = 9 s, flip angle = 45 , acquisition time (AQ) = 2.66 s, FID data points = 64K, spectral width = 20 ppm, number of scans = 32. For experiments using presaturation the transmitter offset was manually set in order to achieve optimal suppres- sion of the residual water signal. The acquired FIDs were Fourier aSt = stevioside; RbA = rebaudioside A; RbB = rebaudioside B; RbC = transformed to yield spectra with 64K data points. Manual phase rebaudioside C; RbD = rebaudioside D; RbF = rebaudioside F; DuA = correction and automatic polynomial baseline correction were always Dulcoside A; Rub = rubusoside; Stb = steviolbioside; Glc = glucose; used. Chemical shift values were referenced to the residual solvent Rha = rhamnose; Xyl = xylose. signals or to the TMS signal. Signal integration was performed without inclusion of 13C satellites. InversionÀrecovery experiments were per- 1 formed using the Bruker t1ir pulse program, with standard acquisition The increasing popularity of quantitative H NMR (qHNMR) parameters. T1 values were calculated using the T1 relaxation routine fi 7 in the eld of natural products analysis is well documented. (Topspin 2.1). 2D COSY, HSQC, and HMBC NMR spectra were Contrary to chromatography-based methods, qHNMR repre- acquired using the Bruker pulse programs cosygpqf, hsqcedetgp, and ff sents a completely di erent approach which is not dependent on hmbcgplpndqf respectively, with standard acquisition parameters. HSQC analyte separation. This enables faster analysis characterized by experiments for samples SR-1 and SR-8 were performed by employing a 8 excellent reproducibility and robustness. Analyte identification reduced chemical shift window (92À106 ppm) in order to obtain better is supported by 2D NMR techniques, and quantification does not resolution of adjacent cross peaks in the anomeric region.9 The acquired rely on external calibration, thus eliminating the need for 2D data was Fourier transformed and manually phase corrected. standard compounds. This work deals with the development NMR Method Validation. All NMR spectra for method validation À and validation of a qHNMR