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J. Agric. Food Chem. 2008, 56, 3738–3744
From Wine to Pepper: Rotundone, an Obscure Sesquiterpene, Is a Potent Spicy Aroma Compound CLAUDIA WOOD,†,§,‡ TRACEY E. SIEBERT,†,‡ MANGO PARKER,†,‡ DIMITRA L. CAPONE,† GORDON M. ELSEY,† ALAN P. POLLNITZ,*,† MARCUS EGGERS,| MANFRED MEIER,| TOBIAS VÖSSING,| SABINE WIDDER,| GERHARD KRAMMER,⊥ MARK A. SEFTON,† AND MARKUS J. HERDERICH† The Australian Wine Research Institute (AWRI), P.O. Box 197, Glen Osmond (Adelaide), SA 5064, Australia; School of Agriculture, Pontificia Universidad Católica de Chile, Casilla 306 Correo 22, Santiago, Chile; and Symrise Scent and Care Division and Symrise Flavor and Nutrition Division, Mühlenfeldstrasse 1, D-37603 Holzminden, Germany
An obscure sesquiterpene, rotundone, has been identified as a hitherto unrecognized important aroma impact compound with a strong spicy, peppercorn aroma. Excellent correlations were observed between the concentration of rotundone and the mean ‘black pepper’ aroma intensity rated by sensory panels for both grape and wine samples, indicating that rotundone is a major contributor to peppery characters in Shiraz grapes and wine (and to a lesser extent in wine of other varieties). Approximately 80% of a sensory panel were very sensitive to the aroma of rotundone (aroma detection threshold levels of 16 ng/L in red wine and 8 ng/L in water). Above these concentrations, these panelists described the spiked samples as more ‘peppery’ and ‘spicy’. However, approximately 20% of panelists could not detect this compound at the highest concentration tested (4000 ng/L), even in water. Thus, the sensory experiences of two consumers enjoying the same glass of Shiraz wine might be very different. Rotundone was found in much higher amounts in other common herbs and spices, especially black and white peppercorns, where it was present at ∼10000 times the level found in very ‘peppery’ wine. Rotundone is the first compound found in black or white peppercorns that has a distinctive peppery aroma. Rotundone has an odor activity value in pepper on the order of 50000-250000 and is, on this criterion, by far the most powerful aroma compound yet found in that most important spice. KEYWORDS: Grape; wine; Shiraz; Syrah; rotundone; pepper; Piper nigrum; aroma; spice; GC-MS-O
INTRODUCTION
Pepper is one of the most potent spices added to food and is found on almost every dinner table. Black pepper (Piper nigrum) has been valued for its aroma and flavor for millennia. It was the principal spice imported by the Roman Empire (1, 2). When Alaric and the Visigoths laid siege to Rome, they demanded 3000 pounds of pepper as part of the ransom (1). Pepper was so important to the European economy in the 15th century that demand inspired an age of exploration, beckoning sailors to embark on voyages of discovery (2, 3). Just as the lure of pepper established the spice route (2) and led Columbus to discover the New World, our research into pepper aroma in Australian red wine has led us to identify an important impact odorant in peppercorns and other plant products. * Corresponding author (fax +61 8 8303 6601; e-mail ajpollni@ senet.com.au). † The Australian Wine Research Institute. § Pontificia Universidad Católica de Chile. ‡ These authors contributed equally to this work. | Symrise Scent and Care Division. ⊥ Symrise Flavor and Nutrition Division.
P. nigrum is a vine belonging to the Piperaceae family; it is indigenous to the southwestern (Malabar) coast of India and today is extensively cultivated in Southeast Asia and Brazil. Unripe fruits can be preserved as green peppercorns or fungally fermented to produce the black peppercorns that are highly valued for their delicious aroma and hot taste. White pepper is produced when the fruit (pericarp) is removed from the ripe red peppercorns. About three-fourths of all pepper sold is black pepper, with white pepper accounting for most of the remainder (3). Wine, made from grapes from the vine Vitis Vinifera, has also been enjoyed since ancient times. The aroma of wine is complex and reflects the grape variety and maturity, site, and winemaking, with hundreds of volatile compounds present. Some of these aroma differences can be related to individual aroma impact compounds. For example, methoxypyrazines are responsible for green capsicum (bell pepper) aromas in some cool climate wines (4, 5), and thiols such as 3-mercaptohexanol, 4-mercapto-4-methylpentan-2-one, and 3-mercaptohexyl acetate are responsible for passionfruit and tropical guava aromas in Sauvignon Blanc and other varieties of wine (6–8).
10.1021/jf800183k CCC: $40.75 2008 American Chemical Society Published on Web 05/08/2008
Rotundone Is the Pepper Aroma Impact Compound
Figure 1. Structures of (-)-rotundone, guaiol, and d5-rotundone.
Shiraz is a major red wine grape variety. Australian Shiraz is a medium- to full-bodied wine, usually oaked, encompassing a flavor spectrum from blackberry, plum, and mulberry to spicy, licorice, cigar box, eucalyptus, and black pepper. Shiraz is grown in Australia in a range of climates from hot sunny regions, which tend to produce full-flavored plum, spice, blackberry, and chocolate flavors, to cooler wetter regions, which sometimes produce more of the distinctive black pepper aroma, depending on the year of vintage (9, 10). Some of Australia’s most famous “icon” wines are made from Shiraz grapes, for example, Penfolds Grange and St. Henri, Henschke Hill of Grace and Mt. Langi Ghiran Langi Shiraz. Various studies on the compositional and sensory analysis of Shiraz, which is also known as Syrah or Hermitage, have been published (9–12). However, as for pepper itself, the compound(s) responsible for the distinctive ‘black pepper’ aroma in Shiraz grapes and wine has (have) remained unidentified. MATERIALS AND METHODS Rotundone (Figure 1) was synthesized from the sesquiterpene guaiol (Figure 1). Guaiol was extracted from guaiac wood oil (an essential oil from the guaiac tree Guajacum officinale) using an n-hexane/acetone mixture (2:1). After acetylation of guaiol using acetic anhydride, pyridine, and 4-dimethylaminopyridine (DMAP) in toluene, an allyl oxidation step was performed with either cobalt acetate, cobalt naphthenate, or cobalt(II) or cobalt(III) acetyl acetonate. Rotundone was then obtained via acetate pyrolysis and purified using column chromatography with silica gel 60 (70–230 mesh). From 1 kg of guaiac wood oil a main fraction of 15.6 g was obtained after chromatography yielding 42% rotundone (6.55 g), which was purified by further chromatography: EIMS, m/z (rel intensity) 219 (17), 218 ([M]+, 100), 204 (14), 203 (89), 189 (10), 177 (11), 175 (27), 163 (38), 162 (28), 161 (50), 147 (37), 137 (39), 135 (19), 133 (25), 121 (18), 120 (27), 119 (37), 107 (21), 105 (32), 95 (17), 93 (20), 91 (30), 79(18), 77 (18), 67 (15), 55 (11), 41 (17); 1H NMR (600 MHz, CDCl3), δ 4.72 (1H, app quint, J ) 0.9, H12a), 4.69 (1H, app quint, J ) 1.8, H12b), 2.98 (1H, ddq, J ) 11.2, 3.4 and 7.3, H10), 2.69 (1H, app quint, J ) 7.2, H4), 2.58 (1H, dd, J ) 18.6 and 6.6, Η3R), 2.56 (1H, dd, J ) 15.6 and 12.0, Η6R), 2.33 (1H, dt, J ) 15.6 and 1.8, Η6β), 2.00 (1H, m, H7), 1.95 (dt, J ) 18.6 and 1.6, Η3β), 1.81–1.74 (3H, overlapping m, H8,9R), 1.75 (3H, br s, H13), 1.51 (1H, m, Η9β), 1.10 (3H, d, J ) 7.2, H14), 0.99 (3H, d, J ) 7.2, H15); 13C NMR (150 MHz, CDCl3), δ 208.1, 176.9, 151.0, 145.4, 109.0, 46.1, 43.0, 37.9, 36.8, 32.6, 30.7, 26.9, 20.2, 19.2, 17.5. 1H NMR (600 MHz, CDCl3, run on a Varian Unity Inova spectrometer, using Varian vnmr6.1c software) and 13C NMR spectra (150 MHz, CDCl3) of the reference compound were in excellent agreement with those reported (13) except that the 1H NMR signal for H15 was not given in that paper. We obtained an optical rotation that was in the same direction and similar to the published value (13) of [R]D31 -76.5° (CHCl3; c 0.09). Retention indices (RI), 1685 (DB-1), 1700 (DB-5), 1890 (DB-1701), and 2280 (Wax), were in agreement with the Symrise GC-MS database and the literature (14). d5-Rotundone (Figure 1) was synthesized and characterized as described in the following paper (15). Samples. The red wine used as nonpeppery control was a young (
