Article pubs.acs.org/JAFC
Flavor, Glucosinolates, and Isothiocyanates of Nau (Cook’s Scurvy Grass, Lepidium oleraceum) and Other Rare New Zealand Lepidium Species Catherine E. Sansom,† Veronika S. Jones,‡ Nigel I. Joyce,§ Bruce M. Smallfield,§ Nigel B. Perry,† and John W. van Klink*,† †
The New Zealand Institute for Plant & Food Research Ltd., Chemistry Department, University of Otago, Box 56, Dunedin 9054, New Zealand ‡ The New Zealand Institute for Plant & Food Research Ltd., Private Bag 11600, Palmerston North 4442, New Zealand § The New Zealand Institute for Plant & Food Research Ltd., Private Bag 4704, Christchurch 8140, New Zealand S Supporting Information *
ABSTRACT: The traditionally consumed New Zealand native plant nau, Cook’s scurvy grass, Lepidium oleraceum, has a pungent wasabi-like taste, with potential for development as a flavor ingredient. The main glucosinolate in this Brassicaceae was identified by LC−MS and NMR spectroscopy as 3-butenyl glucosinolate (gluconapin, 7−22 mg/g DM in leaves). The leaves were treated to mimic chewing, and the headspace was analyzed by solid-phase microextraction and GC−MS. This showed that 3-butenyl isothiocyanate, with a wasabi-like flavor, was produced by the endogenous myrosinase. Different postharvest treatments were used to create leaf powders as potential flavor products, which were tasted and analyzed for gluconapin and release of 3butenyl isothiocyanate. A high drying temperature (75 °C) did not give major glucosinolate degradation, but did largely inactivate the myrosinase, resulting in no wasabi-like flavor release. Drying at 45 °C produced more pungent flavor than freezedrying. Seven other Lepidium species endemic to New Zealand were also analyzed to determine their flavor potential and also whether glucosinolates were taxonomic markers. Six contained mostly gluconapin, but the critically endangered Lepidium banksii had a distinct composition including isopropyl glucosinolate, not detected in the other species. KEYWORDS: Lepidium oleraceum, Brassicaceae, glucosinolate, gluconapin, 3-butenyl isothiocyanate, myrosinase, postharvest
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INTRODUCTION The plant genus Lepidium (family Brassicaceae) comprises at least 175 herb species worldwide, with common names such as peppercress, peppergrass, and pepperwort reflecting their pungent flavors.1,2 In the islands of Aotearoa/New Zealand the first peoples, the Maori, used a plant that they called nau, ̅ ngau, or heketara as a food.3,4 The same plant was one of those used by Captain Cook during his explorations of New Zealand to help prevent scurvy,5 so it became known as Cook’s scurvy grass.6 This strong-smelling herb has been classified as Lepidium oleraceum Sparrm. ex G.Forst., endemic to New Zealand.7 However, the taxonomic status of L. oleraceum has been unclear because of its broad distribution around New Zealand and its morphological variability, and a recent revision recognized a total of 16 Lepidium species endemic to New Zealand, many of which are threatened in their natural environments.8 The Brassicaceae family is well-known for its array of glucosinolates and isothiocyanates, which have been exploited as flavors (e.g., wasabi) and also offer a variety of health benefits (e.g., broccoli).9−13 Physical disruption of cells, e.g., by chewing, leads to reaction between glucosinolates and myrosinase enzymes that are compartmentalized separately in vacuoles within “S-cells” and myrosin cells, respectively.10 This reaction hydrolyzes the sugar from the glucosinolate to produce isothiocyanates, plus nitriles and thiocyanates depending on the conditions and other enzymes and metals present.14 Lepidium © 2015 American Chemical Society
species have a range of reported glucosinolates, including those with alkyl, aromatic, and sulfur-containing side chains.10 However, no glucosinolates have been reported from L. oleraceum or the other New Zealand Lepidium species. The only chemistry-related report describes the antioxidant capacity of L. oleraceum.15 We are investigating endemic New Zealand species with traditional food uses as potential novel commercial flavors.16 We now report our investigation into the flavor potential of L. oleraceum. The leaves were analyzed using LC−MS for the glucosinolates and headspace solid-phase microextraction (HSSPME) GC−MS for the isothiocyanates released by simulated chewing. The L. oleraceum powders were tasted to evaluate their flavor profiles. We also analyzed the glucosinolate content of seven other Lepidium species endemic to New Zealand, all of which have a conservation classification of “threatened/ nationally vulnerable”, including those from the remote Kermadec and Chatham Islands.7,17 Received: Revised: Accepted: Published: 1833
December 3, 2014 January 26, 2015 January 27, 2015 January 27, 2015 DOI: 10.1021/jf505859u J. Agric. Food Chem. 2015, 63, 1833−1838
Article
Journal of Agricultural and Food Chemistry
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glucosinolates by this method compared to using heated MeOH. Extracts were then centrifuged at 20817g for 10 min, and the supernatant was diluted to a final volume of 25 mL with H2O. An aliquot was filtered (0.22 μm) into glass vials for LC−MS analysis. The LC−MS system consisted of a Thermo Electron Corp. (San Jose, CA) Finnigan Surveyor MS pump, Finnigan MicroAS autosampler, and Finnigan Surveyor PDA detector and a ThermaSphere TS-130 column heater (Phenomenex, Torrance, CA). A 2 μL aliquot of each prepared extract was separated with a mobile phase consisting of 0.1% formic acid in H2O (A) and 0.1% formic acid in MeCN (B) by reversed-phase chromatography. The column used was a 150 × 3 mm i.d., 3 μm, 80 Å, Luna C18(2) with a 4 × 2 mm i.d., 10 μm, guard column of the same material (Phenomenex), maintained at 30 °C with a flow rate of 200 μL/min. A gradient was applied: 0−3 min, 98% A; 15 min, 85% A; 20 min, 60% A; 25 min, 50% A; 30−33 min, 2% A; followed by a return to the starting condition over 5 min. The eluent was scanned by PDA (190−390 nm) and LTQ 2D linear ion-trap API-MS (ThermoFinnigan, San Jose, CA) with electrospray ionization (ESI) in the negative mode. Data were acquired for precursor masses from m/z 250 to m/z 1000 with Pulsed Q collision-induced dissociation (PQD) fragmentation at 40 arbitrary units. PQD fragmentation at MS2 allowed collection of lower m/z data (
