Important Odorants of Four Brassicaceae Species, and Discrepancies Between Glucosinolate Profiles and Observed Hydrolysis Products
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foods Article Important Odorants of Four Brassicaceae Species, and Discrepancies between Glucosinolate Profiles and Observed Hydrolysis Products Luke Bell 1 , Eva Kitsopanou 2, Omobolanle O. Oloyede 2 and Stella Lignou 2,* 1 School of Agriculture, Policy and Development, University of Reading, Whiteknights, Reading RG6 6AR, UK; [email protected] 2 Sensory Science Centre, Department of Food and Nutritional Sciences, Harry Nursten Building, University of Reading, Whiteknights, Reading RG6 6DZ, UK; [email protected] (E.K.); [email protected] (O.O.O.) * Correspondence: [email protected]; Tel.: +44-(0)118-378-8717 Abstract: It is widely accepted that the distinctive aroma and flavour traits of Brassicaceae crops are produced by glucosinolate (GSL) hydrolysis products (GHPs) with other non-GSL derived com- pounds also reported to contribute significantly to their aromas. This study investigated the flavour profile and glucosinolate content of four Brassicaceae species (salad rocket, horseradish, wasabi, and watercress). Solid-phase microextraction followed by gas chromatography-mass spectrometry and gas chromatography-olfactometry were used to determine the volatile compounds and odorants present in the four species. Liquid chromatography-mass spectrometry was used to determine the glucosinolate composition, respectively. A total of 113 compounds and 107 odour-active components Citation: Bell, L.; Kitsopanou, E.; were identified in the headspace of the four species. Of the compounds identified, 19 are newly Oloyede, O.O.; Lignou, S. Important reported for ‘salad’ rocket, 26 for watercress, 30 for wasabi, and 38 for horseradish, marking a signifi- Odorants of Four Brassicaceae cant step forward in understanding and characterising aroma generation in these species. There were Species, and Discrepancies between several non-glucosinolate derived compounds contributing to the ‘pungent’ aroma profile of the Glucosinolate Profiles and Observed species, indicating that the glucosinolate-derived compounds are not the only source of these sensa- Hydrolysis Products. Foods 2021, 10, tions in Brassicaceae species. Several discrepancies between observed glucosinolates and hydrolysis 1055. https://doi.org/10.3390/ products were observed, and we discuss the implications of this for future studies. foods10051055 Academic Editors: Franziska Keywords: volatile compounds; odorants; glucosinolate; Brassicaceae; ‘salad’ rocket; wasabi; S. Hanschen and Sascha Rohn horseradish; watercress Received: 8 April 2021 Accepted: 7 May 2021 Published: 11 May 2021 1. Introduction Crops of the Brassicaceae family are grown all over the world, and they form an Publisher’s Note: MDPI stays neutral important part of many different cuisines and cultures [1]. Some species are noted for their with regard to jurisdictional claims in distinctive, and often very strong, tastes and flavours. Armoracia rusticana (horseradish), published maps and institutional affil- Eruca sativa (‘salad’ rocket), Eutrema japonicum (wasabi), and Nasturtium officinale (water- iations. cress) are four such examples, which are noted for their pungent, peppery, and aromatic organoleptic properties [2–4]. Horseradish and wasabi produce large roots that are grated and used as a condiment in many cultures across the world, most notably in Eastern Europe and the United Kingdom Copyright: © 2021 by the authors. (horseradish) and Japan (wasabi; [5]). Horseradish is a vegetative perennial that grows Licensee MDPI, Basel, Switzerland. widely in temperate regions [6], whereas wasabi can only be grown in a very few locations, This article is an open access article owing to its sensitivity to temperature change and root oxygen availability [7]. Wasabi is distributed under the terms and traditionally cultivated in damp river valleys of Japan, although commercial operations conditions of the Creative Commons have been established elsewhere, such as in the UK. Attribution (CC BY) license (https:// Salad rocket originates from the Middle East and has spread throughout the Mediter- creativecommons.org/licenses/by/ ranean basin [8]. It has become naturalised on every inhabited continent and is considered 4.0/). Foods 2021, 10, 1055. https://doi.org/10.3390/foods10051055 https://www.mdpi.com/journal/foods Foods 2021, 10, x FOR PEER REVIEW 2 of 28 cial operations have been established elsewhere, such as in the UK. Foods 10 2021, , 1055 Salad rocket originates from the Middle East and has spread throughout the Medi- 2 of 28 terranean basin [8]. It has become naturalised on every inhabited continent and is con- sidered an invasive weed in some regions. Watercress has similarly become naturalised (for example anin N invasiveorth America weed) in and some grows regions. in shallow Watercress rivers has and similarly streams. become It can be naturalised cul- (for ex- tivated commerciallyample in on North large America) scales using and artificial grows in growing shallow ‘pools’ rivers flooded and streams. with stream It can be cultivated water. Its leavescommercially and shoots on are large a popular scales usingsalad artificialand sandwich growing garnish ‘pools’, and flooded they withcan also stream water. Its be made into leavessoups [9] and. shoots are a popular salad and sandwich garnish, and they can also be made It is widelyinto accepted soups [9 ].that the distinctive aroma and flavour traits are produced by glucosinolate (GSLIt) ishydrolysis widely accepted products that (GHPs the distinctive [10]). GSLs aroma are andsulph flavourur-containing traits are sec- produced by glu- ondary metabolitescosinolate produced (GSL) hydrolysisby Brassicales products plants (GHPs in response [10]). to GSLs biotic are and sulphur-containing abiotic stress secondary [11]. Myrosinasemetabolites enzymes produced are responsible by Brassicales for the plants hydrolysis in response of GSLs to bioticin water and to abiotic form stressa [11]. My- plethora of GHPsrosinase, the enzymes conformation are responsible of which forcan thebe hydrolysisdetermined of by GSLs the inpresence water to of form en- a plethora of zyme cofactorsGHPs,, pH thelevel conformation, metallic ion of concentration which can be, and determined the precursor by the GSLs presence side of-chain enzyme cofactors, structure [8]. pHThese level, products metallic include: ion concentration, isothiocyanates and the (ITCs precursor), nitriles GSLs, epit side-chainhionitriles structure, in- [8]. These doles, oxazolidineproducts-2-thiones include:, and isothiocyanates other diverse (ITCs),products nitriles, that result epithionitriles, from tautomeric indoles, re- oxazolidine-2- arrangementsthiones, (Figure and1). other diverse products that result from tautomeric rearrangements (Figure1) . Figure 1. TheFigure glucosinolate–myrosinase 1. The glucosinolate reaction–myrosinase and hydrolysis reaction and products. hydrolysis Abbreviations: products. ESM1,Abbreviations: epithiospecifier ESM1, modifier protein 1; TFP,epithiospecifier thiocyanate forming modifier protein; protein NSP, 1; TFP nitrile, thiocyanate specifierforming formingprotein; protein;ESP, NSP epithiospecifier, nitrile specifier protein. forming protein; ESP, epithiospecifier protein. Previous work has reported olfactometry data for each of these crops; however, data Previousare work generally has reported very scarce.olfactometry Only data five studiesfor each of ofE. these sativa cropsvolatile; however compounds, data have been are generallypublished very scarce. in theOnly last five twenty studies years of [3E.,12 sativa–15]. volat Veryile little compounds information have regarding been wasabi root published in thevolatile last twenty composition years [3 and,12– aroma15]. Very is available little information outside of regarding Japanese wasabi language root journals [7,16]. volatile compositionSimilarly, and very aroma little is information available outside is available of Japanese for watercress, language with journals only four [7, papers16]. published Similarly, veryin little the last information 40 years [17 is– available20]. Horseradish for watercress is the most, with well only characterised four papers of pub- these four species, lished in the lastbut 40 still, years only [17 six–20 studies]. Horseradish of note have is the been most published well characteri in thes lasted of 50 these years four [6,21 –25]. species, but still, Thereonly six is also studies an ‘elephant of note in have the room’been regardingpublished previous in the last reports 50 ofyears GHPs present in [6,21–25]. aroma profiles of these Brassicaceae vegetables. Many of the reported GHP compounds There is arealso derived an ‘elephant from GSLin the precursors room’ regarding that are previous not regularly reports reported of GHPs as partpresent of the profile for in aroma profileseach respectiveof these Brassicaceae species. In E.vegetables. sativa, for Many example, of the GHPs reported such as GHP methyl com- ITC, 3-butenyl pounds are derivedITC, 1-isothiocyanato-4-methylpentane, from GSL precursors that are not andregularly 5-(methylsulfanyl)pentanenitrile reported as part of the have all profile for eachbeen respective previously speci reportedes. In E. [13 sativa]. The, for GSL example precursors, GHPs to thesesuch compoundsas methyl ITC (glucocapparin,, GCP; gluconapin, GNP; 4-methylpentyl GSL, 4MP; and glucoberteroin, GBT; respectively) have never been reliably or consistently reported as being part of the GSL profile in this species [26,27]. This begs the question whether these identifications are correct