6 methyl 5 hepten 2 one

N-Alkanes (C7–C30) used to calculate the retention indices (RI values) were obtained from Sigma-Aldrich (Milwaukee, WI, USA). All standards (β-pinene, (−)-β-elemene, prenol, 2-methyl-2-butenal, heptanal, octanal, nonanal, 6-methyl-5-hepten-2-one, caryophyllene, and acetic acid) used for qualitative and quantitative analysis of the aroma compounds were purchased from Sigma-Aldrich. Nitrogen (99.9992% purity) and helium (99.999% purity) were purchased from Beijing Haipubeifen Gas Industry Co. Ltd. (Beijing, China). Liquid nitrogen (99.99% purity) was purchased from Beijing Xianheyu Gas Industry Co. Ltd. (Beijing, China).

Super Q (80/100 mesh) was purchased from Alltech (PA, USA). n-Hexane (95%, for pesticide residue analysis) was purchased from Fisher Scientific (Loughborough, Leicestershire, UK). (Z)-3-Hexenyl acetate (98%) were purchased from Alfa Aesar (Heysham, UK). Limonene (97%), cis-jasmone (92%) and linalool (96%) were purchased from TCI America (Portland, USA). α-Pinene (98%), camphene (90%), 6-methyl-5-hepten-2-one (98%), methyl salicylate (99%), ocimene (90%) (mix of (E) and (Z)-isomers), (E)-caryophyllene (80%), benzothiazole (96%), indole (98%), myrcene (97%), octanal (99%), undecane (99%), tridecane (99%), pentadecane (99%), nonanal (95%) and decanal (98%) were purchased from Sigma Aldrich (Steinheim, Germany). Geranylacetone (96%) was purchased from TCI (Tokyo, Japan). (E,E)-α-Farnesene was synthesized in three steps from isoprene and sulfur dioxide (Hassemer et al. 2016 (link)). (E)-4,8-Dimethyl-1,3,7-nonatriene (DMNT) and (E,E)-4,8,12-trimethyl-1,3,7,11-tridecatetraene (TMTT) were synthesized from geraniol and (E,E)-farnesol, respectively (Leopold 1990 ).

Authentic standards of n-alkanes (C₇–C₃₆), acetophenone, octan-3-one, 5-methyl-heptan-3-one, 6-methyl-5-hepten-2-one, 1,4-naphthoquinone and 2-methoxy-1,4-naphthoquinone for a comparison of gas chromatographic–mass spectrometric data to components found in the Carinostoma-secretion were purchased from Sigma (Vienna, Austria). 6-Methyl-1,4-naphthoquinone was prepared according to Bruce and Thomson (1952 (link)); as oxidizing reagent we used CAN (=cerium IV ammonium nitrate from Sigma, Austria). For two further compounds, namely 2-methoxy-6-methyl-1,4-naphthoquinone and 4-chloro-1,2-naphthoquinone, we used known natural sources for reference, namely the secretions of P. quadripunctatum (Raspotnig et al. 2010 (link)) and Cyphophthalmus (formerly Siro) duricorius (Raspotnig et al. 2005 (link)). O-Methyl oximes of ketones in the Carinostoma-extracts were prepared by adding 200 μl of methoxamine (MOX) reagent (=2 % methoxyamine-hydrogen chloride in pyridine, Thermo Scientific, Vienna, Austria) to 60 μl of ketone-rich extracts in hexane, followed by incubation at 70° for 1 h. The products were washed twice with water, dissolved in 60 μl of hexane, and an aliquot (1.5 μl) was directly used for GC–MS.

The VOCs standards β-cis-ocimene, benzaldehyde (≥99%), cis-geraniol (≥97%), hexanal (≥95%), 1-hexanol (≥98%), ethyl benzoate, ethyl butanoate, ethyl hexanoate, ethyl octanoate, eucalyptol (≥99%), methyl benzoate, methyl butanoate (≥98%), 6-methyl-5-hepten-2-one, myrcene (≥90%), (R)-(+)-limonene (~90%), linalool (≥97%), α-pinene, 1-octen-3-ol (≥95%), γ-terpinene, terpinolene, (-)-terpinen-4-ol, trans-2-heptenal (≥95%), and C₇-C₄₀ n-alkanes mix were purchased from Sigma (Sigma-Aldrich, Saint Louis, MO, USA). All standards were of at least analytical grade.

Freeze-dried Finnish mushroom (Craterellus tubaeformis) was obtained from Lyotech Oy Co. Ltd. Company (Helsinki, Finland). The aroma compounds in the freeze-dried mushroom were extracted as essential oil by supercritical CO₂ (FLAVEX Naturextrakte GambH Co. Ltd., Rehlingen, Germany) for commercial purposes and the corresponding residues were used in this study. Cellulase produced by Novozyme Nordisk (Bagsvaerd, Denmark) was purchased from Novo Co., Ltd. (Shanghai, China). Neutral protease was purchased from Longda Bio-Products Co., Ltd. (Yishui, Shandong, China). 1,2-Dichlorobenzene and n-alkane mixtures were all chromatography grade and purchased from Sigma-Aldrich Co., Ltd. (Shanghai, China). Authentic standard compounds including 1-octen-3-ol, hexanal, heptanal, pentanal, nonanal, benzaldehyde, octanal, and 6-methyl-5-hepten-2-one were obtained from Sigma-Aldrich Co., Ltd. The other chemicals used were of analytical grade and were obtained from Shanghai Chemical Reagent Co. Ltd. (Shanghai, China).

The AVA contents in flesh were analyzed by gas chromatography mass spectrometry (GC-MS) as previously described (Zheng et al., 2016 (link)). A solid-phase microextraction (SPME) needle with a 1-cm long fiber coated with 65 μm of polydimethylsiloxane, and divinybenzene (Supelco Inc., Bellefonte, PA, United States) was used for volatile extraction. The identification and quantification of volatiles was performed on an Agilent 6890N GC equipped with a flame ionization detector (FID) detector and a DB-WAX column (0.32 mm, 30 m, 0.25 μm, J&W Scientific, Folsom, CA, United States). All volatiles were quantified according to standard curves of authentic compounds. β-Ionone, dihydro-β-Ionone, β-damascenone, and 6-methyl-5-hepten-2-one were obtained from Sigma (St. Louis, MO, United States). Extracts from three triplicate tissue samples were analyzed.

The compounds studied and their structures are presented in Table 1. (E)-β-ocimene was purchased from Bedoukian Research Inc. (95.7% (E)-β-ocimene and 2.8% (Z)-β-ocimene). Isomers of α and β-farnesene, α-terpinene (≥ 95% purity) and 6-methyl-5-hepten-2-one (99% purity) were purchased from Sigma-Aldrich®/Merck at high purity. Compounds were degassed before use by repeated freeze-pump-thaw cycles.
Table 1

Structures and synonyms of the volatile organic compounds used in this study for the identification of gas phase oxidation products