2 4 6 trimethylpyridine

Standard compounds of the highest available purity were used. Pentanal (PEN), hexanal (HEX), 2-pentyl-furan (2PF), 2,4-nonadienal (NON), pyridine (PYR), 1-octen-3-ol (OCT), (E)-2-octenal (OCA) and 2,4,6-trimethylpyridine (TMP) were purchased from Sigma-Aldrich (St. Louis, MO, USA). Water was purified with a Milli-Q purification system A10 Gradient/Elix System (Millipore, Bedford, MA, USA). A standard stock solution of TMP at 0.1 mg L⁻¹ was prepared in Milli-Q water, stored in a sealed vial at 4 °C, and used as internal standard.

Commercial chicken seasoning (78 packages, a certain brand with the same ingredients, containing sodium glutamate, salt, sugar, rice flour, chicken, whole egg liquid, flavors, onion, garlic, disodium 5′-ribonucleotide and riboflavin) were purchased from dozens supermarkets in Zhengzhou and Shanghai, and the deterioration was determined by sensory evaluation and divided into different levels of deterioration based on the intensity of rancid flavor, where 30 packs of fresh ones (F), 9 packs of light deteriorated ones (L), 16 packs of medium deteriorated ones (M) and 23 packs of serious deteriorated ones (S). 2,4,6-trimethylpyridine, the mixed standards of 37 kinds of fatty acid methyl esters and C7-C40 saturated alkanes were purchased from Sigma-Aldrich Co. (St. Louis, MO, USA). All the other chemical reagents were of analytical grade.

Diethyl ether (anhydrous, 99.7%), sodium chloride (NaCl) and sodium sulfate (granular, anhydrous 99%) were purchased from Sangon Biotech (Shanghai) Co., Ltd. (Shanghai, China). n-Alkane standards (C7-C30) and internal standards: 2-ethylbutanoic acid (acid fraction), 2-methyl-3-heptanone (neutral fraction), 2-nonanol (alcohol fraction) and 2,4,6-trimethylpyridine (basic fraction) were purchased from Sigma-Aldrich Chemicals Co. (St. Louis, MO, USA). Authentic reference compounds listed in (Tables 1–3) were obtained from commercial sources, as follows: nos. 1–4, 8, 10, 12, 14–18, 20–25, 30, 33 (Aladdin, Shanghai, China); no. 5 and 11 (TCI, Shanghai, China); nos. 6, 7, 19, 27, 32, 34, 35, 37, 41 (Sigma-Aldrich, Shanghai, China).

The volatile compounds in fillets were assayed by the method described in our previous study [17 (link)] using the gas chromatography-mass spectrometry (GC-MS, QP2020, Shimadzu, Japan) coupled with automated solid-phase microextraction (SPME) system (AOC-6000, CTC, Zwingen, Switzerland). Volatile compounds were identified by chemical standards (Sigma, Shanghai, China) and linear retention indices (calculated using a series of n-alkanes). The concentrations were quantified by the ratio of the peak area to the internal standard (2, 4, 6-trimethyl-pyridine, Sigma, Shanghai, China). Odor activity value (OAV) was used to evaluate the contribution of each volatile compound. When the OAV was equal to or greater than 1, it was considered that the substance was an odor-active compound contributing to the overall aroma [18 (link)]. The OAV was calculated as: $${\mathrm{OAV}=\mathrm{C}}_{\mathrm{i}}/{\mathrm{OT}}_{\mathrm{i}}$$
where C_i is the concentration of the volatile compound, OT_i is the odor threshold of the compound reported in the literature [17 (link)].

2-ethyl-3,5(6)-dimethyl pyrazine (EDP, a mixture of the 3,5- and 3,6-dimethyl isomers, herein referred to as pheromone isomers), E-β-Farnesene (EBF), 2,4,6-trimethylpyridine (TMP), and ylang ylang oil were purchased from Sigma-Aldrich (Sigma-Aldrich, St. Louis, MO, USA). The purities of these four chemicals were >95%.