*2.6. OAVs and Contribution Rate Analysis*

The OAVs of aroma compounds were determined by dividing concentrations with their threshold [13]. The contribution rate was the OAV ratio of single aroma compound to total aroma compounds.

#### *2.7. Aroma Recombination and Omission Experiments*

The recombination and omission experiments were performed by a triangle test of sensory evaluation in a climate-controlled (26 ± 1 ◦C) sensory room [3]. A total of 50 sensory panelists aged 24–49 years old were screened and selected based on GBT 16291.1–2012. The panelists were trained for flavor recognition based on ISO 4121:2003. All panelists had been trained weekly and could describe and recognize odor qualities. Flavor profiles were determined using a scale from 0 to 5, which represented not detectable (0), very weak (1), weak (2), moderate (3), strong (4) and very strong (5) odors, respectively. The recombination model (model 1) was constructed by the above odorless matrix and authentic flavor standards with OAVs greater than 1. The sensory panelists evaluated the aroma similarity between model 1 and roasted mutton by a triangle test. Afterwards, the omission model (model 2) was prepared by omitting one aroma compound from model 1. The panelists estimated the aroma difference between model 1 and model 2. Finally, the recombination model (model 3) was prepared by an odorless matrix and aroma compounds that significantly affected the aroma profile of the samples. The panelists evaluated the aroma similarity between model 3 and the samples.

### *2.8. Flash GC E-Nose Analysis of Aroma Profile*

A Heracles II e-nose (Alpha M.O.S., Toulouse, France) equipped with MXT-5 and MXT-1701 flame ionization detectors (FIDs) was used for the analysis of aroma compounds and aroma differences. The samples were treated as reported by Melucci and co-workers [7]. Briefly, the sample was heated at 50 ◦C for 30 min. Then, 3000 μL of headspace gas was injected into the GC port at a speed of 125 μL/s. The column temperature was 50 ◦C, rose to 250 ◦C at 2 ◦C/s and was maintained for 10 s. The temperatures of the GC port and FID were 200 ◦C and 260 ◦C, respectively. The aroma compounds were identified by retention indices from MXT-5 and MXT-1701 columns and determined by comparison with GC-MS data. The aroma differences in the pre- and postrigor roasted mutton were determined by PCA.

#### *2.9. Statistical Analysis*

All analyses were conducted in 12 measurements. Comparisons among roasted mutton of different aging times were performed using independent-samples t-test. The statistical analysis of aroma compounds in the roasted mutton were conducted at a level of *p* < 0.05 with SPSS 19.0 software (IBM Corporation, Armonk, NY, USA). Origin 2017 software and SIMCA 14.1 were used to perform plotting figures.

#### **3. Results**

#### *3.1. Identification and Quantitation of Aroma Compounds in the Roasted Mutton*

As presented in Tables 1–3, 33 aroma compounds were identified by GC-O-MS, among which 33 and 30 compounds were detected in the pre- and postrigor roasted mutton, respectively. Butanoic acid, pentanoic acid and 2,6-dimethylpyrazine were only found in prerigor samples. The aldehydes (10) and alcohols (7) with maximum types were the major odorants in the samples (Table 3). 3-Methylbutanal, pentanal, hexanal, heptanal, octanal, nonanal, and 1-octen-3-ol might be important odorants based on their high odor qualities (O) from GC-O-MS. The characteristic ion fragments of aroma compounds were obtained according to the identification of authentic flavor standards (Table 2).


**Table 1.** Aroma compounds, linear retention indices (LRIs), and identification methods in the pre- and postrigor roasted mutton.

<sup>a</sup> The aroma compounds in the pre- and postrigor roasted mutton. <sup>b</sup> Reported data in literatures. <sup>c</sup> Data calculated based on the retention time of n-alkanes (C7–C40) by linear interpolation. <sup>d</sup> Identified methods. MS, mass spectrometry library of GC-MS; LRI, linear retention indices; O, odor qualities; S, authentic flavor standards. <sup>e</sup> Not found or calculated.

> To better understand the contributions of odorants to the aroma profile in samples, quantitation analysis was performed (Tables 2 and 3). The pre- and postrigor roasted mutton both contained 15 compounds (OAVs > 1), which were quantitated based on the standard calibration curves of 5 points (Table 2). The major aroma compounds in the samples were propanal (105.86–152.67 ng/g), pentanal (1398.14–1407.06 ng/g), 2,3 pentanedione (115.22–208.95 ng/g), hexanal (3218.71–4383.43 ng/g), heptanal (744.04–1294.82 ng/g), 1-pentanol (162.93–165.20 ng/g), octanal (264.68–506.82 ng/g), 2,5-octanedione (170.57–537.81 ng/g), nonanal (119.41–197.01 ng/g), and 1-octen-3-ol (219.01–498.46 ng/g). In particular, the concentration of only 3-methylbutanal of 15 aroma compounds (OAVs > 1) in the prerigor roasted mutton was significantly higher (*p* < 0.05) than that of postrigor mutton. The concentrations of the 13 key aroma compounds in the prerigor roasted mutton were significantly lower (*p* < 0.05) than postrigor mutton, except 3-methylbutanal and pentanal.


**Table 2.** Ion fragments and standard calibration curves of aroma compounds (OAVs >1) in the preand postrigor roasted mutton.

<sup>a</sup> Selected ion fragments based on the authentic flavor standards. <sup>b</sup> Equations of standard calibration curves, where x is the concentration ratio of authentic flavor standards to internal standard and y is the peak area ratio of authentic flavor standards to internal standard. The pre- and postrigor roasted mutton both contained the 15 aroma compounds.

**Table 3.** Concentrations, OAVs, and contribution rates of aroma compounds in the pre- and postrigor roasted mutton.


<sup>a</sup> Concentrations of aroma compounds were calculated according to standard calibration curves of 5 points. <sup>b</sup> OAVs were calculated by dividing concentrations with their threshold. <sup>c</sup> Contribution rates were the OAV rates of individual compound to all compounds.
