*2.2. Sample Preparation*

All animal procedures performed in this study were approved by the Animal Care and Use Committee of the Institute of Food Science and Technology, Chinese Academy of Agricultural Sciences (Beijing, China). A total of 120 sheep (6-month-old, small-tail sheep × Mongolian sheep with 27.40 ± 2.64 kg carcass weight) were pastured together in Inner Mongolia Province in China. All sheep had the same genetic background and were fed the same diet. 12 Sheep were randomly selected from 120 sheep. The *back strap* was obtained according to the cutting technical specification of mutton, which was same with *backstrap 5101* in the seventh edition of Handbook of Australian Meat [11]. The pre- and postrigor muscles were applied in each carcass. The left carcass was treated with prerigor, and the right half was treated with postrigor in each carcass. The prerigor *back straps* were the muscles from 12 carcasses (pH: 6.42 ± 0.08), which were quickly frozen at −35 ◦C within 45 min after slaughter. The postrigor *back straps* were the muscles from the 12 carcasses (pH: 5.42 ± 0.27), which were kept at 4 ◦C for 72 h and thereafter frozen at −35 ◦C. All samples were wrapped with nylon/polyethylene, transported to our lab by cold-chain

logistics and stored at −80 ◦C. The muscles were incubated (MIR-154-PC, Panasonic, Japan) at 4 ± 1 ◦C overnight thaw until the core temperatures dropped to the range of −3 and −5 ◦C. After being trimmed off connective tissue and surface fat, the samples were cut into cubes (3 × 1.5 × 1.5 cm3). The samples were roasted for 10 min by traditional burning charcoal. The roasting process ended when the core temperature reached 77.6–79.9 ◦C in the samples (surface temperature: 85–97 ◦C).

### *2.3. GC-O-MS Analysis*

Aroma compounds were extracted by the headspace solid-phase microextraction (HS-SPME) with a carboxen−polydimethylsiloxane fused silica (CAR/PDMS, 75 μm) coating fiber [12]. The minced sample and 2-methyl-3-heptanone (internal standard, 1.5 μL, 1.7 <sup>μ</sup>g·μL−1) were put into a 20 mL vial sealed with a PTFE-silicon stopper. The vial was incubated at 55 ◦C for 10 min and the aroma compounds were extracted at 55 ◦C for 45 min. Immediately, the coating fiber was desorbed at 250 ◦C for 3 min. The analysis was prepared on an Agilent gas chromatograph (7890B) coupled with an olfactometry (ODP C200, Gerstal, Mulheim an der Ruhr, Germany) and 5977A mass selective detector. The aroma compounds were separated by a fused-silica capillary column (60 m × 250 μm × 0.25 μm, DB-Wax capillary column, Agilent Technologies, Santa Clara, CA, USA). The temperature program of the GC oven was 40 ◦C for 3 min, raised to 70 ◦C at 2 ◦C/min, increased to 130 ◦C at 3 ◦C/min, ramped to 230 ◦C at 10 ◦C/min and maintained for 10 min. The helium (99.99%) was prepared as a carrier gas with a flow rate of 1.4 mL/min. The injector temperature was kept at 250 ◦C with a splitless inlet. The electron ionization mode was positive ion (70 eV) with an acquisition range from 40 to 500 *m*/*z* in full-scan mode.

#### *2.4. Identification Analysis of Aroma Compounds*

Aroma compounds were identified by mass spectrometry library, linear retention indices (LRIs), odor qualities, and authentic flavor standards. LRIs were obtained according the retention time of n-alkanes (C7–C40) by linear interpolation. The aroma compounds were also determined by professional panelists using GC-O. Meanwhile, the authentic standards of aroma compounds were analyzed with the same detection procedure as that used for the samples. The aroma compounds were confirmed by retention times between authentic flavor standards and samples.

### *2.5. Quantitation Analysis of Aroma Compounds*

Aroma compounds were quantitated by calibration curves of authentic flavor standards following semiquantitation of an internal standard. First, the concentrations of aroma compounds in the samples were evaluated according to the ratio of the concentration and peak area of the internal standard. In particular, the aroma compound concentrations with OAVs greater than 1 were calibrated by a 5-point standard curve of authentic flavor standards. Prior to quantitation analysis, the roasted mutton was prepared to obtain an artificial odorless matrix based on previous studies [12]. The calibration curves of aroma compounds in the roasted mutton were constructed by the above odorless matrix and authentic flavor standards with different concentrations. 2-Methyl-3-heptanone was put into the mixture to calibrate the peak area of aroma compounds. The odorless matrix without flavor standards was considered the control. Authentic flavor standards were analyzed by GC-SIM with the same detection procedure as that used for the samples. Authentic flavor standards, scanned ions (*m*/*z*) and calibration equations were obtained. The calibration curves of aroma compounds all have great linear correlations, where x is the ratio of the concentration of aroma compound to the internal standard and y is their peak ratio.
