*3.2. Determination of Key Aroma Compounds in the Roasted Mutton*

As shown in Table 3, the pre- and postrigor roasted mutton both contained 15 aroma compounds with OAVs greater than 1, including propanal, 3-methylbutanal, pentanal, 2,3-pentanedione, hexanal, heptanal, 2-pentylfuran, octanal, (E)-2-octenal, nonanal, 1 octen-3-ol, 1-heptanol, benzaldehyde, (E)-2-nonenal, and (E)-2-octen-1-ol. This result was also in agreement with the analysis of odor qualities. Among them, the highest OAVs were determined for hexanal (715.27-974.10), followed by 3-methylbutanal (243.96–426.56), octanal (378.12–724.02), 1-octen-3-ol (219.01–498.46), heptanal (248.02–431.61), and nonanal (119.41–197.01). The OAV of only 3-methylbutanal of the 15 aroma compounds in the prerigor roasted mutton was greater (*p* < 0.05) than that of postrigor mutton. The 13 aroma compounds had the reverse trends (*p* < 0.05). The changes in the contribution rates of aroma compounds were in accordance with those of OAVs, among which hexanal (29.92–31.62%) presented the highest contribution rate, followed by octanal (16.31–22.17%), 3-methylbutanal (7.40–18.85%), 1-octen-3-ol (9.57–15.32%), and heptanal (10.88–13.21%). These results preliminarily indicated that the 15 aroma compounds (OAVs > 1) with high contribution rates might be key odorants from the difference of aroma profiles in pre- and postrigor samples.

#### *3.3. Confirmation of Key Aroma Compounds in the Roasted Mutton*

The odorless matrix was constructed with 74.67% ultrapure water and authentic flavor standards (OAVs > 1) in the samples. The recombination model (model 1, 15 odorants) with all aroma compounds with OAVs greater than 1 revealed an extremely high similarity with the original roasted mutton in terms of the aroma profile by the triangle test. The results of the omission experiments (model 2, 14 odorants) indicated that 8 odorants significantly affected the overall aroma profile (*p* < 0.05) of the samples, including 3-methylbutanal, pentanal, hexanal, heptanal, 2-pentylfuran, octanal, nonanal, and 1-octen-3-ol. In particular, the model without hexanal and 1-octen-3-ol presented a noticeable difference (*p* < 0.01) compared to the aroma profile in model 1. Finally, the recombination model with the 8 aroma compounds mentioned above (model 3) showed a high similarity (4.51 out of 5 points) in comparison with roasted mutton, as illustrated in Figure 1. In particular, the pre- and postrigor roasted mutton both had fatty, roasty, meaty, grassy, and sweet odors. The intensity of the aroma profile in the postrigor roasted mutton was significantly greater (*p* < 0.05) than prerigor sample.

**Figure 1.** Aroma profiles of roasted mutton compared with the aroma recombination model 3.

### *3.4. Potential Markers Analysis for Discriminating the Pre- and Postrigor Roasted Mutton Based on Aroma Compounds*

As presented in the score scatter plot of OPLS-DA (R2X = 0.92, R2Y = 0.99, Q2 = 0.99) (Figure 2), the pre- and postrigor roasted mutton were obviously separated. R2 and Q<sup>2</sup> revealed the fitness and predictive ability of the model, respectively. The prerigor roasted mutton was in the second and third quadrants, in which aldehydes, acids, esters, alkanes, and nitrogen-containing compounds were the predominant chemical families, such as 3-methylbutanal, pentanoic acid, and 2,6-dimethylpyrazine. The postrigor roasted mutton was located in the first and fourth quadrants of the model, among which alcohols, aldehydes, ketones, and furans had an important contribution, including 1-octen-3-ol, hexanal, 2,3-pentanedione, and 2-pentylfuran. The aroma compounds (variable importance for the projection ≥ 1) were generally considered as potential markers to discriminate samples. A total of 20 aroma compounds were identified to show differences between pre- and postrigor mutton (Figure 2c), such as 2,5-octanedione, 2,6-dimethylpyrazine, 1-octen-3-ol, and hexanal. The results also indicated that the postrigor roasted mutton had richer aroma compounds than the prerigor roasted mutton.

**Figure 2.** OPLS-DA of aroma compounds in the pre- and postrigor roasted mutton. (**a**) score scatter plot. (**b**) loading scatter plot. (**c**) VIP plot.

To further quickly determine the differential aroma compounds in the pre- and postrigor roasted mutton, flash GC e-nose and PCA were used. As illustrated in Table 4, 11 aroma compounds were identified in the two samples by flash GC e-nose. Among these, hexanal had the maximum peak area, followed by pentanal, 1-octen-3-ol, and heptanal. In particular, the peak areas of most aroma compounds, including hexanal and 1-octen-3-ol, in the postrigor samples was significantly greater (*p* < 0.05) than that in the prerigor samples, which was consistent with the GC-O-MS results. PCA of the flash GC e-nose was performed to determine the correlation pattern with individual composition variables in the discrimination between the two samples. As presented in Figure 3, the accumulative variance contribution rate of the first two PCs was 98.81% higher than 85% (PC1 of 97.68% and PC2 of 1.13%), which was sufficient to discriminate between these two samples. The general aroma feature could be well distinguished by a flash GC e-nose coupled with PCA. Based on above analysis, hexanal and 1-octen-3-ol might be potential markers to discriminate the pre- and postrigor roasted mutton.

**Table 4.** Peak areas of aroma compounds detected by flash GC e-nose in the pre- and postrigor roasted mutton.


Data with different superscripts (a, b) within each row indicate significant difference (*p* < 0.05).

**Figure 3.** PCA of aroma compounds in the pre- and postrigor roasted mutton by flash GC e-nose. R and L represented the prerigor roasted mutton and postrigor roasted mutton, respectively.

#### **4. Discussion**

*4.1. Aldehydes and Alcohols Were Key Aroma Compounds in the Pre- and Postrigor Roasted Mutton*

It was reported that aldehydes and alcohols were the most important aroma compounds in roasted meat [14]. It was clearly observed that these compounds mainly con-

tribute to the overall aroma of samples, such as hexanal (OAVs: 715.27–974.10) and 1-octen-3-ol (OAVs: 219.01–498.46). This result was in agreement with previous studies [1,2], which showed that hexanal had the most abundant concentration in roasted mutton, followed by 1-octen-3-ol, nonanal, and octanal. In particular, 8 of 15 odorants (OAVs > 1) comprising 6 aldehydes and 1 alcohol were confirmed as key odorants by the recombination and omission experiments. This result also corresponded to the studies, in which hexanal, heptanal, octanal, nonanal, and 1-octen-3-ol had the higher concentrations and OAVs in grilled goat meat [15]. The roasted mutton had strong roasty, fatty, grassy, meaty, and sweet odors, which were mainly caused by aldehydes and alcohols derived from the degradation of lipids and Strecker degradation of amino acids [16]. The phospholipids contained more unsaturated fatty acids than triacylglycerols, which caused the former's predominant contributions to the formation of fatty aldehydes and alcohols [17]. Pentanal, hexanal, heptanal, and 1-octen-3-ol could be generated from the oxidation of unsaturated fatty acids, which were responsible for the grassy note [3,18–20]. Aldehydes containing octanal and nonanal might predominantly contribute to fatty aromas [12]. 3-Methylbutanal, a Strecker aldehyde, was detected in the Maillard reaction with a seasoning-like odor [21]. In addition, the ketones and alkylfurans, including 2,3-pentanedione and 2-pentylfuran, could also be generated from the decomposition of lipids, which could generate roasty and meaty notes, respectively [22–24]. In particular, the aroma profile of roasted mutton was formed by the synergistic effect of key odorants rather than a single component [25]. Meanwhile, the concentrations of most key aroma compounds in the postrigor roasted mutton were significantly higher than those of the prerigor mutton, such as hexanal, heptanal, octanal, nonanal, 1-octen-3-ol, and 2-pentylfuran. This result indicated the postrigor *back strap* was more suitable for roasting than the prerigor *back strap*. This phenomenon was also consistent with the study reported by Coppock and Macleod, who clarified that the aging time generated more aroma compounds in the boiled beef [26]. Both thermal oxidation and autoxidation could produce the aldehydes and alcohols in meat and meat products. The richer aroma compounds in the postrigor roasted mutton could be explained by the autoxidation during aging [27,28].

### *4.2. Pre- and Postrigor Roasted Mutton were Discriminated Based on Key Aroma Compounds by GC-O-MS and GC E-Nose*

In this study, GC-MS provided reliable and comprehensive diagnostic information for the detection of 8 key compounds, among which the concentration differences of 8 key odorants were responsible for the discrimination of the overall aroma profile of pre- and postrigor roasted mutton. In particular, hexanal and 1-octen-3-ol predominantly contributed to the aroma profile and caused the aroma difference of samples by using GC-O-MS. Meanwhile, the aroma profiles were obviously separated in the pre- and postrigor roasted mutton by using flash GC-O-MS, GC e-nose, OPLS-DA, and PCA, which was in agreement with aroma analysis of other food [29]. The characterization and discrimination of aroma compounds in the pre- and postrigor roasted mutton could also be successfully identified by GC-O-MS, among which hexanal and 1-octen-3-ol were key odorants and resulted in the difference of aroma profile in samples. The flash GC e-nose performance in the discrimination was consistent with respect to GC-O-MS, which was identical to previous studies [8,28]. These results indicated that hexanal and 1-octen-3-ol might be potential markers for discriminating the pre- and postrigor roasted mutton. This was in accordance with previous studies, among which hexanal and 1-octen-3-ol were indicators of oxidative stability and flavor acceptability in foods [30,31]. In addition, the combination of GC-MS with an e-nose could provide a comprehensive analysis for the characterization and discrimination of aroma compounds.

#### **5. Conclusions**

In this study, a total of 33 and 30 odorants were identified in the pre- and postrigor roasted mutton, among which they belonged to 8 chemical classes, such as aldehydes, ketones, alcohols, furans, acids, esters, and nitrogen-containing compounds. Eight odorants

were confirmed to be the key aroma compounds in the roasted mutton, including hexanal, octanal, 1-octen-3-ol, nonanal, heptanal, pentanal, 3-methylbutanal, and 2-pentylfuran. The sensory evaluation of the recombination model including 8 key aroma compounds scored 4.51 out of 5 points. Only the concentration of 3-methylbutanal of 8 key aroma compounds in the prerigor roasted mutton was significantly higher than that of the postrigor mutton. Other 6 key aroma compounds, including hexanal, octanal, 1-octen-3-ol, nonanal, heptanal, and 2-pentylfuran, had the reverse trends. The pre- and postrigor roasted mutton could be discriminated based on the aroma compounds by GC-O-MS, flash GC e-nose, OPLS-DA, and PCA. Hexanal and 1-octen-3-ol might be potential markers to discriminate the preand postrigor roasted mutton. This study confirmed the key aroma compounds in the roasted mutton. Most importantly, this study provided the scientific data to clarify that the postrigor *back strap* was more suitable for roasting.

**Author Contributions:** Conceptualization, D.Z. and Z.W.; methodology, H.L.; software, T.H.; validation, H.L.; formal analysis, H.L.; investigation, H.L. and S.L.; resources, H.L., Q.M. and S.L.; data curation, H.L.; writing—original draft, H.L.; writing—review & editing, H.L., T.H., F.F., D.Z. and Z.W.; visualization, T.H. and F.F.; supervision, D.Z. and Z.W.; project administration, Z.W.; funding acquisition, D.Z. All authors have read and agreed to the published version of the manuscript.

**Funding:** This study has been financially supported by Agricultural Science and Technology Innovation Program (CAAS-ASTIP-2021-IFST-SN2021-04), National Agricultural Science and Technology Innovation Program (meat science and nutrition) (CAAS-ASTIP-2018-IAPPST-2), Meat Processing Key Laboratory of Sichuan Province Open Fund Project in 2021 (21-R-01).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

