## 3 Results and Discussion
### Key Aroma Compounds in Non-Smoked Bacon Identified via GC-O-Ms
Previous investigations revealed that organic acids could exhibit pleasant aromas in minimal amounts [31]. Other organic acids might partially contribute to the overall aroma of bacon, including acetic acid, butanoic acid, pentanoic acid, octanoic acid, nonanoic acid, and decanoic acid. Others: A total of eight hydrocarbons, six aromatic compounds, and two other compounds were detected in bacon via GC-O-MS. Hydrocarbons have minimal impact on the overall aroma of meat products because of their high threshold [32]. Hydrocarbons including 2,6,10-trimethyl-dodecane, 3-methyl-tridecane, tetradecane, pentadecane, and hexadecane were detected via MS, but none could be sniffed by assessors in the three samples. Alkanes with carbon chains of less than ten carbons may originate from lipid oxidation, whereas other hydrocarbons may accumulate in the fat from feeding [22]. Aromatic compounds could be sniffed by assessors in the three samples but not the key aroma compounds. Benzaldehyde, butylated hydroxytoluene, and p-cresol might contribute to the overall aroma of Yunnan, Chongqing, and Sichuan bacon, respectively. 2-Pentyl-furan was commonly found in meat products and provided a fruity aroma. This compound was derived from lipid oxidation or a Maillard reaction but has little effect on the aroma of bacon in this study [21]. The results indicate that the differences in aroma compounds between different types of bacon are minimal. The variations in flavor primarily stem from differences in the content of each substance. Aldehydes and alcohols are produced from lipid oxidation, while ketones and esters may result from the Maillard reaction. Acids may be derived from microbial metabolism. The variation in volatile content may be attributed to differences in processing and raw material selection, the fatty acid composition of pork from various regions, pig feeding methods and growing environments, and the presence of induced oxidative and antioxidative factors in pork. These factors may be the primary reason for the differences in aroma. The distinctions among the aroma compounds identified by GC-O-MS, GC \(\times\) GC-TOFMS, and GC-IMS are illustrated in Figure 3. A total of 239 volatiles were identified in the three non-smoked Bacon samples, and 74, 188, and 72 volatiles were detected via GC-O-MS, GC \(\times\) GC-TOFMS, and GC-IMS, respectively. However, only nine aroma compounds could be detected by all three methods, including hexanal, heptanal, 1-pentanol, acetic acid, 1-octen-3-ol, 1-heptanol, methyl isovalerate, dihydro-5-propyl-2(3H)-furanone, and hexanoic acid. This might be attributed to the different extraction, separation, and detection methods used in the three methods. SPME was applied to extract volatile compounds in GC-O-MS and GC \(\times\) GC-TOFMS, while GC-IMS just injected 0.5 mL headspace gas into the instrument via an 85 \({}^{\circ}\)C heated syringe. GC \(\times\) GC-TOFMS separated volatiles via two columns with different polarities so that analytes with similar physical and chemical properties could be separated better than GC-O-MS. GC-IMS ionized the volatiles in an ion transfer tube and migrated to a Faraday disk for secondary separation after initial separation via a column. The mass spectrometer identifies volatiles according to the mass-to-charge ratio (\(m/z\)) of the ionic fragments of compounds, even though TOFMS has a fast scanning rate and is capable of handling tiny peaks flowing quickly [33]. However, GC-IMS analyzed the compounds based on differences in the mobility of gas-phase ions [11]. Therefore, the types and contents of compounds detected by different methods will be different. Only by combining various methods to characterize the aroma of beacon can more comprehensive results be obtained. In this study, even with the same extraction method, 79.73% of volatiles detected by GC-O-MS could also be identified via GC \(\times\) GC-TOFMS. However, half of the volatiles detected by GC-IMS were not identified via the other two methods.