## 3 Results and Discussion
### Key Aroma Compounds in Non-Smoked Bacon Identified via GC-O-Ms
Ketones and esters: Although ketones also play a significant role in the development of the bacon aroma, these compounds were less important than aldehydes and alcohols due to their higher threshold [26]. A total of 12 ketones were determined by GC-O-MS, and 9 ketones could be sniffed by assessors. Among the three bacon samples, six, four, and six ketones could be sniffed in the Yunnan, Chongqing, and Sichuan bacon, respectively. A total of eight, nine, and four ketones were detected by mass spectrometry, and the ketones' concentrations were 276.82 \(\pm\) 4.36, 197.15 \(\pm\) 1.33, and 68.35 \(\pm\) 3.3 \(\upmu\)g/kg in Yunnan, Chongqing, and Sichuan bacon, respectively. Four ketones were identified as having the potential to contribute to the overall aroma due to their high FD factors, including 2,3-octanedione, 5-ethyldihydro-2(3H)-furanone, dihydro-5-pentyl-2(3H)-furanone, and 5-butyldihydro-2(3H)-furanone. However, the results showed that the OAVs of these compounds did not exceed 1, and they could still be sniffed by assessors. Previous research has shown that acetoin is the primary ketone present in the loin, salchichon, and shoulder [22]. This might be related to the material used. Various pathways could generate ketones such as the oxidation of free fatty acids and Maillard reactions [27]. It was worth noting that three of the four key ketones were furanone. 5-ethyldihydro-2(3H)-furanone, dihydro-5-pentyl-2(3H)-furanone, and 5-butyldihydro-2(3H)-furanone--known as \(\upgamma\)-caprolactone, \(\upgamma\)-nonanolactone, and \(\upgamma\)-octanoic lactone--contributed to the overall aroma with a sweet, fatty, and butter aroma. Lactones were mainly from fatty acid oxidation [24]. In general, esters play a vital role in the overall aroma of dry-cured meat due to their low threshold. However, esters were not considered as key volatile compounds in this study because only four esters were sniffed with low FD factors. Although 13 esters were detected via GC-O-MS, only one or two esters could be sniffed in each sample, so esters could have a low influence on bacon samples. The total concentration of esters in Yunnan, Chongqing, and Sichuan bacon was 358.4 \(\pm\) 29.91, 156.11 \(\pm\) 1.74, and 15.8 \(\pm\) 1.53 \(\upmu\)g/kg, and the OAV of esters did not exceed 1 except hexanoic acid ethyl ester. The esters seem to be separated well on the non-polar column, because six ketones could only be detected via a non-polar column (SH-Rxi-5 Sil MS). The primary origin of esters is the esterification of acids and alcohols in dry-cured meat products [28]. Acids: A total of ten acids were detected in bacon samples, of which nine, eight, and six acids could be sniffed by assessors in Yunnan, Chongqing, and Sichuan bacon, respectively. The acids were abundant in bacon samples, reaching 518.38 \(\pm\) 16.47, 267.06 \(\pm\) 7.57, and 118.29 \(\pm\) 0.28 \(\upmu\)g/kg in Yunnan, Chongqing, and Sichuan bacon, respectively. However, the OAV of the acids did not exceed 1 except 3-methyl-butanoic acid. For AEDA tests, 3-methyl-butanoic acid and hexanoic acid had higher FD factors and could be considered potential key aroma compounds in non-smoked bacon. This finding contrasts with prior research which pointed to acetic acid as the primary source of organic acid in meat products, notably sausages [23]. Acetic acid is usually generated in meat products that have a fermentation stage, so it could come from the microorganism [29]. Straight-chain acids typically originate from the hydrolysis of fatty acids, while branched acids can be produced by the oxidation of their respective Strecker aldehydes [30]. Therefore, 3-methyl-butanoic acid could originate from the oxidation of 3-methyl-butanal. Another study suggests that aminotransferase could be involved in the degradation of isoleucine and leucine, resulting in the production of 3-methyl-butanoic acid [31]. 3-methyl-butanoic acid contributes to the feet and cheese aroma, while hexanoic acid and heptanoic acid provide a sweaty and sour aroma.