## 3 Results and Discussion
### Analysis of VOCs Obtained by GC-IMS and GC-MS  
According to the data from the company's built-in NIST gas phase retention index database and IMS migration time database, the volatile components were qualitatively analyzed according to the gas chromatography retention time and IMS migration time of volatile substances. A total of 44 volatile compounds were identified by GC-IMS, as detailed in Table 1, and included 5 esters, 8 alcohols, 12 aldehydes, 3 ketones, 1 furan and 2 sulfides. Thus, the volatile organic compounds in bacon are mainly aldehydes, alcohols, and esters. Due to the different concentrations of these compounds, some single compounds may produce multiple signals or spots (dimers or trimers) [31]. The retention indexes of the majority of the volatile compounds ranged from 200 to 400, which indicated that weakly polar and small-molecule volatile compounds were detected in the samples. This may be because the evaluation of advanced nondestructive testing of GC-IMS were similar to the sensory conditions of the samples [32]. Among the abovementioned compounds, 17 compounds, including acetone, 1-propanol, and 3-methylbutanal, were not detected by GC-MS. This suggested that GC-IMS technology can be used to comprehensively determine the real aroma of a sample without sample pretreatment [33]. In previous studies in Huangjiu [34], on sorghum [35], chicken [36], wheat [37], and kiwifruit [38], the differences of aroma components among samples were also revealed by GC-MS and GC-IMS. In GC-MS, 40 volatile organic compounds, 4 ketones, 7 phenols, 8 alcohols, 6 esters, 6 aldehydes, and 6 other compounds were detected (Table 2). It could be clearly seen from Figure 4 that each color represents a different type of volatile compound, and the length represents the number of species. Among them, the fresh sample only exhibits one kind of VOCs (hexanal) and XZ sample had similar flavor substance types with the CP sample. The YZ sample has more alcohol compounds and aldehyde compounds. As one of the characteristic flavor products of fat degradation, aldehydes have a low threshold value and complex taste, so their contribution to the overall flavor of Zhenba bacon is more prominent, in particular, the impact of low carbohydrate aldehydes on the flavor of bacon is more significant. Among them, saturated straight chain aldehydes generally have a strong taste of raw oil and make people vomit, which is a major characteristic flavor substance in fresh meat. In addition, hexanal can be detected in every sample. As a common flavor substance in fresh meat, hexanal has been reported many times, and it has a grass-like taste [39]. In the fresh group, only one volatile flavor substance of hexanal was detected, and in the YZ group, the maximum value was 3.496. During the curing period, a total of five aldehydes were detected, namely pentanal, hexanal, 1-nonanal, 5-methyl furfural and acetal. Some studies have found that the appropriate amount of aldehyde substances (acetal) in liquor can improve the flavor of liquor, so acetal may be used in the liquor flavor during the YZ stage. The types and contents of aldehydes are the most similar in the smoked period and finished product period, which may be caused by the covalent  
Figure 3: Fingerprints of VOCs isolated from different stages using GC–IMS. Each colored point represents a VOC, and different colors indicate varying concentrations, red representing higher intensity, blank meant not present. Fresh: raw sample; YZ: cured sample; XZ: smoking sample; CP: finished sample; VOCs: volatile organic compounds. combination of aldehydes and proteins. With the extension of storage time, aldehydes' flavor substances are not volatile.