## 3 Results  
### Volatile Compounds in PM-EFB  
The volatile components in the EFB during six enrichment rounds (with 12 samples in total) were identified by HS-SPME-GC-MS. In particular, changes in the contents of the organic acids in the EFB in different rounds of enrichment were evaluated (Figure 1A). The total acid, acetic acid, butyric acid, and caproic acid contents were within the ranges of 18.22-70.10 g/L, 9.41-29.21 g/L, 7.78-33.37 g/L, and 5.61-17.47 g/L, respectively. During the six rounds of enrichment, the organic acid contents in the EFB initially increased and then decreased. The total acid, acetic acid, and butyric acid contents peaked between rounds 3 and 4 of the enrichment, whereas the caproic acid content peaked in round 2. More specifically, during rounds 1-4 of the enrichment, the total acid contents in the P13 increased from 36.17 g/L to 70.10 g/L, whereas those of the acetic acid and butyric acid increased from 9.41 g/L to 29.21 g/L and 19.69 g/L to 33.37 g/L, respectively. Until round 6 of the enrichment, the total acids, acetic acid, and butyric acid decreased to 34.96 g/L, 11.11 g/L, and 16.79 g/L, respectively. In the P13, the caproic acid content initially increased from 7.57 g/L to 17.47 g/L, during rounds 1-2 of the enrichment, and then decreased to 6.48 g/L at the end of round 6 of the enrichment. In the P14, the total acid, acetic acid, and caproic acid contents in the P14 increased from 18.22 g/L to 61.39 g/L, 9.57 g/L to 26.12 g/L, and 5.61 g/L to 9.69 g/L, respectively, during rounds 1-3 of the enrichment, and then decreased to 30.58 g/L, 10.63 g/L, and 6.10 g/L, respectively, at the end of round 6 of the enrichment. Simultaneously, the butyric acid content in the P14 increased from 7.79 g/L to 26.19 g/L during rounds 1-4 of the enrichment, and then decreased to 14.11 g/L at the end of round 6 of the enrichment.  
Figure 1: Organic acid content of enrichment-solution sample. (**A**) Changes in organic acids in P13 and P14 (total acids are weighted by volatile acid contents, including acetic acid, butyric acid, caproic acid, valeric acid, heptanoic acid). (**B**) Difference in total acids in EFB between heat-shock and non-heat-shock treatments in the 1 st-6 th rounds of enrichment period. (**C**) Difference in total acid accumulation between different enrichment cycles of P13 and P14 (more significant difference: * \(p\leq 0.05\); significant difference: * \(p\leq 0.01\); extremely significant difference: * \(p\leq 0.001\)).  
In the first enrichment round, the total acids accumulated in the P13-1 and P14-1 reached significantly different concentrations (\(p\leq 0.01\)) due to the influence of the PM and the heat shock. In the subsequent enrichment rounds, however, although the total acid content in the P13 was slightly higher than that in the P14, the difference was not significant (Figure 1B). Thus, it can be speculated that the heat-shock treatment had little influence on the acid production during the PM-enrichment rounds. In contrast, differences in the total acid content between the different enrichment rounds were observed (Figure 1C). In particular, a significant difference in P13 and P14 was found between rounds 1 and 6 of the enrichment and rounds 2, 3, 4, and 5 (\(p\leq 0.01\)). In addition, a significant difference was found between rounds 4 and 5 of the enrichment (\(p\leq 0.01\)). Overall, it can be inferred that the enrichment rounds contributed more significantly to the enrichment of the organic acids in the PM compared to the heat-shock treatment.