## 3 Results and Discussion
### Effect of Different Types on Volatile Compounds in Beers  
To examine the characteristics of volatile compounds in the different types of beer, six types of beer samples were analyzed and identified using gas chromatography-mass spectrometry (GC-MS), and three groups of each sample were examined in parallel. A total of 162 volatile substances were detected (Figure 4), among which the contents of alcohols, esters, and acids were relatively high. Among these compounds, higher alcohols and esters are essential flavor substances in beer [52]. Specifically, the contents of alcohol substances in classic beer and pure beer were relatively high, accounting for 63% of the total volatile substances. In addition, the content of esters in the draft beer was high, accounting for 54% of the total volatile substances, primarily due to the high proportion of ethyl octanoate. Following pasteurization, the content of esters in all types of beer decreased, whereas the content of alcohols in pure beer and ale increased. The content of isoamyl alcohol in classic ripe beer (CRB) was lower than that in classic draft beer (CDB). Our results show that the change in the content of higher alcohols is not closely related to the pasteurization process [53]. Furthermore, aldehydes are the key compounds involved in the aging process  
Figure 3: The content of the different functional compounds in the examined beers. (**A**) The concentrations of B vitamins in the different types of beer; (**B**) the concentrations of phenolic acids in the different types of beer. CRB—classic ripe beer; CDB—classic draft beer; PRB—pure ripe beer; PDB—pure draft beer; ARB—ale ripe beer; ADB—ale draft beer. The different lowercase letters (a–e) are used to indicate significant differences in the different groups of data; i.e., the same letters indicate that there is no significant difference between the two groups of data, and different letters indicate that there is a significant difference between them (\(p<0.05\)). of beer, and the levels of aldehydes increase to certain degrees in different types of beer after pasteurization [54]. Following pasteurization, the relative content of aldehydes in classic beer increased from 7.73% to 9.16%, and the relative content of aldehydes in pure beer and ale also increased. Regarding terpen and other volatile substances, we noted little change in their contents. The volatile compounds in the lager and ale were identified and analyzed, and 56 common flavor compounds were obtained, including 18 alcohols, 3 aldehydes, 24 esters, 5 organic acids, 2 ketones, 3 phenols, and 1 heterocyclic compound (Table S1). Through principal component analysis (PCA) and orthogonal partial least squares discriminant analysis (OPLS-DA) using multivariate statistical methods, the total volatile substances in the different beers were statistically verified. In the OPLS-DA model (Figure 5A), the dependent variable was the 56 total volatile substances, and the six different types of beer samples were taken as independent variables. The figure shows that the classic beer and pure beer exhibited obvious aggregation, and there was no significant difference in volatile components between the two layers. The substances in lager and ale were effectively separated, and there were significant differences between the lager and ale. To verify the validity of the model, the order of Y values was randomly permuted 200 times, and the X matrix remained unchanged. The blue regression line of Q\({}^{2}\) in Figure 5B intersects below the zero scale line of the vertical axis, indicating that the original model is valid. This result can, therefore, enable us to effectively distinguish the lager from the ale. Among the common substances, potentially functional compounds such as farnesol and caprylic acid are present in all types of beer (Table S1). As a natural sesquiterpene alcohol, farnesol has the potential to strongly alleviate inflammation, oxidative stress, and lung injury [55; 56], and its content in all types of beer ranges from 0.82 to 4.69 mg/L.