Screening of a 4-Ethylguaiacol-Producing Bacillus coagulans JN11 and Biochemical Characterization of Its Phenolic Acid Decarboxylase BcPAD

4-Ethylguaiacol (4-EG) is a volatile phenolic compound associated with smoky, woody, and spicy aroma notes in fermented foods and beverages, including Baijiu. In this study, a 4-EG-producing strain, designated JN11, was obtained by screening isolates from Baijiu pit mud and identified as Bacillus coagulans based on morphological, physiological, biochemical, and 16S rRNA analyses. In sorghum juice medium, strain JN11 produced 271.6 ± 2.7 μg/L 4-EG. To investigate the upstream decarboxylation step involved in volatile phenol formation, the phenolic acid decarboxylase gene, BcPAD, was cloned and heterologously expressed in Escherichia coli BL21(DE3). The BcPAD gene comprises 504 bp and encodes a 167-amino-acid protein. Recombinant BcPAD exhibited maximal activity at pH 6.0 and 50 °C and retained more than 60% residual activity after 5 h at 30–40 °C. Fe3+ increased enzyme activity to 115.36% of the control, whereas Zn2+ markedly inhibited enzyme activity and SDS completely inactivated the enzyme. BcPAD showed the highest activity toward p-coumaric acid, with a specific activity of 460.6 ± 18.3 U/mg and a catalytic efficiency (Kcat/Km) of 12.1 ± 1.4 mM−1·s−1, while lower activities were observed toward caffeic acid and ferulic acid, and no activity was detected toward sinapic acid. Homology modeling and molecular docking suggested that the superior catalytic performance toward p-coumaric acid may be related to favorable hydrogen-bonding interactions and substrate orientation within the active site. Although 4-EG production was observed during fermentation by strain JN11, BcPAD was biochemically characterized as a phenolic acid decarboxylase likely responsible for the upstream formation of vinyl derivatives in the proposed pathway. These findings improve our understanding of phenolic acid decarboxylases from B. coagulans and provide a basis for further investigation of the roles of strain JN11 and BcPAD in volatile phenol formation during Baijiu production.