Final answer:
Phenylacetic acid is added to the penicillin fermentation to allow incorporation of the Benzyl side chain group.
Step-by-step explanation:
The side chain group incorporated into penicillin fermentation by adding phenylacetic acid is the Benzyl group.
In the penicillin biosynthesis process, phenylacetic acid is used by penicillin acylase (PA) to perform substitutions of the side chain, resulting in the formation of semi-synthetic penicillin.
Penicillin antibiotics, including penicillin G, have different R groups connected to a common structure. By adding appropriate organic compounds to the culture medium, different kinds of penicillin can be produced.