Theodor Kleemann firm specialized in "colonial goods"
In Sue Key's book, the Theodor Kleemann firm in Danzig that Gustav Levi worked for reluctantly was in the business of tea and coffee import and wholesale.
Here we see a selection of address book listings for the Kleemann family in the years from 1869 to 1905, which tells us more about the family business.
First, the business description does not mention tea and coffee specifically, but instead labels its products as "colonialwaaren en gros," or wholesale colonial goods. Wikipedia tells us this term was widely used to designate goods imported from European colonies, in particular coffee, tea, spices, rice, sugar, cocoa and chocolate, and tobacco.
The earlier listings identify Theodor and Jacob Kleemann as goods agent and business agent. The colonial goods description begins in the 1880s, during the time of the new German empire, which was busily acquiring African and Oceanic colonies. However, the term was in use well before that to describe the trade in English and Dutch imported goods. As a cosmopolitan seaport city, Danzig was an ideal location for a colonial goods import business serving the German market.
Second, though the firm name was Theodor Kleemann, it was owned mutually by several Kleemann brothers. After the death of Jacob Kleemann, his share was owned by his widow Rosa Kleemann, nee Paechter. Gustav Lewi's mother, Emilie Kleemann Lewi, was presumably another owner. She does not appear in any of these listings because she married Hugo Lewi in 1870.