Rabbermann surname

I am researching my great great grandparents Christan & Caroline (nee Meyer) Rabbermann. Christian was born about 1824 & Caroline about1825 in/or around the Minden, Germany area. They had one son, Wilhelm (Wm), who was born 25 Feb 1851 in (Pennusetz) Minden, Germany. The only records that I've found from the old people is a death recording from the old German Evangelical Church, (now UCC) in Waterloo, Il stating Caroline died 28 July,1854 & Christian died 17 Sept 1854. I've been unable to find any other records from either of them including their graves. I am assuming that they sailed out of Bremen, Germany and it had to have been sometime between when Wm was born and and the old people died.