I got my engineering professor to write my second science LOR. None of the schools I applied to had any problems with this except for USC which specifically asked for a science LOR from Bio, Chem, or Physics. I was fortunate enough to get an exception to this rule after I explained my situation (out of school for 3+ years). Best advice would be to check each school's websites for specific wording of what a science LOR needs to be. If it is not specific, it should be okay. If you really want to make sure, just call each school and ask. Good luck.