By Leslie Josephs The Wall Street Journal April 4, 2012 NEW YORK (Dow Jones)--A coffee trade group Wednesday warned importers and exporters about the use of bags made of synthetic fiber, a material that could fail to show damaging water content in beans. "With the increasing availability and use of synthetic bags, we feel it is critical to caution suppliers about GCA contract requirements so that they can continue to conduct lawful commerce and preserve the integrity of the U.S. commodities market," said Brian W. Loughman, chairman of the Green Coffee Association. The New York group, whose contract templates are used widely in the coffee trade, said some green, or unroasted, coffee suppliers were using bags of woven polypropylene and other non-natural fibers to ship coffee. Under GCA contract rules, bags must be made out of natural materials such as jute or burlap, and both parties must agree on the alternative packaging material at the time of contrac...