By Curtis Skinner

NEW YORK, Nov 8 (Reuters) - Microwave popcorn makers could face a long and difficult task ridding their snacks of trans fats, if a U.S. Food and Drug Administration proposal to ban the additives goes into effect. Just ask Orville Redenbacher.

Redenbacher's, a division of ConAgra Foods Inc, spent six years changing its leading line of popcorn, company scientists said on Friday, a day after the FDA made its proposal, which the government said would save 7,000 lives a year.

The Popcorn Board, an industry trade group, said Americans munch 16 billion quarts of popped popcorn a year, and more than two-thirds of that is eaten in the home. $985.7 million worth of unpopped kernels were sold in 2010, down 2.2 percent from five years earlier. Popcorn also is the source of a substantial amount of the trans fats consumed by Americans.

Diamond Foods Inc - owner of Pop Secret - and American Pop Corn Company - owner of Jolly Time - still use the suspect fat in some products. Diamond Foods fell 4.6 percent from its open on the news Thursday, but pared losses before Friday's close. American Pop Corn Company is not publicly traded.

Redenbacher's ditched the fats in all of their products starting in 2006, because of the health concerns.

Initial research and development of switching to a trans fat free oil was four years. It took two years more to change the entire product line.

"We've mastered it, and I'm not going to tell you how we did it," laughed Pamela Newell, a senior director of product development at ConAgra. It took "a lot of money," she added, since many replacement oil blends limited or reduced the flavor of the popcorn.

Partially hydrogenated oils, the primary source of the fats in foods, have long been prized by microwavable popcorn companies for their high melting point. The fat keeps oil solid until the package is heated, so unpopped bags don't ooze.

It also provides a taste and texture in the mouth which isn't easy to replicate, popcorn makers say. But when consumed, trans fats increase bad cholesterol, a leading cause coronary artery disease.

Since 2005, trans fat usage has fallen precipitously - the Grocery Manufacturers Association said manufacturers have voluntarily lowered the amounts of trans fats in their food products by more than 73 percent. But further reduction could prevent 20,000 heart attacks as well as the 7,000 deaths from heart disease a year, the FDA said.

Sales from ConAgra's consumer food segments rose 8 percent in fiscal 2013, due in part to Redenbacher's, according to the company's most recent annual report.