Recent instances of contamination have raised new doubts about the safety of the nation's food supply system and forced government and industry to take new actions to combat the problem.

Producers and environmentalists offer various solutions, but most agree that tighter controls must be put into effect."The food supply system is vulnerable and faces more threats," said Jeffrey Simon, a terrorism expert at the Rand Corp. in California.

Last week alone, American consumers faced three major food contamination scares:

- Chilean produce was banned in the United States for five days after traces of cyanide were found in two grapes in a shipment in Philadelphia.

- Apple consumption dropped off after environmentalists raised alarms that fruit treated with Alar might pose a cancer risk. Federal officials announced later that the apples were safe.

- Up to 400,000 chickens had to be destroyed because the cancer-causing chemical heptachlor turned up in chicken feed.

If one positive thing resulted from these incidents, it was the demonstration that the system to catch such abuses is working, a U.S. Chamber of Commerce official said.

Stuart Hardy, a chamber agriculturalist in Washington, said the Hazard Analysis Critical Control Point concept developed during the Reagan administration to monitor the food chain was working.

The program was begun by the U.S. Food and Drug Administration and industry officials to test produce for harmful pesticides and ensure that the public receives safe food.

But the FDA program may not be enough. Hardy said continual vigilance by industry and government officials is needed and new methods and strategies are essential.

Such steps are under way, he added. The United States is sending a team of American experts to Chile to inspect produce. American officials in the meat-packing industries have been going to foreign countries routinely for some time to inspect products and facilities, he said, and the same technique could be applied to foreign produce.

The concept is gathering some support. Sen. John Heinz, R-Pa., said last week he supports the idea of sending produce inspectors to foreign countries to conduct inspections.

Studies point to such a need for increased controls. Reports of food tampering have increased 14-fold since 1985, according to a report by Brian Jenkins, another terrorism expert at Rand. FDA statistics show there were 1,720 food tampering reports in 1986, compared to 120 in 1985.

Tampering with consumer products is one of the fastest-growing crimes of the decade, Jenkins said, adding much of the product contamination occurs in developed countries because of the publicity such cases draw.

Last week's decision by FDA Commissioner Frank Young to ban Chilean fruit may have been extreme, industry officials said, but many supported the action.

Produce dealers estimated they lost at least $200 million during the ban and said they could lose another $600 million if U.S. consumers are scared away from buying Chilean fruit.

The FDA takes similar actions frequently, said Mary Snyder of the agency's office of regulatory guidance. She would not identify which countries are the worst pesticide violators, but said the FDA routinely bans a foreign shipper or country for the growing season if it finds residues of an illegal pesticide.

For the most part, U.S. crop samples tested for pesticide contamination are negative, Snyder said. They occur on minor crops such as parsley or endive. But often such contamination is unintentional.

"Sometimes illegal pesticides come from wind drifts, or a farmer who used pesticides one year in a field of potatoes, (then) plants a minor crop there the next," she said. Chemicals stay in the soil and contaminate the crop.

Intentional or not, when illegal chemicals are found, the consuming public worries.

"There may not be an answer to the overall problem," said John McClung of the United Fresh Fruit Association. "It's an enormous industry with many countries and people who want to threaten or poison the food supply."

If they want to do it badly enough, he said, no one can stop them.