The Agriculture Department Monday is to propose that within a year major changes be made in the processing of bacon in an effort to prevent the formation of a suspected cancer-causing substance when the bacon is fried.

Until the proposal becomes final, the department says it will order bacon processors to reduce the amounts of sodium nitrite used to cure bacon.

It has been found that when bacon is cooked, sodium nitrite combines with substances called amines to form nitrosamines. Nitrosamines have been found to cause in a variety of animals.

Sodium nitrite is used in bacon and other processed meats as a preservative and to provide color and flavor. The hazzards of its use have been under discussion at the Agriculture Department since 1973.

The interim requirement, which is to take effect in 30 days, calls for a reduction in the amount of sodium nitrite used to cure the meat to 120 parts per million in and requires that it be used only in combination with 550 ppm of sodium erythrobate or sodium ascorbate, both agents used to block nitrosamine formation. This is expected to lower the lever of nitrosamines in fried bacon to 10 parts per billion (ppb).

Under this new regulation USDA is to analyze 300 samples of bacon each month. Any companies whose products more than 10 ppb nitrosamines will not be permitted to sell any more bacon until their samples are in compliance.

Dr. John Birdsall, a spokesman for the American Meat Institute, a trade association, said that kind of enforcement "will hurt some people," but he said "over 90 per cent of the companies" are already in compliance.

The proposed regulation, which could become final in May 1979, would permit only 40 ppm of nitrite in combination with 0.26 ppm of another preservative, potassium sorbate. Bacon would not be permitted to contain more than 5 ppb of nitrosamines.

The way the proposal is worded it will take effect unless the industry can demonstrate that the method does not work.

Monsanto Co., which developed the method, says it work in the laboratory but has never been tested in a manufacturing plant. Birdsall said the method "looks promising but still has too many unanswered questions about it."

But neither of USDA's regulations, the interim nor the proposed, deals with the problem of nitrosamine for mation in the stomach, nor the possibility, still under investigation, that nitrites alone are carcinogens. Nitrites combine with amines found in tobacco, drugs and other foods, to from nitrosamines in the stomachs of test animals.

An Agriculture Department official said these regulations are "based on the best scientific knowledge available to the department at this time. As additional information becomes available, we'll take further action."