LAST WEEK the Department of Agriculture warned of potential problems if microwave oven cooking instructions for pork do not result in the product reaching a uniform 170 degrees. That warning, coupled with an announcement by Rep. Albert Gore (D-Tenn.) that he would hold a hearing this week -- later rescheduled for June -- to explore the problem, has caused serious concern in the pork and microwave industries.

Dr. C. Donald Van Houweling, lobbyist for the National Pork Producers Council, has urged members to use their influence to postpone the hearing, which was originally scheduled for Friday. Van Houweling said his organization feels the science involved in the findings would be better studied out of the limelight of a public hearing.

USDA believes that "cooking instructions in some cookbooks may not guarantee sufficiently high temperatures to kill all organisms" such as trichinae, which cause trichinosis in humans. Trichinosis, acquired from eating insufficiently cooked pork from an infested hog, is characterized by fever, nausea, diarrhea and muscular pains. Occasionally death has resulted. USDA estimates that 150,000 to 300,000 people may have sub-clinical (mild) cases of trichinosis. Only 135 cases of trichinosis were actually reported to the Center for Disease Control in 1979. Not all of them were from eating pork. The incidence of trichinosis has been decreasing over the last 20 years because fewer hogs are infested with the parasites, less than one percent, according to USDA.

USDA is aware of only two studies, both of them unpublished, which show that microwave cooking of pork can leave live trichinae in pork chops, roasts and meatloaf made of a combination of ground pork and beef. Both of the studies were conducted for USDA. The microwave industry has not done any studies itself. But Robert Schiffman, director of the International Microwave Power Institute, an association of manufacturers of microwave ovens as well as other products, is "disturbed" because USDA's announcement "is all based on preliminary investigation. You can't tell if it is a serious problem or if it's a result of the way the experiments were done," he said.

One USDA project was conducted by Dr. Anthony Kotula, chief of the Meat Science Research Laboratory at Beltsville, the other at Iowa State University by Prof. William Zimmermann. Both researchers theorize that microwave ovens have "cold spots" which produce unevenly heated food.

The March issue of Consumer Reports, along with a number of scientists, supports the "cold spot" theory: "Sometimes the microwaves intersect, strengthening their effect," the magazine explains. "Sometimes they cancel each other out. Parts of the food may be heavily doused with energy. Other parts may receive very little.

"If that sounds pretty hit or miss," the magazine goes on to say, "it can be. Microwave ovens aren't renowned for cooking food evenly."

In order to compensate for the problem, newer microwave ovens come equipped with Lazy Susans to rotate the food, or a fan-like apparatus to distribute the microwave energy more evenly.

Kotula has also advanced another theory for his findings: The speed with which a microwave oven cooks, Kotula said, may prevent it from having the "come up and come down" time for heat that is found in a conventional oven. If that is so, it may affect the final temperature of the meat in places where it takes the heat longest to penetrate, such as an area near the bone or surrounded by fat.

For that reason the National Livestock and Meat Board has recommended cooking pork at low or medium power (200 to 350 watts) for a longer period of time, 18 to 20 minutes per pound.Some microwave cookbooks recommend cooking pork at high power for a shorter period.

The National Pork Producers Council was in the midst of revising its pork cookery pamphlet when it was contacted by USDA about Kotula's findings. The revision will now wait until USDA completes a second study in an effort to replicate Kotula's work. "Perhaps," said Chris Herbert, director of the council's nutrition services, "we will recommend cooking pork at lower power setting for a longer period of time."

Kotula's study was completed in January. His finding that infective trichinae survived the cooking process was unexpected. Kotula's study had been designed to test quicker methods for cooking pork, using a microwave oven. The idea was to put pork on more menus in the food-service industry.

Kotula infected 40 pigs with trichinae and then cooked chops from the infected animals. "Much to our surprise," wrote Kotula in a memo, "we found when we thawed frozen pork chops in a microwave oven and then cooked them in a charbroiler, motile [live] trichinae were observed in the digest from the cooked chops and the trichinae were found to be [infective] when confirmed in rats."

The laboratory also partially cooked pork chops in a microwave and then finished cooking them by deep-fat frying to make them brown. In those chops live, but not infective, trichinae were also found.

USDA was preparing to announce these findings at a meeting of the Institute of Food Technologists in June. According to Food Chemical News, the abstract of the paper to be presented said: "Cooking to 170 degrees F. in the conventional oven, convection oven and flat grill produced palatable pork chops which were free of trichinae . . . Additional research is needed to determine the cause of viable trichinae in chops cooked by the roasting-and-hold and microwave oven."

The abstract was withdrawn. According to Dr. Terry B. Kinney, head of the department's Science and Education Administration, "We found something we were not expecting. We wanted to recheck our data to be sure it is not a fluke." But publication of the department's internal memos and Gore's decision to hold a hearing appear to have forced USDA to issue the press release.

Coupled with the Zimmermann study, Kotula's findings have raised serious questions. The Zimmermann study, completed in 1978, remains unpublished, which critics have been quick to point out. But even Van Houweling said Zimmermann is "a good scientist." Zimmermann said his study had not been published because his collaborator retired before he found a journal to accept the study and the matter hadn't been pursued. In Zimmermann's work, 57 pork chops and roasts were cooked in six different models of microwave ovens from five manufacturers. Zimmermann followed the cooking methods recommended by the National Pork Producers Council and microwave oven manufacturers. Ten of the samples contained infective trichinae. Zimmermann also found a wide variance in final internal temperatures of the food, as much as 20 to 30 degrees and in one case 50 degrees. Zimmermann theorizes that even following manufacturers' directions, "cold spots" remain in the cooked products and in these spots the temperature was not sufficient to kill the trichinae.

The actual temperature at which the parasites are killed is 131 degrees F. Allowing for error, the official USDA figure is 137 degrees F. USDA's recommended internal cooking temperature of 170 degrees F. is to ensure that every part of the meat reaches at least 137 degrees F. Recently some food authorities have been recommending an internal temperature of 150 degrees F. to produce a moister roast. USDA is very concerned about this recommendation. It is also concerned about a statement from the National Pork Producers Council that "a tinge of pink throughout port roast is perfectly acceptable and would be perfectly safe." USDA says pork should be cooked until it is white or grayish.

In the press release warning about potential hazards connected with cooking pork in a microwave oven, USDA made the following suggestions:

Rotate dishes during cooking period.

Let product sit for several minutes after cooking, as recommended by the manufacturer, to assure more uniform temperature distribution in all parts of the product.

The effectivenes of this equalization or "dwell time" can be increased if food is wrapped in aluminum foil after cooking.

After the dwell time check various places with meat thermometer. If every part of pork chops or roast has not reached at least 170 degrees F., cook the meat more thoroughly.

USDA emphasized that the "potential problem is unique to microwave cooking because of uneven cooking that may result if extra care is not taken."