Americans are now devouring hamburgers at the rate of 50 billion a year.

Hamburgers have become the people's food: the equivalent of the Italian's spaghetti, the Indian's curry or the Russian's borscht.

Americans spend more than $25 billion a year for beef, and a substantial amount of that money is for hamburgers.

If any proof of the entrenched position of the hamburger in the country's diet is needed, McDonald's Restaurants can give it. The chain, with 23 billion sold already, expects to open about one new outlet a day for the next 10 years.

In Woody Allen's film fantasy "Sleeper," it had taken two centuries for McDonald's to get over the trillion hamburger mark, but at the rate things are going that may be an overly cautious estimate.

The steak, that other great national symbol, has not yet been relegated to the scrap heap of history. Americans still eat slightly less than half their beef in the form of ground meat; the rest is steaks, roasts, and other prime cuts.

But cultural and economic developments have been paving the way for the eclipsing of steak, altering the tastes of consumers and changing the structure of the beef industry.

The preference of Americans for meals and snacks in restaurants and fast food stores shows no signs of letting up. Consumer Reports magazine predicts that by 1980 half the meals eaten in this country will be away from home, if present trends continue.

The fast food industry had sales of $13 billion in 1976, and that figure may double by 1980.

Many of the chains are expanding their menus and offering more salad, seafood, pizza and Mexican recipes, but hamburgers are still the staple of the fast food industry.

The other development that seems destined to speed up the evolution of the country into a hamburger society is economic.

Sometime soon - if not this year, then surely in 1978 and 1979 - the price of beef is going to rise, possibly sharply.

The stage for this already has been set on the nation's ranges and farms and in cattle fattening yeards, and there is no changing it. The size of the country's beef herd is now being rapidly reduced, as ranchers suffer some of their worst losses since the 1930s. So the supply of beef is bound to drop and prices are going to rise.

The Livestock Business Advisory Services, Inc., of Kansas City, Mo., predicts that a porterhouse steak that averaged $2.39 a pound in 1976 will cost as much as $2.50 this year and $2.70 in 1978.

Most of the steaks that Americans eat are carved from steers that have grown fat from substantial quantities of corn - as much as a ton of the grain, and sometimes more.

Corn prices are now the lowest they have been since 1972, but they are still high enough to substantially increase the price of raising choice, grain-fattened animals. As the price of that kind of meat increases, more consumers are likely to switch to cheaper hamburger meat, economists say.

"After 1977, steaks are going to be something we eat only on special occasions - on those evenings when we say, 'Let's get out the Lowenbrau,'" said an economist.

In this changing situation, hamburger has a special role to play in the beef economy.

Unlike choice steaks, hamburger can be made from ground-up dairy cows, imported boneless beef, or from older breeding cows of the beef herd that have been fed mostly grass. Some hamburger meat comes from animals that have been fattened on costly corn, but only a small amount of it.

Hamburger prices haven't varied much since January, 1974. Ground beef was recently on sale for under 70 cents a pound in Washington. The average price nationwide has never gone over $1.05 a pound and has usually been considerably lower.

The price of sirloin steak also is about what it was three years ago, too, except that there was a sudden 50-cents-a-pound jump in early 1975. The average price hit $2.25 on July 4 of that year before sliding down again.

Understanding the reason for those developments is the key to understanding what has happened, and what will happen, to beef prices.

The price of corn has a powerful impact on everything that happens with beef, since the American beef animals that make tomorrow's steaks eat nearly one-sixth of the entire corn crop every eyar.

Cows don't eat much corn, but they do produce calves that will. Their heifers and steers grow up to eat corn in fattening yards, so the price of corn is an important economic indicator.

In 1974, the United States had the poorest corn crop in five years and prices rose sharply. And that event, overshadowed at the time by Watergate and the resignation of President Nixon, still has import for American consumers.

When the corn prices went up, cattle fatteners who buy 450- to 750-pound calves and yearlings from ranchers reduced the prices they were willing to pay for the young animals as an economy measure.

Ranchers responded by sending an unusually large number of their calves to the slaughterhouse instead of to the fattening yard. They also sent many of their cows to slaughter, because they figured there was not point keeping cows for breeding calves that couldn't be sold to the fatteners at a profit.

There was plenty of bargain hamburger meat, as well as beef cuts labeled "grass fed."

But since fewer animals were going to the feedlots to eat expensive corn, there was a shortage of choice, corn-fed beef. That is when the brief, 50 cents-a-pound price increase for sirloin occurred.

With that spurt as the incentive, cattle fatteners began buying up all the calves and steers they could get. What they found was that the American beef herd, which had been growing in size since 1967, was too large.

Too much beef was going to market, so retail stores have been holding down prices to sell it all.

That, in a nutshell, is why prices of both steak and hamburger are about what they were three years ago, despite a long period of inflation.

The large supply of beef coming to market has cushioned American consumers against the economic blows that have hit the beef industry in the last three years.

But the time is coming when consumers will feel the impact of higher corn, fertilizer, fuel and farm machinery prices.

"It's set in concrete," said Howard Madsen, economist with the American Meat Institute.

The price of choice beef is likely to rise because many of the calves that this year should be growing into mature steers in fattening yards were slaughtered in 1975 and 1976.

Many shoppers are expected to switch from steak to hamburger to save money. But also the hamburger society is almost sure to feel the impact of what has happened.

In 1975 and 1976, a record 11 million and 10 million cows respectively were slaughtered. Most of them were ground up for hamburger meat. This "factory," which produces tomorrow's steers, has been reduced in size. At the same time, that killing rate can't continue, which means the supply of hamburger meat on the market will be less. The American Meat Institute estimates that only 8 million cows will be killed in 1980 - 30 per cent fewer than in 1975.

By that time, the expansion of the fast food industry will mean strong bidding for the available supply of hamburger. The number of fast food outlets is expected to increase from 28,000 to 43,000 by then.

Prices at fast food outlets and restaurants rose much more in 1976 than did the price of food in supermarkets.

Some nutritionists feel that a sharp increase in the price of both steak and hamburger would not be such a bad thing. They argue it might encourage people to eat more balanced meals.

Even though most hamburger meat comes from ground-up cows whose meat is lean, hamburger makers add substantial amounts of pure fat to give juiciness and bond the meat together. McDonald's hamburgers are only 19 per cent fat, but some hamburgers are 30 per cent fat.

Surveys of meals in fast food outlets show that they are extremely high in calories. According to Consumer Reports, a snack consisting of a Big Mac hamburger, french fries and a chocolate shake at McDonald's provided 1,100 calories. That is 40 per cent of the recommended daily allowance for a man.

Higher prices might persuade some Americans to switch from steaks and hamburgers to pork chops and pork patties. That would cause packers and retail stores to offer farmers more for hogs. That, in turn, could persuade some Midwestern farmers to feed the corn they raise to a new litter of hogs instead of selling the grain. If foreign countries such as the Soviet Union wanted the corn badly enough, they might react by bidding up the price, and that could throw off the calculations of the cattle fatteners again.

The beef industry is reacting to such possibilities by inventing new ways to stretch consumers' beef-buying dollars. A new process debones chunks of the tough parts of the carcass, tenderize them with needles, molds them together and freezes them into "loaves" that can be cut into steak-like cuts and thawed later.

There is no real evidence, though, that Americans' love affair with beef is ending. They have eaten more and more per person for the last 50 years. If higher prices slow the growth down for a while, it is likely to be only a brief pause in the expansion of the hamburger society.