As Congress reconvened yesterday, an announcement from the other end of Pennsylvania Avenue upstaged the buffets and receptions on Capitol Hill for food lovers who practice politics. The White House has a new pastry chef.

He is Albert Kumin, a 59-year-old, Swiss-born master widely regarded as one of the world's leading practitioners of the fine art of pastry-making. Kumin is leaving the kitchens of Windows on the World in New York City's World Trade Center and taking a sizable cut in pay to come to Washington.

His tenure begins officially on Feb. 1, but he will be on hand earlier in the week when President and Mrs. Carter entertain Chinese Vice Premier Teng Hsiao Ping.

With his countryman, head chef Henry Haller, Kumin will make up the other half of a Swiss Guard intent on protecting the president from charges that the White House fare hasn't always been on as high a level as that served by other world leaders.

"To me it is an honor," Kumin said yesterday. "It's about as high as you can go in our trade." He also added he had reached an age where "you have to slow down a bit." While the White House has a busy schedule of official dinners and receptions, the pressure there will not require the 16- to 18- hours days Kumin has put in at the World Trade Center overseeing the preparation of breads and pastries for several thousand dinners and retail customers each day.

Kumin and Haller have been friends since they both worked in the kitchens of Montreal's Ritz Carlton in the early 1950s. Haller came to the White House from a New York City hotel, the Sheraton East, on Feb. 1, 19669

According to Haller and others, there was no politics involved in the change. Pastry chef Heinz Bender, who had a long and distinguished tenure in the White House kitchen, requested retirement. Haller approached Kumin, who was cleared by the White House.

For some time, the White House has employed three full-time chefs. Extra cooks and helpers are brought in for state dinners and other major productions.This staff will provide food for conferences, luncheons, teas, receptions and official dinners. If there is a need for pastries and coffee for 20 in the morning, the White House doesn't call Watergate Pastry or Ridgewell's. Chef Bender (and now, presumably, Chef Kumin) makes the danish, or the cookes for a group of foreign visitors.

Private meals for the first family are prepared in a separate kitchen on the second floor of the mansion.

While Kumin has yet to meet Mrs. Carter, the president or daughter Amy (who is said to have made Chef Bender aware of her favoriter among his cookie repertoire), he expects to provide some of his specialties for them. "I think so," he said yesterday. "I hope so. But I don't know exactly yet what the procedure is there."

He has, however, worked in the White House kitchen. At Haller's request he was on hand for parties for the king of Morocco, the prime minister of Tunisia and did the cake for the party last fall at which Mickey Mouse was guest of honor.

"You need an experienced person here," Haller said. "There's a lot of production to do and sometimes it must be very fancy. Also, Albert is a very kind and generous man. The character of a person is very important here."

Former associates in New York joined Haller in praising Kumin.

Joseph Baum, who employed Kumin at the Four Seasons and other Restaurants Associates landmarks such as La Fonda Del Sol, the Forum of the 12 Caesars and Zum-Zum, called him "the ultimate pastry chef of our time.

"He had the creativity and precision needed in his art," Baum said, "and he was able to carry it over into largescale production; to work on a scale needed today if modest prices are a goal. He has the blessing of tast as well as the skill of an architect or decorator."

"Albert is one of the finest teachers I've ever seen," added William Primavera of the Culinary Institute of America, the chefs' training school in Hyde Park where Kumin taught from 1974 to 1977. "I've neve seen a man so consistently willing to share his knowledge with yougn people."

Kumin said yuesterday he would like to do some teaching while at the White House, "if it is feasible," and hopes to create some new recipes in time that will be identified with the White House in the way his domed chocolate cake is with the Four Seasons or his lemon tart is with Windows on the World.

So in the White House, as the sometimes hostile Congress organized itself, President Carter could take comfort yesterday in a line from the English Book of Common Prayer: "Thou shalt prepare a table before me against them that trouble me..."