Rabbi Shemtov says his approach is so strict that no one can take issue. “We are very careful, we are meticulous but we are not O.C.D.,” he says. “Otherwise, no one would ever get to eat.”

He peers at a countertop. “Why so loose, the Saran wrap?”

Will this affect the first family’s meals? A White House aide explains that the Obamas rely primarily on a personal kitchen in the residence upstairs. Even so, a refrigerator has a sign on it that says “Family,” indicating it must remain shut.

Rabbi Shemtov continues his inspection while talking to Chef Tommy and working his iPhone and a BlackBerry. He glances up. Workers are even wrapping lights that dangle over counters. “You don’t have to do that!” he says. “We don’t use lights for cooking.”

It’s 11 p.m. The rabbis start the cleansing by fire, for ovens, racks, stove tops. Rabbi Steinmetz cranks up the heat on the wall ovens to 500 degrees, to burn off impurities.

How long should they remain heated?

The rabbi, who says he turned down a similar job at the Waldorf-Astoria that week for the privilege of working in the White House kitchen, sighs gently. “An hour, minimum,” he says. “Minimum, minimum.”

He covers the stove tops with foil, pokes holes above the burners and turns up the flames. The foil keeps the heat from escaping. Minutes pass. Finally, the metal turns red hot.

“It’s kosher,” Rabbi Shemtov says.

Midnight looms. More White House staff members are helping, at least a dozen people in the kitchen. Daniel Shanks, a White House usher, peeks in, wearing a Christmas tie. He waves at Rabbi Steinmetz, a diminutive man with a silvery beard, standing on a crate next to the brazier.