In the process, the fat globules are cracked open to release yellow butterfat, which clumps together into butter. The liquid that remains is buttermilk: naturally defatted milk, with microscopic traces of butter that leave a haunting, rich flavor and a creamy mouth feel. Real buttermilk contains natural diacetyl, the same compound that makes melted butter so aromatic and infuses some Chardonnays with buttery flavors.

“My buttermilk has pieces of butter floating in it, which it’s probably not supposed to,” said Ms. St. Clair, who has a herd of eight Jersey cows at her farm (called Animal Farm and located in the town of Orwell, Vt.), and makes butter and buttermilk for the chef Thomas Keller’s restaurants. “But it certainly tastes good that way.”

She, Mr. Patry and a few other dedicated dairy producers here and in the South have just begun to bring old-school buttermilk to greenmarkets and groceries, as small-scale bottling operations become more affordable.

Their efforts fit neatly into several culinary trends: working with traditional agricultural products, and embracing the once-rejected byproducts and odd bits of favored ingredients. Buttermilk even manages to represent both the American South and Scandinavia, two of the liveliest influences in food today.

Ambitious chefs all over are suddenly wallowing in buttermilk. In New York City alone, Roberto Mirarchi is saucing earthy sweet potatoes with tangy buttermilk at Blanca; Wylie Dufresne of WD-50 glazes sweetbreads with nasturtium-infused buttermilk; and the young gun Matthew Lightner strains the stuff till thick and uses it to fill crisp-fried sunchoke skins at Atera.

At Earth, in Kennebunkport, Me., the chef Ken Oringer adds a drop of tart, icy buttermilk to each oyster on the half-shell, contributing the usual sourness of lemon but also the richness of dairy. And all over the South, high-profile chefs like Linton Hopkins of Eugene in Atlanta and Josh Feathers of Blackberry Farm, near Knoxville, Tenn., are devising ways to use buttermilk, a defining ingredient of the Southern culinary tradition.