OLD ORCHARD BEACH, Me.


THE first time the Food and Drug Administration sent an inspector to check out the dairy operation at Kate’s Homemade Butter, things did not go well.


“The guy wouldn’t even get out of his car,” said Daniel Patry, the company’s good-humored founder. “He refused to believe it was a real operation.”


Mr. Patry, who had worked in commercial dairies for decades, started making butter here in 1981, in his garage. He had no cows and little capital, but a lofty goal: to reproduce the fresh-tasting butter, made from high-quality cream, that he remembered from growing up on a dairy farm in nearby Minot, Me.


Today, Kate’s produces more than a million pounds of butter a year, all from the same tiny garage. And last year, the company became the first large-scale bottler of a dairy product that has almost disappeared from American tables: real buttermilk, the creamy liquid that remains in the churn after the butter comes together.


“People have no idea how good this stuff is, but they are about to find out,” said Mr. Patry, 62, who is possibly the most optimistic and talkative native Mainer in history.


Many home cooks keep buttermilk on hand for pancakes, ranch dressing or corn bread. They might know that it makes more tender cakes (because it softens the gluten in flour), loftier biscuits (its acid boosts leaveners like baking soda and baking powder) and thicker dressings (lactic acid in buttermilk gently curdles proteins into a smooth mass).


But what few cooks know is that commercial buttermilk isn’t really buttermilk. It is made from regular low-fat or skim milk, usually low-grade rejects from cheese and butter companies. The milk is inoculated with cultures to make it acidic, and thickened with additives like locust bean gum and carrageenan. The result is a flattened facsimile of the real thing, as a ring tone is to a song.


“There’s nothing wrong with it, but I wouldn’t want to drink it,” said Diane St. Clair, a dairy farmer in Vermont who, like many of her peers, prefers the tart, light, yet rich flavor of genuine buttermilk.


That’s what poured out of the bottom of Mr. Patry’s churn at 6:45 on a recent morning. Real buttermilk is what’s left of heavy cream once it has been churned (here, knocked around 1,000 pounds at a time, dropping from top to bottom of a 13-foot-high butter churn with great thwacks and thumps) to break its natural emulsification.


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.


“My whole family bathes in it,” said Colleen Cruze, 25, the creamy-skinned scion of the Cruze Dairy Farm outside Knoxville. Her father, Earl, 69, has been spreading the real-buttermilk gospel around the South for decades. The farm now makes about 4,000 gallons a week.


Ms. Cruze graduated from the University of Tennessee with a degree in agricultural science and serves as the family’s buttermilk ambassador, selling golden buttermilk biscuits, buttermilk ice creams in flavors like lime-cardamom and salty caramel, and flavored buttermilks like fig and strawberry at the Market Square Farmers’ Market in Knoxville every Saturday.