“I would taste it and just throw it away,” said George Eng, 67, who served 36 years for murder and several stints in Special Housing Units, as solitary confinement is formally known. “You’d rather be without food than eat that.”

Ms. Murtagh called the loaf “a disgusting, torturous form of punishment that should have been banned a century ago.”

“Most people are appalled at using food as punishment,” she said, adding that many people believe “such behavior went out with the stocks, whips and shackling to the wall.”

The inglorious gastronomical history of the loaf dates back decades, and legal challenges to it and other prison food have percolated through the courts for years. In a 1978 Supreme Court case, Hutto v. Finney, a group of Arkansas prisoners successfully sued over their conditions, including being fed “grue,” an exotic dish made by mashing together meat, potatoes, syrup, vegetables, eggs and margarine, and then hard-baking it in a pan. (The court found the “grue diet” had caused many prisoners to lose weight, perhaps because they did not want to eat something called “grue.”)

For its part, New York’s recipe — baked at each prison that so punished inmates — was simple, vegetarian and often industrial scaled: two types of flour (whole wheat and all-purpose), two types of milk (1 percent and dried), yeast, salt, potatoes, carrots, margarine and a big scoop of sugar (presumably to counteract the intense blandness of the other ingredients).

Indeed, for those who have tasted the loaf, cleansing the palate can be tough.

“It’s like dense, heavy, not bread, but something akin to bread, but with no leavening,” said Jean Casella, a co-director of Solitary Watch, a watchdog group on solitary confinement, who tried the loaf and was surprised by how sugary it was. “It’s something that would sit in your stomach like a rock. I ate a tiny piece of it, and wouldn’t want to go any further than that.”