Officials of Kayco, which distributes Kedem, declined to comment beyond an email from its chief executive, Mordy Herzog, that said: “We welcome the competition. Our primary emphasis has been to deliver a quality product at fair prices for six decades, and we are confident of the continued loyalty of our customers.”

Mr. Brach, a Satmar Hasid, said Kedem juice sold far better than Welch’s Manischewitz. “People are still afraid to take Welch’s because it’s new,” he said.

Welch’s Manischewitz has not yet instilled the comfort zone Orthodox Jews require to consume new kosher products. The biblical laws of kashrut (the rules for permissible foods) specified the types of animals that could and could not be eaten, forbade the mixing of milk and meat and, on Passover, prohibited the eating of leavened bread (as a way of commemorating the Israelites’ hasty flight from Egypt, which did not allow time for dough to rise).

In the following millenniums, the sages expanded these prohibitions with a welter of interpretations intended to fortify the taboos against forbidden foods. A commandment in Exodus and Deuteronomy not to cook a young goat in its mother’s milk became, in modern times, an insistence on separate dishes, cutlery, sinks and dishwashers for meat and milk products.

Grapes are inherently kosher, but the rabbis of the first centuries of the first millennium wanted their religion to avoid any resemblance to cults whose followers would pour wine on the ground as an offering to idols. They specified that wine — or nonalcoholic juice of the grape — be watched over by observant Jews from the time of the grapes’ crushing to the juice’s bottling. They also recommended cooking the wine, because removing flavor would assure that it would never be used for idol worship.

Observant Jews are assured a food is kosher by a seal — known in Hebrew as a hechsher — on the label. The most common imprint is the letter U circled by the letter O, the symbol of Orthodox Union, the world’s largest kashrut certifier, which is based in New York. Its imprint appears on 800,000 products in 100 countries, including cans of Coca-Cola and Hershey bars. But Hasidic sects and ultra-Orthodox Jews prefer to see certifications from their own tribes. “It gives them a sense of comfort and independence,” said Rabbi Menachem Genack, chief executive officer of OU Kosher.