MINSTER, Ohio — Every other day in the month before Passover, Rabbi Mordechai Teren drives the 440-mile round- trip between his home in suburban Cleveland and The Dannon Co. yogurt plant in rural western Ohio.

It's not a pleasure trip. As a representative of the Orthodox Union, which certifies that food is kosher, he is responsible for making sure kosher-for-Passover yogurt meets additional dietary requirements.

He spot-checks nozzles, tubes, tanks and other pieces of equipment to make sure they have been properly sanitized and don't have any buildup of non-Passover material that might mix in with the yogurt.

He checks every container of culture and flavoring to make sure each carries a stamp certifying that the contents are kosher for Passover. He goes through log books and matches the cultures with the times they were set to further assure that only Passover cultures are being used.

Teren, who lives in Cleveland Heights, or another inspector are at the plant every day for several hours at a stretch during Passover production season in March and April.

"I was here this morning at 3 a.m.," he said.

Passover, the most widely celebrated Jewish festival, commemorates the ancient Israelites' liberation from Egyptian slavery, when they fled so quickly they didn't have time to let their bread rise. During Passover, the eating of leaven bread is forbidden.

For Passover yogurt, the cultures and flavors must be free of any grain or byproducts from the fermentation of grain because of their connection to bread. They must be free of legumes because of their similarity to grain.

When Passover season ends, Teren travels to other sites, inspecting baked goods, fish products, food oils, chocolate, condiments and other products to make sure they are kosher.

Dannon, based in White Plains, N.Y., produces kosher yogurt year-round, but only Passover yogurt for a month before and during the eight-day holiday, which begins at sundown Saturday. The company also makes Passover yogurt at its plant in Fort Worth, Texas.

Stainless-steel tanker trucks pull up to the plant in Minster and empty raw milk from their bellies. The milk is pumped into shiny silver silos and rides through coils of steel tubing to be turned into yogurt.

Yogurt is milk that has fermented under the action of bacterial cultures at a specific temperature.

As Teren walks the plant floor, nozzles pump yogurt into cups, which drop onto a conveyor belt with a "thup, thup, thup." He strolls by the maze of steel tubing and 6,000-gallon tanks three stories high, then sticks his head into a control room, where workers monitoring the process are hunched over computer screens.

Mistakes can happen. A company might start production without first notifying the agency, use equipment that isn't properly sanitized, use unacceptable ingredients such as certain flavors in beverages, or label the products incorrectly, said Rabbi Moshe Elefant, chief operating officer of the Orthodox Union.

In such cases, the agency either keeps the product from leaving the factory or has it recalled.

Teren said he hasn't found anything wrong in 15 years of inspections at Dannon.

"When we make kosher for Passover, we use a specific culture that meets clear rabbinical criteria," said John St. Clair, quality and food safety manager at the Dannon plant in Ohio.

Flavors are limited to plain, vanilla, lemon and coffee because some other flavorings include ingredients of grain origins that don't meet the dietary standards.

The market for Passover yogurt is expected to be up about 4 percent this year over 2007, said Dannon spokesman Michael Neuwirth. He said the demand is growing, in part, because consumers are increasingly using the yogurt as a substitute for higher fat ingredients in cooking.

About 40 percent of kosher food sales occur in the five-week period surrounding Passover, according to Menachem Lubinsky, president of LUBICOM Marketing Consulting, which tracks the kosher food industry. Some Jewish families buy kosher food only during Passover, and many non-Jewish people eat kosher food with their Jewish friends during Passover, he said.

In 2005, the kosher foods niche grew 15 percent to become an $11.5 billion industry, according to LUBICOM.

The Orthodox Union, the nation's largest kosher-certifying agency, oversaw production at about 750 plants in 1980. Today, it oversees production at more than 7,000 factories in more than 80 countries. That means things can get busy for Orthodox Union rabbis who travel the globe to inspect products and facilities.