At Zero Otto Nove, a stalwart trattoria in the Bronx, Mr. Bloomberg sought to pre-empt criticism by citing statistics showing an uptick in restaurant revenue and a reduction in salmonella infections since the grading system began in 2010. He denounced critics as “people that complain because they don’t want to keep their restaurants clean.”

“They think it’s O.K. to have mice and roaches and dirt and not have people wash their hands before they come back from the bathroom,” the mayor said, his voice rising. “That’s just simply unacceptable, and their complaints are going to fall on deaf ears, I can tell you that. We’re not going to change.”

The Council speaker, Christine C. Quinn, who is rarely at odds with the mayor, later offered a firm, if measured, rejoinder.

“The mayor is appropriate to defend the idea of a grading system; I defend the idea of a grading system,” Ms. Quinn said in an interview. “I also have to respond when I’ve heard this many complaints from constituents.”

“All I want to make sure is that the grades really tell the true story, and they do it in a way that doesn’t overly or inconsistently fine,” she added.

For health officials, the Council’s hearing came at an inopportune moment. The New York Post reported over the weekend that Per Se, consistently rated among the city’s best restaurants, had avoided a “B” grade with a telephone call to a city official, in which the restaurant successfully argued that the inspection report had contained errors.

City Hall officials quickly pointed out that several dozen restaurants, of varying degrees of prestige, had taken the same route to contest alleged violations and avoid a protracted adjudication process. Mr. Bloomberg, at his news conference on Tuesday, referred to any suggestion of undue influence as “an outrage.”

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“It’s just so unfair,” the mayor said. “No wonder sometimes it’s just so hard for everybody to keep working in this city and trying to do what’s right.”

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About 72 percent of restaurants have an “A” grade, and Mr. Bloomberg noted that an imperfect rating was not necessarily a reason to avoid a restaurant: he said he had continued patronizing a coffee shop near his Upper East Side town house despite its recent drop from an “A” to a “B.”

The health department has been spot-checking restaurants for safety and sanitary issues for decades, but the unappetizing findings — a roach in the kitchen, unrefrigerated, raw food — were published only in hard-to-find documents, and fines for violations were often quietly paid, with the consumer none the wiser.

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Mr. Bloomberg and Ms. Quinn agreed on Tuesday that the grading system provided diners with more access to information, but some restaurant owners argued that the grading system was far too blunt and was frequently based on relatively minor issues.

“It could be a cracked toilet cover; it could be a gap around a pipe,” said Peter Hansen, the director of operations at Benchmarc Restaurants, which owns several expensive dining establishments in the city.

“But,” Mr. Hansen added, “what your customer is thinking is: old tuna.”

Elizabeth Meltz, who oversees food safety at Mario Batali’s Italian restaurants, said that she supported a grading system and that it could improve public health.

But, echoing other restaurant workers, she said some city health inspectors seemed inconsistent in their standards, asking about certain elements of the kitchen on some visits and not on others. Sometimes, the inspectors appeared unfamiliar with complex dishes like terrine and kimchi, Ms. Meltz said, and on one occasion, she believed that an inspector was disrespectful to her because of her gender.

“There can be a lack of ability to communicate whatever expertise they may or may not have,” Ms. Meltz said. “I’m all for the grading system. If we could help the health department get the inspectors that they need and deserve, it should work for everybody.”

Ms. Meltz said she wished inspectors and restaurant workers better understood each other’s needs. “Inspectors could come in and dine, and see what our food is like, and why our antipasti are at room temperature,” she suggested. “And a couple of our sous chefs could take a week of the inspectors’ courses, to see how they inspect, so it’s not this guessing game.”

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In the case of James, which serves artisanal American cuisine on the ground floor of a Prospect Heights brownstone, one of the reasons for its “B” rating was the lack of a ventilating fan in an employee bathroom.

“The bathroom was well ventilated, and I tried to explain that to the inspector, but I guess the law’s the law,” said Bryan Calvert, the chef and one of the owners of the restaurant, who later installed a fan.

Mr. Calvert, who said he prided himself on employing a highly trained staff, said that he missed the “A” cutoff by one or two points, and that he was disheartened when he initially received the lower grade.

“It’s very frustrating,” he said. “We definitely had people who would ask, ‘What happened?’ ”