New York is the first American city to adopt such a stringent rule.

Starting this week, the ban extends to almost all prepared food in restaurants, bakeries, cafeterias, salad bars and food carts.

There will be a three-month grace period before big fines are slapped on violators. Those who reject the ban and get caught face a US$2,000 fine starting Oct. 1.

So chefs who relied on trans fats to make their products flaky or crispy have worked overtime finding substitute ingredients.

They are trying to retool old recipes without damaging flavor, texture or color. There are plenty of substitutes, including natural fats like butter or lard, palm oil, and a growing list of new oil blends.

Dunkin Donuts secretly sold 50 million trial doughnuts with a new blend of palm, soybean and cottonseed oil, to see how customers would react, before announcing it had made the switch.

Dunkin Donuts said customers didn't notice the change.

But for some bakers, adjusting has been painful.

"We're banging our heads against the wall right now," said Manny Alaimo, an owner of the respected Villabate Pasticceria in Brooklyn.

Italian breads and cookies made with the zero-trans-fat shortening just haven't come out right, he said, and a few demanding customers have complained about subtle changes in taste and texture.