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Food companies big and small are struggling to replace genetically modified ingredients with conventional ones.

Pressure is growing to label products made from genetically modified organisms, or “G.M.O.” In Connecticut, Vermont and Maine, at least one chamber of the state legislature has approved bills that would require the labeling of foods that contain genetically modified ingredients, and similar legislation is pending in more than two dozen other states. This weekend, rallies were held around the globe against producers of genetically altered ingredients, and consumers are threatening to boycott products that are not labeled.

And so, for many businesses, the pressing concern is just what it will take to gain certification as non-G.M.O.

Lizanne Falsetto knew two years ago that she had to change how her company, thinkThin, made Crunch snack bars. Her largest buyer, Whole Foods Market, wanted more products without genetically engineered ingredients – and her bars had them. Ms. Falsetto did not know how difficult it would be to acquire non-G.M.O. ingredients.

ThinkThin spent 18 months just trying to find suppliers. “And then we had to work to achieve the same taste and texture we had with the old ingredients,” Ms. Falsetto said. Finally, last month, the company began selling Crunch bars certified as non-G.M.O.

The Non-GMO Project was until recently the only group offering certification, and demand for its services has soared. Roughly 180 companies inquired about how to gain certification last October, when California tried to require labeling (the initiative was later voted down), according to Megan Westgate, co-founder and executive director of the Non-GMO Project.

Nearly 300 more signed up in March, after Whole Foods announced that all products sold in its stores would have to be labeled to describe genetically engineered contents, and about 300 more inquiries followed in April, she said.