No fatalities or serious injuries from Chinese food products have been reported in the United States, although counterfeit Chinese glycerine has been linked to at least 100 deaths in Panama. In May, senior members of the Bush administration, including the secretary of agriculture, Mike Johanns, raised the food safety issue with Chinese officials during trade talks in Washington. And last week, in a step designed to reassure Western customers, the Chinese government said it had closed 180 food plants and identified 23,000 safety violations.

Although they affect only a fraction of imports from China, the rising tempo of alerts, including an F.D.A. restriction imposed on Thursday on sales of five types of Chinese-farmed seafood, has called attention to China’s sudden emergence as a major agricultural exporter. Between 2002 and 2006, F.D.A.-regulated imports of food from China rose from just over 100,000 shipments to nearly 235,000. Experts predict those shipments will reach 300,000 this year.

The spate of recalls and the rising volume of exports have highlighted another worry: the increasing dependence of the United States’s biggest food manufacturers on China for basic additives like apple juice, a common sweetener, and preservatives like ascorbic acid.

These little-known additives form the building blocks of many popular staples in American kitchens, keeping fruit from turning brown or providing the sweetness in breakfast bars. Food experts note, for example, that China supplies more than half of all the apple juice imported to the United States, up from a fraction a decade ago.

Other critical but common additives have followed an even sharper trajectory, according to Peter Kovacs, the former chief executive of NutraSweet Kelco and now a food industry consultant. More than 80 percent of ascorbic acid, better known as vitamin C and also used as a preservative, comes from China, Mr. Kovacs said. Chinese imports of xanthan gum, used to thicken dairy products and salad dressings, account for at least 40 percent of United States consumption.

“This is a problem for the whole food chain, but it was a blank spot,” Mr. Kovacs said. “They’re doing it now, but companies weren’t testing these additives before.”

Although Kellogg and General Mills disclosed these additional steps, they were reticent to provide additional details. And many food makers are nervous to discuss what is emerging as an issue that could threaten the trust of shoppers in long-established brands.