MADERA — Raw, organic almonds form the basis of Karyn Calabrese's garlicky nut pate, her vegan pie crusts and vanilla ice cream custards.

But under a new federal rule requiring that virtually all almonds be pasteurized to prevent foodborne illness, the Chicago restaurateur will have to substitute a new nut, or go to vast lengths to import her raw almonds from across the globe.

Industry representatives say tightening food safety rules to subject almonds to heat treatment will help expand the market for California farmers, who grow about 80 percent of the world's almonds in a flat strip of land sandwiched between the Pacific coast and the Sierra Nevada mountains.

But the regulation, set to take effect Sept. 1, has also angered everyone from organic farmers to followers of the restrictive raw foods diet.

"The almond is the king of the nut world and a main staple for raw foodists," said Calabrese, whose restaurants feature raw, vegan food, none of which has been heated above 110 degrees. "I haven't even thought out what I'll do because it's just such a mind-blowing situation."

Almonds have become increasingly lucrative as they've gained popularity with health-conscious consumers. California farmers expect to harvest 1.3 billion pounds of almonds this year, a bumper crop worth more than $1.4 billion. Following salmonella outbreaks in 2001 and 2004 that were traced to raw almonds, the Almond Board of California rallied for a federal rule requiring all almonds in the state to be pasteurized to keep bacteria from infecting the nuts while they dry in the orchard or while they're processed.

"We consider it unacceptable to continue shipping a product that could contain a microorganism that could make somebody sick," said Richard Waycott, president and chief executive of the board, a marketing arm of the U.S. Department of Agriculture. "We're really confident that this program is a win-win for everybody because it does not alter the product."

In pasteurization — a process also used for milk, juice and eggs — the shelled and hulled nuts typically are laid out on a conveyor belt that passes them through a moist burst of steam to heat the kernels' surface to about 200 degrees, killing any pathogens present. An alternative process sends the nuts into a chamber where they're sprayed with propylene oxide gas.

Major almond buyers such as Mars Inc., Kraft Foods Inc. and Hershey Co. reviewed a study by the board to determine if the process impacted the nut's quality, taste, texture and appearance, and found it had no effect, Waycott said.

Once treated, the pasteurized almonds are ready for sale and can be legally shipped throughout the U.S., Canada and Mexico, said Michael Durando, chief for the marketing order administration branch at USDA.

Growers can apply for exemptions if they can prove that their manufacturing process — be it dry roasting, blanching or any other traditional treatments — achieves pasteurization. They also can sell small quantities of raw, unpasteurized almonds direct to customers at farm stands or at certified California farmers markets, but can face penalties if they're caught selling more than 100 pounds a day to any one person.

That's not enough volume for Berkeley-based Living Tree Community Foods, which soon will start importing its raw almonds from Spain to make its "living" nut butter. Company officials said its customers are concerned about the health effects of propylene oxide, a gas listed as a possible carcinogen by the International Agency on Cancer Research.

Federal guidelines found that extremely low residue levels of the gas had no harmful effects, Waycott said. But the Cornucopia Institute, a Wisconsin-based farm policy research group, asked the USDA to hold off on implementing the rule to solicit an independent study on the chemical.

The rule was developed over three years of careful discussions between industry representatives and agriculture officials, and won't be reconsidered, Durando said.

Madera-based farmer Mike Braga, whose organic nuts are favored by live food fans and grocery chains such as Trader Joe's, said he won't break the law by continuing to sell raw almonds. But if customers aren't demanding it, he said he doesn't see why he shouldn't be able to freeze his almonds instead of pasteurizing them.

"We're going to lose our entire raw market," Braga said. "If such good almonds are available here, why should our customers have to import them from Europe?"