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In many parts of the world, it’s the most normal thing: selling food that you make at home. But when Hai Ngoc Lam arrived in the United States five years ago from Vietnam, he found out what other would-be food entrepreneurs here already knew: It’s illegal in California.

That changed this week with the signing of a law that authorizes small-scale home food businesses. (It’s been a busy week for food news in California: We reported on Thursday that food carts were also legalized.)

“It’s amazing for me,” said Mr. Lam, who with his husband, Joe Acanfora, runs an under-the-radar business selling home-cooked Vietnamese food in the Bay Area.

The law, which limits licenses to those with maximum revenue of $50,000 from home cooking, comes into effect in January and will apply only to counties that choose to participate. A small category of food considered the riskiest — oysters and raw meats among them — are not allowed.