By default, the CBP bans unprocessed meat, including seafood. The list also includes various vegetables, fruits and grains as those substances are prone to potentially containing germs. That is also why frozen meats are banned in airplanes.
Raw rice has also been banned as well as walnuts.
In addition, various seeds and live plants are not allowed on airplanes.
However, sealed food products are allowed on planes if the traveler notifies the CBP prior to traveling. That is how many Koreans have been able to carry kimchi and pickled vegetables.
Processed foods are also allowed. Homemade, marinated perilla leaves are banned, but the ones sold in stores inside of cans are permitted on planes.
“We end up picking out about 4,000 banned items from travelers in a day,” the CBP said. “We encourage Korean travelers to prepare the English names of the items they’re carrying. If they have trouble communicating in English, they should download the form on our website in advance and have it filled out, so that our agents can screen them accordingly.”