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Sodium nitrite is used to speed up the curing of meat and also impart an attractive colour. A 2018 study by the British Meat Producers Association determined that legally permitted levels of nitrite have no effect on the growth of the "Clostridium botulinum" bacteria which causes botulism, in line with the UK’s Advisory Committee on the Microbiological Safety of Food opinion that nitrites are not required to prevent Clostridium botulinum growth and extend shelf life. In the U.S., meat cannot be labeled as "cured" without the addition of nitrite. In some countries, cured-meat products are manufactured without nitrate or nitrite, and without nitrite from vegetable source. Parma ham, produced without nitrite since 1993, was reported in 2018 to have caused no cases of botulism.