To pasteurize a liquid is to heat it in a special, gentle way so harmful bacteria die, without boiling it. Dairies pasteurize milk before it goes into cartons; juice can be pasteurized too. It is mostly a factory and science verb, not something most families do at home — though actually some careful cooks pasteurize eggs or cream in the kitchen. American spelling keeps the z (pasteurize); British spelling often uses s (pasteurise). The word was borrowed from French pasteuriser and carries the name of Louis Pasteur, who invented the process.