that: used for defining (restrictive) clauses (information that defines the subject and cannot be omitted from a sentence without greatly changing the meaning)
The residues that were detected in beef were at very high levels. [Omitting the italicised words (a restrictive clause) would change the meaning of the sentence.]
which: used for nondefining (nonrestrictive) clauses (incidental information that does not significantly limit the meaning of the principal clause)
CFZ residues, which have been detected in beef, are not permitted in human food. [The italicised words (a nonrestrictive clause) may be omitted without affecting the meaning of the sentence.]