Milk containing 0.005 U of benzyl 14C-penicillin G was used to make Cheddar cheese, and the whey was subsequently treated with ultrafiltration (UF) and reverse osmosis (RO). The distribution of labelled penicillin was monitored throughout milk and whey processing. The antibiotic concentrated in the moisture phases. Eight-six percent was recovered from the whey fraction, and approximately 12% from the cheese. Similar distribution patterns were detected for the whey UF permeate and retentate; 84% of the penicillin was in the permeate, and 19% remained in the retentate. The balance of penicillin was recovered primarily in the RO retentate fraction.

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