Use of Microbial β-Lactamase to Destroy Penicillin Added to Milk¹

Abstract

A simple method is described for destruction of penicillin residues in bulk milk to an undetectable amount (less than .003 U/ml) with commercially available crude β-lactamase enzyme. Milk containing .1 or .5 U/ml penicillin G was treated with .01 to 1.0 mU/ml of β-lactamase (Bacillus cereus) for up to 96 h. The Bacillus stearotbermophilus var. calidolactis assay was used to quantify penicillin in milk between .003 to 1.0 U/ml. The .5 U/ml of penicillin G was reduced to an undetectable amount within 18 h at 4°C by 1.0 mU/ml of β-lactamase. The development of titratable acidity over 5 to 6 h in contaminated milks treated with β-lactamase and inoculated with Streptococcus tbermophilus GH, Streptococcus cremoris, Streptococcus lactis, or a commercial starter culture was the same as for control milk samples containing no additives or only enzyme.

Pilot-scale manufacture of Swiss and Cheddar cheeses from contaminated milks treated with β-lactamase yielded cheeses of comparable quality, to control cheeses produced from penicillin-free milk. There were no delays in acid production as judged from pH measurements during production and ripening of the cheeses. About 50% of β-lactamase activity added to milk remained after pasteurization at 63°C for 30 min. The safety for human consumption of cheese containing small quantities of penicillin degradation products from milk treated with β-lactamase remains to be established.