The Effect of Penicillin on the Methylene Blue Reduction Time and Keeping Quality of Milk

Abstract

Summary: The addition to 32 samples of freshly produced milk from 16 farms of 0.25, 0.5 and 1.0 unit/ml. of penicillin tended to increase the methylene blue reduction time at 37—38°, and to improve the keeping quality at 22°, irrespective of season. In no case was the reduction time decreased or the keeping quality made worse. In general, the effects were almost as marked with 0.25 unit as with the higher concentrations. The increases in the reduction time ranged from 0 to over 3 hr. in winter and from 0 to 2.5 hr. in summer. The delay in reaching the end‐point of the keeping quality test ranged from 0 to 15 hr. in winter and from 0 to over 18 hr. in summer. The response to penicillin differed with the two tests in individual samples. In some cases the methylene blue reduction time was markedly increased but keeping quality was little affected: in others, the reverse was observed. In both winter and summer streptococci predominated in the control samples at the keeping quality end‐point, but with penicillin added Gram‐negative rods often gained the ascendancy. At the time of reduction of methylene blue the flora of the winter control samples consisted mainly of streptococci and micrococci, but in the presence of penicillin the proportion of Gram‐negative rods generally increased: in summer Gram‐negative rods predominated in the control samples and in the presence of penicillin. Practically all the Gram‐negative rods were resistant to penicillin. All the micrococci and most of the streptococci were sensitive to penicillin; the resistant streptococci were strains of Str. faecalis. The variable distribution of resistant and sensitive organisms. coupled with the possible production of penicillinase, probably accounts for the variable effect of penicillin on the methylene blue and keeping quality tests in different samples.