Effect of Copper on Microorganisms in the Manufacture of Swiss Cheese1,2

Abstract

Commercial strains of Lactobacillus bulgaricus, Streptococcus thermophilus, lactic streptococci, and propionibacteria were grown in the presence of 0, 2, 4, 8, and 16ppm copper to simulate conditions that might exist in Swiss cheese manufacture. In general, loss of viability and inhibition of acid production was hastened by an increase in copper concentration although genus, species, and strains differed. During the first 3h of incubation in milk, mixed-strain lactic cultures were not affected by the addition of copper up to 16ppm. Later, inhibition and increased death rates occurred depending upon copper concentration. S. thermophilus strains exhibited a wide variety of sensitivities to copper. However, they seemed to overcome partly their sensitivity to copper after about 16h incubation. Two of three strains of L. bulgaricus showed slight reduction in cell numbers and acid production with increasing copper concentration. The third strain more closely resembled the lactic streptococci in its greater sensitivity to copper. Two of four strains of propionibacteria were more sensitive to copper than were the other two. Carbon dioxide production in broth at pH 7.0 differed among the strains. Incubation periods up to 20 days, however, tended to overcome copper inhibition. At an initial broth pH of 5.4, carbon dioxide production was delayed markedly by 16ppm copper, but, with prolonged incubation, two of the four strains attained carbon dioxide production equal to copper-free controls.