Nisin in milk sensitizes Bacillus spores to heat and prevents recovery of survivors.

ABSTRACT

Decimal reduction times (D values) were determined for Bacillus cereus T spores and B. stearothermophilus ATCC 12980 spores in skim milk supplemented with various concentrations (0, 2,000, and 4,000 IU/ml) of the bacteriocin nisin by using an immersed, sealed capillary tube procedure. For both organisms, the addition of nisin lowered the apparent D values. For B. cereus, the addition of 2,000 IU of nisin per ml to skim milk before heating significantly (P< or =0.05) lowered the apparent D value compared to the control treatment. The D values at 97 degrees C were 7.0, 4.8, and 4.7 min for the control and 2,000- and 4,000-IU/ml nisin treatments, respectively. At 103 degrees C, the D values were 1.5, 0.85, and 0.88 min for the control and 2,000-and 4,000-IU/ml nisin treatments. When calculated across both nisin treatments, the mean reductions in apparent D values at 97, 100, and 103 degrees C due to addition of nisin in comparison to the controls were 32, 20, and 42%, respectively. The zD values for B. cereus ranged from 8.0 to 8.9 degrees C. With B. stearothermophilus, the apparent D values at 130 degrees C were reduced by 13 and 21% respectively, because of the presence of 2,000 or 4,000 IU of nisin per ml. The D values were 16.0, 13.8, and 12.5 s for the control and 2,000- and 4,000-IU/ml nisin treatments, respectively. There was a significant (P< or =0.05) decrease in the apparent D value between the control and 4,000-IU/ml treatment. Overall, log populations of survivors for B. stearothermophilus compared to the control were lower at any given sampling time due to the presence of nisin. The results of these studies suggest that spore control is likely due to enhanced sensitivity of spores to heat and the presence of residual nisin in the recovery medium that could prevent outgrowth of survivors.