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

Decimal reduction times (D values) were determined for Bacillus cereus T sp ores 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 organ isms, the addition of nisin lowered the apparent D values. For B. cereus, t he addition of 2,000 IU of nisin per ml to skim milk before heating signifi cantly (P less than or equal to 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, respec tively. 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 acros s both nisin treatments, the mean reductions in apparent D values at 97, 10 0, and 103 degrees C due to addition of nisin in comparison to the controls were 32, 20, and 42%, respectively. The z(D) values for B. cereus ranged f rom 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 p resence of 2,000 or 4,000 IU of nisin per mi. 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, re spectively. There was a significant (P less than or equal to 0.05) decrease in the apparent D value between the control and 4,000-IU/ml treatment. Ove rall, log populations of survivors for B. stearothermophilus compared to th e control were lower at any given sampling time due to the presence of nisi n. 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.