Growth of Brochothrix thermosphacta in ground beef following treatments with nisin in calcium alginate gels

Sterilized, lean and adipose beef carcass tissues were inoculated with Brochothrix thermosphacta, left untreated (U), or treated with 100 μg/ml nisin (IV), calcium alginate (A), or 100 μg/ml nisin immobilized in a calcium alginate gel (AN). Treated tissues were aseptically processed into ground beef and populations of B. thermosphacta and nisin activity were determined during refrigerated storage (4°C) at 0, 7, and 14 days. At day 0, bacterial populations of U- and A-treated ground beef were 3.24 and 3.17 log₁₀ CFU/g respectively. Ground beef treated with N exhibited populations of 2.80 log₁₀ CFU/g while AN significantly suppressed the organism to undetectable levels (<1.30 log₁₀ CFU/g) at day 0. In contrast to high nisin titers from AN-treated ground beef at day 0, nisin titers were undetectable in N-treated ground beef. By day 7, B. thermosphacta had grown to 7.18, 7.04, and 6.92 log₁₀ CFU/g in U-, A-, or N-treated ground beef, respectively, while AN-treated ground beef exhibited significantly different (P≤0.05) populations of 6.56 log₁₀ CFU/g. By day 7, nisin titers from AN-treated ground beef were considerably diminished. At day 14 of the study, all treatments exhibited bacterial populations >7 log₁₀ CFU/g and nisin titers were virtually undetectable in any of the ground beef samples. While the growth of B. thermosphacta could not be effectively suppressed for 14 days, the application of nisin in alginate gels to meat surfaces does afford some immediate protection against undesirable bacteria when these surfaces are processed into ground beef.