High pressure processing (HPP) and in-situ nisin biosynthesis by lactococcus lactis: A hurdle approach to improve listeria spp. Inactivation in bovine milk

Abstract

Cold pasteurization of raw milk may be achieved by combining high pressure processing (HPP) with other (a)biotic factors. In this study, a combined approach involving HPP and biopreservation by in-situ nisin synthesis was evaluated. UHT and raw bovine milk were inoculated with Listeria innocua and treated with HPP (350-600 MPa), or HPP+nisin synthesized in-situ by Lactococcus lactis Rif L+ (LUQ2). HPP treatments of commercial UHT milk yielded two decimal reductions (S VHPP = 2 log10 reductions) of L. innocua with 550 MPa/3min, while the HPP+nisin approach yielded <10 cfu ml−1 under the same pressure-time conditions. Native Listeria monocytogenes was found in raw milk (~102 cfu ml−1) and the total Listeria spp. counts increased to ~109 cfu ml−1 with the L. innocua inoculum. HPP milk pasteurization (SVHPP > 5 log10, aerobic plate count; <1 cfu ml−1, Listeria spp.) was viable by using 600 MPa/12 min, whereas HPP+nisin resulted in milk pasteurization at holding times between 6 and 9 min for the same pressure level. Furthermore, LUQ2 synthesized 9.75±0.54 IU ml−1 of nisin, which is below the FAO/WHO limit (500 IU g−1) suggesting that the promising HPP+nisin approach could be optimized to pasteurize milk at lower pressure levels and/or shorter pressure holding times.