| The efficacy of 2% lactic acid (LA) and 2% low molecular weight polylactic acid (LMW PLA) against spoilage and pathogenic bacteria on vacuum-packaged fresh raw beef stored at refrigeration temperature was investigated. Nisin was tested as an additional hurdle to microbial growth. In the first experiment, raw beef cubes were dipped in a solution of 2% LA, 2% LMW PLA, 200 IU Nisaplin(TM)/mL, 200 IU Nisaplin(TM)/mL in 2% LA (2%NLA) or 200 IU· Nisaplin(TM)/mL in 2% PLA (2%NPLA) for 5 minutes. The beef cubes were drip-dried for 15 minutes, vacuum-packaged, and stored at 4°C for up to 56 days. The surface pH values and population reduction of psychrotrophic aerobic bacteria, psychrotrophic and mesophilic Enterobacteriaceae, Pseudomonas, and Lactobacillus were investigated at days 0, 7, 14, 21, 28, 42, and 56. Compared to control, 2% LA significantly reduced surface pH values of the beef samples by approximately 0.6 units while 2% LMW PLA only reduced them by approximately 0.4 units at day 0 (P < 0.05). Efficacy of 2% LA at lowering the surface pH of raw beef at day 0 was significantly greater than 2% LMW PLA (P < 0.05). However, neither 2% LA nor 2% LMW PLA maintained a low pH on the meat surface beyond 7 days. The 2% LA and 2% LMW PLA showed immediate inhibitory effects on psychrotrophic aerobic bacteria (2.36 and 1.94 log10 reduction), psychrotrophic Enterobacteriaceae (1.86 and 1.37 log10 reduction), mesophilic Enterobacteriaceae (1.00 and 1.00 log10 reduction) and Pseudomonas (1.57 and 1.77 log10 reduction) on fresh raw beef ( P < 0.05). The reduction was evident up to 56 days as seen by the numbers of Enterobacteriaceae and Pseudomonas (P < 0.05). In the second experiment, inoculated beef cubes were treated with a solution of 2% LA, 2% LMW PLA, 400 IU Nisaplin(TM)/mL, 2% NLA, or 2% NPLA as described above. These five treatments showed immediate bactericidal effects on Listeria monocytogenes Scott A (1.56, 1.22, 1.57, 1.64, and 1.94 log10 reduction, respectively) (P < 0.05). The reductions of Escherichia coli O15:H7 numbers by 2% LA, 2% LMW PLA, 2% NLA, and 2% NPLA were only 0.95, 0.89, 1.07, and 0.80 log10, respectively (P < 0.05). These treatments, combined with vacuum-packaging and refrigeration temperature, succeeded more in inhibiting the growth of L. monocytogenes Scott A than E. coli 0157:H7· during storage. These findings suggested that bactericidal and bacteriostatic effects of 2% LA and 2% LWM PLA against spoilage microflora, E. coli O157:H7· and L. monocytogenes Scott A were not significantly different. Furthermore, low concentrations of nisin solutions (200 or 400 IU Nisaplin(TM) per mL) did not have efficacy at enhancing 2% LA and 2% LMW PLA against these four groups of spoilage bacteria or E. coli O157:H7 during storage. |
