Microbial inactivation and shelf life comparison of 'cold' hurdle processing with pulsed electric fields and microfiltration, and conventional thermal pasteurisation in skim milk

Abstract

Thermal pasteurisation (TP) is the established food technology for commercial processing of milk. However, degradation of valuable nutrients in milk and its sensory characteristics occurs during TP due to substantial heat exposure. Pulsed electric fields (PEF) and microfiltration (MF) both represent emerging food processing technologies allowing gentle milk preservation at lower temperatures and shorter treatment times for similar, or better, microbial inactivation and shelf stability when applied in a hurdle approach compared to TP. Incubated raw milk was used as an inoculum for the enrichment of skim milk with native microorganisms before PEF, MF, and TP processing. Inoculated milk was PEF-processed at electric field strengths between 16 and 42 kV/cm for treatment times from 612 to 2105 μs; accounting for energy densities between 407 and 815 kJ/L, while MF was applied with a transmembrane flux of 660 L/h m². Milk was TP-treated at 75°C for 24 s. Comparing PEF, MF, and TP for the reduction of the native microbial load in milk led to a 4.6 log₁₀ CFU/mL reduction in count for TP, which was similar to 3.7 log₁₀ CFU/mL obtained by MF (P≥0.05), and more effective than the 2.5 log₁₀ CFU/mL inactivation achieved by PEF inactivation (at 815 kJ/L (P<0.05)). Combined processing with MF followed by PEF (MF/PEF) produced a 4.1 (at 407 and 632 kJ/L), 4.4 (at 668 kJ/L) and 4.8 (at 815 kJ/L) log₁₀ CFU/mL reduction in count of the milk microorganisms, which was comparable to that of TP (P≥0.05). Reversed processing (PEF/MF) achieved comparable reductions of 4.9, 5.3 and 5.7 log₁₀ CFU/mL (at 407, 632 and 668 kJ/L, respectively (P≥0.05)) and a higher inactivation of 7.1 log₁₀ (at 815 kJ/mL (P<0.05)) in milk than for TP. Microbial shelf life of PEF/MF-treated (815 kJ/L) and TP-treated milk stored at 4°C was analysed over 35 days for total aerobic; enterobacteria; yeasts and moulds; lactobacilli; psychrotroph; thermoduric psychrotroph, mesophilic, and thermophilic; and staphylococci counts. For both PEF/MF and TP-treated milk an overall shelf stability of 7 days was observed based on total aerobic counts (P≥0.05). Milk hurdle processing with PEF/MF at its most effective treatment parameters produced greater microbial inactivation and overall similar shelf stability at lower processing temperatures compared to TP. With higher field strength, shorter treatment time, larger energy density, and rising temperature the efficacy of PEF/MF increased contrary to MF/PEF. Thus, PEF/MF represents a potential alternative for 'cold' pasteurisation of milk with improved quality.