Factors affecting the efficacy of pressure inactivation of Escherichia coli O157:H7 on alfalfa seeds and seed viability

The application of high hydrostatic pressure technology as a seed decontamination technology was evaluated. Alfalfa seeds inoculated with approximately 105 CFU/g of Escherichia coli O157:H7 were subjected to oscillatory pressure treatments at 600 MPa and 20 °C for up to five cycles with a holding time of 2 min/cycle. However, oscillatory pressurization was not able to eliminate E. coli O157:H7. The application of pressure treatment at 600 MPa for 2 min at 20 °C in the presence of chemicals such as calcium hypochlorite, calcium hydroxide, lactic acid or sodium acid sulfate was subsequently investigated and it was demonstrated that this “multiple hurdle” approach was unable to decontaminate alfalfa seeds. Soaking seeds prior to pressure treatment was found to play a critical role on enhancing the pressure inactivation of E. coli O157:H7; seeds soaked in water for 60 min followed by treatment at 600 MPa for 2 min at 20 °C were decontaminated and had a germination rate of 91% which was 4% lower than that of the untreated seeds (not statistically significant, P > 0.05). It was further demonstrated that a process involving soaking seeds in water for ≥ 10 min followed by treatment at 600 MPa for 15 min at 20 °C was equally effective with respect to E. coli O157:H7 elimination and viability retention of the seeds.