Purpose: The purpose of this study was to assess the effects of gamma irradiation on D10 values of P. fluorescens surface inoculated on romaine lettuce and baby spinach, and the sensitivity of the bacteria (D10 values) suspended in buffer peptone water.
Methods: Gamma irradiation was applied on romaine lettuce and spinach following surface inoculations with P. fluorescens (Pf) strains (2-79, Q8R1, and Q287) at doses of 0 (control), 0.25, 0.5, 0.75 and 1.0 kGy and the leafy greens were subsequently stored for 24 hrs at 5°C. The effects of gamma irradiation were also determined on bacteria cells suspended in buffered peptone water (BPW) in tubes. The Pf strains are non-plant pathogenic, non-pectolytic, and saprophytic in nature.
Results: The initial Pseudomonas populations ranged from 9.08 ± 0.21 to 9.69 ± 0.16 log CFU/ml of BPW in the non-irradiated control. The bacterial counts on the non-irradiated spinach and romaine lettuce ranged from 8.14 ± 0.34 to 8.95 ± 0.06 and from 8.32 ± 0.05 to 8.63 ± 0.25 log CFU/g, respectively. The radiation D10 values of P. fluorescens suspended in BPW ranged from 0.09 ± 0.00 to 0.12 ± 0.01 kGy when enumerated on Pseudomonas Agar F and Trypticase Soy Agar. However, on spinach and romaine lettuce, the D10 values of P. fluorescens were significantly (P < 0.05) lower and ranged from 0.04 ± 0.01 to 0.05 ± 0.02 and from 0.05 ± 0.03 to 0.06 ± 0.03 kGy, respectively.
Significance: There was high sensitivity of P. fluorescens (the biocontrol microbe) to irradiation when inoculated on surfaces of romaine lettuce and spinach. In future, combinations of low dose irradiation and biocontrol treatments will be investigated to enhance the potential synergistic inactivation of foodborne pathogens.