length for maximiaing UVC LED intensity as well as the optimum irradiance overlaps on the coupon treatment surfaces, taking into account the 120° optical angle of UVC LEDs. Intensity of the three UVC LEDs array was measured with a spectrophotometer (AvaspecULS2048-USB2-UA-50; Avantes, Netherlands) which was calibrated for the entire UV spectrum. While distance between the optical probe and UVC LEDs array was maintained at 3cm, the irradiance flow rates at eight points covering the abiotic material surfaces (2 by 5cm) were scanned with the spectrometer and measured. To calculate the petri factor indicating even distribution of UV irradiation over the surface, the measured intensity value of each spot was divided by the maximum intensity and averaged. Modified UV intensity was calculated by multiplying the maximum intensity by the petri factor, so that averaged UVC LED intensity was represented by the calculated value (Kim et al., 2017a, b; Kim et al., 2016; Shin et al., 2016).2.4. UV treatment and combined treatment with mild heatTheinoculatedabioticcouponsweretreatedinthechamberatroom temperature with the UVC LED array at dosages of 0.5, 1, 2, and 3mJ/ cm2. Treatment times for each dosage were calculated by dividing UV doses by the modified intensity with an appropriate conversion factor. To enhance the bactericidal effect, mild heat was combined with UVCLEDtreatment.Hotairgeneratedbyaheattoolwasblownintothe treatment chamber through a silicon tube. Circumstance temperature was measured with a fiber optic temperature sensor (FOT-L, FISO Technologies Inc., Quebec, Canada) connected to a signal conditioner (TMI-4, FISO, Technologies Inc., Quebec, Canada) and presented in Fig.2.Hotairwasdeliveredfor13suntilthecircumstance temperature reached 50°C, after which the heat tool was turned off for 7s to allow residual circumstance heat to increase to 60°C. During UVC LED irradiation for calculated times, temperature was maintained along with a slight temperature drop: 3°C drop at 50s and 10°C drop at 75s.2.5. Bacterial enumerationAfter UVC LED treatment, coupons were transferred to sterile 50ml centrifuge tubes containing 30mlPW and 3g glass beads (425–600μm, Sigma-Aldrich Corp, St. Louis, MO, USA), and vortexed at maximum speed for 2min to detach foodborne pathogens from coupon surfaces. Cell suspensions in the tubes were tenfold serially diluted in PW, and 100μl aliquots of selected dilutions were spread-plated onto selective media; Sorbitol MacConkey agar (SMAC, Oxoid) for E. coli O157:H7, Xylose lysinedesoxycholate agar (XLD,Oxoid) for S.Typhimurium, and Oxford agar base with antimicrobial supplement (OAB, MB Cell) for L.