Antimicrobial mist pretreatment for enhancing superheated steam efficacy in inactivating Enterococcus faecium NRRL...

Superheated steam (SHS) is an emerging sanitation technology for dry food processing environments. This study investigates the efficacy of antimicrobial pretreatments in conjunction with SHS to inactivate microorganisms on stainless steel surfaces. Stainless steel coupons were inoculated with Enterococcus faecium (11.1 ± 0.4 log CFU/cm²), equilibrated to a water activity of 0.2 (a_w), and subjected to various antimicrobial pretreatments prior to SHS exposure. This includes soaking for 10 or 20 s in sterile water, 70 % ethanol, 0.05 % peracetic acid (mist applied for 5 s), or acidified oil emulsion (100 μL). SHS experiments were conducted using both bench-scale (100 % steam content) and pilot-scale equipment (5 % steam, 95 % hot air). The nozzle-to-surface distance was maintained at 3 cm. All experiments were performed in triplicate, and surviving microorganisms were enumerated using plate counts (detection limit: 0.2 log CFU/cm²). Antimicrobial pretreatments alone resulted in microbial reductions of <2.0 ± 0.5 log CFU/cm². SHS treatment using the bench-scale system achieved >6.4 ± 0.3 log CFU/cm² reduction regardless of pretreatment. Pilot-scale SHS treatment (with or without antimicrobials) achieved lower reductions (<2.8 ± 0.2 log CFU/cm², p < 0.05) possibly due to lower steam content. Increasing the treatment intensity (195 °C for 30 s) resulted moderate improvement in microbial inactivation (3.3 ± 0.6 log CFU/cm²). This study demonstrates that antimicrobial pretreatments can enhance the efficacy of SHS sanitation and highlights the importance of steam content in the treatment environment.