- Food Control
- Volume 125, July 2021, 107988
Abstract
Listeria monocytogenes forms biofilms on food-contact surfaces providing this pathogen with the potential to serve as a constant cross-contamination source. The objective of this study is to examine the efficacy of steam treatment against the biofilm of L. innocua, a well-known L. monocytogenes surrogate, on common food-contact surfaces using a pilot-scale steam treatment blancher. Saturated steam at 100 °C was effective in inactivating L. innocua in biofilms on all tested food-contact surfaces with a 6-sec steam treatment attaining a 2.4–3.1 log10 CFU/coupon (1.5 cm × 1.5 cm) reduction depending on the type of surface. However, the effectiveness of steam decreased dramatically during prolonged steam treatment with tailing effects more pronounced on rubber, low-density polyethylene (LDPE), polyvinyl chloride (PVC), followed by polyester (polyethylene terephthalate, PET), and then stainless steel (SS). A 30–180 s steam exposure at 100 °C caused a 4.0–6.4 log10 CFU/coupon reduction of L. innocua biofilm on SS, and 3.0–4.8, 2.8–4.2, 2.7–4.5 and 2.6–3.3 log10 reductions on PET, LDPE, PVC, and rubber surfaces, respectively. Organic soil from 1:10 diluted apple juice did not compromise the bactericidal effects of steam against L. innocua biofilm on all tested surfaces. Repeated steam exposure did not impact hydrophobicity and roughness parameters of SS, PET, and rubber coupons, but decreased hydrophobicity of PVC and LDPE, increased the Rz value of PVC, and decreased Rp and Rz values for LDPE surface. Data suggested that a short time steam exposure alone or in combination with other interventions likely provides effective mitigation treatments to control Listeria biofilm on SS, PET, and rubber surfaces.
Introduction
Fresh produce is a common vehicle causing human foodborne pathogen outbreaks or illnesses. Listeria monocytogenes is identified as a major source of foodborne illness due to its ubiquitousness in processing environments (Almeida et al., 2013; Beno et al., 2016; McCollum et al., 2013; Murugesan et al., 2015), ability to grow at refrigerated temperatures (Bardsley et al., 2019; Gonzalez-Fandos et al., 2001; Salazar et al., 2017), and a high mortality rate (Scallan et al., 2011). This microbe has been implicated in multistate outbreaks associated with various fresh and frozen commodities, including cantaloupe (McCollum et al., 2013), caramel apples (FDA, 2015), stone fruits (Chen et al., 2016), frozen vegetables (CDC, 2016a), packaged salads (CDC, 2016b) and Enoki mushrooms (CDC, 2020).
L. monocytogenes forms biofilms on different food-contact surfaces. L. monocytogenes in biofilm showed enhanced resistance to antimicrobial interventions (Ayebah et al., 2006; Chaturongkasumrit et al., 2011; Hua et al., 2019; Korany et al., 2018; Somers & Wong, 2004), providing a continuous source of contamination to foods that come into contact with contaminated surfaces. In fact, contaminated food-contact surfaces were the main causes for recent listeriosis outbreaks linked to cantaloupes (McCollum et al., 2013), stone fruits (Chen et al., 2016) and caramel apples (Angelo et al., 2017). In light of the caramel apple listeriosis outbreak, multiple food-contact surfaces including polishing brush, drying brush, conveyor, and wooden bin inner surface were confirmed to be L. monocytogenes positive (Angelo et al., 2017). These types of contamination on commonly utilized surfaces highlighted the importance of effectively sanitizing food-contact surfaces. Direct food-contact surfaces have been required to be fully cleaned as to prevent contamination/cross-contamination of “covered” produce (e.g. produce items that are “covered” under the provisions of this regulation are those commonly consumed raw) and packing environments regulated under the Food Safety Modernization Act (FSMA) Produce Safety Rule (FSMA, 2016).
L. monocytogenes cells contained within a biofilm on different food-contact surfaces are difficult to remove (Fagerlund et al., 2017; Frank & Koffi, 1990; Hua et al., 2019). We previously found that the bactericidal effects of commonly used commercial sanitizers were compromised by organic soils. Peroxyacetic acid, the most effective sanitizer among those tested at a 200 ppm and 5.0 min exposure only resulted in a 3.0–3.7 log reduction of L. monocytogenes in biofilm on the soiled surfaces (Hua et al., 2019), which highlights the need for more effective surface disinfection methods. Heating in the form of hot air, hot water or steam is a traditional method for microbial reduction. A 6-min of hot water immersion treatment at 60 °C reduced 7-day-old L. monocytogenes biofilm on stainless steel (SS) by 3.2 log10 CFU (Tobin et al., 2020). A 15-sec of hot water treatment at 95–100 °C provided ~7 log reductions of L. monocytogenes attached to the inner surface of model drainpipes (Berrang et al., 2014). Steam carries latent heat and is more efficient for microbial inactivation than hot air, or water. Steam application (>93.3 °C for at least 5 min) has been approved by FDA to disinfect water-contact surfaces in bottled drinking water facilities (FDA, 2019). Steam offers various advantages over sanitizers and other intervention methods. It can heat surfaces/target materials quickly and reach into crevices/cracks while leaving no chemical residue on treated surfaces. Steam pasteurization is effective against pathogens on meat surfaces with a 15-sec steam exposure leading to a 3.4-log10 CFU/cm2 reduction of L. monocytogenes on beef carcasses (Phebus et al., 1997). Steam effectively eliminated L. monocytogenes biofilms on SS surface during a 20-sec treatment at 85 °C (Park & Kang, 2014). However, steam was less bactericidal against L. monocytogenes biofilm on polyvinyl chloride (PVC) surfaces with a 50-sec exposure at 85 °C only achieving ~1.7 log10 CFU/coupon reduction (Park & Kang, 2014). These data indicate that the bactericidal efficacy of steam against L. monocytogenes biofilm depends on the material on which the biofilm is formed.
Besides SS and PVC, polyester (polyethylene terephthalate, PET), polyethylene, and rubber are extensively used in conveyor belts and brush beds of fresh produce packing lines. These materials can tolerate temperatures up to 100 °C (Kiskan et al., 2008; Li et al., 2019; Mogheiseh et al., 2019; Nimanpure et al., 2019). The antimicrobial efficacy of steam against L. monocytogenes biofilms on these surfaces is largely unknown. Therefore, this study aimed to 1) evaluate and compare the antimicrobial efficacy of steam at 100 °C against L. innocua, a well-known surrogate of L. monocytogenes (Fairchild & Foegeding, 1993; Tobin et al., 2020), biofilm on SS and PVC, PET, low-density polyethylene (LDPE) and rubber surfaces using a pilot-scale steam intervention system; 2) investigate the impact of diluted apple juice on steam disinfection efficacy against L. innocua biofilms on above-mentioned food-contact surfaces; and 3) examine the potential links between surface properties and steam inactivation efficacies.
Section snippets
Bacterial strains and inoculum preparation
L. innocua NRRL B-33197 was obtained from USDA-ARS Culture Collection of National Center (NRRL) for Agricultural Utilization Research (Peoria, IL, United States), and the L. innocua strains of TVS 470 and TVS 471 were obtained from University of California, Davis (Davis, CA, United States). Each strain was stored at −80 °C in Trypticase Soy Broth (Becton, Dickinson and Company (BD), Sparks, MD, United States) supplied with 0.6% Yeast Extract (TSBYE, Fisher Scientific, Fair Lawn, NJ, United
Steam and food-contact surface coupon temperatures
The steam temperature was maintained at 100 °C with a minor fluctuation (Fig. 1C). The temperature of the treated surface coupons rapidly reached 92 °C within 6 sec. The surface temperature of SS coupons at 6-sec of exposure was higher than that of other surfaces reaching 94.7 ± 0.2 °C compared to PET (93.2 ± 0.3 °C), LDPE (93.9 ± 0.6 °C), PVC (93.3 ± 0.5 °C), and rubber (92.3 ± 0.4 °C) surfaces (Fig. 1D). During subsequent steam exposure, the mean surface temperatures of treated surface
Rapid kill of L. innocua biofilm on different food-contact surfaces by steam disinfection
SS, PVC, PET, LDPE and rubber are common food-contact materials used in fresh produce packing houses. Each surface has a unique chemical composition, physical properties, and thermal conductivity. Thermal conductivity represents the ability of a material to conduct heat and depends on thermal diffusivity, heat capacity, and density of a specific type of material (Nabelek et al., 2010). SS had a high thermal conductivity at 13.8 W/mk (Jalaluddin & Miyara, 2012), while PET, LDPE, PVC, and rubber
Conclusion
Steam exhibited a fast killing kinetic against L. innocua biofilm on different food-contact surfaces; a 6-sec steam treatment attained a 2.4–3.1 log10 CFU/coupon reduction depending on surface materials. However, the killing rate of steam decreased dramatically during subsequent steam treatment and exhibited a tailing effect which was more pronounced on rubber, PVC, and LDPE surfaces, followed by PET and then SS surface. Organic matter soils did not compromise the bactericidal effects of steam
CRediT authorship contribution statement
Zi Hua: Investigation, Formal analysis, Writing – review & editing. Frank Younce: Writing – review & editing. Juming Tang: Writing – review & editing. Dojin Ryu: Writing – review & editing. Barbara Rasco: Writing – review & editing. Ines Hanrahan: Writing – review & editing. Mei-Jun Zhu: Conceptualization, Visualization, Writing – original draft, Writing – review & editing, Supervision.
Declaration of competing interest
None.
Acknowledgments
This study was supported by Washington Tree Fruit Research Commission. We would like to express our gratitude to Dr. Trevor Suslow at University of California, Davis for kindly providing L. innocua isolates from the produce packing facility. We thank Mrs. Tonia Green and Ms. Ritu Sharma for their assistance in the preparation of experimental materials.