ABSTRACT

Food manufacturers often use squeegees as a tool to remove condensation from overhead surfaces. This practice is done to reduce the likelihood of environmental pathogen contamination by eliminating condensed-water droplets that could fall from overhead surfaces during production. However, this practice may actually spread environmental pathogens across these surfaces, defeating its purpose and further increasing the risk for contamination in the processing area. To understand the risk associated with this common practice, test pipes inoculated with Listeria innocua ATCC 33090 were exposed to steam to produce condensation, which was then removed by squeegees. The pipe surfaces, droplets, and squeegees were subsequently analyzed for Listeria to determine the distance the organism spread across the pipe and how many organisms were transferred to the droplets and the squeegees. Results showed that Listeria traveled as far as 16 in. across the surface of the pipe, and bacterial transfer to the droplets decreased as the squeegee traveled further from the contaminated area. Sanitizers alone were able to remove about 1 to 2 log CFU of Listeria per in2 from the squeegee blades when materials were contaminated with Listeria (>6 log CFU/in2). Among the cleaning protocols evaluated, an extensive cleaning regimen was able to remove 3 to 4 log CFU/in2, which would be recommended to reduce the risk associated with environmental pathogen transfer. This study provides evidence that supports recommendations for minimizing the cross-contamination risk associated with condensation management practices.

HIGHLIGHTS
  • Food manufacturers remove condensation from overhead surfaces manually.

  • Condensation removal may increase the risk of spreading environmental pathogens.

  • Listeria bacteria traveled at least 16 in. on surfaces due to condensation removal.

  • Rubber squeegees are more sanitary than sponges and easier to clean.

  • Cleaning regimes applied to cleaning tools are critical to prevent transmission.

The food processing environment has been recognized as a risk factor for bacterial contamination of final products. Human pathogenic bacteria, such as Listeria monocytogenes, can survive, grow, and potentially contaminate ready-to-eat (RTE) food if they are found in the processing environment. RTE food processing facilities are often designed to use low-temperature processing rooms to decrease bacterial growth; however, L. monocytogenes is a psychrotrophic bacterium with the ability to grow under these conditions. In fact, the Food Safety Modernization Act, through its “Preventive Controls for Human Food” rule, and other food safety programs consider it necessary to establish an environmental monitoring program to identify bacterial niches and control them immediately to reduce potential contamination of RTE products (13, 14).

Condensation is well known to be a potential carrier for these pathogenic bacteria (2, 11). During hot-water sanitation procedures in refrigerated RTE food processing facilities, the relative humidity in the environment increases greatly. After cleaning and lower-temperature operations are resumed, condensation droplets are formed on overhead surfaces due to temperature differences and high relative humidity in the processing environment. As a result, the hanging drops can often fall on to food or food contact surfaces, potentially introducing contamination and pathogenic bacteria, such as L. monocytogenes. In facilities where wet cleaning is used, water availability in the environment allows the growth of L. monocytogenes, increasing its chances to survive long term in the processing environment.

The U.S. Food and Drug Administration and the U.S. Department of Agriculture Food Safety Inspection Service, according to their current good manufacturing practices and guidelines, require manufacturers to control condensed water when it can contaminate utensils, food contact surfaces, or finished products in bulk (13, 14).

Currently, the food industry is using two condensation management strategies: (i) the use of equipment to dehumidify the air and/or cover surfaces of overhanging pipes and (ii) the use of manual interventions, such as squeegees, to physically remove the condensate from different surfaces. Owing to the economic implications and capital investment needed, new or additional air dehumidifiers are not often used, particularly in small operations.

Manual intervention to physically remove condensation is a less expensive solution, but it may lead to unforeseen risks of bacterial contamination. It has been documented that L. monocytogenes can adhere to multiple types of surfaces, including rubber, plastic, stainless steel, and aluminum (1). Due to this ability, it is likely that bacterial contamination and spread can occur not only from drops falling from overhead surfaces but also due to the cleaning tools (i.e., squeegees). These tools can facilitate the spreading of the organism, or they could become a source of contamination if they are not cleaned and sanitized properly after each use.

Current best practices for cleaning these tools include a water rinse and the use of alkaline detergent, followed by rinsing and sanitizing procedures. Even though these are the recommended procedures, food processing facilities may choose to use only a sanitizing dip, because a full cleaning procedure is time‐consuming and perceived as impractical. Currently, there is a lack of information about the bacterial transfer and spread of contamination in the processing environment by cleaning tools. With no scientific evidence suggesting that these practices should be discouraged, the implementation of the lengthy protocol is not favored. Also, information is not available on the effectiveness of the simpler cleaning procedure and the potential contribution to transmitting L. monocytogenes in the food processing environment. Therefore, this study was designed (i) to determine the potential for the spreading of Listeria bacteria across overhead surfaces and the levels of transference to condensed-water droplets when applying manual condensation management techniques, (ii) to understand the effectiveness of sanitizers in removing Listeria bacteria from squeegee blades, and (iii) to determine the best cleaning regime that will reduce contamination on these cleaning tools.

MATERIALS AND METHODS

Inoculum preparation

Listeria innocua ATCC 33090 was used as a surrogate for L. monocytogenes for safety reasons because of the potential for aerosols to be generated during the steaming process. This organism has been utilized as an appropriate surrogate for L. monocytogenes in different studies owing to its similar ecological, cohabitation, and thermal characteristics, among others (5, 8, 15). L. innocua was obtained from the frozen culture collection at the Food Processing Center at the University of Nebraska–Lincoln, and it was initially obtained from the American Type Culture Collection. This strain was checked for purity using nonselective medium before use (tryptic soy agar, Acumedia, Lansing, MI). For inoculum preparation, 10 μL of the stock culture was transferred to test tubes containing 9 mL of brain heart infusion broth (Acumedia). The inoculated brain heart infusion broth was incubated for 24 h at 35°C. After three consecutive transfers using the same procedure, the inoculum was ready to be used.

Transfer of Listeria onto overhead surfaces and condensation generation

After bacterial cells were grown, they were harvested by centrifugation (6,000 × g for 10 min at 4 °C) and resuspended into dilution water containing a small amount (5%) of fetal bovine serum (Gibco value FBS [ThermoFisher, Waltham, MA], total protein 3 to 5 g/dL). This procedure allowed us to have a concentration of 8 log CFU/mL in the prepared inoculum. The presence of proteins (fetal bovine) in the inoculum had been recommended previously, to allow the survival and attachment of bacterial cells to stainless steel materials on the condensation areas (6) and also to represent a real-world scenario, where soiled surfaces are often present. The prepared culture was used to inoculate aluminum tape located on the surface of galvanized pipes (Fig. 1). Galvanized pipes (2-in. internal diameter) were selected since they are often located over food processing areas and commonly found in food processing facilities. Additionally, their shape induces condensation droplets to form on their bottom surface, which may lead to the accumulation of bacteria and condensed water that could fall to processing floors or food contact surfaces. To facilitate bacterial detection, the pipes were covered with aluminum tape (aluminum tape 425, 48 in. [122 cm] long by 2 in. [5.08 cm] wide; 3M, St. Paul, MN). The microbial inoculation was then performed on the surface of the aluminum tape, which allowed cutting and removal of the tape for microbial analysis and enabled us to represent different areas of the pipe.

FIGURE 1

Detection methods for Listeria bacteria in condensed water (a) and on the surface of pipes (tape sections) (b).

FIGURE 1

Detection methods for Listeria bacteria in condensed water (a) and on the surface of pipes (tape sections) (b).

L. innocua (ATCC 33090) was used to inoculate an area of 2 in2 (12.9 cm2) of the pipe. This section was located at the end of the pipe (approximately 1 in. [2.5 cm] from the end). The parts of the pipe that should remain uninoculated were covered entirely with plastic bags, exposing only the 2-in2 portion to inoculation with L. innocua. A hand-held spray device (spray pump bottles, SP Bel-Art Scienceware, Wayne, NJ) attached to a sterile conical tube (15-mL Corning, Falcon tube, Glendale, AZ) was used to apply the inoculum evenly on this exposed area. A known amount of inoculum (2 mL) was delivered to each pipe used in the experiment. This inoculum was dispersed on the exposed area. Finally, the pipes were allowed to dry (1 h) inside a biosafety cabinet. The inoculated portions served as the point of origin to evaluate the spread of Listeria contamination in the longitudinal direction of the pipe. For each replicate, three inoculated areas (2 in2 each) on pipes were used as controls to quantify the initial level of contamination (actual microbial counts varied from 5.5 to 6 log CFU/in.).

Additionally, the number of bacteria transferred to the condensed-water droplets falling off the pipe (as the squeegees removed them) was evaluated. The inoculated galvanized pipes (48 in. [122 cm] long, with a 2-in. [5.1 cm] internal diameter and a 2.4-in. [6.1 cm] outer diameter), which were locally sourced and were manufactured by UTP (Dubai, United Arab Emirates), had been previously filled with water and capped on both ends. The water inside the pipes was used to create the temperature differential necessary to facilitate the formation of hanging droplets during the simulated cleaning cycle. After inoculation, the pipes were then held overnight in a walk-in cooler (4°C) to allow attachment of the microorganisms on the surface of the tape and to allow the water inside the pipe to come to refrigeration temperatures.

The treated pipes were transported to a cold room and hung horizontally on wooden racks located near the ceiling to mimic the overhead pipes often seen in processing facilities. To simulate hot-water cleaning cycles used in food processing facilities, steam was generated inside the cold processing room (5.5°C) to produce condensation on the pipes. To generate steam, a steam line from the facility was placed inside a stainless steel barrel. This barrel was located 2 m from the hanging pipes (in the corner of the cold processing room). By opening the steam valve, the cold processing room was saturated with steam, thus increasing the relative humidity. As a result, hanging droplets were formed on the bottom surfaces of the inoculated pipes. Preliminary trials were used to determine the proper steaming cycle (i.e., the time necessary to generate enough condensation). Those conditions were then used during experiments to produce hanging drops along the bottom surface of the aluminum tape located on the underside of the pipes.

As part of the cleaning cycles, squeegees are used to remove condensation from surfaces. To replicate this practice, a rubber blade squeegee (Buna-N rubber; ULINE, model H-6490, Pleasant Prairie, WI) was used to remove all of the condensed-water droplets attached to the bottom surface of the pipe, starting at the inoculated section. Those droplets were collected to evaluate the spread of Listeria on the pipe. In general, for every 4 in. (linear) of pipe, 2 or 3 drops were formed, which represented 300 to 500 μL of condensate. In addition to condensation droplets, tape sections were also collected. Microbial analysis was performed to determine the ability of Listeria to travel longitudinally on the surface of the pipe and to the droplets falling from the squeegees.

Sanitation of the squeegees

To evaluate the sanitation procedures used for squeegees, cleaning tools were inoculated and then subjected to a series of cleaning and sanitizing procedures to assess the efficacy of the procedures. The inoculum was prepared by transferring 100 μL of an active Listeria innocua culture into three Erlenmeyer flasks, each containing 600 mL of brain heart infusion broth. Flasks were incubated overnight at 35°C for 24 h. The inocula were pooled into a sterile bag and mixed using a Stomacher (240 rpm for 1 min; Stomacher 400 circulator, Seward, Worthing, England). The final inoculum had a bacterial population of 8 to 9 log CFU/mL.

Fifteen equally sized pieces (1 by 1 in. [2.5 by 2.5 cm]) of squeegee material were inoculated by submerging a single piece in a 300-mL beaker containing 100 mL of Listeria culture. Foam blades (60 to 65% styrene butadiene rubber and 35 to 40% ethylene-propylene-diene-monomer rubber; ULINE, model H-2848]) and rubber (Buna-N rubber; ULINE, model H-6490) pieces (coupons) were submerged in the prepared inoculum for 1 h inside a biosafety cabinet as performed by Nyarko et al. (9) and Mafu et al. (7). After this inoculation procedure had been done, inoculated pieces were placed on a sterile petri dish and allowed to dry (for 1 h) inside a biosafety cabinet.

Blade materials were selected based on the most common squeegee types on the market. Based on preliminary findings, the most common materials are rubber and double foam blades. Both materials were inoculated to understand if the nature of the materials may have an effect on the sanitation of these tools and to determine which material type is more sanitary for use in food processing facilities.

To determine the bacterial reduction achieved by different detergent and sanitizer solutions, the solutions being evaluated were prepared according to the manufacturers' recommendations. The cleaning agents are described in Table 1. Deionized water was used to prepare all solutions to make sure that organic and inorganic matter did not interfere with the final concentration of the active compound in solution. A solution of quaternary ammonia containing at least 550 ppm was used in all of the replications for this treatment. This concentration was checked using a titration method, Quat test kit 317, provided by Ecolab (Ecolab, St. Paul, MN). All prepared solutions contained between 550 and 600 ppm of quaternary ammonia, as recommended by the detergent and sanitizer manufacturer.

TABLE 1

Sanitizers in different cleaning and sanitizing solutions applied to squeegees during cleaning procedures

Sanitizers in different cleaning and sanitizing solutions applied to squeegees during cleaning procedures
Sanitizers in different cleaning and sanitizing solutions applied to squeegees during cleaning procedures

The chlorine-based sanitizer solution was made using a 600-ppm concentration. Similarly, the solution was checked using an iodometric titration (back titration) (Ecolab). This method relies on the ability of oxidizing agents to produce a back titration, and a correlation is reported in the method instructions for each chemical (oxidizer kit 322, Ecolab). According to the analyses, all prepared solutions had a concentration of between 590 and 600 ppm of chlorine. A peracetic acid solution (mixture of hydrogen peroxide plus acetic acid plus peroxyacetic acid) was prepared based on the manufacturer's recommendations for nonfood contact surfaces (1 oz/8 gal [ca. 30 mL/30 L]). The same iodometric titration was used for testing the concentration of this solution. The results showed a concentration of 2,025 ppm in all replications for peracetic acid solutions. Chlorinated alkaline detergent was prepared using the concentration recommended by the manufacturer (20 mL/L).

Sanitation protocols

Different sanitation protocols were tested to simulate cleaning procedures used in food processing facilities for cleaning tools, such as squeegees. Negative controls (no cleaning agent) were included to determine the ability of L. innocua to attach to each material (baseline) and to show the reduction observed by a simple rinsing step. The sanitizer's effectiveness was tested by submerging blade pieces into each sanitizing solution for 1 min. The tested solutions were water (as the negative control), quaternary ammonia, a chlorine-based sanitizer, and peracetic acid. An aliquot (250 mL) of the sanitizer solution was placed into a beaker, and the blade piece was placed in the solution. After the treatment time had elapsed, a microbial analysis, including a neutralization step, was carried out to determine the effectiveness of the sanitizers.

Two more aggressive sanitation protocols were also tested to determine the maximum reduction of Listeria bacteria that could be achieved from the squeegee materials (rubber or foam). The first regime included a water rinse step (10 s), followed by a detergent cleaning solution (10 min), a water rinse (10 s), and a final sanitizer rinse of quaternary ammonia (1 min). The second, additional sanitation regime was a water rinse (10 s), followed by soaking in a detergent cleaning solution (10 min), then scrubbing using a Scotch Brite green pad (two 10-s passes for all piece sides), and finishing with a water rinse (10 s) and a quaternary ammonia rinse (1 min). The concentrations for detergent and quaternary ammonia solutions were the same as described above. For both protocols, the bacterial reduction was calculated based on the initial population attached to the material after inoculation.

Microbiological analyses

After manually removing the condensed water from the pipe using the rubber squeegee, the transfer of Listeria bacteria was determined by plating condensed-water samples using a selective Oxford medium (Acumedia). Samples of condensed water were taken along the pipe at 4, 8, 12, 16, 20, 24, 36, and 48 in. (10.2, 20.3, 30.5, 40.6, 50.8, 61.0, 91.4, and 121.9 cm) from the contaminated area. To collect these samples, while squeegees were removing the hanging droplets, sterile recipients (10- to 50-mL conical tubes) were used to collect the condensed water when droplets were running off the squeegee. The collected samples were homogenized by vortexing the content inside the conical tube. Owing to the small amounts (<3 mL) of condensate, especially in the smallest length intervals (4 in. [10.2 cm]), the contaminated water was spread plated onto Oxford medium (100 μL). When necessary, serial dilutions were made (100 μL of condensate into 900 μL of dilution water). Microbiological results were expressed in CFU per milliliter of the collected condensed water. The above-described microbial analysis and collection protocols are summarized in Figure 1a.

To evaluate the longitudinal spread of Listeria on overhead surfaces, samples of the aluminum tape pieces were collected at 4, 8, 12, 16, 20, 24, 36, and 48 in. (10.2, 20.3, 30.5, 40.6, 50.8, 61.0, 91.4, and 121.9 cm) from the contaminated area. Samples were placed inside conical tubes containing Letheen broth (10 mL). After the tape was immersed in broth, sonication was applied for 1 min at 20°C (Bransonic model 1210R, Branson Ultrasonics, Brookfield, CT) to ensure that Listeria bacteria (if present) were detached from the tape. If necessary, the broth was diluted using Butterfield dilution water (1:10). Three milliliters of Letheen broth (Acumedia) or dilution water was then plated on EL (environmental listeria) Petrifilm (3M Microbiology, St. Paul, MN). Plates were incubated at 35°C for 30 h, and counts were recorded as CFU per square inch according to the manufacturer's recommendations. The above-described microbial analysis and collection protocol are summarized in Figure 1b.

To evaluate the sanitation of squeegees, pieces of the squeegee blades were cleaned according to their respective sanitation protocols and then aseptically transferred into a sterile bag containing 100 mL of Letheen broth. This broth was used as a neutralizing agent to improve the recovery of the bacteria during sample analysis. All samples were then placed inside a Stomacher (240 rpm for 1 min; Stomacher 400 circulator, Seward) and blended for 1 min to facilitate the transfer of live bacteria present in the material into the Letheen broth. When necessary, the broth was diluted using Butterfield dilution water (1:10). Three-milliliter amounts of this broth–dilution water mixture were enumerated using EL Petrifilm (3M) plates. Finally, the EL Petrifilm plates were incubated at 35°C for 30 h following the manufacturer's recommendations. After incubation, bacterial colonies were counted, and these data were reported as log CFU per square inch. To determine which treatment was the most effective, the data were evaluated as the reduction in log CFU per square inch using the inoculated untreated pipes as the baseline (initial concentration).

Experimental design and statistical analysis

For the overhead-structure experiments, at least three independent replicates were included. Each replicate represented the average of three pipes (subsamples) inoculated at a single time. These subsamples were used to reduce the sampling error among the pipes. The average value and the standard deviation of each replication were calculated. Similarly, for the cleaning protocol experiments, trials were done using three pieces (subsamples) per replication. The results for all three subsamples were averaged to decrease sampling error. Statistical analysis was done using analysis of variance (ANOVA) with Tukey's test comparison (P < 0.05) utilizing SAS software (version 9.4, SAS Institute, Cary, NC). Control samples were compared with treated samples to understand whether different materials had an influence on bacterial attachment. To determine which material could be cleaned better, a factorial design (two materials by six different regimes) was used with ANOVA with Tukey's test comparison utilizing SAS software (P < 0.05).

RESULTS AND DISCUSSION

Transfer of Listeria bacteria by condensation

The pieces of aluminum tape covering galvanized pipes and the falling droplets were analyzed to detect the transfer of Listeria bacteria. The results are shown in Figure 2. As expected, condensed water falling from the 0- to 4-in. section (0 to 10.1 cm) (close to the inoculation point) was heavily contaminated with Listeria bacteria (5 log CFU/mL). This concentration is comparable to the initial bacterial population obtained from the inoculated (∼5.3-log CFU/in2) area, suggesting a high transfer risk in that section of the pipe. As the squeegee moves across the galvanized pipe, the transfer of Listeria bacteria to the water drops decreases because the concentration of the organism becomes diluted as the squeegee gets further from the inoculation point. At 12 in. from the inoculation point, droplets still showed Listeria bacteria consistently (close to 2.3 log CFU/mL). After 16 in., Listeria bacteria were still detected (higher than the detection limit of 1 log CFU/mL) in the condensed water, indicating that the risk of environmental contamination of food processing areas from overhead structures was still present. However, the risk was significantly lower at 16 in. than at closer sampling points (4-log reduction from the initial inoculation site). It is worth mentioning that Listeria bacteria were still detected in some subsamples from condensed water obtained at 36 in. from the inoculated site. As shown by the results in Figure 2, the higher standard deviation (almost 2 log) indicates that bacterial spread and detachment from the squeegee is not always consistent, as it reflects the interaction of bacterial cells with the surface of the pipe (aluminum tape) and the blades of the squeegee.

FIGURE 2

Transfer of Listeria innocua ATCC 33090 to condensed-water droplets and to linear sections of the pipe when using a squeegee as a manual intervention for condensation removal. Zero inches represents the inoculation point, with 48 in. being the total length of the pipe. Detection method limits are highlighted by the dotted lines (yellow for condensed water, and dark gray for overhead surface).

FIGURE 2

Transfer of Listeria innocua ATCC 33090 to condensed-water droplets and to linear sections of the pipe when using a squeegee as a manual intervention for condensation removal. Zero inches represents the inoculation point, with 48 in. being the total length of the pipe. Detection method limits are highlighted by the dotted lines (yellow for condensed water, and dark gray for overhead surface).

This study also analyzed the transfer of Listeria bacteria across the surface of the galvanized pipes by evaluating pieces of aluminum tape attached to the underside of the pipes where condensation is formed. As shown by the results in Figure 2, the inoculation point (2 in2) had a bacterial population of around 5.30 log CFU/in2. This tape section served as an inoculum point when using rubber blade squeegees to remove condensation from the pipes. It was observed that Listeria bacteria were able to be transferred linearly to uninoculated portions of the aluminum tape, as well as to the squeegee. The first linear section, corresponding to 4 in. from the inoculation point, showed high counts of Listeria bacteria (approximately 3 log CFU/in.) after condensed water was removed using the squeegee. Based on this study, the number of organisms decreased continuously as the squeegee moved across the pipe, finally reaching a level below the detection limit (0.40 log CFU/in.) after 16 in. (40.6 cm). Similar to the results described for the condensed-water-droplet experiments, even past 36 in. (91.4 cm) from the inoculation point, Listeria bacteria were sporadically detected in pieces of the aluminum tape. This result suggests that this pathogenic bacterium could be slowly spreading across the overhead surfaces as routine environmental cleaning practices are carried out. The relevance of these findings should take into consideration how long bacteria would survive under these conditions.

L. monocytogenes has been reported to have the ability to contaminate condensation-forming surfaces made from stainless steel (6). Some research suggests that survival could occur for extended periods (6), especially if the bacterial cells could survive in biofilms on those surfaces. In fact, these investigators suggest that multispecies biofilms, as well as the presence of proteins on the surface, could enhance the attachment of L. monocytogenes. Environmental monitoring specialists from the food industry and regulatory agencies have recognized overhead pipes as niches for L. monocytogenes based on their experience, and they strongly recommend monitoring these sites when implementing environmental programs (11). Therefore, both studies suggest that environmental contamination with L. monocytogenes is possible in processing facilities due to its ability to associate itself with condensation that may be formed during cleaning and processing operations.

Based on these observations, it is recommended that the condensation removal practices used by food processing facilities should be reviewed to reduce the potential for transferring contamination. Condensed water should be collected and disposed of as appropriate to decrease this risk. Currently, there are commercially available squeegees with the ability to connect to a plastic reservoir at the bottom, which would allow employees to collect and treat the potentially contaminated condensed water before its disposal. By properly cleaning and sanitizing squeegee blades and using reservoirs prefilled with detergent and/or sanitizer, this risk associated with manual removal of condensation from overhead structures could be reduced significantly.

Finally, the enumeration of Listeria bacteria was also performed on the used squeegees. This analysis showed that 3.51 ± 0.16 log CFU of Listeria was transferred from the pipes to the squeegees used to removed condensation across 40 in. of pipe. This result suggests that this tool could continuously spread Listeria bacteria through the facility. Therefore, effective cleaning interventions for cleaning tools should be used to decrease this risk.

Understanding the effectiveness of sanitizers to reduce Listeria bacteria on cleaning tools

Squeegee blades are often composed of either of two different materials, rubber and foam. Blades of these two materials were contaminated with L. innocua in these experiments. The microbial analyses showed that the bacterial populations in rubber and foam blades were 6.53 ± 0.12 and 7.47 ± 0.22 log CFU/in2, respectively. This difference in the levels of bacteria in the inoculated materials suggests that foam blades allow more bacterial attachment than rubber, potentially due to the foam structure. This observation suggests that rubber would be a more sanitary material than foam squeegees.

The reduction of L. innocua was evaluated by using different cleaning regimes, and the results for rubber and foam materials are shown in Figures 3 and 4, respectively. As can be seen from the results in both figures, water rinsing reduced L. innocua populations attached to rubber or foam squeegees by 0.71 ± 0.37 and 0.41 ± 0.11 log CFU/in2, respectively. On average, all sanitizers (quaternary ammonia, chlorine, and peracetic acid) were able to reduce L. innocua by around 1 log CFU/in2 on foam blades and less than 2 log CFU/in2 on rubber blades. The results for quaternary ammonia or chlorine sanitizer solutions were not statistically different from the results for samples treated with water, thus showing a minimal effect from these products. This observation is not unusual: Ronner and Wong (10) observed that different sanitizers, including quaternary ammonia (200 ppm) and chlorine (100 ppm), were not able to significantly reduce the presence of L. monocytogenes or Salmonella enterica serovar Typhimurium on rubber (Buna-N). A recent study showed that quaternary ammonia detergent was able to decrease L. monocytogenes by 1.4 to 1.8 log CFU on rubber using the manufacturer's recommended concentration (4). These reductions are similar to those observed in this study.

FIGURE 3

Reductions of Listeria innocua ATCC 33090 on rubber blades achieved by the cleaning and sanitizing solutions and tested protocols. Different letters represent statistical differences among the results for the applied treatments.

FIGURE 3

Reductions of Listeria innocua ATCC 33090 on rubber blades achieved by the cleaning and sanitizing solutions and tested protocols. Different letters represent statistical differences among the results for the applied treatments.

FIGURE 4

Reductions of Listeria innocua ATCC 33090 on double foam blades achieved by the cleaning and sanitizing solutions and tested protocols. Different letters represent statistical differences among the results for the applied treatments.

FIGURE 4

Reductions of Listeria innocua ATCC 33090 on double foam blades achieved by the cleaning and sanitizing solutions and tested protocols. Different letters represent statistical differences among the results for the applied treatments.

The solution containing peracetic acid was able to reduce bacteria by greater amounts (1.87 ± 0.32 and 1.41 ± 0.33 log CFU/in2) than water rinsing alone and other sanitizers tested on rubber and foam blades, respectively. Even though the reduction using this solution was slightly higher than the reductions by other sanitizers, the results for this hydrogen peroxide solution were not statistically different from the results for the other sanitizers tested. It is essential to highlight that this study used sanitizers directly after the inoculation procedures, and ideally, these sanitizers should be used after a cleaning procedure. However, sanitizing without cleaning can be a common practice for these tools in food processing facilities. The direct application of the sanitizer represents the worst-case scenario in which the squeegees are only dipped into the sanitizer for a brief amount of time (1 min).

Determination of the best sanitation regime to significantly reduce Listeria bacteria on cleaning tools

Complete sanitation regimes using alkaline detergent, rinse, and sanitizer were statistically more effective than the majority of the sanitizers by themselves, providing Listeria reductions of 2.87 ± 0.70 log CFU/in2 on rubber and 2.11 ± 0.60 log CFU/in2 on foam (Figs. 3 and 4, respectively). The addition of a scrubbing step led to further bacterial removal or reduction on both materials. Using this extensive cleaning protocol, the reductions in L. innocua were 3.56 ± 0.53 log CFU/in2 on rubber and 3.54 ± 0.51 log CFU/in2 on foam blades.

As mentioned above, by using a rubber squeegee to remove condensate from overhead structures, up to 3.51 log CFU of Listeria was transferred to the squeegee. This result suggests that in order to significantly minimize the risk of transferring Listeria bacteria, a sanitation protocol must be applied. Limited studies are available that have evaluated the risk of condensation in food processing facilities. Brashears et al. (3) is one of the few articles available that analyzed the microbial quality of condensation in meat processing and RTE food processing facilities. According to their findings, aerobic plate counts varied from 0.9 to 2.7 log CFU/100 mL of condensation. These data indicate low microbial counts, since these values are equivalent to 10 CFU/mL. Based on these in-plant data, the use of the sanitizers by themselves could potentially be enough to control the risk if the contamination were that low on the surface of the materials. However, those data were for samples collected from large surface areas (1.8 m for overhead pipes and 0.8 m2 on overhead surfaces), and based on the transmission data provided in this article (first objective), the contamination may not be homogeneously distributed across pipes and overhead surfaces or on the surface of the squeegees. Even when Listeria bacteria were present at low levels in the condensed water (below the detection limit), the squeegee had higher levels (3.5 log CFU per squeegee) of bacterial contamination. Because squeegees and other tools used to control condensation may have higher counts, the use of sanitizers by themselves would be ineffective. In addition, a single-sanitizer protocol does not comply with the U.S. Environmental Protection Agency–approved usage for sanitizers, which encourages the use of a multistep protocol.

According to the statistical analysis performed, the rubber material can be cleaned more effectively than the foam material (P < 0.05). This result is noteworthy because even though the difference is small (0.40 log CFU/in2), it may help the food industry reduce the risk of cross-contamination events. For a cleaning protocol to be considered an effective strategy, food manufacturers should be able to remove at least 3 log CFU of bacterial contamination (12). Based on this research, only the intensive cleaning protocol was able to achieve ≥3 log CFU/in. Therefore, extensive cleaning protocols should be recommended for the cleaning of squeegees to reduce the risk of cross-contamination from these tools.

In conclusion, this study was designed to determine the potential transfer of Listeria bacteria on overhead pipes when squeegees are used as a manual intervention to remove condensation. Based on this study, when a high concentration of Listeria bacteria is present (5 log CFU/in2) on overhead-pipe surfaces, Listeria bacteria can transfer longitudinally up to 16 in. from the contaminated area and to the condensed water that falls from those surfaces. Therefore, it is recommended that this condensed water should be removed, collected, and disposed of appropriately when manual interventions are used. Some cleaning tools available in the market may accomplish this function, and food manufacturers should be encouraged to use these tools to reduce the risk of bacterial contamination spread in the food processing environment.

According to this study, rubber blades are more sanitary than foam blades since they can be cleaned more efficiently, possibly due to reduced bacterial attachment to the structure. To efficiently sanitize squeegee blades, it is recommended that a full procedure (detergent and rinse, followed by sanitizers) that includes a scrubbing step be applied to achieve 3 log of bacterial contamination. Information provided by this research will allow manufacturers to make informed decisions related to cleaning protocols to reduce the risk associated with condensation in food processing facilities.

ACKNOWLEDGMENT

This study was supported through funding from the Alliance For Advanced Sanitation at the University of Nebraska–Lincoln.

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