Trends in Food Science & Technology 20 (2009) S45eS47

Review

* Corresponding author.

0924-2244/$ - see front matter � 2009 Published by Elsevier Ltd.doi:10.1016/j.tifs.2009.01.042

Removal of biofilms

and stubborn soil by

pressure washing

D. Burfoot*, K.E. Middleton and

J.T. Holah

Campden BRI, Chipping Campden, Gloucestershire

GL55 6LD, United Kingdom (Tel.: D44 1386 842000;

fax: D44 1386 842100; e-mails: d.burfoot@campden.

co.uk; k.middleton@campden.co.uk;

j.holah@campden.co.uk)

Pressure washing is widely used in the food industry. The

results of an earlier study on microbial removal are reported

here. This paper also reports on the removal from surfaces of

a stubborn soil of cream substitute. Washing with cold water,

even after the application of disinfectant, had no visible effect

on the soil. Washing with hot water alone caused some

removal from the edges of the soil but only when using

water at high pressure. Applying a disinfectant, followed by

hot water at high pressure produced the greatest, but not

complete, soil removal. The study indicates that mechanical

abrasion is also required for the removal of stubborn soils.

IntroductionPressure washing is the most common approach to the

cleaning of production equipment and environmentalsurfaces in the food industry. In practice, the surfaces arerinsed with hot or cold water, a detergent is then appliedfor the required contact time, the surfaces are then rinsedagain before a disinfectant is applied. The pressures usedfor cleaning can vary greatly from mains water pressure,generally around 5� 105 Pa, up to about 100� 105 Pa.Higher pressures are available when using commercialwash-down systems but a 100� 105 Pa maximum is themost commonly used in food factories. To date, there hasbeen no scientific evidence to demonstrate which pressuresare the most economical to use for cleaning. This paper out-lines the results of studies to examine the effects of

operating conditions of a pressure washer, along with theapplication of a detergent, on the removal of biofilms orstubborn soil from stainless steel surfaces.

Materials and methodsThe details of the experimental method to assess the re-

moval of biofilms have been described elsewhere (Burfoot& George, 2007; Burfoot & Middleton, 2009). The methodinvolved growing bacterial biofilms of Pseudomonasaeruginosa onto stainless steel surfaces (5 mm� 5 mm)that were then cleaned using specific cleaning treatments.The biofilms on treated and untreated (control) surfaceswere then stained using acridine orange and the number oforganisms on the surfaces were counted under a microscopeusing a method known as direct epifluorescent microscopy(Holah, Betts, & Thorpe, 1988; Holah, Betts, & Thorpe,1989). The microbial reduction achieved using each clean-ing treatment was then calculated. The cleaning treatmentsconsisted of spraying with cold (8 �C) or hot (60�) watersupplied from a pressure washer (KEW 1702K, KEWCleaning Systems Ltd., Penrith, UK) at 25� 105 or95� 105 Pa (25 or 95 bar). Distances between 20 and120 cm were set between the nozzle of the washer andthe target surface. The exposure time was also variedbetween 5 and 30 s. In tests with a 5 s exposure, the spraywas moved across and back over the sample surface to sim-ulate industry practice. In tests with a 30 or 60 s exposuretime, the nozzle was static. Tests were carried out withthe spray perpendicular or at 45� to the target surface. Insome tests, a detergent was applied (Maxifoam, an alkalinedetergent from Holchem Laboratories Ltd., Haslingden,Lancs., UK, 4% v/v solution) as a foam using the pressurewasher. The chemical was left on the surface for 20 minprior to pressure washing.

In the studies to examine the removal of stubborn soils,a cream substitute (Elmlea from Van de Bergh Food Ltd.,Crawley, UK) was spread onto the stainless steel surfaces(5 mm� 5 mm) and then left in an oven for 4 h at200 �C. After cooling in ambient air for 24 h, the surfaceswere photographed before and after a cleaning treatment.These treatments consisted of spraying with cold or hotwater (8 or 60 �C) at 5� 105, 25� 105, or 95� 105 Pa(5, 25, or 95 bar). The spray was moved across and thenback over the target surface in a 5 s exposure period. Insome tests, a chemical solution was applied as describedfor the tests of microbial removal.

S46 D. Burfoot et al. / Trends in Food Science & Technology 20 (2009) S45eS47

ResultsThe results of the microbial studies have been presented

by Burfoot and Middleton (2009). They showed no signif-icant effects of the distance between the nozzle and target,the nozzle pressure, or the angle of the spray, on theremoval of microorganisms from the target surface. Afterusing water at 8 �C and 95� 105 Pa, with no chemicaltreatment, the average microbial reduction with the sprayperpendicular to the target was 2.9-log10 (s.d.¼�0.3)and it was 3.3-log10 (�0.4) with the spray at 45� to the tar-get. With water at 60 �C and 95� 105 Pa, with no chemicaltreatment, the average microbial reductions with the sprayat 90 or 45� were 4.9-log10 (�1.0) and 4.1-log10 (�0.6),respectively.

Fig. 1 summarises the key results from Burfoot and Mid-dleton (2009). Microbial removal increased with cleaningtime when using cold water ( p< 0.001) but not when usinghot water. Microbial reduction was greater after using hotrather than cold water ( p< 0.001) when no detergent hadbeen used. However, there was no significant differencein microbial reduction achieved using hot or cold waterafter a chemical treatment as both cleaning treatmentsproduced a 5.6-log10 reduction. Pre-treating the targetsurfaces with detergent, followed by pressure washingwith water at 8 �C and 95� 105 Pa, increased the microbialreduction from 2.9-log10 to 5.6-log10 ( p< 0.001). All treat-ments, except cold water applied alone for less than 30 s,produced an approximately 5-log10 microbial reduction orgreater.

Fig. 2 shows the appearance of target surfaces beforeand after cleaning to remove the stubborn soil of creamsubstitute that had been baked on to the surface. Washingwith cold water, even after the application of detergent,had no visible effect on the stubborn soil (Fig. 2a). Washingwith hot water alone caused some removal of material from

Fig. 1. Effect of operating variables on the removal of microorganisms from taand the standard deviations. The numbers show the number of points specifie

(2009)

around the edges of the soil with the most removal resultingfrom the highest water pressure (95� 105 Pa). The use ofhot water at mains pressure (5� 105 Pa) had no apparentvisible effect on the stubborn soil (Fig. 2b). Using hot waterafter a detergent treatment gave greater solid removal thanhot water alone but it did not remove all of the soil evenwhen using a pressure of 95� 105 Pa (Fig. 2c). Forcomparison, the results from another study are shown inFig. 2d. In that study, high speed ice particles were directedat the target surfaces with the aim of producing an abrasiveaction on the soil. The use of ice particles provided muchgreater removal of the stubborn soil.

Discussion and conclusionsAlthough the biofilm of P. aeruginosa is known to be

very robust (Gibson, Taylor, Hall, & Holah, 1999), all ofthe cleaning treatments produced around a 5-log10 reduc-tion in the number of organisms, with the exception ofcleaning with cold water at application times less than60 s. Microbial removal increased with cleaning timewhen using cold water, however, the longer cleaning timesare not practical in a factory environment. Gibson et al.(1999) did not find any relationship between cleaningtime and microbial removal but that was at much shortertimes between 1 and 10 s.

Interestingly, microbial removal did not vary with dis-tance between the nozzle and target indicating that impactpressure does not affect microbial removal over the rangetested. Gibson et al. (1999) also found that nozzle pressure,between 17.2� 105 and 68.9� 105 Pa, did not affectmicrobial removal. However, Burfoot, Brown, Ashwell,and Wilkinson (2004) found that the removal of Staphylo-coccus aureus dried onto target surfaces increased withspray pressure over the range 2� 105e12� 105 Pa. Theseorganisms would be expected to be more easily removed

rget surfaces using hosing. The bars show the average log10 reductionsd at half the level of detection. Data taken from Burfoot and Middleton.

Fig. 2. Photographs of target surfaces before cleaning and after cleaning with a range of treatments including pressure washing with hot or cold waterat 3 pressures and with or without pre-treatment with chemical detergent. For comparison, the photographs at the bottom show the effect of treatingthe surfaces using high speed ice particles launched from a nozzle located 5 or 10 cm from the target surface and using exposure times of 1, 3, or 5 s.

S47D. Burfoot et al. / Trends in Food Science & Technology 20 (2009) S45eS47

than organisms in a biofilm hence the removal at lowerspray pressures. The finding that microbial removal waslittle affected by spray pressure when using cold water sug-gests that some organisms are easily removed whilst othersare more difficult and require additional cleaning actions,such a chemical application, long treatment times or hightemperatures, to be removed.

Temperature or detergent, alone or in combination, wasineffective in totally removing the stubborn soils. However,high speed ice particles were more effective indicating thatmechanical abrasion is also required for the removal ofsuch stubborn soils.

References

Burfoot, D., Brown, K., Ashwell, B., & Wilkinson, D. Systematic designof a disinfection tunnel for use in high-care food areas. Paper 7 on

CD ROM Proceedings of the International Conference on Engi-neering and Food, 2004, ICEF 9.

Burfoot, D., & George, M. (2007). Decontaminating surfaces usinghigh speed ice particles. Food Manufacturing Efficiency, 1(3),1e8.

Burfoot, D., Middleton, K. (2009). Effects of operating conditionsof high pressure washing on the removal of biofilms fromstainless steel surfaces. Journal of Food Engineering, 90(3),350e357.

Gibson, H., Taylor, J. E., Hall, K. E., & Holah, J. T. (1999). Effectiveness ofcleaning techniques used in the food industry in terms of removal ofbacterial biofilms. Journal of Applied Microbiology, 87, 41e48.

Holah, J. T., Betts, R. P., & Thorpe, R. H. (1988). The use ofdirect epifluorescent microscopy (DEM) and the directepifluorescent filter technique (DEFT) to assess microbialpopulations on food contact surfaces. Journal of AppliedBacteriology, 65, 215e221.

Holah, J. T., Betts, R. P., & Thorpe, R. H. (1989). The use epifluores-cence microscopy to determine surface hygiene. InternationalBiodeterioration and Biodegradation, 25, 147e153.