Pig scalding and dehairing

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Scalding is undertaken principally to soften hairs on pig skin making them easier to remove. There has been relatively little published work on the effect of pre-cleaning of pig carcasses prior to scalding on carcass hygiene and scalding water quality. 

Commercial whipping and brushing machines similar to those used for post-singe scraping and polishing for use as carcass cleaners prior to scalding are marketed by a number of equipment manufacturers worldwide, and are known to be used by some pork producers in the UK (Tinker et al 2007).  A limited number of evaluation studies have been published on their use (Woltersdorf and Mintzlaff, 1996; Rahkio et al., 1992) with uncertain results.

An alternative pre-scald treatment is hot water washing, as recommended by Kotula (1987) and used for pre (Gill et al., 1997; Gill and Jones, 1998) and post-evisceration carcass treatment (Gill et al., 1998; Aabo, 2007).  To our knowledge, despite being recommended over 20 years ago as of in need of investigation (Kotula, 1987), no evaluation of such a system used before scalding, appears to have been previously reported in the scientific literature.  As yet unpublished work from the FSA project M01038 indicates that pre-scald washing of pig carcasses with hot water prior to scalding and dehairing has a small beneficial effect on surface Enterobacteriaceae numbers on pork carcasses prior to evisceration.  Thus, pre-scald washing may be seen to have some benefit (Q43).

Plugging the anus to prevent the escape of faeces into scalding water, or during de-hairing, scraping or polishing, has been recommended in a number of additional reports (Richmond, 1991; ICMSF, 1998; Wong et al., 2002 Q42; Q51).  Robust evidence that effective plugging has considerable microbiological benefits in terms of reducing Enterobacteriaceae numbers in UK plants is reported by Purnell et al., 2010.

A microbiological bonus of effective scalding is that the external surface of newly-scalded carcases contains relatively few microorganisms.  Furthermore, pork carcasses at this stage of processing are rarely contaminated with readily detectable quantities of human pathogens (Berends et al., 1997; Bryant et al., 2003).  Pearce et al. (2004) report that in an Irish plant; hot water or steam scalding caused a reduction to the levels of total aerobes and coliforms of around 4 log units.  The reported decrease for total aerobes has subsequently been confirmed in a Swiss plant (Specha et al., 2006). Pearce et al. (2004) also found scalding (61±1°C, 8 min) to reduce the incidence of salmonella on carcasses from 31% to 1%, and hence considered it an important CCP (however the incidence increased to 7% after dehairing).  Dockerty et al. (1970) found that higher temperatures for shorter times were more effective at achieving TAVC reductions.  Scald water temperatures of 54 or 60°C for 9 or 7 min reduced counts by 0.9 and 2.5 log10 CFU cm-2 respectively (Q45-46; Q52-Q56).  Scalding using condensed water vapour has been shown to offer a number of advantages, in terms of bacterial numbers reductions, over conventional tank scalding (Nickels et al., 1976; Nerbrink and Borch, 1989; Woltersdorf and Mintzlaff, 1996; Q44).  Additionally, the risk assessment study by Delhalle et al. (2008) established a correlation between the operation of condensed vapour scalding systems and a reduction in Salmonella prevalence (1.68±0.67), and counts of Escherichia coli (mean 0.65±0.29 log CFU cm-2) and aerobic bacterial counts (mean 0.61±0.19 log CFU cm-2) on the finished carcass.

Consistently high numbers of aerobes were observed after bleeding compared with scalding (Warriner et al., 2002), underscoring the previous reports that significant reductions occur as a consequence of immersing a carcass in a tank of water with a temperature or more than 60oC (Q46).  The reductions in bacterial numbers as a consequence of scalding reported by Warriner and colleagues were observed for every hour of the plant’s 8-hour processing period; tending to suggest that although the quality of water in scald tanks deteriorates visibly during processing, numbers of bacteria transferred to the carcass do not significantly increase. 

A consistent theme across publications that have investigated scalding is that it is effective at reducing bacterial numbers and the incidence of detection of potential human pathogens such as Salmonella.  However, after scalding, carcass surfaces have been shown to become re-contaminated with bacteria mainly as a consequence of de-hairing using flails and the optional decapitation stages of processing (Bryant et al., 2003).  After de-hairing, both the APC and coliforms counts have been shown to increase by approximately 2 log units (Pearce et al. 2004).  Although it should be kept in mind that after the scalding and dehairing processes, the APC were still significantly lower than those measured on the same carcasses after bleeding.  A number of authors have reported similar observations and it is now well established that numbers of bacteria follow trends similar to that depicted in Figure 5 below for the Enterobacteriaceae during each of the different processing stages.

Enterobacteriaceae on pigs

isolated from pig carcasses by swabbing at each stage of processing (Berends et al., 1997)

In a small-throughput plant (80 pigs per day) Bolton et al. (2002) found roughly equivalent results that APC after the scalding and dehairing processes were significantly lower than those recorded after bleeding.  In addition, useful pathogen reductions occurred because no Salmonella were detected after the scalding and dehairing processes compared to 50% of the carcasses being Salmonella-positive after stunning and bleeding. 

The prevalence of Salmonella after scalding in a higher throughput plant (1000 pigs per day) was found to be 1%, which increased to 7% after dehairing (Pearce et al., 2004).  It has been speculated that the level of contamination at dehairing might be reduced if a plastic cone was applied to the anus to reduce the opportunity for faecal contamination from that region (Bolton et al., 2002). 

A build up of contamination may be at least partly counteracted by providing sufficient overflow of scalding water (Smulders and van Laack, 1992; Q45).  However Troeger (1994) found that there was no direct connection between bacterial numbers in tank scalding water and the hygienic status of carcasses that have passed through the water.  Conversely, in a survey of abattoirs across Europe, Hald et al. (2003) found that salmonella contamination during pluck removal was related to the effectiveness of scalding and crucially found that Salmonella in the scald water increased the likelihood of isolating Salmonella from the carcass after evisceration (Q46).

Gill and Bryant (1993) suggest that the scald tank water should be as hot as possible but not more that 60°C. A short communication by Johanson et al. (1983) showed that combined dehairing and singeing systems (e.g. horizontal tumbling units) were less efficient than lines where the two operations were separated (Q60).

A more-recent Swiss study (Spescha et al., 2006), has reported changes in total aerobes and Enterobacteriaceae in two abattoirs for a number of process stages.  The sampling and testing methods used were as described by EU commission decision 471/2001.  The sampling method is known to generate highly variable results (Hutchison et al., 2005; 2007).  Thus a potential criticism of the study is that the relatively small numbers of samples collected (n=100) may not high enough to overcome the variation associated with sampling, although the work involved in testing the numbers of samples taken was significant.  However, for some process stages, the observed reductions are high enough to be recognisable.  On average, from both plants, scalding caused a reduction in the measured numbers of total aerobes of around 4 log CFU cm-2.  As has been reported in other plants, dehairing restored numbers of total aerobes by 1.5 log CFU cm-2 (plant A) and 2.5 log CFU cm-2 (plant B).  Overall the combined effect of both process stages was a decrease of roughly 2.5 log CFU cm-2.  In contrast to some of the other publications in this area, the Spescha study provides information on the changes in numbers of total aerobes at the EU-specified sites of ham, back, belly and jowl.  For scalding and dehairing, there were no statistically-significant differences between any of the sites.

In a high-throughput plant Warriner et al. (2002) did not find a significant reduction in APC between carcasses sample after bleeding and those collected after de-hairing.  Thus at this particular plant, flail dehairing did not significantly contribute to the carcass’s bacterial load.  Although there was no measured increase in numbers of bacteria on carcasses throughout the working day, the total aerobes isolated from the scald tank water were increased by two log units during the course of processing day (Warriner et al., 2002).  Furthermore, the levels of Enterobacteriacae and E. coli increased on the de-hairing equipment during the course of the day’s processing.  The slightly-increased numbers of aerobes in the scald tank water were a consequence of dirt, faeces, ingesta and some of the surface bacteria from the pig carcass surface (Q50).  Warriner and colleagues measured a uniform tank temperature of 69 degrees) which was warm enough to achieve a 1 log reduction in the numbers of Salmonella in scald tank water using the calculations provided by Bolton et al. (2003; Q46).  The Bolton calculations are supported by a second study by the same research group in a working high-throughput plant (Pearce et al., 2004).  This second study collected 108 samples of scald tank water maintained at 61°C from various locations along and within the tank over the course of several days processing.  Surprisingly, the study did not isolate any Salmonella (Pearce et al., 2004).  However, Letellier et al. (2009) determined, from analyses of pigs from 312 production lots, that presence of Salmonella in the scald tank water was a significant risk factor which correlated with Salmonella contaminations of the final carcasses (Q46; Q48; Q50).

A later publication by Warriner (Namvar and Warriner, 2006) studied dehairing by scraping.  There are differences between scraping and flail dehairing.  In the Canadian system encountered by Warriner, pigs are laid on the horizontal between a pair of flanged rubber rollers which are then rotated, rotating the carcass and stripping the hair from its surface (Keith Warriner-Mike Hutchison, personal communications).  Based on the movements of generic E. coli typed using ERIC-PCR, the study concluded that in terms of cross contamination, the scraping stage was the second most important cause for carcass-to-carcass contamination (Q57-Q59).  It should be kept in mind that samples were not taken after all of the processing stages and that there are some fundamental differences between scraping and flail dehairing. 

Most of the work that has been undertaken on scalding has concentrated on the surface microbiology of the carcass.  The process, when undertaken using a scald tank however, affects other parts of the carcass.  Borch et al., 1996 have reviewed the literature in this area and conclude that bacterial penetration into the sticking wound during scalding is not significant.  In addition, scald tank water can fill the lungs.  Although the microbiota of the water itself is well-controlled by the temperature of the scald tank, there exists a possibility that the water could be contaminated during passage through the mouth and pharynx (Q49).  There is not much information on significance of such a potential hazard.  More generally, Borch et al. (1996) believe that the significance of the pharynx as a processing hazard is overlooked and that it should be actively considered when assigning control points to a porcine slaughter process.


Bolton, D. J., Pearce, R. A., Sheridan, J. J., Blair, I. S., McDowell, D. A. and Harrington, D. (2002) Washing and chilling as critical control points in pork slaughter hazard analysis and critical control point (HACCP) systems. Journal of Applied Microbiology 92, 893-902.

Bolton, D. J., Pearce, R., Sheridan, J. J., McDowell, D. A. and Blair, I.S. (2003) Decontamination of pork carcasses during scalding and the prevention of Salmonella cross-contamination. Journal of Applied Microbiology 94, 1036-1042.

Berends, B., R., VanKnapen, F., Snijders, J., M., A., and Mossel D., A., A. (1997) Identification and quantification of risk factors regarding Salmonella spp. on pork carcasses.  International Journal of Food Microbiology 36, 199-206.

Borch, E., Nesbakken, T. and Christensen, H. (1996) Hazard identification in swine slaughter with respect to food-borne bacteria. International Journal of Food Microbiology 30, 9–25.

Bryant, J., Brereton, D. A. and Gill, C. O. (2003) Implementation of a validated HACCP system for the control of microbiological contamination of pig carcasses at a small abattoir. Canadian Veterinary Journal 44, 51-55.

Dockerty, T. R., Ockerman, H. W., Cahill, V. R., Kunkle, L. E. and Weiser, H. H. (1970) Microbial level of pork skin as affected by the dressing process.  Journal of Animal Science.  Vol. 30,884-890.

Delhalle, L., De Sadeleer, L., Bollaerts, K., Farnir, F., Saegerman, C., Korsak, N., Dewulf, J., De Zutter, L. and Daube, G. (2008) Risk Factors for Salmonella and Hygiene Indicators in the 10 Largest Belgian Pig Slaughterhouses. Journal of Food Protection 71, 1320-1329.

Gill, C. O. and Bryant, J. (1993) The presence of Escherichia coli, Salmonella and Campylobacter in pig carcass dehairing equipment.  Food Microbiology. 10, 337-344.

Hald, A. Wingstrand, M. Swanenburg, A. Altrock and Thorberg, B.M. (2003) The occurrence and epidemiology of Salmonella in European pig slaughterhouses, Epidemiology and Infection 131, 1187–1203.

Letellier, A., Beauchamp, G., Gue'Vremont, E., D'Allaire, S., Hurnik, D., and Quessy, S. (2009) Risk factors at slaughter associated with presence of Salmonella on hog carcasses in Canada.  Journal of Food Protection. 72,2326-2331.

ICMSF (1998) Microorganisms in Foods 6: Microbial Ecology of Food Commodities. Blackie Academic and Professional, London.

Johanson, L., Underdal, K. G., Grøsland, K., Whelehan, O. P. and Roberts, T. A. (1983) A survey of the hygienic quality of beef and pork carcasses in Norway. Acta Veterinaria Scandinavica. 24, 1-13.

Kotula, A. W. (1987) Control of extrinsic and intrinsic contamination of pork. pp181-201.  In Elimination of Pathogenic Organisms from Meat and Poultry edited by F. J. M. Smulders. Elsevier: Amsterdam-New York-Oxford.

Namvar, A. and Warriner, K. (2006) Application of enterobacterial repetitive intergenic consensus-polymerase chain reaction to trace the fate of generic Escherichia coli within a high capacity pork slaughter line. International Journal of Food Microbiology 108, 155-163.

Nickels, C., Svensson, I., Ternstrom, A. and Wickberg, L. (1976) Hygiene and economy of scalding with condensed water vapour and in tank.  Proceedings of the 22nd European Meeting of Meat Research Workers. Malmo, Sweden.  Vol. 1  C1:1 - C1:10.

Nerbrink, E. and Borch, E. (1989) Bacterial contamination during the pig slaughtering process.  Proceedings of the 35th International Congress of Meat Science and Technology (ICoMST 89), Copenhagen, Denmark2, 356-362.

Pearce, R. A., Bolton, D. J., Sheridan, J. J. McDowell, D. A., Blair, I. S. and Harrington, D. (2004) Studies to determine the critical control points in pork slaughter hazard analysis and critical control point systems. International Journal of Food Microbiology 90, 331– 339

Purnell, G., James, C., Wilkin, C.-A. and James, S. J.  2010.  An evaluation of improvements in carcass hygiene through the use of anal plugging of pig carcasses prior to saclding and dehairing.  Journal of Food Protection 73, 1108-1110.

Rahkio, M., Korkeala, H., Sippola, I. and Peltonen, M. (1992) Effect of pre-scalding brushing on contamination level of pork carcasses during the slaughtering process. Meat Science. Vol. 32, pp173-183.

Richmond, M. (1991) The Microbiological safety of food - Part II.  Report of the Committee on the Microbiological Safety of Food.  HMSO.

Spescha, C., Stephan, R., and Zweifel, C. (2006) Microbiological contamination of pig carcasses at different stages of slaughter in two European Union-approved abattoirs.  Journal of Food Protection 69, 2568-2575.

Smulders, F. J. M. and van Laack, R. L. J. M. (1992) On the quality of pork. 1. Microbiological concerns. Fleischwirtschaft. 72, 888-891.

Tinker, D. B., Dodd, C. E. R., Richards, P., James, S. J., James, C., Wilkin, C-A., Burfoot, D., Howell, M., and Purnell, G. (2007) Assessment of processes and operating conditions in UK pork abattoirs.  7th International Symposium on the epidemiology and control of foodborne pathogens in pork (SAFEPORK 2007), Verona, Italy, 9-11 May 2007, pp337-340.

Troeger, K. (1994) Development of bacterial count in scalding water during slaughter: Effect on surface bacterial counts on pig carcasses. Fleischwirtschaft. 74, 518-520.

Warriner, K., Aldsworth, T. G., Kaur, S., and Dodd, C. E. R. (2002) Cross-contamination of carcasses and equipment during pork processing. Journal of Applied Microbiology 93, 169-177.

Woltersdorf, W. and Mintzlaff, H. J. (1996) Condensation scalding of pigs: A practicable method .1. Scalding effect and surface bacterial count.  Fleischwirtschaft.  Vol. 76:3, pp274-277.

Wong, D., Hald, T., van der Wolf, P. J. and Swanenburg, M. (2002)  Epidemiology and control measures for Salmonella in pigs and pork.  Livestock Production Science.   76, 215-222.