29 Apr 2020

Reduction of Salmonella contamination during processing

plantas de faenado avícola


Scott M. Russell

Diamond V

Content available in: Español (Spanish) Português (Portuguese (Brazil))

Problems with litter quality and the use of foggers to combat heat can cause birds to become wet and dirty with litter and droppings, which can create a serious pollution problem in the processing plant as there is a large amount of organic matter in the scalding tank.
Contamination of skin and feather follicles by bacterial agents in the water can be reduced by 90% by installing a brush and a chlorine shower before and after the scalding tank. The aim is to reduce the faecal matter present on the skin of the carcasses as much as possible before they reach the chiller.

Scalding tank

The scalding tank is one of the most important contamination points of Salmonella in any plant. Most of these tanks do not have a true counter-flow of water.

Figure 1. Substitution of multiple blanching tanks to dilute organic matter

The water must flow against the direction of the carcasses in order to wash them effectively. The water flow must be abundant so that the organic matter in the tank can be diluted effectively. A good practice is to install multiple scalding tanks in succession to dilute the organic matter as much as possible, as illustrated in Figure 1.

The objective is to minimise the amount of organic matter entering the chiller, where the efficiency of any oxidizing agent such as chlorine is drastically reduced as organic material increases.

Control of Salmonella cross-contamination in the scalding tank

Paradoxically, the action of removing organic matter from contaminated birds actually results in increased contamination of the rest of the carcasses, because the removed organic matter contaminates the water in the scalding tank, thus creating a serious opportunity for cross contamination.

The use of multiple scalding tanks is very important to reduce contamination.

Some companies reduce the scalding tank temperature from approximately 130°F to 100-110°F. This practice is counterproductive because it encourages organic matter to sink to the bottom of the tank, facilitating contamination with Salmonella, which lives and even multiplies at 100°F.

Evaluation of the scalder’s efficiency

Microbiological evaluation of the scalding tank is critical to determine if it is a high risk point for Salmonella contamination. An example of a scalding tank not operating properly is illustrated in Figure 3. Possible reasons for this problem include: Tank temperature too low, Insufficient water supply, Water flowing in the same direction as the carcasses, rather than against.

chickens slaughterhouse

Figure 2. Scalder operating as a bath and not as a backwash station.

Older scalding tanks do not have a reverse flow of water and this is the worst situation for Salmonella control. The situation is aggravated when the processing plant has water supply limitations.

Figure 3. Scalding tank with inadequate operation and where Salmonella contamination increases as the carcasses pass through it.

Correcting the problem

This problem can be corrected by one or more of the following actions:

  • Establish a counter-current water flow
  • Ensure a water temperature of at least 123 ° F
  • Install a chlorine shower and brush system immediately after bleeding to remove excess stool before scalding
  • Consider introducing a chemical into the tank

Once these corrections are implemented, it is important to conduct a new microbiological evaluation. Figure 4 illustrates a realistic scenario in a properly operating process plant.

Figure 4. Properly functioning scalding tank in which contamination is reduced between before and after scalding of the casings.

In conclusion, the scald is an important point to implement interventions that allow effective control of Salmonella, being especially relevant when microbiological evaluations show that the scalding tank is a major point of risk.

Adding chemicals to the scalding tank and managing temperatures

A survey conducted by the United States Poultry and Egg Production Association (USPEA) revealed that approximately 50% of companies use chlorine in the tank and the other 50% use sodium hypochlorite.

Neither of these products are effective because chlorine is immediately inactivated upon contact with organic matter, and sodium hypochlorite does not have a significant impact on bacterial contamination levels.

Figure 5. Effect of a chemical to sanitise scalding water (reduction of aerobic bacteria)

Figure 6. Effect of a bacteriological index reducing product on the concentration of E. coli (colony forming units) detected in carcasses immediately after scalding.


Figures 5 and Figure 6 illustrate the concentrations of bacteria before and after the blanching when using water without chemicals and when using products to reduce the bacteriological index.

The addition of an acidic disinfectant can be considered, which allows the water temperature to be reduced, although it has a tremendous impact on the efficiency and cost of scalding.

Benefits of using chemicals in the scalding tank

When chemicals are used in the scalding tank, it becomes possible to reduce the water temperature, which can generate the following benefits:

  • Feather removal is facilitated (when water is acidified)
  • Less bacterial growth, including Salmonella
  • Fewer over-scalding problems, especially in the breast muscle
  • Less energy cost to maintain the temperature of the scalding tank
  • Less overcooking of subcutaneous fat, allowing better yields
Control of cross-contamination in the plucker

The rubber fingers of plucking machines represent one of the greatest risks of cross-contamination in processing plants because they come into direct contact with contaminated surfaces and then come into contact with the rest of the carcasses.

Research carried out at the University of Georgia has determined that the water that accumulates in the bottom of these machines can easily contain up to 1 million bacteria per millilitre, even when using water with a chlorine concentration of 40 ppm. Figure 7 shows the increase in salmonella prevalence during plucking.

Figure 7. Salmonella prevalence before and after plucking

Cross-contamination evaluation in pluckers

Figure 8 illustrates the data resulting from evaluations made in scalding tanks, pluckers and New York type spray stations, where chlorinated water (controls) or the product “Tasker Blue” were added. The effects were truly dramatic as the addition of “Tasker Blue” resulted in a 2.7 log10 decrease (more than 99%) of bacteria on the carcass surface upon arrival at the re-processing station.

Subsequent studies have shown that this action also has a dramatic effect in reducing salmonella in carcasses as they exit the chiller.

Figure 8. Aerobic bacteria count before plucker and after pre-processing station.


A major problem regarding the use of chemicals in the scalding tank or plucking machine is that managers are not used to investing at any point “at the beginning of the process”, but more towards the end. The investment at the beginning of the process by the managers can change rapidly, as the legislation changes regarding permissible Salmonella contamination levels.

Chemical disinfection of the processing chain

There are three general methods to reduce pathogenic bacteria in the processing chain excluding:

  • Scalding tank
  • The plucker
  • The cooling system
  • Post-Cooling Immersion Systems



The purpose of this equipment is to wash and sanitize the processing equipment that comes into contact with the carcasses so that contamination is not transferred to other stations and that cross-contamination between carcasses is reduced. It is important to have high water pressure and to add chemicals such as chlorine. Chlorine dioxide, peracetic acid, or products such as Zentox or TOMCO (acidified hypochlorous acid) can also be used.

Cl Effectiveness

An important limitation to the effectiveness of chlorine is the accumulation of fat or other organic materials on the equipment, which prevents the penetration of the product so that it can destroy bacteria. The bacteria are easily released when they come into physical contact with the carcasses, which can easily happen with equipment such as brushes dedicated to removing fecal matter after evisceration.

A high risk point is the machine that removes the crop, as this organ often contains pathogens such as Salmonella and Campylobacter

pH in washing systems

The pH of all washing systems should be kept below 6.5 and a quality program should be in place to frequently check the acidity of the water if good control over bacterial contamination is to be maintained. Shower pressure should be checked and it should be ensured that spray nozzles are not clogged.

Figure 9 illustrates some sprinklers that are not working and others that have a very irregular water distribution pattern. Sprinkler nozzles should be positioned in such a way that they can actually wash the surface of the equipment that comes into contact with the carcasses, as well as the carcasses themselves.

slaughter plants

Figure 9. Malfunctioning washing equipment.

Showers for washing inside & outside the carcasses.

These showers should be checked frequently for their levels of:

  • Chlorine
  • Concentration of other products
  • Chemicals
  • pH
  • Water pressure & distribution

General recommendations for washing stations

Some general recommendations for these washing stations include:

  • Maintaining the water pressure.
  • Maintaining the correct pH level.
  • Maintaining the correct concentration of chlorine or any other chemical.
  • Maintaining the correct distribution of water on the carcasses.

Several chemicals that are being experimentally tested for this purpose include:

  • Chlorine dioxide
  • Peracetic acid
  • Zentox & TOMCO (acidified hypochlorous acid)

Most companies use chlorine in washing systems.

A study conducted by Northcutt et al. –USDA / ARS 2005– suggested that the addition of chlorine to the internal and external washing showers of the carcasses does not have any positive effect on aerobic bacterial counts of E. coli, Salmonella or Campylobacter on carcasses, since in many circumstances Chlorine is unable to penetrate organic matter if it is too abundant on the carcasses.




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