Research Digest: February 2008

Additional research reviewed by Casey Owens, Ph.D.

Stunning with low atmospheric pressure

Recent pressures on the poultry industry come from animal rights activists to use gas stun-to-kill (irreversible) systems in order to improve welfare of the animals. Current electrical systems are humane methods to render birds unconscious but require hanging of the live birds, which is a point of focus for activist groups.

Controlled atmosphere (gas) stun systems generally bypass the step of hanging a live bird, because the hanging step occurs after the stunning process when the birds are no longer conscious and therefore minimize handling of the live bird. A limitation of using gas-stunning systems is achieving uniform concentration of gases, which can cause problems with the stunning effectiveness. Removing air by reducing atmospheric pressure may be an alternative stunning system, as it would reduce the partial pressure of oxygen in the bird.

This study examined the optimum pressure level required to stun and subsequently kill broilers resulting in an irreversible cessation of respiratory ventilation movements. Various levels of pressures were selected for use in this study, ranging from 70.9 kPa (based on allowable range for human habitats in long-term space exploration) to 17.8 kPa (based on oxygen pressures shown to induce unconsciousness in hens). In experiment 1, after subjecting birds to target pressure levels, birds were observed for loss of posture (LOP) and cessation of respiratory movements (CRM). From this experiment, the operating pressures were determined and selected pressures, ranging from 35.3 kPa to 17.8 kPa, were used for experiment 2 where elapsed time to LOP and CRM was recorded.

In experiment 1, the higher-pressure treatments (40.5 kPa to 70.9 kPa) elicited no response in the birds. However, the lower level pressures (29.5 kPa and 17.8 kPa) resulted in 100 percent of birds exhibiting LOP and 70 to100 percent mortality (29.5 and 17.9 kPa, respectively). In experiment 2, pressure levels ranging from 26.6 kPa to 17.8 kPa had significantly shorter times to LOP compared to pressures of 29.5 to 35.3 kPa. The lower pressure levels (£26.6 kPa) required approximately 34 seconds to LOP where the higher-pressure levels (31.1-35.3 kPa) required greater than 46 seconds. Using lower pressures (£29.5 kPa), 100 percent LOP was observed, but only 75 percent LOP was observed with higher pressures.

Time to CRM was also measured and the shortest times (79-85 seconds) were observed for the lowest pressures used, 23.6 kPa to 17.8 kPa, and this was similar to other gas stunning systems (time to death). Increasing pressures resulted in increased time to CRM (128-142 seconds). There was 100 percent mortality in the lower pressures studied (£26.6 kPa), but this percentage was drastically reduced to 12.5 percent with higher pressures of 32.1 kPa and 35.3 kPa. Using 29.5 kPa, 62 percent mortality was observed. Based on this information and a regression equation from previous literature, the researchers determined that a maximum pressure of 19.4 kPa should be used; this would result in 99.99 percent mortality. A high mortality rate such as this would be imperative when using any irreversible stun (stun-kill) system.

The authors stated that the vessel and system performed well in this study; there was very little variation in the process. Evacuation of the vessel was highly repeatable and the pressure achieved and time to achieve it had little variation. For example, after 34.5 seconds of evacuation, the average pressure was 21.1 kPa (SE=0.08).

While decompression is not on the list of acceptable means of euthanasia for animals by the American Veterinary Medical Association, the respiratory physiology of the birds is different from mammals. Therefore, the concerns (e.g., trapped air in body) related to decompression and mammals are not applicable to birds. Furthermore, reduced atmospheric pressure is approved in Europe for slaughter of farmed game avian species.

The results of this study suggest that a low atmospheric pressure stunning system may be an alternative to traditional gas or electrical stunning systems. The authors state that this low-pressure system provides economic and safety advantages over other gas stunning systems. However, more research is needed to determine the effects of such systems on physiological responses, process efficiency, carcass and meat quality, as well as animal welfare aspects.

J.L. Purswell, J.P. Thaxton and S.L. Branton; 2007. Identifying process variables for a low atmospheric pressure stunning-killing system. Journal of Applied Poultry Research, 16: 509-513. (

Worn picker fingers susceptible to bacterial contamination

The defeathering process requires the use of scalding and picking. Mechanical pickers with rubber picker fingers are used in this process to remove feathers from poultry. Cross contamination from bird to bird via processing equipment is a food safety concern. While the rubber picker fingers are designed with materials that resist bacterial attachment better than stainless steel, the materials can degrade with usage and result in cracks and crevices, which are areas for harboring bacteria. Therefore, these worn fingers may be more susceptible to bacterial attachment and biofilm formation, which can result in more cross contamination. These fingers are replaced almost entirely based on visual inspection and judgment of the evaluator.

In this study, rubber picker fingers were collected from six locations within the second commercial defeathering machine in the processing line of three broiler processing plants. The fingers had been in service for approximately five days, or ten shifts. New, unused rubber picker fingers were also evaluated as a control. A three-rib section of the rubber picker fingers were subjected to microbial analysis as well as scanning electron microscopy for visual analysis.

Fingers that were collected after five days in operation showed signs of wear including cracks and broken tips. However, sampling for microbial analysis was not directly at the tip of the rubber finger. In plants 1 and 2, bacterial levels ranged from 0 to 5.41 log10 cfu/mL while plant 3 had higher bacterial levels (3.23 to 7.33 log10 cfu/mL). The authors did state that plant 3 had a management change during the experiments and that the levels of bacteria were 100-fold less after the management change, suggesting better sanitation practices. Scanning electron microscopy confirmed the level of bacterial contamination. Plants 1 and 2 had lower levels of bacteria than plant 3 overall. The new, unused rubber picker fingers had no bacteria detected as indicated by the same microbial analysis as for the used fingers.

There were few differences observed in bacterial levels on the picker fingers due to location. In most cases, there were no differences among the 3 locations (front, center and exit of picker) with the exception of plant 2 where the front location had significantly higher bacterial levels compared to the exit location. The authors explained that this could be expected because as the broilers move forward, it is expected that it will become progressively cleaner. There were no differences in bacterial levels on rubber fingers from the right and left banks of the picking machines.

After a thee-day and extended incubation, the bacterial levels on the picker fingers increased suggesting that the picker fingers are susceptible to bacterial growth. There was a 4-log increase after extended incubation at 23C. The authors did explain that these increases in bacterial levels (due to incubation) would not necessarily be typical in plants because of regular sanitation practices. However, there is a possibility of plant downtime (for repairs, holidays, weekends, etc.) that could contribute to increasing levels of bacteria on picker fingers.

The results of this study suggest that bacterial levels on rubber picker fingers can increase due to usage in the picking machines. However, a combination of appropriate material and design for rubber picker fingers and effective sanitation practices will be helpful in preventing significant contamination. Regular and thorough evaluation of picker fingers for worn/damaged fingers is also necessary as an intervention step for pathogen control.

Comment: It seems that the actual worn/cracked areas were not necessarily tested in this study as the samples taken were further down on the rubber picker finger. The tips of the rubber picker fingers would have much more contact with carcasses and therefore, may be a greater source of bacteria for cross-contamination. Further research to evaluate the bacterial levels of the rubber picker finger tips may provide more complete information.

J.W. Arnold; 2007. Bacterial contamination on rubber picker finders before, during, and after processing. Poultry Science, 86: 2671-2675. (

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