What do we know specifically about Controlled Atmosphere Stunning (CAS)?



When I owned, and operated my own smoked ham processing company, I originally purchased my fresh hams (raw materials) from a slaughter operation that electrically stunned hogs.  The occurrence of broken bones and blood splash in the hams was way too frequent; it was costing me in yield and production efficiency.  I then switched to a supplier that utilized CO-2 stunning.  Broken bones and blood spots were then almost nonexistent.  Based on this I always wanted to see Controlled Atmosphere Stunning (CAS) operation.  This year I had that opportunity.


The hogs were placed in the CO-2 chamber (approximately eight at a time) with a powered gate.  They emerged insensible.


What do we know specifically about Controlled Atmosphere Stunning (CAS)?


In a commercial process, animals experience their introduction into CAS atmospheres gradually, either through transport at a controlled rate into a controlled atmosphere gradient or through controlled introduction of stunning gases into an enclosed space.  The transport of introduction rate may be slow or relatively quick depending on the process, gases used and specific species.


Per Temple Grandin at NAMI 2016 Animal Welfare Conference, genetics may affect pig’s reaction to carbon dioxide.


Regarding determination of insensibility in the use of CAS:


·         Nystagmus (vibrating) eye is permissible


·         Gasping is permissible (not to be confused with rhythmic breathing which is not permissible)


A best practice in CAS is to have an inspection port to observe anesthesia induction.  This obviously is for observation of any potential animal welfare issues.


Per the Danish Meat Institute, the order of events during return to sensibility in CO-2 stunned hogs (average time):


·         Corneal reflex (touch eye)                                      42 seconds


·         Rhythmic breathing                                               68 seconds


·         Excitation                                                              76 seconds


·         Nystagmus (vibrating) eye                                     86 seconds


·         Spontaneous natural blinking                                 93 seconds


·         Conscious movement (righting reflex)                    171 seconds


·         Attempt to stand up                                               387 seconds


Here are some key points for successful CAS stunning:


1.   Maintain a CO-2 concentration of over 80%.  A 90% concentration at either the bottom of the pit or at the final stage of the process is strongly recommended.


2.   Increase dwell time if there are problems with return to sensibility.


3.   Undersized equipment that has insufficient capacity is often the cause of insufficient dwell time or handlers overloading the gondolas with animals.  Either a larger piece of equipment or an additional unit will be required to increase system capacity. 


4.   The animals must have sufficient room in the gondolas or containers to stand or lie down without being on top of each other.


5.   When automated gates are used to move pigs up to and into the chamber, they must be equipped with pressure limiting devices.  This prevents the gates from knocking animals over or dragging them along the floor.  Often, powered gates work best when they are equipped with a push button or other control that allows the handlers to control forward movement of the gate.  When the handler lets go of the control, the gate stops.  An automated control works well to return the gate to its start position after it has moved the animals.


6.   Ventilation problems in the plant building can sometimes cause CO-2 to be sucked out of the chamber.  Some commercial CO-2 equipment holds CO-2 in a pit that is not sealed and sometimes air pressure changes in the plant building can cause sensible hogs to emerge from the chamber.  Some of the factors that can suddenly lower CO-2 concentration is either turning off or turning on large ventilation fans in the plant building, wind blowing around the plant building, or leaving certain plant doors open.  Careful observation will be required to correct this problem.  It is often correctible and no equipment purchases are required.


Here is some additional information regarding CAS stunning:


FSIS received a petition request in November 2015 for the use of argon and nitrogen (inert gases) in swine stunning.  It was granted in 2016.  This does not mean a regulation change but allows an establishment to ask for a waiver.




What is an inert gas?  An inert gas is a gas which does not undergo chemical reactions under a given set of conditions.  They often do not react with many substances.  Inert gases are used generally to avoid unwanted chemical reactions.  These undesirable chemical reactions are often oxidation and hydrolysis with the oxygen and moisture in the air.  Nitrogen and argon are the most commonly used inert gases due to their high natural abundance (78% nitrogen and 1% argon) in the air and low relative cost.




Much of the information in this article came from AVMA Guidelines for the Humane Slaughter of Animals: 2016 Edition




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