LPT 312: Abattoir Practices and Animal Products Technology (1+1)

2) Stunning By Means of Gas

STUNNING BY MEANS OF GAS

Stunning by Carbon-di-oxide gas

• Carbon dioxide was first used to induce preslaughter anaesthesia in animals in 1904, but not successfully on a commercial scale until 1950.
• Since then the method has been modified in several different ways and is now used fairly widely throughout the world although not as extensively as it could be, probably because of the high cost of installation and operation.
• Some authorities suggest that the struggling witnessed in pigs for a period of some 15-20s when they first come into contact with high concentrations of gas is due to the very irritant properties of carbon dioxide.
• The others are of the opinion that the struggling is equivalent to the induction stages of anaesthesia and that the pig is in the unconscious state during this period.  
• The recommended concentration of CO₂ should be at least 70%.
• Experiments have shown that nitrous oxide (NO₂) may be an alternative anaesthetic gas for stunning pigs.
• Its density is close to CO₂ and so it can be used in similar equipment.
• The indications are that although meat quality may be improved the induction time is unacceptably long.
• However, currently, CO₂ is the only gas that is widely used for stunning animals commercially.
• It is usually stored in cylinders or bulk tanks as a liquid under pressure.
• It is also available in solid form for which a converter is necessary.
• The gas is non-inflammable and has a higher specific gravity than air, sinking to the bottom of any container, a fact that has to be borne in mind when it is being used for anaesthesia or euthanasia purposes.
• When properly used it presents no hazard to the operative.
• A concentration of 80-95% CO₂ in air is the most suitable for pre-slaughter anaesthesia.
• If the concentration is too low the pigs will not be properly stunned and if it is too high there is a tendency of the pigs to become stiff, show reflex muscular activity and bleed poorly.
• If the exposure period is too long, superficial congestion of the skin occurs and when pigs are scalded the skin is bluish in colour.
• Pigs subjected to CO₂ anaesthesia will regain consciousness if they are not subsequently bled, the recovery time varying with the concentration of the gas, but averaging about 90%.
• It is important, therefore, that in addition to the concentration of gas, the period of exposure should be 45s and bleeding should take place as soon as possible and certainly within 30 seconds of the pigs leaving the gas chamber.
• It is possible that some adverse effects observed on occasions are due to incorrect concentration of gas and air and/or inadequate or too long exposure times.