CAS anesthetization – the way of the future around the world

The popularity of CAS SmoothFlow is rocketing worldwide


Globally, the interest in controlled atmosphere stunning (CAS) systems for poultry is growing, driven by multiple factors. Governmental legislation and consumer preferences concerning animal welfare are key considerations.

Product quality, however, is also a main driver for choosing CAS. Marel Poultry was the first company to produce such an anesthetization system. At the moment, the popularity of the current generation Stork CAS SmoothFlow system is rocketing worldwide.

There are two types of stunning systems commercially used for poultry, a water bath system and controlled atmosphere stunning. Worldwide, water bath stunning is the most used system, but it is losing ground.

Loss of quality

Since 2013, EU legislation 1099/2009 has been in place. This legislation protects animals at the time of killing and describes in detail what and how systems must be used for stunning and killing animals. For water bath stunning systems minimum currents (i.e. milliAmpères) per bird are determined to ensure loss of consciousness for all birds. These required currents, however, cause loss of meat quality. This (partly) explains why the majority of European processors has shifted or is shifting to controlled atmosphere stunning.


In other parts of the world, legislation is gradually becoming stricter. In some countries, this will lead to a faster switch to CAS stunning. We see this happening in the US, where the “Joint Animal Protection Organization Statement on Broiler Chicken Welfare Issues”, a group including the US Humane Society and Compassion in World Farming, asks poultry companies to use CAS by 2024.

The same year is marked as a deadline by the Global Animal Partnership organization for the broiler industry to have taken measures which significantly improve animal well-being. It is not without reason that several major US poultry processors recently decided to abandon electrical stunning and implement the CAS SmoothFlow system in their broiler processing plants. 

The world’s first

Tim van Schaik, product specialist live bird handling at Marel Poultry, tells about the world’s first Controlled Atmosphere Stunning (CAS) system for poultry, which was produced by Marel Poultry. “Based on intensive research in the early nineties, we launched the first generation CAS stunning system in 1996. CAS SmoothFlow, our latest generation system, is the result of 30 years of research as well as customer and market experience.”

Based on science

“We have co-operated with numerous research institutes worldwide and the technology we apply is supported by dozens of scientific publications. Our Stork CAS SmoothFlow system has a unique feature in the world of anesthetization; the regulated addition of O2, next to CO2. Tim van Schaik continues: “Based on scientific research, we found that a mixture of CO2 and O2 is the best way for broilers to get smoothly induced into unconsciousness.”

The SmoothFlow system can consist of up to five equal sections, each with its own programmable controlled atmosphere. The broilers lapse gently into unconsciousness in the induction phase. Perfect control over the air mixtures in subsequent sections results in a smooth transfer to the completing phase. “Birds lapse gently into unconsciousness, without any stress, pain or convulsions.”

The most humane

Taking into account the entire process, including shackling, CAS is the most humane way of anesthetizing. “Certainly when the CAS SmoothFlow system is combined with the ATLAS live bird handling system, broilers can stay calmly in their trays all the way from the farm, during transport to the anesthetization area.” Contrary to electrical stunning, broilers aren’t shackled live; only after being stunned, they are shackled in an unconscious state, which is also less strenuous for the shackling team. Tim van Schaik adds, “Looking through windows along the entire tunnel, we can check the broilers’ behavior during the inline stunning process. The new generation Marel Poultry systems stun the broilers within five minutes. We see that the birds go to sleep gently, without any stress, pain or convulsions. Atmosphere stunning results in irreversible anesthesia.”


Once birds have been transported smoothly to the stunning system, they are exposed to less than 40% CO2 for one minute. After that, the CO2 level is slowly built up to a higher percentage. The improved product quality yield of CO2 anesthesia, compared to water bath stunning, will be one of the main drivers for processors to switch to CAS.


The CAS SmoothFlow system processes poultry according to the first-in-first-out principle. Van Schaik: “The continuous process assigns exactly the same stunning time and atmosphere level to every broiler. Another advantage of this linear setup is the possible removal of DOA’s (birds dead on arrival) before they enter the stunning process.” Certain countries have regulations that forbid DOA’s from entering into the stunning device.

Decisive arguments

According to Tim van Schaik, plenty of arguments are important when deciding to use CAS. “Many considerations have to be taken into account, such as animal well-being, food safety, regulations, hygiene, religious requirements and product quality. Additionally, atmosphere stunning systems have larger footprints than water bath stunning systems, so available space can also be a decisive aspect. Our customers choose their most appropriate system with different priorities and needs in mind. It’s our mission to advise the solution which best meets each individual situation.”

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