Chicken batches slaughtered later in the schedule, pre-thinning flocks prior to slaughtering and temperatures in the evisceration room topping 15C are all factors likely to increase the risk of campylobacter in poultry processing, said a new French research.

The study highlighted evisceration as the "operation contributing most to the spread of contamination".

It added that dirty marks visible on carcasses after the process also heightened the threat from the bacteria - which could be caused as processing equipment cannot adapt to differing carcass sizes. The equipment and the machines could also be dirtied and therefore could contaminate the following carcasses.

The risk of Campylobacter climbed when a previous batch-thinning of a flock had taken place - a practice used for staggered slaughtering - particularly if the sanitary barrier was not rigidly maintained. Transport crates were identified as being a major source of potential contamination. The length of time between thinning and slaughtering was also found to be an issue given the bacteria's ability to spread rapidly from bird to bird.

The risk of infection is lowest at the start of the day when equipment is at its cleanest and this increased with flocks not first on the slaughter schedule, said the study. The more batches that go for slaughtering, the higher the risk of contamination.

This also occurs during the evisceration process - when the rupture of the viscera releases high number of the bacteria onto the carcass. The research suggested a systematic washing procedure might be an effective means of significantly reducing bacteria numbers on carcasses.

But the study cautioned infection can occur throughout the entire process – including the chilling room. It added that the number of carcasses on a trolley during chilling tended to be associated with increased contamination and recommended that chilling would yield better results if trolleys carried fewer birds. Contamination also increased when trolleys were in close contact - although more research would be needed to quantify this, said the scientists.

The contamination of equipment and surfaces, process water and air also increases risk, which was also boosted when the evisceration room was higher than 15C. The group noted that this was "difficult to interpret" as other studies had shown Campylobacter could survive at lower temperatures as well.