Heat pick up in chilled chicken carcasses held at ambient
Heat pick up in chilled chicken carcasses held at ambient
Friday, 10 April 2009
Chris James and Steve James comment on recent work on heat pick up in chicken carcasses.
We are often asked to give an expert opinion regarding the temperature of food. Many of these problems result from the question of heat pick up caused by transferring products from a controlled refrigerated condition to an ambient condition for a short period of time such as an unrefrigerated loading bay. One recent enquiry was regarding trays of chicken carcasses where temperatures exceeding 4°C had been found, contrary to regulations 18,1, (b) and (d) Schedule 8, (9); Schedule 12: Schedule 14 of the Poultry Meat, Farmed Game Bird Meat and Rabbit Meat (Hygiene and Inspection) Regulations 1995.
Previous work and experience on temperature rises during loading and unloading in refrigerated and unrefrigerated environments and the results from predictive modelling show that:
•Temperatures in bulk products respond very slowly to changes in environmental conditions.
•Exposed surfaces can respond quickly to changes in environmental conditions that produce a high heat transfer (high air velocity) and/or a large temperature difference between the surface of the product and the environment.
•Even thin slices of meat take many minutes for the core to change significantly in temperature.
•Any form of wrapping will substantially reduce the rate of temperature rise.
However, we had no specific data on the response of a chilled chicken carcass to exposure to ambient temperatures. We therefore had a simple experiment carried out to confirm what we would expect to happen.
Calibrated thermocouples were placed immediately below the skin of the breast muscle and 50 mm deep into the breast of two chilled chickens (1.3 and 1.9 kg stated weight) purchased from a supermarket (see above). The unwrapped birds were placed next to each other in a perforated tray and a 5th thermocouple sensor positioned to measure the temperature of the air immediately above the carcasses. All 5 thermocouples were attached to a squirrel data logger and temperatures recorded at 1 minute intervals. The tray was transferred to a chill room where it remained until the deep temperatures were within ±0.5°C of +3°C. The tray was then moved to a laboratory bench and the temperatures recorded for 1 hour.
Temperatures measured immediately below skin and in deep breast of chickens moved from chill to ambient conditions, back chill, then back to ambient
For the first 7 minutes the air temperature was relatively constant at approximately 20°C before gradually rising to approximately 24°C after 1 hour (see above). After 3 minutes one of the surface temperatures had reached 5°C and the other 3°C. Both of the surface temperature probes were positioned immediately under the surface of the breast skin at a depth of approximately 1 mm. It was observed that the skin thickness on the larger carcass was less than that on the smaller. At that time no response was noticeable in the two deep temperatures. It required 29 minutes for the deep temperature in the 1.3 kg bird to rise 2°C to a value of 5°C and 33 minutes to reach the same value in the 1.9 kg bird (see graph above). After 60 minutes the core temperatures were 7.8 and 7.4°C for the 1.3 and 1.9 kg bird respectively. A total temperature rise of 4.8 and 4.4°C respectively. To get an idea of how well these temperatures recover the carcasses were returned to the chill store and temperatures monitored. As can be seen the carcasses take substantially longer return to return to the chilled temperatures they started at.
These results are a worst-case scenario. In reality the rises seen will be much lower. To be able to change the temperature of trays of meat in pallets is a very difficult operation unless air is specifically directed over the surface of the meat. This requires large fans and purpose built air-handling system. Just putting the pallets in an area at a different temperature has little effect. The meat remains surrounded by a layer of still air at a similar temperature to that of the surface of the meat. The rate of heat flow into or out from the meat is very slow and the temperature change consequently very slow.
These data can also be applied to a domestic situation. The conditions being analogous to the effect of taking a chilled chicken out of the refrigerator and placing in on a kitchen work surface.