Chilling without the pressure


Researchers at UCD are working on the use of a vacuum chamber to chillcooked foods, a method that could make refrigeration in food productionunnecessary. Dick Ahlstrom reports.

A team at University College Dublin has turned the science behind the pressure cooker on its head. Instead of using pressure to cook faster, they use an absence of pressure to chill faster.

The process is known as vacuum cooling and is under study by the Food Refrigeration and Computerised Food Technology Group within UCD's Department of Agriculture and Food Engineering. Dr Da-Wen Sun, who leads the group, believes it holds great promise as a way to chill precooked foods much faster than conventional refrigeration.

This is not just a matter of convenience, Dr Sun says. Strict EU guidelines demand that cooked meat joints including ham, turkey, chicken, pork and beef for delicatessen, catering and industrial use must be cooled within tight time limits post cooking.

Irish and UK guidelines on cook-chill systems recommend that cooling of meat joints should begin within 30 minutes of leaving the oven, Dr Sun says. "They require cooked meats to be taken from 74 degrees C to 10 degrees C or less in 2.5 hours and that is from the microbiological point of view. That is the danger zone for microbes to grow."

Current practice involves using air blast chilling, immersion chilling and slow air chilling, with food temperatures reduced due to conduction, the transfer of heat from inside the joint to its cooler surface. Unfortunately for the producers, food isn't very cooperative. It gives up its heat very slowly and is even less conductive than the glass providing thermal insulation in windows, he points out.

For this reason, the guidelines suggest meat products should not exceed 2.5 kilos and 100 mm in thickness. "But when you make the product very small, the cooking losses will be high," Dr Sun says, and the producers would prefer to cook larger, 5-8 kilo, joints for economic reasons.

Dr Sun is originally from Guangzhou (formerly Canton) in China, graduating from university with a PhD in chemical engineering. He moved to the UK to pursue research in agricultural engineering and refrigeration technology and then brought this expertise to UCD in 1995. He now oversees two EU funded vacuum chilling projects jointly with the National Food Centre worth a combined €889,000 and a third worth €350,000.The first showed that large cooked joints could be chilled 400 per cent faster using vacuum cooling rather than conventional techniques. "Because the technology was so good, we decided to research other projects," Dr Sun says.

A pressure cooker works by using pressure to raise the boiling point of water well above 100 degrees C, he says. "That is why we cook foods quickly. Vacuum cooling is the opposite of this process. We reduce the pressure in the chamber."

When the pressure drops, so too does the boiling point of water. At sea level, atmospheric pressure is 1,013 millibars (mbar) and water boils at 100 degrees C. When pressure reaches 23 mbar, boiling point drops to just 20 degrees C.

"We go for lower than that," says Dr Sun. "We go for six to eight mbar. The water boils at between zero and five degrees C."

The boiling point is so low that the latent heat inside the cooked joint provides enough warmth to drive rapid evaporation and the cooling it causes. The moisture is evacuated from the chamber to keep the evaporation rate as high as possible.

"That is why the cooling technol-ogy is so different," says Dr Sun. "Conventional cooling mechanisms are based on conduction. The reason why we cannot rely on conduction is the thermal conductivity of food. It is very, very low." Using conventional methods it is impossible to meet the guidelines for large joints, he adds.

Tests showed that vacuum cooling could bring down the temperature of large joints in less than the 2.5 hour limit, he says. Ham and beef joints weighing between 4.5 and 4.9 kilo were brought to the required 10 degrees C in 2.3 hours using vacuum chilling. Blast chilling of similar sized joints took 9.8 hours and immersion chilling and slow air chilling both took 14.3 hours to reach the target temperature.

The latest research project with the National Food Centre using funding from the EU Food Institutional Research Measure looks at quality issues. While eating quality is "quite similar" the overall quality is lower with vacuum chilling compared to refrigeration based chilling, he says. The team hopes to find ways to maintain quality using this technology.