Chilling without the
pressure
Dick
Ahlstrom, The Irish Times
Researchers at UCD
are working on the use of a vacuum chamber to chill cooked
foods, a method that could make refrigeration in food production unnecessary.
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 technology 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.
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