« August 2007 Table of Contents 
Processing & Services: Freezing methods
Manufacturers using mechanical, cryogenic systems have it down cold
By Lauren Kramer
August 01, 2007
The quality debate of fresh vs. frozen seafood has been a
long one, and like produce, the frozen stigma has plagued the
seafood industry for many years.
"Fresh seafood is good initially but begins to degrade as it
remains in an unfrozen state - such that product purchased by
consumers days later is not as good as what it might have been
in the first hours after it was caught," says David Weber, VP
of operations at Gorton's in Gloucester, Mass. "At Gorton's we
are strong believers in the quality of our frozen product. The
bulk of our raw material, Alaska pollock, is caught and
processed into blocks within an hour or two of its being caught
- thereby locking in the product attributes."
There is no question that seafood and freshness go hand in
hand. But how to best keep seafood fresh by freezing is an
ongoing debate with two methods, cryogenic and mechanical, at
different ends of the spectrum.
Which one makes the most sense depends on a variety of
factors, not the least of which are product type, size and
variety, processing space, geographical location, available
capital and anticipated production volume.
Mechanical freezing
Mechanical freezing relies on a heat exchanger wherein a
refrigerant like ammonia or Freon passes through a closed
refrigeration system of compressors, pumps and coils. In the
process, the temperature of air around those coils, usually
about minus 35 degrees Fahrenheit, is transferred to the
product as it makes its way through the freezing
compartment.
In cryogenic freezing, by contrast, the minus 320-degree F
temperature of liquid nitrogen is applied directly to the
product, removing heat at typical operating temperatures of
minus 80 to minus 100 degrees F.
Once processors have made their choice of freezing system,
they typically defend it passionately.
"We've used ammonia-based freezers from day one, back in the
1950s," says Weber. "This cold refrigeration cycle is very
efficient, very inexpensive and keeps things at the right
temperature for us - minus 70. That's plenty cold enough to
cool our products."
Gorton's has avoided cryogenic freezing because while it
cools product fast, it is more than twice as expensive,
depending
on the product being frozen,
says Weber.
"The increased cost is related to the freezing method and
chemical used as opposed to ammonia-based freezing," he
explains. "You would only use that for something that had high
value and needed to be frozen very quickly," he explains. "For
our product, we're starting out at pretty cold temperatures to
begin with. To bring our products down to a shippable
temperature costs about a penny a pound, so it is very
inexpensive."
Indeed, capital costs constitute a prime motivating factor
for processors choosing a freezing system. Mechanical systems
have high initial costs as processors are required to invest in
a range of equipment, but their ongoing maintenance costs are
relatively low.
"It can cost anywhere from $400,000 to $1 million for a
mechanical freezer, depending on the unit you choose, and
that's just for the freezer," says Art Christianson, VP of
operations at High Liner Foods in Portsmouth, N.H. "The other
associated equipment costs extra."
Some argue that the quality difference between seafood
frozen cryogenically versus seafood frozen mechanically is
indiscernible. At FMC FoodTech, the company's mechanical
super-chill technique uses its Advantec impingement freezer,
whereby temperatures of minus 30 degrees F can be achieved in
only 3 to 4 minutes, says Food Engineer Staffan Sundsten.
"Super-chill means lowering the temperature just below the
freezing point of water, but not freezing the water in the
fish," he says. "By keeping product at that temperature,
compared to the normal temperature of, say, salmon fillets on
ice, you get a 30 percent increase in shelf life." That is
because the microorganisms that start to grow on the surface of
the fish are slow starters at those temperatures, Sundsten
explains. "It means you can fit more product when you ship, and
you don't have to deal with the ice."
Products super-chilled using the impingement technique are
frozen an inch on their surface with high-speed jet air. "We
make a crust around the whole product and utilize that coldness
to level out the product," explains Sundsten. "We don't change
the quality of the product."
FMC developed its Advantec impingement freezer in the early
1990s. "The hamburger industry asked us to make this for
hamburger producers, to replace the liquid nitrogen tunnels
they were using," says Sundsten. "We were asked to make a
tunnel freezer with the same footprint, temperature and
capacity as the nitrogen tunnels, but which were cheaper to
use. Within a couple of years our impingement technique
replaced hundreds of these nitrogen tunnels."
When FMC exhausted the hamburger market, it turned to
seafood and the freezing of fillets. To date, it has 200
modules of impingement freezers worldwide in Holland, Canada,
Vietnam, Thailand, Indonesia and other Asian countries.
Cryogenic freezing
But while FMC's Advantec operates at minus 40 degrees F,
nitrogen works at minus 320 and carbon dioxide at minus 109
degrees F.
"You get a greater yield with cryogenic freezing, which is
why so many shrimp processors use this method," says Bill
Baker, VP of food and beverage marketing for BOC, a member of
the Linde Group, which produces and sells industrial gases
including nitrogen and carbon dioxide. "Cryogenics freezes
faster, which means moisture is retained in the cells of the
product. While Advantec offers a faster freeze than a standard
mechanical freeze, and thereby diminishes the gap, there still
is a gap," he insists.
That gap can make a significant difference - not in the
taste of the end product, but in the yield, because with the
escape of water, the weight of the product is reduced. In a
high-value product like shrimp, for example, the yield savings
helps pay for cryogenic freezing over mechanical.
Baker estimates that 50 percent of all further processed or
value-added shrimp in the United States is frozen
cryogenically. Whether that freezing process involves nitrogen
or carbon dioxide depends on the customer's proximity to a gas
source.
"Where there are a lot of
[carbon dioxide] sources, the
prices can be less expensive than nitrogen, so it depends on
your geography," says Baker. "In Thailand, for example, the
majority of processors use [carbon dioxide] for freezing,
because there is an enormous [carbon dioxide] industry close to
where the prawns are located."
Another reason for the popularity of cryogenic freezing
among seafood processors is that it is easy to transfer from
one product line to another.
"The ability to switch from product line to product line is
probably equal to the yield
advantage of cryogenics," says
Baker. "Cryogenics give processors the flexibility to run
multiple products on the same line, and the fact is that most
seafood processors have a tremendous number of SKUs and
product switches required in a single day."
While cryogenic systems cost far less to set up, the cost of
replenishing liquid nitrogen is significant and ongoing. At
Phillips Foods, Process Improvement Manager Greg Kraft says the
company receives daily shipments of nitrogen, particularly in
the high season from August through February.
"But we require liquid nitrogen for the quality aspect of
it," he insists. "At the end of the day, by using nitrogen to
freeze, you have a higher-quality product."
Cryogenic freezing systems, depending on the application,
can be as low as 20 percent of the cost of a mechanical system
to install, according to Kraft.
Typical liquid nitrogen freezers, capable of processing
about 1,000 to 1,500 pounds per hour, usually cost $75,000 to
$150,000, as compared to a mechanical tunnel freezer or spiral
freezer using a house ammonia system that can run $400,000-plus
for the same throughput, says Kraft.
Phillips Foods freezes up to 70 percent of the product
produced in its Baltimore plant using liquid nitrogen, and the
remaining 30 percent is frozen mechanically.
"We started using nitrogen when we got into crab cakes about
10 years ago," says Bobby Love, global quality control manager
for the company. "It's great for our value-added products and
portion-cut fish - products where you want to protect the
integrity of the product as much as possible."
The advantage of cryogenic freezing, he says, is that the
makeup of the end product is not changed as much as in
mechanical freezing.
"For crabmeat, we need something to instantly freeze it to
protect its size, flavor profile and texture," he explains.
"Because of the quality factor, we can't afford to waste time
with a slower, ammonia-enclosed system, which will take 90
minutes to freeze product solid, versus nitrogen's 2
minutes."
That time difference means cryogenic systems can keep
moisture loss down to almost nothing. With a slower freeze, by
comparison, moisture can be withdrawn from the product.
In deciding which freezing technique to use, processors
usually find that the central issues are quality, product type,
flexibility and cost. Cryogenic methods freeze faster, and as a
result are more flexible. They also keep moisture loss to a
minimum.
Proponents of mechanical systems argue that thanks to recent
advancements in technology, dehydration and a difference in
quality in the end product
are impossible to detect. But Baker
disagrees.
"When we do a yield test with the customer, we can prove
that there is less yield loss in a cryogenic than a mechanical
system," he says. "There is a difference between the two
systems, and though advancements have been made in mechanical
freezing, those advancements cost more and require more
sophisticated pieces of equipment to run."
The system you use is dictated largely by the product you
are freezing, Baker insists. "The smaller your product is, the
greater the dehydration."
Contributing Editor Lauren Kramer lives in British
Columbia