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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

 

 

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