Thawing Out the Details About Frozen Food

By Mark Crowell, CRC


In 1954, Gerald Thomas, of C.A. Swanson & Sons, was trying to figure out what to do with 270 tons of leftover Thanksgiving turkey. Out of this daunting dilemma came something that would change America forever: the TV dinner. Paired with mashed potatoes, cornbread and carrots, all in a box that looked like a TV, it ushered in a new era of convenience.

From its heady growth years, today’s conventional frozen meal market has become a mature, slow-growth behemoth with sales of $7.9 billion, according to Mintel’s 2004 “Frozen Foods” report. This segment still offers great convenience but is increasingly pressed to compete with fresh, exciting offerings on the store perimeter. The exception to this trend is natural and organic foods.

The Hot Market of Frozen Foods
In contrast to conventional, the organic and natural frozen food market is nowhere near cooling off. According to Nutrition Business Journal, sales of natural and organic frozen entrees, including dinners, pies, pizzas, burritos and other prepared frozen items have grown 10 to 14 percent annually since 1997.

And there is still plenty of room in the freezer! While there are many conventional companies competing in the frozen food section, the organic market still has quite a bit of room to grow. The organic share of the frozen entrée market is still less than five percent.

Organic sales today are dominated by one company, Amy’s Kitchen, which holds a 38 percent share of the organic market, and according to Information Resources Inc., grew their frozen entrée business 17 percent last year. And this growth will continue. In October they opened a brand new 176,000- square-foot plant in White City, OR. The plant has four frozen pizza lines and will be able to produce 4.5 million cases of product annually. An entrée line will begin production in January.

Other leaders include General Mills’ Cascadian Farms, which does the second largest volume in frozen prepared organic meals, Cedarlane Natural Foods and Fairfield Farm Kitchens, producers of the Organic Classics line of “comfort foods,” as well as vegetarian entrees for the world-renowned Moosewood Restaurant in New York.

Organic consumers desire the same convenience long enjoyed by non-organic frozen meal consumers. By combining astute equipment selection and proper operation, organic frozen food processors will ensure both high quality products and convenience for their customers.

Avoiding Freezer Burn
Organic may taste better when it comes out of the field, but what about on the fork? In a recent competitive study of frozen Indian-style entrées, CuliNex, a product development consultancy, evaluated 28 meals from eight manufacturers to assess product quality in the category. More than half of the products suffered from defects related to improper freezing: broken sauces, watery rice, tough meat. Due to the interaction of multiple ingredients, frozen entrees present one of the most complex challenges in frozen food manufacturing. How can these problems be avoided? By starting with an understanding of what can go wrong and why.

The first area to look at is formulation. The use of functional ingredients like starches and gums are almost always necessary to ensure the stability of frozen meals, particularly the sauce or gravy component. Organic options in this area are getting better all the time, but choices are still limited compared to conventional.

When it comes to frozen meals, great ingredients are certainly necessary, but they won’t guarantee a high quality product. For that you also need great processing. The first thing to understand is the importance of water. Our bodies are composed of 60 percent water. Raw meat is about 75 percent water. Fruits and vegetables are up to 95 percent water. Understanding and controlling water is the key to maintaining product quality in frozen foods.

Freeze it Cold. When we freeze food, it doesn’t freeze uniformly. It happens gradually with the formation of the first ice crystals nearest the freezing source. Those first ice crystals are pure water. As water continues to freeze out of the product, the remaining constituents that make up the food, things like minerals, vitamins, and proteins, become more concentrated in the water that remains. This concentration of dissolved solids serves to lower the freezing point of the water that is left, giving the process its name, the concentration effect.

Eventually, if the temperature is low enough, the entire product will freeze, usually by the time it reaches 0°F. Leaving any water unfrozen can be problematic because water that contains high concentrations of dissolved food solutes is prone to chemical reaction causing a loss of product quality. Examples include solutes precipitating out of solution (the cause of gritty ice cream), dehydration of adjacent tissues in meat (loss of texture) and destruction of colloidal suspensions (as in the separation of cream).

Freeze it Fast. Solid food is made up of cells. Within and between those cells is water. When water freezes rapidly it forms minute ice crystals—but when it freezes slowly, large ice crystals are formed that can rupture and separate cells. Generally, the faster a product can be frozen, the better. Fast freezing can also reduce the impact of the concentration effect by minimizing the time dissolved food solutes are in contact with one another. Freezing products to a lower temperature than they will eventually be held at is another way to ensure no unfrozen water remains in the product.

Keep it There. The industry standard of holding temperatures between -10°F and 0°F is a compromise between quality and cost. Shelf life decreases rapidly above 0°F. Raw beef has a shelf life of 13 to 14 months at 0°F but only 5 months at 10°F. Straw-berries have a shelf life of 12 months at 0°F but only 2.4 months at 10°F.

Maintaining storage temperatures also makes a huge difference in quality. Fluctuations as small as three degrees can be damaging over a period of time as concentration effects are intensified and ice crystals grow larger.

Microbiologically, 0°F is not strictly required for food safety and food spoilage organisms do not grow below about 14°F. Even at 0°F food enzymes as well as non-enzymatic chemical reactions that cause discoloration and off-flavors are slowed, but not stopped. Fruit is almost always treated with ascorbic acid and vegetables are blanched before freezing to inactivate enzymes and kill surface bacteria.

Processors should also note that while bacteria will not grow while food is frozen, some of it may survive the freezing process. Thus, care should be taken to ensure that bacteria is killed and the product is not recontaminated before it’s frozen. Labeling instruction should clearly state the importance of proper heating.

One Goal, Many Paths
For the best quality the goal is the same—get it really cold as fast a possible and keep it there. How to do that in the most cost-effective manner, however, is the key decision for every plant operator producing frozen food. While the equipment choices are many, there are only three basic methods of commercial freezing: freeze it in air, freeze it by indirect contact with the refrigerant, or freeze it by direct immersion in a refrigerating medium.

Air Freezing. This is the oldest and least expensive method from an equipment standpoint. Sometimes known as “sharp” freezing, food is simply placed in an insulated cold room, usually kept between -10°F and -20°F. Air moves by natural convection inside the unit, sometimes aided by fans.

The next step up in cost and effectiveness are blast freezers which operate with forced air velocities of 2000 to 3000 ft/min at -20°F to -50°F and can reduce freezing time by 75 to 85 percent. Blast freezers come in many designs, from rooms where food is frozen in batches, to continuous feed tunnels with multiple conveyor systems. Food may also be conveyed vertically by means of a spiral belt.

To illustrate the different uses, John Weaver, purchasing manager, relates that Fairfield Farm Kitchens formerly used a 20’ x 40’ blast freezer to cool small runs of soups and sauces. “The blast freezer ran 24/7 and it was very expensive and inefficient to operate. We saved about $40,000 a year by eliminating the blast freezer and using a chill tank and storage freezer instead,” he said. “The chill tank rapidly brings these bulk products down to 40°F. From there we move products directly to our 35,000-sq.-ft. holding freezer where it freezes solid. We tested both methods and found there was no loss of product quality.”

Their spiral blast freezer is better put to use with entrees where it helps minimize handling, taking the product from the deposit line all the way to cold storage. The -20°F temperature freezes entrees in about 90 minutes, helping preserve the quality in these more delicate products. The freezer can easily handle 20,000 to 50,000 units per day and can be shut down on days it’s not needed, Weaver notes.

The fluidized-bed freezer is a specialized air freezing method used to produce individually quick frozen (IFQ) products such as seafood, vegetables and fruit. The food product rests directly on a mesh belt which conveys it through the freezer. Chilled air is blown up through the belt at a velocity just great enough to lift the product up and separate it. The product freezes quickly in air while flowing along the belt like a fluid. Because food that is directly exposed to chilled air is rapidly dehydrated, pre-chilling or glazing is often used to form a protective surface crust on the product prior to it’s entering the fluidized bed.

Indirect Contact Freezing. This method consists of a number of metal shelves or plates through which refrigerant flows but does not come into contact with the food. The food, usually in a flat package, is placed between refrigerated shelves and pressure is applied to squeeze the shelves into contact with the packages. A typical 1” to 2” thick package is frozen within one to two hours. Liquid foods and purees are generally frozen in a double walled, tubular, scraped-surface heat exchanger, in which refrigerant runs between the walls while the liquid food is pumped through the inside of the inner tube containing a rotating shaft. Food is forced to pass through the thin space between the rotating shaft and the refrigerated wall of the vessel. It’s followed by scraper blades attached to the rotating shaft that shave away the frozen food from the inside of the tube.

Immersion Freezing. This places the product or package directly into contact with the refrigerant. The refrigerants are of two broad classes: low-freezing-point liquids, which are chilled by indirect contact with another refrigerant; and cryogenic liquids, such as liquefied nitrogen or carbon dioxide.

Sojo Food’s 25,000-sq.-ft. plant near Seattle, WA uses a small two compartment nitrogen freezer; one side to pre-chill product, the other side to freeze it. The unit can freeze 500 to 600 lbs. of product per hour and operates at -60°F. Liquid nitrogen, approved for use in organics, is sprayed into the freeze side of the unit and then circulated over to the pre-chill side to maximize use of the gas. Each rack spends 15 to 25 minutes on the pre-chill side, then a similar amount of time on the freeze side.

“We defined these times by trial and error for each product,” said Joe Ertman, president. “Since the use of liquid nitrogen is expensive, it doesn’t pay to over-cool the product before transferring it to the holding freezer.”

For maximum freezing effect, nitrogen should contact the food surface as liquid droplets. If it contacts the surface as a gas it loses more than half of its cooling effect. Spent gas is still cold and is usually vented to contact and pre-chill incoming food, as in the two-chamber unit at Sojo Foods.

Sojo also has a larger nitrogen tunnel freezer, which can handle up to 2000 lbs. per hour. Liquid nitrogen vapor pre-chills the product to about 0°F and when the product passes directly under the spray the surface is chilled to about -185°F. It then moves into another zone to bring the product to about -50°F. Liquid CO2 works in a similar fashion but is normally sprayed throughout the length of the tunnel and contacts the product as dry ice snow at -79°F.

At Sojo, Ertman said he chose the smaller nitrogen unit for two reasons, first, because of its lower initial cost. This unit was $15,000 to $20,000 and a comparable ammonia refrigerant system would have been about $100,000. However, some of that savings is offset by the higher operating costs of using nitrogen. It costs $.08 to $.10 per lb. while an ammonia or CO2 system would have only cost $.03 to $.05 per lb. “Also we get superior product quality with this system,” Ertman said. “All of our products are frozen prior to packaging and nitrogen minimizes dehydration since it is the fastest freezing method.”

Mark Crowell, CRC, is the founder and principal culinologist at CuliNex, a consultancy specializing in the development of organic and natural food products. Mark is the former director of product development for Olive Garden Restaurants and Starbucks. CuliNex assists manufacturers, ingredient suppliers and foodservice operators by developing and bringing products to market. Mark can be reached at mcrowell1000@msn.com