J. Peter Clark

Freeze drying is enjoying a recent burst of popularity due to the success of breakfast cereals containing freeze-dried berries, such as strawberries, raspberries, and blueberries. Every major cereal manufacturer either has such products or is about to introduce them. According to the Wall Street Journal of May 15, 2003, fruit-containing cereals are the most successful product innovation since sugar-added cereals in the early 1950s.

PROCESSINGHerb Aschkenasy, CEO of Oregon Freeze Dry, Albany, Ore. (phone 541-926-6001) described the recent success of freeze-dried fruit as due, in part, to better packaging. Freeze-dried strawberries were introduced in breakfast cereal in the 1960s, but they were very expensive and gradually became soggy as they picked up moisture. Improved packaging helps prevent that moisture pickup. Also, cereals in general have become more expensive, and the manufacturers are making smaller packages, so the berries have less time after a package is opened to pick up moisture.

Oregon Freeze Dry is celebrating its 40th anniversary. The company freeze dries many items, primarily materials used by others as ingredients. They are proud of their front-end capabilities—preparation, cooking, and freezingb�14;as these are often critical to a product's quality. Aschkenasy also described his line of four dozen entrees aimed at the backpacking and survival markets. Branded as Mountain House, the products are freeze dried as complete products, rather than mixed from components, because Oregon Freeze Dry believes this approach retains better flavor and rehydration characteristics.

Freeze Drying Basics
Freeze drying, or lyophilization, is a preservation process most often associated with coffee, pharmaceuticals, food for space travel, and food for backpackers. It yields a high-quality, lightweight, and easily rehydrated product that retains the original shape of the starting material, unlike conventional drying, in which shrinking and surface hardening can occur.

Freeze drying uses sublimation, the direct conversion of ice to vapor, to remove water from the starting material. Sublimation of water ice occurs below the triple point, which is about 4.5 mm Hg water vapor pressure and about 0°C. At or below these conditions, ice becomes vapor without passing through a liquid state. The solids retain their shape, and the vaporizing ice leaves pores behind. In conventional drying, the water is liquid and the solids are soft and collapse upon removal of the water, causing shrinkage. Soluble solids also can migrate to the surface with water and create impervious surface layers, or case hardening, which impedes rehydration.

Because freeze drying takes place at low temperatures, volatile flavors are retained better than in conventional drying, as is color. Freeze-dried fruits look much like the fresh item and rehydrate quickly in milk or in the mouth, releasing a surprising burst of flavor.

Freeze drying can occur at atmospheric pressure and used to be commonly observed when housewives put wet laundry on outside lines during winter. Typically, the wet clothes would eventually dry but be stiff, as if still frozen. The low humidity of a cold day could fall below the triple point, and the energy needed for sublimation would be provided by the sun.Ernest Hemingway once wrote about a freeze-dried tiger carcass found high on an African mountainside, and freeze drying is still used to preserve biological specimens and even whole animals. Freeze-dried microbial cultures can remain viable, and freeze-dried proteins are used as therapeutic and diagnostic agents.

Normally, freeze drying occurs under vacuum in an attempt to enhance mass transfer of moisture from the drying material. Often, the limiting rate is actually heat transfer. External heat transfer is usually limited by the surface temperature of the materialb�14;if this is too high, local melting may occur or there may be discoloration. As material dries, the ice front shrinks away from the surface, leaving a porous and insulating layer of dried material. This becomes the limiting resistance for internal heat transfer.

Research at the University of California at Berkeley in the late 1960s and early b�19;70s showed that the presence of gas such as nitrogen or helium in the pores of the dried layer could enhance heat transfer in some materials. Too much gas could impede water vapor transfer, so there is an optimum total pressure for freeze drying, which depends on the material but often is higher than normal practice.

The energy of sublimation is a little more than the heat of vaporization of water and can be supplied to the surface by radiation or conduction. In the low pressures usually used, convective heat transfer is not very effective.

Water must be removed by exhausting with a vacuum pump or ejector or by condensing to ice. The condenser temperature must be below the ice temperature in the drying material for water to move from the material. Noncondensable gases are removed by vacuum pump or ejector.

Equipment Innovations
One of the major suppliers of commercial freeze drying equipment is Atlas, now a major division of Niro A/S, Copenhagen, Denmark, having been purchased from Atlas-Stord in late 2001, according to Doug Thrasher, Sales Engineer in the United States (phone 410-997-8700). Atlas offers both Ray batch and Conrad continuous freeze dryers with several novel features.

All of the companyb�19;s dryers use radiant heating from hollow metal platens heated with hot water. By not having direct contact with the heating surface, Atlas provides more-uniform heating of the product, compared to trays sitting directly on platens. Trays almost always have irregularities, which makes direct contact nonuniform; thus, the temperature of the platen must be limited to prevent melting or scorching. All Ray batch freeze dryers utilize trolleys to insert and remove trays of product from the cabinet (see photo). This minimizes product handling and equipment cleanup labor.

The larger batch dryers feature a Continuous De Icing (CDI) system, which utilizes two condensers, cooled by ammonia refrigerant, within the cabinet. One condenser is in communication with the product, while the second is closed off and slightly warmed to melt the accumulated ice. The melting condenser is only slightly elevated in pressure over the operating chamber. The low pressure is regenerated by cooling the condenser prior to its being put back into service.

A similar system called Dry Condensing can be used in edible oil refining to condense evolved steam and free fatty acids to a more-concentrated waste stream than that produced by steam stripping with steam eductors.

Atlas also offers continuous freeze-drying systems in which airlocks are used to introduce and remove trays of material. The trays are pushed along tracks between heated platens. The CDI system is also used. Most of the 40 Conrad units around the world (none in North America) are used to freeze dry coffee. Concentrated coffee is prepared, frozen, granulated, sorted by size, and then placed onto trays for drying. Capacity is quoted in water removal capability and ranges from 3,300 to 16,000 kg/24 hr. Corresponding costs are $4b�13;10 million, excluding the refrigeration system.

Other Freeze-Dried Materials
Van Drunen Farms, Momence, Ill., is a major supplier of freeze-dried herbs, vegetables, and fruits. According to Kevin Van Drunen, General Manager (phone 815-472-3100), the family farm still grows chives, parsley, and basil, which were once sent to other firms for freeze drying. Now the company has almost 50 freeze-drying chambers, plus several drum dryers, and performs sugar-infusion (osmotic) drying of fruits.

In addition to the herbs they grow, they purchase frozen fruits and vegetables for drying and sale to the baking, cereal, and confectionery industries. Their spices and herbs are repackaged in retail and foodservice sizes.

A recent development is high-ORAC blends for health foods. ORAC, an acronym for oxygen radical absorbance capacity, refers to the ability of foods or nutrients to protect the human body against free radical damage and to the antioxidant power of foods such as fruits and vegetables.

Another pioneer in the venerable industry of freeze drying is Ed Hirschberg, president of Innovative Foods, South San Francisco, Calif. (phone 650-871-8912). He has been involved in freeze drying for nearly 40 years and at one time owned the largest number of chambers in the country. Now, he has sold or scrapped most of them and spends his time developing new food concepts that draw on his past experience. One example is freeze-dried salami as a crisp snack. Another is infused nutraceuticals in fruits and vegetables.

Hirschberg was involved in making compressed freeze-dried vegetables and fruits when these were components of military rations. In this process, compressed products are dried, then slightly moistened to make them flexible, compressed to reduce density, and dried again. It is not an inexpensive process. He also owned a novel continuous freeze dryer, developed by a coffee company, which had a chamber 13 ft in diameter and 70 ft long. It used a vibratory conveyer to move coffee granules under radiant heaters. Unfortunately, it had a tendency to make coffee powder that was indistinguishable from that made by spray drying at much less cost, so it was scrapped.

Domestic freeze dryers face competition from international sources, as do many other food segments. Because many freeze dryers are batch, there is labor involved in loading and unloading, as well as in preparation and packaging. Regions with low labor cost thus have some advantage. On the other hand, freeze-dried items have low bulk density and so incur high shipping costs. Also, as Aschkenasy points out, precisely because freeze drying is such a good preservation process it is best to use the highest-quality raw material possible. Freeze-dried materials are very hygroscopic because of their low water content and must be packaged with care.

While a stable and mature process, freeze drying is receiving more attention as both food and nonfood applications are developed.

Contributing Editor
Consultant to the Process Industries
Oak Park, Ill.