Freeze-drying technology, a high-tech method to dry juice

Freeze-drying is a very gentle dehydration process used for the preservation of high-quality foods. It was invented by Jacques-Arsene d’Arsonval at the College de France in Paris in 1906. Later, during World War II, it was widely implemented to preserve blood serum. Since then freeze-drying has become one of the most important processes for the preservation of heat-sensitive biological materials. During the 1950s, industrial freeze-drying of foods began. Freeze-drying is currently used as a preservation method for foods, pharmaceuticals, and a wide range of other products.

Lyophilization is the technical term for freeze-drying. Freeze-drying is the process by which the solvent (usually water) and/or suspension medium are crystallized at low temperatures and removed by sublimation. Sublimation is the direct transition of water from a solid state to a gaseous state without melting.

There are three stages in the process: freezing, sublimation drying, and desorption drying. The freezing phase is the most critical in the entire freeze-drying process. It is important to freeze foods rapidly to avoid the formation of large ice crystals, which deteriorate the final product quality. During the primary drying phase, the pressure is lowered through the application of a high vacuum, and heat is applied to provide the energy needed for the ice to sublime. This initial drying phase removes about 95% of the water present in the food. This slow step can take anywhere from several hours to two days. The application of too much heat during this phase could result in a loss in final product quality.

Another component of the primary drying process is a condensation of the sublimed water vapour. Sublimation accounts for about 45% of the total energy consumption for the process, while application of vacuum and condensation each represent about 25% of the total energy consumption (Ratti 2001). A secondary drying phase is then needed to remove unfrozen water molecules remaining after primary drying. During this phase, the temperature is raised higher than in the primary drying phase to vaporize the water molecules. Pressure is frequently lowered during this phase, but not in all cases.

The final freeze-dried food typically contains between 1% and 4% moisture. Final food products are nitrogen-sealed and packed in polybags or cans. They can be stored between 6 months and 3 years in polybags and 25 years or longer in cans.

Freeze-dried foods possess superior quality as compared with food dehydrated by other methods. The high quality is due to the absence of a liquid phase, as well as the low temperature of the process. Freeze-drying preserves flavour, colour, and appearance while minimizing thermal damage to heat-sensitive nutrients. In addition, the texture is well preserved due to the process occurring in a solid state. Freeze-dried products are typically crisper and have rehydration ratios four to six times higher than conventional air-dried foods. Other key benefits of freeze-drying include the high recovery of volatiles, retention of structure and surface area, high yield, long shelf life, and reduced weight for storage, shipping, and handling.

The main disadvantage of freeze-drying is cost, due to its high energy consumption and high costs for both operation and maintenance. The energy required for freeze-drying is almost double that required for conventional air drying. In addition, the cost of freeze-drying is four to eight times higher than conventional air drying (Fink 1977). As a result, the process is only economically feasible for high-value food products.

Some foods freeze-dry very well, but not all food is suitable for freeze-drying. Freeze-drying is commonly used for coffee, fruit, juice, vegetables, herbs, food flavourings, fish, seafood, eggs, and dairy. Individual food types can be freeze-dried, as can dishes with multiple ingredients, such as stews or soups. As a result, freeze-dried foods can be used for backpacking, camping, military rations, survival food storage, and space exploration. Generally, larger-sized foods (such as fruit or meat) need to be cut into smaller pieces prior to freeze-drying. Thermally sensitive materials, such as fruits, are good candidates for freeze-drying. These have become popular as snacks and ingredients in other foods, such as breakfast cereals. Meat and seafood require cooking before freeze-drying. For some foods, freeze-drying is not cost-effective.

1. Flink, J. M. 1977. “Energy Analysis in Dehydration Processes.” Food Technol. 31(3): 76–83.
2. Ratti, C. J. 2001. “Hot air and freeze-drying of high-value foods: a review.” Food Eng. 49(4): 311–319.
3. Wikipedia, Freeze drying, <https://en.wikipedia.org/wiki/Freeze_drying>.