FREEZE-DRYING
Freeze drying is a relatively recent advancement in the food preservation world. Frreze-drying freezes the food, then it removes most of the moisture in a vacuum chamber, before finally sealing the food in airtight conditions. Freeze-dried foods can be easily transported at normal temperatures, stored for a long period of time, and consumed with a minimum of preparation. Once prepared, freeze-dried foods retain the same look and taste as the original, natural products.
There are a number of steps involved in the process of Freeze-Drying. They are as follows.
1- The food is first checked for contamination to ensure that it is safe to consume.
2- Some kinds of food, like seafood and meats, must be cooked before freeze drying however this is not the case for all products.
3- The food pieces are spread out on flat, metal trays which are stacked in slots in a wheeled cart. Some foods have different needs before they can be frozen, for example, for food that has been pre-cooked and frozen, the trays are pre-chilled to prevent partial thawing during handling. With liquids like coffee, the pre-brewed coffee is poured into shallow pans. The carts are wheeled into a large, walk-in cold-room where the temperature can be as low as -40°C. In this extremely cold temperature, the food is quickly frozen.
4- The carts are wheeled out of the cold-room and into a vacuum drying chamber. Sometimes before this step can commence the food is first ground up into small particles in a low-temperature grinder, or two products are combined together. When the trays of frozen food pieces are inside, the chamber is closed and sealed.
There are a number of steps involved in the process of Freeze-Drying. They are as follows.
1- The food is first checked for contamination to ensure that it is safe to consume.
2- Some kinds of food, like seafood and meats, must be cooked before freeze drying however this is not the case for all products.
3- The food pieces are spread out on flat, metal trays which are stacked in slots in a wheeled cart. Some foods have different needs before they can be frozen, for example, for food that has been pre-cooked and frozen, the trays are pre-chilled to prevent partial thawing during handling. With liquids like coffee, the pre-brewed coffee is poured into shallow pans. The carts are wheeled into a large, walk-in cold-room where the temperature can be as low as -40°C. In this extremely cold temperature, the food is quickly frozen.
4- The carts are wheeled out of the cold-room and into a vacuum drying chamber. Sometimes before this step can commence the food is first ground up into small particles in a low-temperature grinder, or two products are combined together. When the trays of frozen food pieces are inside, the chamber is closed and sealed.
5- The drying procedure involves a process known as sublimation. In sublimation, a solid material is forced to change state into a gas without ever becoming a liquid. In the drying chamber, the air is evacuated with a vacuum pump to reduce the pressure to about 0.036 psi. The temperature of the food is raised to about 38°C by such methods as direct conduction through the bottom of the trays, radiation from heat lamps, or microwave heating. When the chamber is evacuated of air, the pressure is below the point where water can simultaneously exist in a solid, liquid, and gas state. This is known as the triple point of water.
6- The dried products are removed from the drying chamber and tested for moisture content and purity to make sure the process has been effective.
7- Some food pieces may be ground to a smaller size or may be reduced to a powder. Others may be screened to separate them by size. Two or more different products may also be blended together.
8- Freeze-dried foods must be sealed in airtight containers to prevent them from absorbing moisture from the air. Several types of containers may be used: plastic laminated foil pouches, cans, or drums for bulk packaging. Some freeze-dried food is vacuum packed, in which the air is evacuated from the container before sealing. The packaging is done in the freeze-dry plant almost as soon as the foods come out of the drying chamber so they do not absorb moisture..
7- Some food pieces may be ground to a smaller size or may be reduced to a powder. Others may be screened to separate them by size. Two or more different products may also be blended together.
8- Freeze-dried foods must be sealed in airtight containers to prevent them from absorbing moisture from the air. Several types of containers may be used: plastic laminated foil pouches, cans, or drums for bulk packaging. Some freeze-dried food is vacuum packed, in which the air is evacuated from the container before sealing. The packaging is done in the freeze-dry plant almost as soon as the foods come out of the drying chamber so they do not absorb moisture..