The long history of fruit drying has taken on a new dimension with modern technologies. Freeze drying (lyophilization) technology, which overcomes the limitations of traditional methods and better preserves nutrients and flavors, has become the preferred method for producers in recent years. This method works by first freezing the fruit and then allowing the water to evaporate under low temperature and vacuum. The result is dried fruits that are high in nutrients, retain their color and texture, and have a long shelf life. This process, also known as lyophilization, occurs through sublimation, where water is transformed directly from solid (ice) to gas.
To fully understand the freeze-drying process, which also involves the use of dehumidifiers, it is essential to know the relationship between temperature (T) and pressure (P). The connection between reducing air pressure under vacuum and the sublimation of water (solid ice to vapor) is key to this process. The stages in a freeze dryer application are as follows:
Freezing: In the first step, the fruit or other material is frozen to a sufficiently low temperature to crystallize the water. This stage allows the water to turn into a solid state (ice).
Primary Drying (Sublimation): The frozen product is placed under vacuum. The vacuum keeps the pressure low enough to prevent the ice from melting. Then, heat is applied to encourage the ice to turn directly into gas (sublimation). In this stage, the solid water (ice) evaporates directly into vapor without passing through the liquid phase.
Secondary Drying: In the final stage, any remaining bound water (that did not freeze) is removed. The temperature is further increased, and the vacuum level is adjusted to remove most of the water molecules in the product.
After the process, the products are held in rooms with controlled temperature and humidity before packaging. During this stage, the products are very sensitive to moisture. The areas where the products are temporarily stored before packaging must be controlled for humidity using dehumidifiers. Freeze-dried products become very light and crisp because most of the water is removed during this process. This crisp texture is due to the high porosity of the products. Because of their porous structure, lyophilized products tend to absorb moisture from their surroundings easily. Moisture can affect the products' crisp and dry texture, causing them to soften and lose their original texture. Without the use of dehumidifiers, high humidity levels can create a suitable environment for the growth of microorganisms such as bacteria and mold. Lyophilized foods are not conducive to microbial growth due to their low moisture content; however, as moisture levels increase, these products become more susceptible to spoilage. Therefore, when storing finished freeze-dried products, low-humidity, cool, and dry environments achieved with dehumidifiers are preferred.
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