Freeze dried

Freeze-drying is a complex process used to preserve perishable materials, to make them more convenient for transport. Freeze-drying works by freezing the material, then reducing the pressure and adding heat to allow the frozen water in the material to sublimate.

Process

The freeze-drying process consists of three stages: pre-freezing, primary drying, and secondary drying.

Pre-freezing

The material is cooled below its triple point, the lowest temperature at which the solid and liquid phases of the material can coexist. This ensures that sublimation rather than melting will occur in the following stages.

Primary drying

During the primary drying phase, the pressure is lowered (to the point of a vacuum) and enough heat is supplied to the material for the water to sublimate. The amount of heat necessary can be calculated using the sublimation curve.

Secondary drying

The secondary drying phase aims to remove any unfrozen water molecules, since the ice was removed in the primary drying phase. This part of the freeze-drying process is governed by the material's adsorption isotherm. In this stage, the temperature is raised higher than in the primary drying phase, and can even be above 0 °C, to break any physico-chemical interactions that have formed between the water molecules and the frozen material.

Applications

Freeze-drying is used in a variety of sectors, from pharmaceutical industries to food processing. In the pharmaceutical industry, it is used to dry pharmaceutical products like antibiotics and vaccines, as well as other biological materials. In the food industry, it is used to dry foods like fruits, vegetables, and meats to preserve them, increase their shelf life, and make them lighter and more convenient for transport.

Advantages and Disadvantages

Freeze-drying has many advantages over other drying methods. It maintains the nutritional content of the food, as well as its color, shape, and size. It also results in a lightweight, easy-to-store product. However, it is a complex process that requires a deep understanding of the material being dried and its physical and chemical characteristics. It also requires precise control of the process conditions.