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In order to understand the concept of freeze-drying, we need to understand the meaning of the word sublimation. Sublimation is the process of changing the state of matter, directly from solid to gas, without going through an intermediate liquid. Under atmospheric pressure, most chemical compounds and elements exist in 3 different states at different temperatures.
In these cases, the transition from solid to gas requires an intermediate liquid state. However, the pressure mentioned here is the partial pressure of the matter and not the total pressure of the system. All solids with a measurable vapor pressure at a given temperature can usually sublimate in air (like water ice below 0°C).
Therefore, we can understand the process of sublimation-drying, also known as freeze-drying, as the process of separating water from raw materials in a freezing state and low vacuum (at the state below the three-phase point of water at point O).
Here, the water, in its solid state, will convert into vapor withouth going throug the liquid phase
SUMMARY OF THE FREEZE-DRYING PROCESS
For machines with separate freezing and drying systems, fresh food is quickly frozen at very low temperatures (-30 to -50 degrees Celsius), then put into a vacuum chamber. Most of Bach Ma group’s sublimation dryers have a combined drying and freezing chamber, so the material only needs to be put into the drying chamber where both processes occur.
2/ Freeze-drying takes place in two stages. Stage 1 is the primary drying stage. In the vacuum-drying environment, the ice crystals in the material will sublimate without going through the liquefaction stage. About 90% of the moisture in the material is removed.
Then comes the secondary drying stage. In the secondary drying stage, to evaporate the residual moisture, the temperature is adjusted gradually (possinly > 0 degrees Celsius). The moisture content left in the raw materials is about 1~4%.
3/ Freeze-dried food is vacuum-packed, preventing moisture & oxidation.
4/ When freeze-dried food is processed and soaked in water, it will return to its original state of taste, nutritional value, structure & shape.
ADVANTAGES OF THE FREEZE-DRYING TECHNOLOGY
• The freeze-drying has created products with good quality. Nutritional components (proteins, lipids, carbohydrates), vitamins, enzymes and biologically active ingredients, colors, odors, tastes, etc., are almost entirely preserved.
• Especially, the products after drying have a soft porosity, when soaked in water, they swell up and return to almost the same state as the original material.
• After drying, the products are put into a bag and then vacuum-sealed, which enable them to be stored at room temperature, have a long shelf life, low storage costs, and little change in quality.
• The moisture content after the freeze-drying process is very low, about 1-4%, which helps preserve the food for a long time without the use of preservatives (can be up to 20 years).
Freeze-dryers have many sizes suitable for every production scale of small or large units.
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