Freeze-drying remains one of the most gentle methods of drying various perishable food products. It is also known as lyophilization or cryodessication. Freeze drying is an excellent alternative to evaporation and reverse osmosis for the concentration of many liquid food products. Since low temperatures are used and no vapour-liquid interface occurs, therefore high-quality products are produced. It is a typical dehydration process of freezing the material, reducing the surrounding pressure and then adding enough heat to allow the frozen water in that material to sublime it directly. The principle of freeze-drying is sublimation i.e. direct transition of a substance from the solid-state to the gaseous state. Initially, the product is frozen and then dried by sublimation by applying reduced pressure. The low pressure enables direct changeover of the frozen solvent into a vapor.
Product stability is highly increased as the water content is reduced which is related to microbiological and chemical activities, which are the prime reason for product degradation. Compared to other drying methods, freeze-drying causes less damage to the product and also avoids shrinkage of the material. Common preservation methods use high temperatures that directly affect the shape and colour of the product, followed by its flavour, smell and nutritional profile in the other hand, freeze-drying is a very gentle process which is particularly suitable for highly heat-sensitive and precious products.
Figure (a) presents common preservation techniques which damage structural changes, loss of colour and sensory profile as well as nutritional profile (change in proteins). Figure (b) represents freeze-drying, ice directly transitions into water vapour by sublimation allowing to maintain product original texture and nutritional value. Mainly, it is used for preserving superior quality heat sensitive food products like beer, wine, fruit juices, vinegar, proteins, enzymes, microbes, blood plasma, product conditioning for further use (e.g. fruits in yoghurt) etc. It is extensively applied for food products where the aroma is to be protected and there is a need for ensuring high quality of food products. However, more commonly this technique is used in the pharmaceutical industry. Further, it is used to make lightweight food (like ‘freeze-dried ice cream’, ‘astronaut food’, etc) so that astronauts and hikers can easily reconstitute it with available water.
Advantages of Freeze-Drying:
The advantages of freeze-dried products are aplenty. Freeze drying generates very high quality finished products along with financial as well as operational advantages by minimal product loss, reduce energy costs and ease of usage. It works over a wide range of food and dairy products such as fruits, vegetables, fish, meat, coffee, flavour essence, milk, cream, cheese, yoghurt, ice cream and many more. By applying this technique original flavour, colour, texture, proteins and vitamins are retained. This technology results in durable rapidly rehydrated stable and long-life products. Further, no cold chain distribution required for lyophilised products. It also reduces shipping costs (as lightweight products are produced).
Process of Freeze Drying:
The process of freeze drying mainly comprises freezing, primary drying and secondary drying.
As the name indicates this process involves the operation of freezing. Most liquid products, or formulations, freeze by the formation of ice crystals. Size and shape of the ice crystals depend on the cooling speed, rapid cooling results in the formation of small ice crystals while slower cooling leads to larger ice crystals. In the lab (at small scale), it is done by placing the material in a freeze-drying flask and rotating the flask shell freezer, which is cooled by mechanical refrigeration, dry ice and methanol, or liquid nitrogen on the other hand, in industries freezing is usually done by using a freeze-drying machine. In this step, the material is cooled to the lowest temperature at which the solid and liquid phases of the material can coexist. However, the freezing temperature of this formulation highly depends on characteristics and composition. There are two ways to freeze formulations.
Eutectic mixtures contain substances that freeze at low temperature than water surrounding them. During cooling such mixtures, water is the first to separate from the substances and freezes to ice. This formulation may appear to freeze completely but still remaining substances are liquid. They form concentrated areas that freeze eventually at temperatures below the freezing point of water. The temperature where all components of the mixture are properly frozen is called eutectic temperature. This is the critical temperature of the formulation and the maximum temperature the formulation can endure during the freeze-drying process.
Applying a vacuum to an incompletely frozen eutectic mixture may result in the destruction of the product as unfrozen components expand when placed under vacuum. In case of amorphous or glassy mixtures, with a decrease in temperature, the mix becomes more and more viscous and eventually freezes to a vitreous solid at the glass transition point. For amorphous products, the critical temperature in terms of stability is called collapse temperature. The collapse temperature is typically slightly lower than the glass transition point. Amorphous products are very challenging to freeze dry.
2. Primary Drying
During primary drying, pressure is kept low (few milibars). This phase removes the bulk of water from the product. Enough heat is supplied to material in order to cause sublimation, this heat can be calculated using molecules latent heat of sublimation. At this level heat input must be controlled. The ideal product temperature should be as high as possible so as to maximize the vapor pressure difference (to break interactions between frozen material and water molecules) and at the same time below the product’s critical temperature to preserve the frozen character. By using heated shelves, the set temperature is slowly approached at the defined heating rate. Heat is supplied via conduction or radiation. Simultaneous measurement of the actual product temperature allows for end point detection of primary drying. In this phase, 95% of water is removed by sublimation so partial vacuum is kept. This process is comparatively slow. Product is exposed to less heat intensity so that it does not suffer any deformations in its texture.
3. Secondary Drying
The most of the water content is removed in first phase. There exists 5-10% residual moisture in the product which is due to water bound to the matrix. At this stage, ice should be remove completely. The secondary drying step removes the adsorbed water molecules by desorption. This process is governed by material adsorption isotherm. In order to achieve ideal conditions for desorption, the lowest possible pressure as well as a further increase of the shelf temperature is required. Again, product stability must be considered when choosing the shelf temperature. Secondary drying is usually performed for shorter time periods. After completion, vaccum is broken by addition of inert gas like N2 before packaging material. The final moisture should range between 1-4%.
Freeze-drying is a relatively expensive process compared to other preservation processes. The equipment is about three times as expensive as the equipment used for other separation processes, and the high energy demands lead to high energy costs. Furthermore, freeze-drying also has a long process time, because the addition of too much heat to the material can cause melting or structural deformations. Commercially, freeze concentration technology is not economically competitive with more established alternatives. Freeze concentration is applied only where the focus is on aroma retention and high-quality precious products. It is especially suited for heat sensitive products.
Student, Maharana Pratap University of Agriculture and Technology
Ever heard of cryogenic freezing? Ice-cream made using this technology is slowly becoming a rage. To read more, you can check out our article on it.