Ever wondered how your instant coffee or tea lasts as long it does after you purchase it? Don't overthink it because your answer lies in a mind-blowing preservation technique known as freeze-drying or lyophilization which is the cooler term. Buckle up as we dive into what exactly freeze-drying entails.
Freeze-drying (lyophilization) is a drying process in which the solvent which is usually water is crystallized at a low temperature and thereafter sublimated from the solid state directly into the vapor phase. It is like magically removing all the water from your food while keeping its shape, flavor, and nutrients intact, minus the magic and plus a whole lot of cool science. This explains why among the dehydration methods, freeze-drying is the ideal method for the maintenance of the desirable functional and palatability characteristics (Harper and Tappel, 1957). Freeze-drying is a process that occurs in three stages and the stages include the freezing stage, the primary drying stage and the secondary drying stage.
Stages in freeze-drying process
In the freezing stage, the food product to be processed is cooled down to a temperature at which all the material is in a frozen state. Larger ice crystals are produced by slower freezing rates and vice versa. Because larger ice crystals are easier to sublimate and so speed up primary drying, the size of the crystals affects the rate of drying (State and State, 2023). The frozen solvent is eliminated during the primary drying stage by sublimation, which requires that the system pressure at which the product is dried be less than or close to the frozen solvent's equilibrium vapor pressure.
During the primary drying stage, the temperature of the frozen layer is most often at 2mmHg or less because water usually exists in a combined state and the food product must be cooled below 0°C to keep the water in the frozen state. When the ice sublimates, the interface that initially occurred at the exterior surface moves inward, leaving behind a porous layer of dried material. During the primary drying stage, some of the nonfrozen water in the dried layer may be desorbed.
Removal of the water that did not freeze is part of the secondary drying stage. When the primary drying stage comes to an end, the material that has been dried's pores allow the desorbed water vapor to be transferred during the secondary drying step (Duan et al., 2016). For maximum desorption efficiency, this last step might be performed at a temperature over the shelf but below the glass transition temperature of dry materials. This would remove any remaining unfrozen or bound water.
Factors that affect the freeze-drying process of food
1. Freezing rate and temperature: The initial freezing step affects the final quality of the product and should be chosen based on the type of food being frozen. Rapid freezing leads to better preservation of the food's structure but is more energy intensive. Slow freezing produces larger ice crystals which speeds up the primary drying process but can destroy the cell structure of the food. With regards to temperature, using the wrong temperature for the operation can cause unintentional structural changes in the food product. Increasing the temperature of the heating plate during the freeze-drying process reduces costs but could lead to product deterioration (Duan et al., 2016). The balance between the input and the necessary heat should be taken into consideration when choosing an appropriate shelf temperature.
2. The composition of the food affects how it behaves during freeze-drying: Foods with high moisture content generally take longer to freeze-dry and those with high sugar concentrations can lower the freezing point and affect drying rate.
3. The thickness, surface area and uniformity of the food product affects the efficiency of the freeze-drying process: Food products with thicker pieces and small surface areas take longer to dry and reduce the efficiency of the freeze-drying process.
4. The working pressure of the pressure chamber or drying chamber affects freeze-drying process: A low working pressure might lead to the development of a dry product with a texture that is more fracture-resistant and can withstand mechanical stress.
Applications of freeze drying to food
1. In the meat industry, freeze-drying is being utilized for beef, chicken and pork due to the long-term storage stability and the ability to not cause physical and chemical changes in these products. Some food products undergo changes in color and flavor as result of heating altering the nature of the food.
2. Freeze-drying is used in the fruits industry because of the storage stability of freeze-dried fruits.
3. Eggs are products that enjoy the benefits of freeze-drying since freeze-dried eggs have a long shelf life and good functional properties.
4. Freeze-drying is employed in the production of instant coffee and tea because the aroma and flavour compounds are preserved which leads to high-quality instant coffee.
In the dairy industry, freeze-drying is used to produce freeze-dried cheese powders which are used in snack foods.
Advantages of freeze-drying
The thermo-labile components of lyophilized foods (proteins, flavor, and color) are preserved than other dehydrating processes. Food products like vegetables which contain heat-sensitive compounds can be preserved by freeze drying. Since proteins are denatured by heat, freeze-drying is a beneficial method in the meat industry.
Freeze-dried foods can be rehydrated more quickly and easily because a lot of microscopic pores are available. The porous nature of lyophilized food allows the addition of water to the food products.
Freeze drying does not usually cause shrinkage and toughening of food and so lyophilized foods are able to keep their three-dimensional shape. This makes them appealing to sight compared to other dehydrating methods.
Freeze-dried foods keep long before deterioration because when food products are lyophilized, water is removed and the rate at which food deteriorate depends on water activity.
Freeze-dried foods are light in weight and portable since the removal of moisture reduces the weight of the food by up to 90%, making it much lighter and more compact than its fresh or canned counterpart. This makes freeze-dried foods ideal for outdoor activities like camping, hiking, backpacking where weight and space are critical factors.
Taste of freeze-dried foods are preserved, and re-refrigeration is not required because the freeze-drying process removes moisture by sublimation and so avoids the use of heat which can alter the taste, color and texture of the food. Also, lyophilized foods can be stored at room temperature because they are sealed in airtight containers and packages, eliminating the need for refrigeration and making it easier to take on the go.
Conclusion
Freeze-drying is unique to other dehydrating method in a sense that it removes water through solid-gas phase transition (sublimation) while the other methods do so by liquid-gas phase transition. Take a moment to recognize the amazing science that made it possible the next time you have a freeze-dried snack or mix some lyophilized berries into your yogurt in the morning. Freeze drying preserves more than just food—it preserves opportunities for a future that is more appetizing, nourishing, and sustainable.
References
Harper, J.C. and Tappel, A.L., 1957. Freeze-drying of food products. Advances in Food Research, 7, pp.171-234.
State, K. and State, B. (2023) ‘Available online : www.ncribjare.org DOI : https://doi.org/10.35849/BJARE202302/97/003 Journal homepage : www.ncribjare.org Application of Freeze-Drying in Food Processing and Storage - A Review’, 05(02), pp. 21–35.
Duan, X. et al. (2016) ‘Technical aspects in freeze-drying of foods’, Drying Technology, 34(11), pp. 1271–1285. Available at: https://doi.org/10.1080/07373937.2015.1099545.
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