The terms drying and dehydration both refer to the removal of water. Drying is the oldest method of food preservation. The sun, wind, and Smokey fire have all been used throughout history to remove water from fruits, meats, grains, and herbs. The practice of drying foodstuffs, both perishable and non-perishable, for preservation is centuries old. Food dehydration is defined as the process of removing water from food by circulating hot air through it, preventing the growth of enzymes and bacteria. Dried foods are tasty, nutritious, lightweight, simple to prepare, store, and use. Sun drying, solar drying, fluidized bed drying, spray drying, and drum drying are all methods of drying. Among all of these methods, freeze drying or lyophilization yields the highest quality products.
What is freeze drying?
Freeze drying/lyophilization is the process of freezing water and then removing it from a sample, first by sublimation (primary drying) and then by desorption (secondary drying). Freeze drying is an efficient method of drying materials without causing them harm. When aqueous solution stability is an issue, lyophilization is the most commonly used method for producing parenteral. It is critical for the preservation of materials that require low moisture content (less than 1%) to ensure stability and necessitate a sterile and gentle preservation process. The term ‘lyophilization’ refers to a process those results in a product that ‘loves the dry taste.’ This term, however, does not include the freezing process. As a result, while the terms lyophilization and freeze-drying are interchangeable, freeze-drying is a more descriptive term.
Freeze dried foods
History of freeze drying:
Since the 13th century, the Inca have been freeze drying potatoes to make chuo. Multiple cycles of exposing potatoes to below freezing temperatures on Andean Mountain peaks in the evening and squeezing water out and drying them in the sunlight during the day were used in the process.
Richard Altman invented a method to freeze dry tissues (either plant or animal) as early as 1890, but it went virtually unnoticed until the 1930s. Jacques-Arsene d’Aronval invented freeze drying for pharmaceutical applications in 1906. There was no more freeze-drying information until Tival in 1927 and Elser in 1934 had patented freeze drying systems with improved freezing and condenser steps.
During World War II, blood plasma and penicillin were required to treat the wounded on the battlefield, which marked a watershed moment for freeze drying. Many serum supplies spoiled before reaching their recipients due to a lack of refrigerated transport. The freeze-drying process was developed as a commercial technique to render blood plasma and penicillin chemically stable and viable in the absence of refrigeration. It was developed by Jacques-Arsene d’Arsonval. He is more famous in pharmacy field. In the 1950s and 1960s, freeze drying was recognized as a versatile tool for both pharmaceuticals and food processing.
Sublimation is a phenomenon that occurs when water passes directly from the solid state (ice) to the vapour state without passing through the liquid state. Water sublimates at pressures and temperatures below the triple point, i.e., 4.579 mm Hg and 0.0099 0 C. The material to be dried is first frozen, then heated (via conduction, radiation, or both) under a high vacuum so that the frozen liquid sublimates, leaving only solid, dried components of the original liquid. The driving force of water during lyophilization is the concentration gradient of water vapour between the drying front and the condenser. Below the triple point (6.1 mbar at 0 0 C for pure water), only solids and gases exist. In figure we can the triple point.
The principle of freeze drying is based on this physical fact. Sublimation of water can take place at pressure and temperature below triple point i.e., 4.579 mm of Hg and 0.0099 0 C. Water in food is subjected to sublimation for removal.
Process to produce product that loves the dry state:
There are 4 stages in the complete drying process:
Any treatment of the product prior to freezing. This could include concentrating the product, revising the formulation (adding components to increase stability and/or improve processing), reducing a high vapour pressure solvent, or increasing the surface area.
On a small scale, this is often accomplished by placing the material in a freeze-drying flask and rotating the flask in a bath cooled by mechanical refrigeration, dry ice, and methanol or liquid nitrogen. On a large scale, freezing is usually accomplished with the aid of a freeze-drying machine. It is critical to cool the material below its triple point during this step. This ensures that sublimation occurs. Rapid freezing is used to quickly lower the material below its eutectic point, avoiding the formation of large ice crystals. Large ice crystals may cause cell wall damage, resulting in the destruction of more cells and a poor texture and nutritive content. The freezing temperatures range from -50 to -80 degrees Celsius. Because the product can be spoiled if the freezing phase is not done properly, it is the most critical phase of the entire freeze-drying process.
• Primary drying:
During the primary drying phase, the pressure is reduced (to a few millibars) and sufficient heat is applied to the material to allow the water to sublimate. During this drying phase, approximately 95% of the water in the material is sublimated. The application of partial vacuum controls the pressure. A cold condenser chamber and/or condenser plates, in addition, provide a surface(s) for the water vapour to re-solidify on. The temperature of the condenser is typically less than -50 0 C. the heat is brought mainly by conduction or radiation.
• Secondary drying:
Since the ice was removed during the primary drying phase, the secondary drying process aims to remove the unfrozen water molecules. The adsorption isotherms of the materials govern this section. In this phase, the temperature is raised to above zero degrees Celsius in order to break any physicochemical bonds between water molecules and the frozen material. In this stage, the pressure is usually reduced to encourage desorption.
Following the completion of the freeze-drying process, the vacuum is usually broken with an inert gas, such as nitrogen, before the material is sealed.
The final residual water content in the product is extremely low at the end of the operation, ranging from 1% to 4%.
Equipment in freeze-dryer:
The refrigeration system, vacuum chamber, drying chamber, and condenser are the four basic components of freeze drying. The refrigeration system keeps the condenser inside the freeze-dryer cool. It has the ability to cool shelves in the product chamber in order to freeze the product. The vacuum system is made up of a separate vacuum pump that is linked to an airtight condenser and an attached product chamber. The drying chamber is typically a horizontal cylinder with the condenser installed near the trays.
Commercially available freeze dryers include both continuous and batch models.
Commercial freeze dryer
Working of freeze dryers:
To freeze the product, the refrigeration system cools the shelves in the product chamber. The food is placed on the trays, and heat is delivered to it via radiation or conduction. The condenser attracts and condenses the vapours that are being sublimated off the product.
Applications of freeze-drying in food industry:
Freeze-drying is a method of preserving food while making it very light. As an example of astronaut food, the process has been popularized in the form of freeze-dried ice cream. It is also widely used to make essences or flavourings for food. Hikers also like and appreciate freeze-dried foods. It has a longer shelf life than wet food, which tends to spoil quickly. The coffee in instant is freeze-dried. Freeze-dried fruits are used in breakfast cereal or sold as a snack, and are a popular snack choice among toddlers, pre-schoolers, and dieters, as well as being used as a treat for pet birds by some pet owners. Herbs for cooking are also freeze-dried.
Advantages of freeze – drying:
- Under vacuum conditions, oxidizable substances are well protected.
- Long preservation period due to 95% – 99.5 percent water removal.
- Accurate loading quantity and uniform content
- Because of the aseptic process, there is little contamination.
- Minimal loss of volatile chemicals, as well as heat-sensitive nutrients and aromatic components.
- Minimal changes in properties because microbe growth and enzyme action are not possible at low temperatures.
- Normal temperature transportation and storage
- Quick reconstitution time.
- The dried material’s constituents remain uniformly dispersed.
- The product is processed in liquid form.
- Product sterility can be achieved and maintained.
Disadvantages of freeze –drying:
- Volatile compounds can be removed using a high vacuum.
- The most expensive unit operation is the single one.
- Individual drug-related instability issues
- Some issues with dryer chamber sterilization and sterility assurance, as well as aseptic loading of vials into the chamber.
Key takeaway from market study:
The freeze-dried food market is expected to grow at a CAGR of 9.5 percent between 2021 and 2026. Growing food-processing sector and increased demand for ready-to-eat food products among consumers worldwide, ease of processing heat-sensitive food products, and growing consumer interest and participation in outdoor sports and expedition activities are the major factors driving market growth between 2021 and 2026. Pet food is the most common type of freeze-dried product. Pet food is expected to be the most popular product category in the market, followed by meat, fish, and other vegetables. Asia-Pacific has the highest demand for freeze-dried food products. Furthermore, Asia-Pacific is the market’s fastest-growing region.
Freeze-drying, also known as lyophilization, is a method of dehydrating at low temperatures. Freeze-drying is one of the best methods for drying products while preserving quality and without causing harm to the product. However, due to the high cost of equipment, there is a large class of products for which the use of freeze-drying is not economically justified.
However, freeze-drying is regarded as the best method of food drying in terms of product quality. The global freeze-dried foods market was significantly larger in 2020 and is expected to grow at a CAGR of 9.5 percent. The ease of processing heat-sensitive food products, as well as rising consumer interest, are key factors driving market revenue growth. Rapid growth in the food processing sector and high demand for ready-to-eat or convenience foods is expected to drive demand for freeze-dried foods such as fruits and vegetables for snacking, baking, soups, dressings, and confectionery products.
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Dr. Naseer Ahmed, Jagmohan, Harmeet Singh, Prerana Chauhan (2013) Different Drying Methods: Their applications and recent advances. International journal of Food Nutrition and Safety, 2013, 4(1):34-42.
Deepak Bhambere, Kunal A. Gaidhani, Mallinath Harwalkar, Pallavi S. Nirgude (2015) Lyophilization/ Freeze Drying – A review. World Journal of Pharmaceutical Research 4(8):516-543.
Utpal Kumar Das, Ranjit Bordoloi, Subha Ganguly (2014) Freeze-drying technique and its wide application in biomedical and pharmaceutical sciences. Research Journal of Chemical and Environmental Sciences.
Freeze-drying fundamentals article by Tara McHugh published in Food Technology Magazine
Lyophilization (freeze-drying) in the Food Industry by Reginals Davey published in News Medical and Life Sciences
Freeze-dried Foods Market size, Share and Industry Analysis by product type, by distribution channel and by region forecast to 2028 report published on REPORTS AND DATA
Freeze dried food market size, Share, Trend, Forecast, & Industry analysis- 2021-2026 report published on Stratview Research.