Freeze-drying works on the basic principle of sublimation, the shift from a solid state to a gas state. The process occurs when a molecule gains enough energy to break free from other molecules and form a gas. Water is an example of something that can sublime from a solid state to a gas form. It does so when the conditions are not right for a liquid to form.
Freeze drying, often referred to as lyophilization, removes water frequently used to preserve perishable goods to increase their shelf life and make them travel-ready. Freezing the material, lowering the pressure, and increasing heat, freeze drying causes the material’s frozen water to transform immediately into vapor (sublimate).
What is Freeze Drying?
A dehydration procedure called freeze-drying or lyophilization relies on a product’s water being sublimated.
This indicates that the product’s water content changes directly from a solid to a gaseous state or from ice to vapor without moving through a liquid condition.
For a few reasons, including the following, freeze-drying is regarded as a high-quality dehydration technique:
It functions at low temperatures, which helps maintain a product’s nutritional value, flavor, and warmth.
Sensitive Materials
The product’s deterioration is greatly postponed by freezing, which inhibits chemical and microbiological activities (thereby extending its shelf life)
How does Freeze Drying Work?
There are stages to freeze drying:
- Pretreatment
- Freezing
- Primary Drying (Sublimation)
- Secondary Drying (Adsorption)
Using freeze drying properly can shorten drying periods by 30%.
Pretreatment
Any technique ” improving the product before freezing is referred to as pretreatment. This could entail changing the formulation to add ingredients that will boost stability and improve processing, concentrate the product, dilute the product, reduce the amount of a solvent with a high vapor pressure, or increase the surface area. The choice to pretreat a product is frequently made based on a theoretical understanding of freeze-drying and its needs, which are established by cycle time or product quality considerations.
Freezing Phase
The most crucial step in freeze drying is freezing, and there are numerous techniques for doing it. Freezing can be done on a shelf in the freeze dryer, in a chilled bath (shell freezer), or in a freezer. Rather than melting, sublimation is ensured by cooling the substance below its triple point. This keeps its physical form intact.
Large ice crystals, which can be created by gradual freezing or annealing, are best for freeze-drying. However, the outcomes of freeze-drying biological materials are less than ideal because when crystals are too big, they may rupture the cell walls. The freezing is done quickly to avoid this. An option for materials that precipitate is annealing. The product is quickly frozen, and then the temperature is raised to encourage crystal growth.
The Phase of Primary Drying (Sublimation)
Primary drying (sublimation), the third stage of freeze drying, involves lowering the pressure and heating the material to cause the water to sublimate. The vacuum accelerates sublimation. Water vapor can stick to and solidify on the cool condenser’s surface. The vacuum pump is additionally shielded from the water vapor by the condenser. In this stage, the material’s water content is reduced by about 95%. Drying in the initial stages can take time. The structure of a substance can be changed by excessive heat.
The Phase of Secondary Drying (Adsorption)
Secondary drying (adsorption), the last stage of freeze drying, occurs when the ionically attached water molecules are eliminated. The links between the substance and the water molecules are broken by elevating the temperature above that of the primary drying phase. The materials are still permeable after being freeze-dried. The vacuum can be broken with an inert gas after the freeze-drying procedure is finished before the material is sealed. To 1-5% residual moisture, most materials can be dried.
What are the Issues that Must be Avoided During Freeze Drying?
Too much heat applied to the product can result in melt back or product collapse.
An excessive amount of vapor striking the condenser might produce condenser overload.
- Excessive vapor production
- Excessive surface area
- Inadequate condenser space
- Not enough refrigeration
Vapor choking occurs when vapor production exceeds the vapor port’s capacity (the opening between the product chamber and the condenser), increasing chamber pressure.
What is the Difference Between Dehydration and Freeze Drying?
Removing moisture is the major goal of food preservation to prevent food from spoiling or mold growth. The food’s shelf life and nutritional value are increased by removing moisture without changing the product’s makeup.
The water in food is removed by dehydration in shoddy home dehydrators by roughly 70%. The food is only edible in this situation for a short period. But high-end commercial dryers can get rid of 90 to 95 percent of the moisture. In contrast, using Harvest Right equipment at home to freeze-dry food eliminates 98 to 99 percent of the water content.
Most homemade dehydrated products, such as dried fruit, meat, and vegetables, have a shelf life of one to two years at most due to the quantity of moisture that remains in dehydrated food. The shelf life of the same items dried with a freeze dryer is 15–25 years.
Because of the chilly vacuum, freeze-dried food typically keeps 97% of its nutritional value. Additionally, the nutritious value of dried food is only about 60% of fresh food of the same caliber. The heat used during dehydration, which destroys the food’s vitamins and minerals, is mostly to blame for this loss. According to the American Institute of Cancer Research, most of the vitamins and minerals in freeze-dried meals are still present.
The appearance or flavor of the food is not altered by freeze-drying. Big slices of turkey, mashed potatoes, gravy, stuffing, and corn are all included in the freeze-dried turkey supper, which looks and tastes the same as if it had just been prepared. The best part is that you wouldn’t even know if this dinner had been freeze-dried 15 years ago.
Can you Eat Freeze-Dried Food?
Foods that have been frozen-dried are a good option for your health. Freeze-drying is one of the most often used dehydration methods due to its many benefits.
Freeze-drying is one of the best techniques for maintaining the activity of beneficial plant compounds, including phytochemicals and minerals, while preserving color, flavor, and structure. As a result, it is widely used to make premium food products.
Studies have shown that freeze-drying is the best drying method for retaining antioxidants such as anthocyanins, flavonoids, ascorbic acid, and vitamin C.
Antioxidants are beneficial compounds that aid your body’s defenses against the harmful effects of oxidative stress. They also support the bulk of fruits and vegetables’ health benefits.
The opposite, however, might also be true depending on the fruit, even if freeze-drying rarely raises a fruit’s phytochemical concentration.
Reduced water activity hinders the growth of most bacteria, yeasts, and molds; hence freeze-drying also helps to increase a food’s shelf life.
Fresh plant-based foods might not always be available depending on this.
Finally, removing the water content from a product reduces its volume and weight, making it easier to handle, store, and transport.
Reference: Effect of Freeze-Drying on Quality and Grinding Process of Food Produce: A Review
Freeze-drying is a crucial processing step in the creation of food powders. It provides high-quality dehydrated products with a long shelf life. Unfortunately, the glass transition temperature (during the drying operation) and stress created (during the grinding operation) in the food structure negatively impact food quality attributes and grinding characteristics during the drying process. However, because of its unique benefits, it has been effectively used in a variety of biological materials, including both animal and plant products. Spices, vegetables, and fruits are among the culinary products that are increasingly in demand on the market as freeze-dried and ground goods.
What are Freeze-Drying Potential Drawbacks?
Freeze drying is a fantastic food preservation technique. However, there are a few potential disadvantages to consider.
First, disease-causing bacteria in raw foods can withstand drying and sustain storage, even if eliminating water from a product prevents microbial development. They can cause foodborne sickness if consumed.
Foods that need to be cooked before eating must be prepared before being freeze-dried.
The high porosity of these products makes it easy for oxygen to enter, which could lead to increased levels of oxidation or degradation of bioactive chemicals. Second, freeze-drying preserves the antioxidant content of foods but also increases the risk of oxidizing or degrading bioactive chemicals.
How to Freeze Dry Food with Dry Ice?
Wash, dry, and cook any meat before freezing it. Make sure to boil any poultry, beef, or fish before drying it. Fruit and vegetables should be washed under cool running water before being dried with a paper towel.
Noodles of pasta must also be cooked.
Cheese doesn’t need to be washed before being freeze-dried.
Larger fruit and vegetables should be cut into small pieces ranging from 1 inch (2.5 cm) to 2 inches (5.1 cm) across using a sharp knife. Larger things should be cut into portions that are 2.5–5.1 cm (1.2–2 inches) across. Slice cooked meat into less than 1 inch (2.5 cm) thick pieces if you freeze-dry it. To ensure that the pieces freeze dry at the same rate, try to make them all the same size.
Blueberries, raspberries, and other small fruits can be freeze-dried whole.
Cut up larger produce items like pears, apples, and potatoes into smaller pieces.
Use a serrated knife to cut a loaf of bread into slices about 12 inches (1.3 cm) thick if you plan to freeze-dry it.
Fill freezer bags with the chopped food portions before sealing the bags. Place the frozen bags with the cut-up bits. Make sure you just put 1 type of food per bag rather than mixing other types of food. The air should then be completely expelled from the bags with your hands or by rolling it away from the opening with a rolling pin.
Eliminating the air will guarantee that the meal won’t develop any ice crystals.
Pick a storage container that is big enough so the bags only fill it halfway. It would be ideal to use a sizable plastic container with a lid or a huge cooler made of foam. Remember that the box must fit inside your freezer; therefore, if it’s small, you may only be able to freeze small dry quantities of food at a time.
Pour dry ice into the bottom of the box until it forms an even layer while wearing heavy-duty gloves, such as leather or work gloves. The weight of the food determines how much dry ice you’ll need to utilize. Therefore, you will require roughly 5 pounds (2.3 kilograms) of dry ice if you freeze 5 pounds (2.3 lb) of food. Dry ice weight 0.45 kilograms per pound should be added to the box’s bottom. If it doesn’t, add another 1 pound (0.45 kg) until it does, or until it completely covers the bottom of the box.
Dry ice is layered between the food bags. Be careful not to stack two bags on top of one another. Place the bags on top of the dry ice layer at the bottom, and then add 1 to 2 pounds (0.45 to 0.91 kg) of dry ice to cover the bags completely.
The food bags should be covered with one more layer of dry ice. You might need to alternate layers of dry ice and food bags depending on the size of the box and the quantity of bags you have. Dry ice should be placed both on top of and beneath each layer of food.
Make a few holes in the lid, then fasten it to the container. Cut three to four holes into the top of the box using a box cutter or a good knife. These holes allow gas and moisture to escape for the dry ice to melt and the food to completely dry.
Give the box at least 24 hours in the freezer. When all the dry ice evaporates, the food has finished freezing and drying. Depending on how many layers of food you’re freeze drying (and how much dry ice you’ve used to cover it with), this could take 24 hours or longer. To open the box and examine the contents, put on gloves.
Conclusion
Freeze-drying is an effective method for preserving samples. It has unique preservation properties and is used in many different applications. There is a growing list of new uses for freeze-dried samples. The freezing process removes ionically-bound water molecules from samples. The process involves three main phases: pre-freezing, primary, and secondary drying.
Freeze-dried meals are frequently used for military rations, space exploration, emergency and survival situations, hiking, and camping. They are favored over traditional food due to their portability, ease of preparation, lightweight, and long shelf life.
