Lyophilization, popularly known as freeze-drying is a method of dehydration. Freezing techniques are used on foods to be dehydrated to remove the water content. Whether it’s a ready-to-eat meal or a fruit or just a snack, you may find a variety of freeze-dried foods in your local neighbourhood. It is widely believed that freeze-drying is the best method of drying. Freeze-drying is a widely used practice by dried food manufacturers in the food and pharmaceutical industries.

Freeze-drying is a high-quality dehydration method and is most preferred as it operates at low temperatures which contribute to preserving a product’s nutritional value, taste, appearance, and heat-sensitive compounds. This technique prevents chemical and microbiological processes which significantly delay deterioration and helps extend the shelf life.

Foods to be dehydrated contain roughly 80%–95% water, which is divided into bound and free water. While free water freezes, bound water doesn’t. During the process of dehydration, free water must remain frozen at all times and all free water and most bound water should be removed. This is the most crucial step and is also one of the many differentiating points between vacuum-drying and freeze drying.

The freeze-drying process can be divided into three steps:

1. Freezing: The product is often frozen under atmospheric pressure.
2. Primary drying: Also known as proper freeze-drying, this is the sublimation step in which frozen-free water is removed.
3. Secondary drying: Also known as desorption drying, it’s the drying of products to their desired humidity by removing the remaining bound water.

Freeze-drying should be performed in controlled conditions that do not melt the water, as the presence of liquid water may alter the product’s composition, shape and physical properties. Once frozen water is removed by sublimation, the remaining product develops a highly porous structure.

Impact of Freeze-Dried Foods on Health

Freeze-drying helps retain nutrients and phytochemicals in foods. Therefore, dehydrated foods are a healthy food choice. It also extends a product’s shelf life and facilitates storage and transportation along with handling. Freeze-drying is widely used to produce high-value food products as the procedure helps retain the activity of useful plant compounds namely phytochemicals and nutrients while preserving the color, flavor, and structure.

Decreased water activity inhibits the growth of most bacteria, yeasts and molds and by removing a product’s water content, freeze-drying helps prolong a food’s shelf life. This is especially important for fresh plant-based foods, which are not perennial by nature.

User Friendly Freeze-Drying Foods

Freeze-dried foods are preferred over regular foods due to their lightweight, long shelf life, portability and ease of preparation. These are often used for recreational outdoor activities, emergencies, military ration, space exploration and survival. While plant-based foods are the most popular freeze-dried products, a wide range of foods can be preserved using this method. Fruits, vegetables, meats, grains, frozen meals, dried mushrooms, beverages, spices, dried cheese and sweeteners are some of the foods that can be freeze-dried easily.

Short Comings of Freeze Drying

Although freeze-drying is an outstanding method of food preservation-yet, there exists a set of potential downsides to consider. Freeze-drying doesn’t kill bacteria present in raw food. Thus, you must cook raw foods before freeze-drying to avoid foodborne illness. The high permeability of freeze-dried foods may lead to greater degradation of nutrients.

Despite preserving foods’ antioxidant content, the high porosity of freeze-dried products allows easy access to oxygen, which may lead to higher levels of oxidation or degradation of bioactive compounds.

During the reduction of a product’s water availability, microbial growth diminishes. These disease-causing microorganisms in raw foods can survive the drying process and remain present during storage. When consumed, they can transmit food-borne illnesses. For this, foods that need to be cooked before consumption must also be cooked before being freeze-dried.

Dry-Freezing at Home

You can easily freeze-dry foods at home without purchasing specialized equipment by following the freezer method. This process doesn’t require special equipment but is time-consuming. Given below are the steps you need to follow for the freezer method:

1. Wash, dry, and cook the food item.
2. Chop it as small and even as you’d like. It is best to avoid large chunks of food.
3. Place the food into a single layer on a tray and place the tray in your freezer. Within a week, the food in the freezer will be ready.
4. It is important to test your food by removing a piece from the freezer and letting it thaw. If the food turns black or dark brown upon thawing, it hasn’t freeze-dried completely. If it returns to its natural or standard color, the process is complete.
5. Store your freeze-dried foods in airtight containers to avoid moisture.