Freeze-drying: what is it and what processes are involved?

Freeze-drying is a process that involves freezing a substance to extract the liquid it contains by sublimation.

This process not only increases the shelf life of the product, but also preserves its shape and volume, as well as its nutrients and organoleptic qualities.

At Barnalab Liofilizados we specialize in freeze-drying for third parties. We have extensive experience in the sector and are capable of freeze-drying R&D batches as well as large productions.

Unlike traditional drying processes to preserve food longer (which, due to high temperatures, can cause physical and chemical changes in food, as well as changes in taste or texture, making it less palatable or even inedible), the freeze-drying process, by involving freezing and sublimation, not only increases the shelf life of food, but also preserves its shape and volume, as well as its nutrients and organoleptic qualities.

In general, fruit, vegetables, and mushrooms, both chopped and in pulp, can be freeze-dried. The same applies to meat or offal, which must first be cut into smaller pieces to undergo the process.

The number of additional processes to freeze-drying that each type of food involves, as well as the added value that can be given to it once it has been mixed with other elements (see additives), is what determines whether freeze-drying is profitable or not.

Liquids can be freeze-dried perfectly well, although it should be borne in mind that the smaller the amount of water in the process, the faster the freeze-drying will be.

Freeze-drying milk (cow’s milk, mother’s milk, donkey’s milk, mare’s milk…) and its derivatives (yoghurt, kefir…), as well as bone and vegetable broths and creams, are examples of products that can be very well freeze-dried.

Freeze-dried foods retain up to 95% of their nutritional value, while maintaining their organoleptic characteristics.  This method does not modify the physicochemical structure of the food, so it can be preserved without the need for the control required by the cold chain. In addition, its low moisture content gives the food a high microbiological stability.

• Maintains the taste, colour, smell, and texture of the food.
• Nutritional qualities are maintained: vitamins, minerals, proteins,
• It does not damage heat-sensitive foods, as it is carried out at low temperatures.
•  Food can be rehydrated quickly.
•  As the food will be pre-cooked, it can be quickly prepared for consumption.
•  The food becomes light and easy to transport, making it ideal for travel and extreme situations.
•  Freeze-dried food lasts much longer.

• Some freeze-dried products are somewhat more expensive than canned or dehydrated products.
• Some raw materials must be treated before freeze-drying because they are sensitive to the process. For example, to avoid loss of colour.
• Freezing can damage some foods. For example, it can make them more susceptible to collapse.
• For effective preservation, some freeze-dried foods must be kept at low temperatures.

Freeze-dried foods have a shelf life of 10 to 25 years, as the removal of water removes bacterial activity and with it the degradation of the product, but this will mainly depend on subsequent storage.

Several key storage conditions have a major impact on the shelf life of freeze-dried foods.

• Oxygen: Oxygen can have negative effects on the nutrients, vitamins and colour of food. It can also increase the growth of micro-organisms such as bacteria. It is imperative that stored foods are hermetically sealed to preserve their shelf life.
• Moisture: Moisture also creates a beneficial environment for microorganisms, causing freeze-dried food to deteriorate. Shelf life is shortened considerably when food is stored in a humid environment.
• Light: When food products are exposed to light, the proteins, vitamins and nutrients they contain can be damaged. This can quickly lead to discolouration and bad taste, so be sure to store products in a dark place.
• Temperature: High temperatures cause protein breakdown and destruction of vitamins, which affects colour and smell. Storing food in a hot environment will quickly deteriorate its shelf life.

Freeze-dried product range

Book affected by floods and recovered by freeze-drying

Freeze-dried foods used in haute cuisine

Freeze-drying facilitates the storage of ready meals

Freeze-drying is a dehydration or drying process in which the water is removed from a product without altering the molecular bonds, thus preserving its taste and texture. In food products, this allows the raw material to be preserved for longer periods and to be transported and stored more easily.

Freeze-drying is therefore used by those who wish to preserve a product indefinitely at room temperature.

The sectors in which it is used are very broad: pharmaceuticals, food, agriculture, veterinary, cosmetics, archaeology, libraries, etc. Almost anything can be freeze-dried.

On sailing ships (for example in the Ocean Race) or spacecraft, it has been used to store prepared meals, in which the user only must hydrate the freeze-dried food packaging. This makes storage easier due to the small footprint and guarantees the nutritional value for the consumer.

Barnalab offers freeze-drying services in the following sectors in which we specialise:

• Vegetable extracts: ALOE VERA, SEAWEED, TURMERIC, SAFFRON and other vegetables.
• Truffles, boletus: all kinds of MUSHROOMS.
• Cultures and probiotics: KEFIR, YOGHURT and other FERMENTED.
• Insects: freeze-drying of edible larvae.
• Nutraceuticals: compounds for food supplements that can be of animal, microbial or plant origin.
• Food additives and flavourings: FLAVOURING, AROMATISING, GELLIFYING…
• Recovery of wet documents: service for the recovery of documents and books that have suffered water damage in a flood (see libraries or museums).
• Recovery of archaeological objects: advice and assistance in the conservation of archaeological objects, as well as objects from sunken ships of historical value.
• Pigments and polymers

The freeze-drying time is very important, however, the times vary a lot between different products, as the water content varies from one to another.

The key is to cut the fruit into slices: smaller pieces are easier to freeze-dry. Berries with a thick skin are also generally good for storing water, e.g., blueberries. Strawberries, on the other hand, are very easy to freeze-dry.

The higher the temperature, the faster the drying process, although slower drying retains more vitamins and a better structure.

Therefore, temperatures and drying cycles must be adjusted to the desired quality of the final product, which can be determined by Barnalab Liofilizados.

The following table shows in hours the drying times for medium-sized pieces of approx. 10 mm at a final temperature of 25°C, 40°C and 60°C.:

Freeze-drying consists mainly of three stages, which are carried out with specialised equipment.

The food is introduced in trolleys with trays into the freezing tunnel at -40ºC, so that it freezes as quickly as possible and thus avoids the formation of ice crystals that could damage its organoleptic qualities. Once frozen, they are kept in the freezing chamber until they are transferred to the freeze-dryer.

Once the freeze-dryer is filled with the product trays, the recipe is entered into the control panel.

The vacuum is achieved by extracting the air from the chamber with vacuum pumps to reduce the pressure to below 1.5mbar and at the same time raising the temperature of the food.

Once the chamber is emptied of air, the pressure is below the threshold at which water can exist simultaneously in a solid, liquid and gaseous state, called the triple point of water. This is the point at which the heat causes the ice crystals to turn into water vapour, which is extracted and condensed, separating it from the food.

The resulting product is dry, full of small, sponge-like holes where the crystals used to be, through which the water is reabsorbed when it is prepared for consumption again, having retained its original size and shape.