We humans simply love the idea of preservation. The most common example of this is a refrigerator. Although a refrigerator does serve many other purposes, we love them because they allow us to stash all the goodies that we can get our hands on and then gobble them later. Nothing works better than a refrigerator!

However, when we are dealing with more sensitive materials, such as medicines and drugs, the presence or absence of preservation could mean life or death for people, so we need to be extra careful that materials are not spoiled before they are used. The question is, how do we make that happen?

What is Freeze Drying?

Freeze drying, which scientists might call lyophilization, uses both a vacuum and a freezing process to remove water from perishable foods and medicines. The result is a product that can be stored at room temperature for years without spoilage, or packed in limited storage spaces and reconstituted with water later. Makers of instant coffee often use a freeze drying process, as do dietitians creating meals for campers, soldiers and astronauts. This process is also used in the pharmaceutical industry to preserve the integrity of air or moisture-sensitive medicinal compounds.

Drying foods with heat for preservation is an ancient concept, but it has certain drawbacks. The water inside the food is in liquid form, but the heat of the sun or other source slowly converts it to a gas. As the liquid passes out of the food, the cell walls are often damaged and the essential flavor and texture of the food is lost. Adding water to foods dehydrated with heat does not always restore flavor or texture. This is why freeze drying has an advantage over heat dehydration in the preservation process.

Freeze drying first involves cooling the food or chemical compound, often far below the freezing point of water. At this point, all of the water contained inside the food should be frozen into solid crystals. The basic structure of the fruit, vegetable or meat has not changed, but the water content is in the solid state. Half of the process has been accomplished at this point through temperature reduction.

The drying process involves the use of a vacuum chamber. The frozen foods or chemicals are placed into the vacuum chamber, and the surrounding air is pumped out. If this process were performed at room temperature, the food would most likely be destroyed as the liquid water forced its way into the vacuum chamber. However, the frozen water crystals actually change from solid ice to a gas, bypassing the liquid state altogether.

This process is called sublimation. It’s the same effect that causes solid dry ice to virtually disappear when struck with a hammer. During the freeze drying process, the solid water converts to a steamy gas in the vacuum chamber, leaving all of the solid food materials dehydrated.

Once the water has been removed through freeze drying, the dried foods or chemicals are often stored in vacuum-sealed packs to keep air and moisture from reaching them. These packs can be stored at room temperature, since bacteria and other harmful organisms cannot survive without air anyway. Freeze drying also leaves behind tiny pores where the frozen water crystals used to be. This is why instant coffee grounds mix so quickly and thoroughly with hot water. Freeze drying is also a popular process for creating ‘space age’ ice cream desserts and backpack-friendly meals for hikers and campers.

How Freeze Drying Works?

All required preparatory steps to be done to the product should be done before freezing. Steps such as placing the product in specific shapes for freezing, adjusting concentration or addition of cryoprotectants, etc.

1) Freezing

The product is cooled until frozen throughout. This can be done either in a basic chest freezer, a commerical blast freezer, specialised laboratory equipment for freezing (eg, glycol bath, LN2, etc) or in some types of freeze dryer (our HS/HSL and F series freeze dryers are capable of freezing product in situ).

Freezing a product slowly will create larger interlinked ice crystals which facilitate faster vapour removal during sublimation. This in turn allows for a faster freeze drying cycle. However, large ice crystals are not always desirable, particularly in the case of food or organic cell structures where large ice crystals can rupture the cell walls and damage the end product. In such cases a method of rapidly freezing the product to create smaller ice crystals is advised.

2) Primary Drying

In this phase the condenser drops to its operating temperature and the chamber pressure is lowered to a few millibar with a vacuum pump.

Heat energy is supplied to the product to facilitate the sublimation process (the necessary heat input can be calculated with the latent heat of sublimation of ice or other sublimating material). The water content at the end of the primary drying phase will typically be around 5-8%.

Heat energy for sublimation can be input in several ways, for instance the heat can be ambient temperature in the case of a basic laboratory machine, heated conductive shelves or radiative heat from shelves/product chamber wall.

The speed of this process is important in many cases – some products can be dried as quickly as the machine is capable of with minimal loss of quality, others will require a slow gradual drying to prevent excess heat damaging the product.

It is important to remember that the product will dry from the outside inwards. Therefore the thickness of the material will partly determine the length of this stage – the thicker the material the more difficult it will be for vapour to escape from the product from the central ice core. This will cause the drying process to be slower and less efficient, in a worst case scenario the reduction in sublimation cooling and resultant rise in product temperature may comprimise the structure of the product.

3) Secondary Drying

This phase removes unfrozen moisture that is not ice, but is instead sorbed (bound) to the product. This sorbed water is removed via desorption and the rate at which this occurs is dependant on the product temperature. Once the primary drying phase is complete then the product temperatures in secondary drying phase can be fine tuned to increase the rate of desorption either through gradual controlled temperature rise or simply set at maximum for the fastest possible rate. Take care not to damage your product with excessive heat or increase the rate of vapour release such that the vacuum level dimishes above the eutectic point of the product. If you are unsure when primary drying is complete, see the below section, “How can I tell when Primary Drying has ended?”

By the end of the secondary drying process the water content will be around 1-3%, with lower water content requiring significantly increased drying times.

Depending on your product needs, the vacuum can be broken either with air admittance or with an inert gas (typically Nitrogen).

Freeze-Drying-technology-in-Pharmaceutical-Industry

Advantages of freeze drying

Freeze-drying is viewed as the optimal method of choice for dehydration because of the preservation of quality, meaning characteristics of the food product such as aroma, rehydration, and bioactivity, are noticeably higher compared to foods dried using other techniques.

Shelf-life extension

Shelf-life extension results from low processing temperatures in conjunction with rapid transition of water through sublimation.With these processing conditions, deterioration reactions, including nonenzymic browning, enzymatic browning, and protein denaturation, are minimized. When the product is successfully dried, packaged properly, and placed in ideal storage conditions the foods have a shelf life of greater than 12 months.

Re-hydration

If a dried product cannot be easily or fully re-hydrated, it is considered to be of lower quality. Because the final freeze dried product is porous, complete re-hydration can occur in the food. This signifies greater quality of the product and makes it ideal for ready-to-eat instant meals.

Effect on nutrients and sensory quality

Due to the low processing temperatures and the minimization of deterioration reactions, nutrients are retained and color is maintained. Freeze-dried fruit maintains its original shape and has a characteristic soft crispy texture.

Disadvantages of freeze drying

These all relate to the economy of the process and the storage and distribution of the end product.

(a) Primary cost. – The equipment is sophisticated and expensive.

The chamber (which is required by any mechanical drying method) has to be very strong: it must be capable of withstanding the pressure differences. The seals on the door must be airtight to ensure that low pressures are easily maintained.
The low pressure required necessitates sophisticated vacuum equipment, steam ejectors and/or heavy duty piston pumps. If the latter are used, a vapour removal system is required between chamber and pumps.
Some system is required to provide heat to the food during the drying process.
To maximise the capacity of the system, it may be desirable to freeze the product before drawing a vacuum, either within the chamber or in a separate process.

(b) Operating costs. – In addition to the energy of evaporation (which is required by all drying processes), energy is required to develop and maintain the vacuum and for any refrigeration facility (to pre-freeze and perhaps to recondense vapour to stop it getting through to a piston vacuum pump).

A high level of competence is required in operators and service engineers: there is, therefore, a relatively high labour cost.
The technology is advanced: therefore, repair and maintenance costs (spares) are high, compared to more conventional drying systems.
It is a batch process: there are periods of loading/unloading when equipment is unoperational.

(c) End product.

The high prime and operating costs mean that this process is only suitable for high value foods which can carry a high production cost. However, the consumer expects good quality from high cost goods.

The lack of shrinkage and case hardening associated with a very low moisture content means that freeze dried foods are very brittle and so need to be protected by rigid packaging.
The open structure and low water content mean that freeze dried foods are very vulnerable to oxidative deterioration and so, as already mentioned, gas tight packaging associated with inert gas purging is often adopted.

Therefore, expensive packaging is the norm.

The freeze drying process across industries

Freeze drying isn’t just limited to food. Here are a few other products that can also be freeze dried:

Pharmaceutical – The pharmaceutical industry frequently uses freeze drying to prolong the shelf life of wound dressings, drugs and more.
Biotechnology and biomedical – Freeze drying has been used on vaccines, bacterial cultures, blood plasma, purified proteins, and enzymes.
Nutraceutical – Antioxidants, aloe vera, echinacea, and similar products that have been extracted from foods and plants for their health benefits can also be freeze dried.