Freeze drying

The boiling point of water depends on pressure - at atmospheric pressure, 1 bar, it boils at 100°C.

If water is held in a sealed container in which we can draw a vacuum, the lower the pressure or the better the vacuum, the lower the temperature at which the water boils but more heat is needed to evaporate 1 kg of water:

Pressure (bar)	Temperature (° C)	Latent heat of vaporisation (kJ/kg)
1.0	100	2257
0.5	81	2305
0.2	60	2358
0.1	46	2392
0.01	7	2485

At a pressure of about 0.006 bars, the boiling point of water is 0°C. The latent heat would be 2500 kJ/kg but, in evaporating, the water would take some of the heat from itself causing that remaining to freeze (this releases 334 kJ/kg), leaving ice at 0°C. If the vacuum is maintained at about 0.006 bars, the ice will sublime, that is, not melt but go straight to water vapour. As long as heat is available, the latent heat of sublimation at 0°C is 2834 kJ/kg.

In the same way as the boiling point is depressed by improving the vacuum, so taking the pressure below 0.006 bars will depress the sublimation temperature below 0°C:

Pressure (bar)	Temperature (°C)	Latent heat of sublimation (kJ/kg)
0.0060	0	2834
0.0026	- 10	2836
0.0010	- 20	2837
0 0004	- 30	2838
0.0001	- 40	2838

When a pot of water is being heated at atmospheric pressure, it will boil at 100°C whatever the temperature of the flame. In the same way, ice at a pressure of 0.0001 bar will sublime at - 40°C, as long as the water vapour can escape, whatever the temperature of the source of heat. If the water vapour cannot escape the pressure will quickly increase and the sublimation temperature will rise in proportion until, at a pressure of 0.006 bars, the ice will start to melt.

The process of freeze drying

Developing the above principles, the freeze drying process involves:

1. Placing the food in a chamber which is then sealed, after which a vacuum is drawn so that the pressure is well below 0.006 bars (probably below 0.001 bars).

2. The temperature of the food is above the sublimation temperature and so, in cooling down and freezing, it supplies some heat for water to evaporate and then sublime (this is the same as evaporative cooling).

3. Once the food is at the sublimation temperature which corresponds with the pressure, heat is somehow supplied to the food to provide the latent heat of sublimation so that the frozen water sublimes to water vapour.

4. The water vapour is drawn out of the chamber by the vacuum system, thus maintaining the low pressure in the chamber until all the frozen water has sublimed, leaving the dried food.

The dried food does NOT need to be stored under refrigeration since it contains no water needing to be kept frozen in order to prevent microbial growth. The dried food DOES need to be well packed, however, probably in aluminium foil laminates, to prevent the food from reabsorbing moisture from the air; it may perhaps be packed in a vacuum or inert gas to prevent, otherwise rapid, oxidative deterioration. If the food has been previously frozen, stage 2 does not apply.

Advantages

(a) No shrinkage - the freezing of the food effectively fixes its shape.

(b) No case hardening - there is no water movement to carry solubles to the surface; the ice directly sublimes from within the food as the drying front penetrates.

(c) No thermal damage - no high temperatures to cause loss of flavour or development of 'burnt' flavours.

(d) Rapid rebydration - because a very open texture is obtained.

Disadvantages

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.

Accelerated freeze drying (AFD)

Although freeze drying takes no longer, and is often faster, than more conventional systems, attempts have been made to accelerate the process in order to reduce production costs and increase the capacity of the process. The limiting factor is the transference of heat to the food: convection is impossible in a vacuum. Therefore, radiant heat, a relatively inefficient method is used.

A technique for accelerating the drying process for slabs of food or relatively uniform particles of food (e.g. whole prawns) has been developed and commercially adopted. The food is arranged in single layers between expanded metal mesh, held in a tray and covered by a sheet of stainless steel or aluminium (See Figure 10). This sandwich is placed between hollow plates in the chamber (See Figure 11).

Figure 10 - Food and metal grid arrangement for AFD process

Once the chamber is at the required pressure and the food is at the sublimation temperature, fluid contained within the hollow plates is heated to a temperature between 60 and 100°C. The heat is conducted rapidly through the metal sheets/trays and mesh to the food surface allowing rapid sublimation. The mesh is important because the water vapour is allowed to escape into the chamber; this would not be possible if the food was in contact with a continuous metal surface. If the water vapour could not escape, the pressure at the food surface would increase and the food would melt.

Figure 11 - Arrangement of food in chamber for AFD process

As the sublimation front penetrates the food, the pressure is applied to the plates (generally by a hydraulic ram as indicated in Figure 11) up to a maximum force of about 8 Ib/in², which causes the mesh to penetrate the surface of the food giving more direct heat conduction to the sublimation front. Simultaneously, the temperature of the heating material is reduced since, after sublimation, the surface temperature of the food will be the same as the heat source.

Care has to be taken not to force the mesh beyond the sublimation front because this will cause thawing at the centre. Similarly this process cannot be used for foods containing bone, cartilage or substantial fatty tissue, all of which will conduct heat beyond the sublimation point resulting in thawing.

Where the fat is distributed more evenly, as with fatty fish, a lower temperature is used while, with other products, the risk of mechanical damage limits the pressure that can be applied to the plates.

Adoption of AFD may halve the process time associated with freeze drying (from 10 - 12 hours to 6 - 7 hours) but this depends very much on the thickness of the food, acceptable temperatures and plate pressures. Current research is attempting to further accelerate heat transfer.

Practical investigations often indicate that freeze dried prawns are frequently superior to those frozen individually using liquid nitrogen, and always superior to those frozen by other techniques, but production costs are higher. For fin fish produce, the production cost is too high unless weight and reconstitutability considerations outweigh all others. The end product has to be cooked during or after rehydration and it has been reported that water retention is poor.

Conclusions

Freeze drying and accelerated freeze drying are processes which have high capital, running and maintenance costs and are, therefore, limited to high cost and speciality foods. However, when used for such products, superior quality goods are obtained if the process is carefully controlled.