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?
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 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.
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.