Preservation of bio-degradables is an important aspect. Traditionally Sun drying of pulses and some vegetables constituted preservation. Sunlight not only removed maximum moisture but also its UV rays killed micro organisms facilitating longer shelf life. An advantage of Sun drying is that the natural taste would return upon reconstitution.
Air and water tight packing in polyethylene/PVC lined aluminium foils and aluminium lined tetra packs provide advanced protection from atmosphere. The sea food, meat, pulses etc. can be packed with natural freshness. When stored in deep freezers, these would retain their natural freshness for longer periods. One aspect to note with such packing was that asceptic cleaning or sterilization was difficult in large scale production.
Food grains, in large quantities may be exposed to nuclear(gamma) radiation which destroys micro organisms, germs and rodents. Bhaba Atomic Research Center in India has developed a technic for exposing the food to such radiations. This is now in effective use and is proven a boon for mass scale storage and longer shelf life.
However human organs, corpses or dead pets etc. were stored in cryogenic state (upto -180 degree Celisus produced by LN2), a technique not suitable for vaccines.
Freeze drying removes water content upto 98% (depending on duration of freeze drying) while retaining nature of ingredients.
The argument against freeze drying is that it is destructive of tissue structure and leaves the dried food with a woody texture formed due to ice crystal growth during freeze drying. The leftover moisture leads to deterioration after a few years. Structural deterioration due to porous surface left behind by ice crystals worsen the things.
However, these technical problems can be overcome by common table sugar which inhibits ice crystal growth as well as prevents denaturation. Brine shrimps, nematods, seeds are lyophilized(freeze dried) in presence of sugars to prevent complete desication. Structural decay may be taken care off by chemical fixatives like formaldehyde and glutaraldehyde and storing in oxygen in water tight and air tight containers.
The post production sterilization or autoclaving of containers of injectables being not possible in most cases, freeze drying comes to the rescue.The rise and fall of temperature and pressure being absent in lyophilization, keeps the nature of fluids intact. The freeze dried products can be stored at room temperature.
The principle of freeze drying is sublimation. Meaning that, under low pressure (0.06 atm) or vacuum, solid water converts directly to gaseous form.
The disadvantage with cryogenic preservation is the enormous cost over long periods. But liquid nitrogen (cryogenics) has its own advantage that it prevents frozen tissues from suffering from other water and oxygen dependant deterioration.
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