The Process Of Freeze Drying
From time in memoir there has always been that need to store surplus food for future use. While most cultures have almost similar means to achieve this, an amalgam of storage, options have come up over the years. Such an example of use of traditional and technological means of preservation is freeze drying.
It is rather conspicuous in cultures across the globe that sun drying has always been a preferred choice of preservation. While using the readily available sun rays to rid of moisture and food spoiling micro-organism, may seem so ancient, it still remains a cheaper option. However, not all cultures or people readily have sun shine at their disposal, often cold areas in both north and south pole use freezing as an ideal food preservation method.
Freeze drying can be traced to Andean civilization; however, it is not until world war two that it was exploited heavily. Faced with logistical problems of transporting blood and serum to soldiers in the battle field, there was heavy loss due to spoilage. Freeze drying offered a more reliable means of transportation of serum amongst other biological products without compromising the quality of the substance. This has further been incorporated to the modern day food industry given the long distances required to transport products.
How freeze drying is done
Generally the process entails a four stage process that eventually guarantees longevity of the food or biological serum. These can be essentially divided into pre-treatment, freezing, primary drying, and secondary drying.
As the name suggests, this stage is a precursor to the treatment proper. It involves the manipulation of the food properties before treatment. Food products are made up of ingredients which at times are the ones solely responsible for the spoilage regardless of preservation techniques employed. This is achieved through means like concentration, vaporization amongst other ways. This can be done through addition of ingredients or subsequent removal of active ingredients. The focus is mainly on stabilisation of active ingredients, increase of surface area, and decrease of high vapor solvents in the food.
This is perhaps the most self-explanatory stage of preservation. More often than not, the pre-treated food is subjected to temperatures triple times their actual triple point (Lowest temperature that both the solid and liquid phases can co-exist). While in large scale preservation, this is achieved through a freeze drying machine. However depending on the contents of the food annealing, cyclic freezing of varied temperatures or rapid freezing can be used. Rapid is often the best choice in biological and nutritionally volatile food products that might lose their value due to the cyclic cooling. Liquid nitrogen or temperatures ranging between 50 and – 80 degrees are used in this phase.
The removal of 95 percent of moisture is enabled using pressurized dry air (heat is supplied). Exploiting the latent heat of sublimation, a partial vacuum and generally done over a long period of time brings about the drying process.
This follows the primary phase and often serves as the final removal of frozen water particles in the food. Upon removal of water (this is done at a slightly higher temperature using adsorption isotherms), the vacuum is often filled with inert gases like nitrogen before sealing to avoid spoilage by micro-organisms.