Freeze drying technology recommends itself for removing moisture from food and pharmaceutical products because of the low temperature drying conditions where water is sublimed from solid to vapor phase under vacuum. The high investment cost and exorbitant recurring cost for processing make this technology beyond the reach of most entrepreneurs. Freeze drying or lyophilization is still the preferred mode of drying for highly thermal sensitive materials like live cultures, vaccines, high value flavors etc. Commercial freeze dryers are far and few probably because of the cost factor. Establishment of a Freeze Drying Plant for meat for exclusive use of army personnel in nineteen sixties in Tundla, near Agra in India never took off because of the non-acceptance of the dried product by the end users due to undesirable textural changes.
Spray drying, a popular technology being adopted by food industry, especially dairy products industry, is based on concentration of the liquid products to optimum solid levels followed by spraying the intermediate product into a hot chamber where incoming hot air carries the moisture off while the resultant dry powder settles down at the bottom of the chamber to be drawn off. The concentration step, mostly under vacuum does cause some damage to the biochemically active or thermally sensitive constituents. Though contact with hot air in the spray drying chamber is for a small duration, here again some damage to the end product does take place. Most of the milk powder preparations and instant coffee are manufactured by spray drying. New innovative efforts have enabled the industry to dry highly hygroscopic and heat sensitive materials using carriers for aiding the drying operation, preserve the product from moisture ingress and prevent loss of volatile principles. Cost wise water evaporation through spray drying is much less expensive compared to freeze drying and hence its universal adoption.
Fruit and vegetables are generally preserved by sun drying, especially in rural areas where mechanical drying is not feasible due to severe infrastructure limitations. There are several drying systems using different heat sources and various scientific principles. While low capacity batch dryers use static product configuration and moving hot air systems, continuous dryers with large capacity are operated by large scale processors. It was in nineteen sixties that several continuous hot air dryer of high capacity were imported into India from Bulgaria for processing onions for export. To day there are many indigenous fabricators in India, capable of making hot air driers like cross flow or through flow or fluidized bed driers, using almost all fuels including solar energy for generating hot air. Plant materials like spices, condiments, herbs, nuts, vegetables, some fruits etc are processed in such driers.
If food products are to be protected from losing their nutrients and aroma, freeze drying has been the main choice. If the recent claims by a patent filed in the US are to be believed, a newer option is available to the food processing industry and the Radiant Zone Drying (RZD) technology as it is being called, can protect heat sensitive phytochemicals in fruit juices by controlled heat input and lower product temperature. According to the innovators RZD technology can dry sticky and amorphous products with practically no carriers, it is 95% energy efficient, has low utility cost and processing cost is less than other drying technologies. This liquid drying technology useful for juices, purees, extracts and slurries can yield dry powders of unparalleled quality and purity. The programmed drying under this technology uses consecutive drying zones of varying temperatures and compensates for the change from constant drying period when moisture is high to falling rate drying period at lower moisture levels towards the end.
Under RZD regime liquid products are deposited on moving clear polyester belts under which radiant heaters are positioned and infra red sensors above the belt monitor the product temperature accurately. The Dryer is divided into multiple temperature zones down the length of the drying chamber and the temperature is maintained between 45C and 90C. The zonal arrangement is such that it allows more heat input during the initial stages to allow for maximum evaporation without raising product temperature because of large latent heat content of water present at the beginning. Water vapor is prone to flash off while the product viscosity is still fluid. High production rate and minimum thermal damage are characteristic features of the technology. As the technology is under the Patent regime, RZD plants will have to be procured from licensed fabricators in the US and there fore can be expensive. RZD technology is supposed to bridge the gap between high quality freeze drying and low cost spray drying processes.