By means of freeze-drying (lyophilization), many kinds of biological matter can be preserved without damage or changes in quality or viability for extended periods. In the freeze-drying process, the water contained in the biological structure is frozen; the ice is then removed by sublimation (turning directly into vapor without passing through the liquid stage). By this method the twin advantages of freezing and drying are combined into one favorable means of preservation. Adequately packaged, the resulting product can be kept for years at room temperature.

Freeze-drying is a process which can be applied to three broad categories of biological products:

1. Non-living matter such as blood plasma, serum, hormone solutions, pharmaceutical products and food.
2. Special matter for surgical transplants such as arteries, skin and bone.
3. Live cells intended to be kept alive for long periods of time. This category includes bacteria, viruses and yeast but not mammalian cells.

Freeze-drying is widely usedfor the preservation of blood plasma and food due to the fact that the process prevents the growth of microorganisms such as fungus and mould, it inhibits deterioration due to chemical reactions such as decoloration and taste degradation or staleness, and it prevents loss of organoleptic or physiological properties. The process facilitates the distribution and storage of food because it is not necessary to keep it cold. In the case of food preservation, the process features two additional advantages: the product does not change shape and can readily be re-hydrated.

Freeze-dried products currently on the market include antibiotics and other pharmaceutical products, granulated instant coffee and certain soups.

For Freeze-drying, the product to be treated is placed in a chamber in which vacuum is rapidly produced. As pressure in the chamber drops, temperature also drops and the water containedin the product freezes. Next, still under vacuum, the product is heated and ice sublimation occurs. The vacuum and heating temperature involved in the process vary according to the product.

Conventional industrial installations produce vacuum by means of a combination of vacuum pumps and cold traps which operate at -40 or -50 °C, to freeze water to be extracted from the product and to reduce the pressure within the freeze-drying chamber. The large mechanical vacuum pumps and freezing equipment require specialized labor to run and maintain - which increases cost. In INVAP freeze-drying plants, vacuum is produced by means of vapor ejectors, a technological innovation which simplifies the process and cuts operating costs.

Ejectors produce vacuum by a passive process, and need only vapor to work. They require little maintenance and are very easy to operate, which comes to bear on a decision regarding the location the installation. The system requires only gas, electricity and water to operate.

This Food Freeze-drying Plant was INVAP's first venture into the field of food industry.
INVAP designed and built this plant from the basic principles through operations start-up in the town of Gaiman, in the province of Chubut, for Nutripac S.A., an Argentine food processing company.

The plant built by INVAP to freeze-dry food makes use of a set of supersonic ejectors, a starting ejector and two barometric condensers. Vacuum is maintained by means of a tall liquid column so the structure is 22 meters high.

The process features the following stages.

• The food to be processed is prepared and cooked.
• The product is then pre-frozen (if necessary) and stored in special baskets.
• Next, the product is placed in the freeze-drying chamber to undergo freeze-drying.
• Finally, the product is unloaded and packed in plastic bags under an inertnitrogen atmosphere, to be distributed and marketed.

The volume of production varies according to the product undergoing the process and work shifts. The average daily output is 300 kg of freeze-dried products.

To give an idea of the size of the facility, just the pipe connecting the freeze-drying chamber to the first stage ejector is 1 meter in diameter and nearly 9 meters long.

The plant started operating by the end of 1999.

INVAP offers verification and validation services in its Pilot Plant and Laboratory to determine the freeze-drying parameters for the design and operation of freeze-drying plants.

The facility to do this consists of experimental equipment which closely reproduces the operating conditions of the industrial equipment. This validation is a necessary step towards the design of any industrial facility, since it measures the operating parameters which differ widely between substrates.