Freeze Drying: a Cool Way to Dry Foods

Drying or dehydration is a method of preservation for foods employed from ancient times to prevent or delay the spoilage of fresh food. When fruits and vegetables are harvested, they are consumed fresh or after cooking. When people have more than they can eat immediately, the remainder, if not preserved, will spoil. If these are preserved then they could be eaten some time later when needed. So drying is a very important method of preserving foods.

Drying is carried out very commonly. Fishermen dry fish near the sea. Sometimes papads are seen being dried. Near Nasik grape vineyards, grapes are seen being dried into raisins. Farmers dry grains in fields.

If fish is not dried then it will spoil because of enzymes and microbes and it becomes inedible or even unsafe to eat. Bread can become mouldy if kept out for a few days. Fruits like bananas and mango also become mouldy and milk sours because of bacterial fermentation.

Microbes grow on foods that have plenty of moisture as it needs water to grow. It is possible to slow down or prevent growth by keeping food at very low temperatures or using salt or sugar at high concentrations (like in pickles or jam) or usingpreservatives or removing the moisture by drying. This is how earlier civilisations used to save the excess food for later use after eating what they could.

Drying Methods

Sun drying was the earliest method. This was used by farmers to dry grains in fields as well as fruits in orchards. When plenty of land is available with good sunshine and warm weather, this is ideal. Many dry fruits were prepared in warmer Middle Eastern countries.

However, when land, sun and weather are not favourable then mechanical drying with chambers, with hot air with or without vacuum, has been used. There are many different forms of mechanical dryers, a popular being spray dryer to prepare dry powders. Another form of dryer is now becoming useful because of the high quality of products namely, freeze dryer.

Freeze Drying

This technique was tried first in World War II for blood plasma and penicillin, as they wanted to transport them without spoilage when refrigerated transport was lacking. For foods, this method was used for military and astronauts whose dried ration was difficult to rehydrate. Freeze-dried foods were superior.

While other drying methods remove moisture from foods converting liquid to vapour. While vapour was removed the food would shrink as the remaining food was pliable. The freeze-drying removes the moisture by converting ice to vapour through what is called sublimation. The food remains rock solid throughout the drying process as moisture is present in food as just ice. That resists shrinkage so the food retains its original shape and size. The only change is the loss of moisture.

Triple Point of Water

Water under atmospheric conditions boils at 100ºC and freezes at 0ºC as shown in the above diagram. At a temperature lower than 0ºC, it exists as ice. As it is heated it melts and by further heating beyond 100ºC, it forms steam or water vapour. As one lowers the pressure, by creating a vacuum, the water converts to vapour at a lower temperature and as the pressure is lowered to 0.006 atmospheres, it reaches the triple point of water. Below this pressure, ice directly converts to vapour without going through a liquid state of water. The process is called sublimation.

This phenomenon is used in freeze-drying. Food temperature is lowered by cooling till it freezes. Then the pressure is reduced below 0.006 atmospheres, by applying high vacuum. Then heat is applied which will convert directly the ice to water vapour, without forming liquid water. Slowly the water is removed.

Water is present in foods in two forms, free and bound water. Free water exists without being attached to any other substance and when food is squeezed it oozes out as liquid. It gets frozen when temperature is lowered. Bound water exists as solvent as well as is attached to many large substances like protein and starch and is more difficult to remove from foods. There are 2 stages for drying. In the first stage, free water is removed which is actually in the form of ice. The remaining water, called bound water, is attached to different substances and is more difficult to remove. So, it takes almost a day or more to completely dry the food pieces.

Advantages of Freeze Drying

As the process is carried out at very low temperatures, there is very little loss of vitamins and heat-sensitive materials. It retains beneficial plant compounds e.g. antioxidants such as anthocyanins and flavonoids.

Another advantage is the quality of the rehydrated product with respect to flavour, colour and appearance. As there is very little damage to these since a low level of heating is applied, the quality is very close to the original.

The shelf life of freeze-dried food is also much longer than other methods with most products can last without loss of quality for several years.

Disadvantages

One difficulty is that this method is much more expensive than others. Secondly, since heat is avoided in pre-treatment as it affects the sensory quality, the microbes remain viable in freeze-dried foods. So foods for freeze drying must have excellent microbial quality and should also be stored at the proper temperature and low humidity without allowing moisture to get in as that may trigger the growth of surviving microbes.

Products& Future

There are some freeze-dried products available in the consumer market. Coffee has been freeze-dried for some time. It has an excellent aroma and is also easy to disperse as it is granular and unlike the spray-dried coffee, it does not float on the surface resisting dispersion.

Many fruits are now being freeze-dried and have excellent flavour as well as appearance as they do not undergo shrinkage. Although these are more expensive the cost of freeze drying has come down and also the better sensory properties justify it.

Since the process is more expensive than other drying methods, only the expensive commodities will be freeze-dried as consumers are not going to pay very high prices for ordinary foods. Developments in design such as radiant freeze dryers and microwave-assisted freeze dryers will further enable cost-effective production.