Course:FNH200/Lessons/Lesson 08/Page 08.4

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8.4 Drying Methods

Sun Drying

Sun drying is mostly used in dry, warm climates. This is a very slow drying method (several days). It is mostly used for fruits, vegetables, and fish. An advantage of this method is the fact that it is quite inexpensive; however, disadvantages include long drying periods (up to several weeks) and the risk of invasion by insects, birds, rodents, and microorganisms.

The appearance is shrunken and has poor rehydration capacity.

Sun dried tomatoes

Spray Drying

Spray driers are used to produce the greatest quantities of commercially dehydrated foods. Spray driers are restricted to use with liquid foods since the principle of the operation is the introduction of the food as a spray of small droplets into a high velocity stream of warm air. Because droplet sizes are small, drying rates are very rapid and high quality dehydrated food products can be produced. Foods most commonly dehydrated by spray driers include skim milk, coffee, tea and eggs.

You will note that the equipment is designed to maximize drying rate, to produce dry particles of uniform size and to prevent sticking of partially dry food particles to the walls of the spray driers. You should also note that spray driers operate continuously, that is, the product is sprayed into the drying chamber and the dried product and moist air are separated and removed from the driers.

Figure 8.2 Spray drier. Source: Food Science and Nutritional health by T.P. Labuza and J.W. Erdman. West Publishing Co., St. Paul MN. 1984.
Want to learn more?
  • Fluid milk and egg products are commonly spray-dried into powder form. Milk and eggs are low-acid foods that are susceptible to growth of pathogenic microorganisms as well as to spoilage by microbes and enzymes.
  • Since the dehydration process does not kill microorganisms nor inactivate enzymes, can you think of additional processes or approaches that can be used in conjunction with dehydration for better preservation of milk and egg powders?

Tray (Air) Drying

Food placed on trays or racks is exposed to heated air at a set velocity. This type of drying can be quite fast and requires heated air with a relative humidity (%RH) lower than that of the product to be dried. Water soluble components migrate to surface of food and are deposited as water evaporates. The dried food has relatively poor re-hydration properties, is shrunken in appearance and is very dense. Products dried this way are: pasta, vegetables, fruit, spices. The diagram below shows the sequence of events that occur during dehydration of fruit tissue.

Figure 8.3 Tray Dryer as Adapted from Labuza and Erdman

Drum Drying

During the process of drum drying, food paste is applied to a heated drum in a thin layer to promote rapid drying. As the drum rotates, it picks up a thin film of food material that dries rapidly. The dried food is scraped off the drum near the end of a full rotation of the drum. Dehydrated mashed potatoes and some ready-to-eat breakfast cereals are dried this way. Some popular low fat snack foods and potato chip like products contain drum dried potato flakes as the primary ingredient.

Figure 8.4 Drium Drier. Source: Food Science and Nutritional health by T.P. Labuza and J.W. Erdman. West Publishing Co., St. Paul MN. 1984.

Freeze Drying

Freeze driers are fairly recent innovations as far as food dehydration is concerned. Freeze drying is restricted to high value foods because of the high costs associated with this dehydration method.

During freeze drying, water is removed from food in the frozen state without transition through the liquid state. This phenomenon, called sublimation, is illustrated below:

water(solid) —> water(vapour)

Sublimation of water is accelerated under vacuum conditions. During freeze drying, food is frozen and then placed in the freeze dryer, the dryer is sealed and a vacuum is created and maintained. Application of heat from radiant heaters within the shelves of the freeze drier provides the energy required for sublimation to occur. During freeze drying the amount of heat applied to the food is carefully controlled to maximize the rate of drying without causing transition of water from the solid to the liquid phase. A schematic diagram of a food being freeze dried is shown in Figure 8.5.

Figure 8.5. Schematic drawing of how food is freeze dried.Adapted from: Potter, N. and Hotchkiss, J.H. 1995. Food Science (5th ed), Ch. 10. Aspen Publishers., p. 229.
Figure 8.6 Freeze drierSource: Food Science and Nutritional health by T.P. Labuza and J.W. Erdman. West Publishing Co., St. Paul MN. 1984.

During freeze drying, because the food remains rigid during dehydration, the subliming water leaves voids where the ice crystals were located. There is no translocation of water-soluble constituents because there is no movement of liquid, allowing freeze dried foods to retain their shape. Also, freeze dried foods rehydrate almost completely because the voids left by the subliming water provide channels through which water can enter the food, and the water-soluble components of the food in their original locations provide the driving force for rehydration. Freeze dried foods do not usually exhibit the shrinkage and chemical changes noted earlier to occur in other dehydrated foods.

  • If you have consumed the instant soup mixes that are prepared in a cup with the addition of boiling water, you will have noted that the vegetables have rehydrated within several minutes and that they possess a fresh flavour. Those vegetable pieces were freeze dried.
  • You may wish to compare the rate and extent of rehydration of vegetable pieces in an instant soup mix (in which vegetables were freeze dried) with those vegetables from a soup mix which has to be boiled for about 10 minutes in order to rehydrate the vegetable pieces (these vegetable pieces were tray-air dried).

Vacuum Microwave Drying

Vacuum microwave or radiant energy vacuum (REV) technology is being developed for the dehydration of food, nutraceutical and pharmaceutical products by Dr. Tim Durance in the Food Science program at the University of British Columbia. The technology consists of a combination of vacuum (in order to keep the temperature low) and microwaves (for ultra-rapid energy transfer), producing high quality products with less nutrient loss, better flavour retention, and less colour change. Vacuum microwave dried (VMD) products retain a more natural appearance and have the advantage of complete re-hydration (reconstitution). VMD is a quick drying method when compared to freeze drying and air drying methods (VMD can take only a few minutes, compared to hours in freeze drying).

For more information, visit (Links to an external site.)

Deep Fat Frying

During deep fat frying, the high temperature of hot oil causes water in the food to evaporate rapidly, often accompanied by substantial pick up of oil by the food being dried. Dried foods such as potato chips have a low water content (as well as a low water activity) but a high oil content. Many snack foods and bakery products, such as donuts, are produced by means of deep fat frying.

Do you know how instant noodles are made? After cooking and shaping, the noodles are dried either by deep-fat frying or non-frying (hot-air) drying. Frying is usually done at 140-150°C for 1-2 minutes, while hot-air drying uses a temperature of about 80°C for 30 min. The two drying methods yield products that differ greatly in fat content.

Extrusion (cooking) Drying

Slurry of food is passed though a tube, under pressure, that is heated by steam. The moist heat causes starch gelatinization and cooking of the product. Product is forced though a narrow opening (a die which can produce a product with a variety of shapes) at the end of the tube and escaping steam causes the dehydrating product to puff. Many ready-to-eat breakfast cereals and snack foods are produced this way.

Figure 8.7. Extrusion (cooking) DryingSource:Understanding Food Science and Technology by P. S. Murano. Wadsworth/Thomson Learning Inc. 2003.

Common food products prepared by extrusion drying

Category Examples
Ready-to-eat breakfast cereals Puffed cereals, flaked cereals, high-fiber strands
Snacks Puffed snacks, Crispbreads
Confections Licorice, some chocolates
Texturised protein Soy meat-analogues, "processed" cheese
Infant foods Biscuits, weaning cereals
Want to learn more?
  • Now that you have read about the different drying methods
    • Which methods would lead to the most changes, or conversely the least changes, in the resulting dehydrated foods?
    • How do dehydrated foods obtained by different drying methods compare in terms of the rate and ease of rehydration?