3-15. PRESERVATION BY DRYING
Drying is the loss of moisture. Moisture can be removed from foods by natural
drying, evaporation, or dehydration. Reducing the water content of foods concentrates
the sugar, salts, and minerals and retards the activity of microorganisms. The drying
process is hastened by a greater flow of air over the produce, higher temperatures, a
product with porous surfaces, and a product with large surface areas.
a. Natural Drying. The natural drying method is used for grains, nuts, spices,
legumes such as peas and beans, some fruit and vegetables, and some meats. Food
products dried by the sun include raisins, prunes, figs, dates, beans, peas, and jerky.
The advantages of sun drying are low equipment cost and no fuel cost. There are some
disadvantages, such as a loss of sugar from the food, no control over the temperature,
humidity, and air flow, no control over the weather, and some contamination of food
from the atmosphere.
b. Evaporation. In this method, evaporation, the only variable under control, is
the temperature. The method is limited for the most part to milk.
c. Spray Drying. Spray drying is used with liquids. The liquid is sent down
high-pressure hoses and into a high-pressure nozzle that micronizes the liquid into a
superfine mist. The mist enters a sterilized room that has rapid moving, superheated air
(400F) and settles onto the floor as a powder. Spray drying is utilized to produce milk
and powdered eggs.
d. Drum Drying. Drum drying is used with heavy liquids. The product is dried
on the surface of steam-heated (300F) steel drums. Drum drying is utilized to produce
tomato paste and purees.
e. Dehydration. Dehydration is the most common method used for drying
foods. Under this method, temperature, relative humidity, and airflow (air-movement)
are all controlled. Atmospheric dehydrators and vacuum dehydrators are used to dry
foods. The advantages of the vacuum dehydrator are less oxidation (due to the
removal of free oxygen), operation at lower temperatures, and rapid diffusion from cells.
f. Effects. Some of the effects of drying, including dehydration, are
Nutrients are concentrated through a reduction of water content in the
(2) There is a reduction of water-soluble vitamins such as ascorbic acid,
riboflavin, and thiamin.
(3) Proteins may become somewhat denatured because of high
temperature used in drying.