Chemical Changes During Drying

Equilibrium Relative Humidity

ERH (equilibrium relative humidity) is a term used to describe the availability of the water to the chemical environment and microorganisms present in foods. The percent ERH scale varies from zero to 100. If a food of a certain moisture content is put in a closed chamber which is set to some relative humidity, the food will change its moisture content in an attempt to equal the ERH of the chamber. 

Fresh foods, such as vegetables, meat and fruit products, have an ERH close to 100 percent. This means that the water present in these foods is available for rapid growth of microorganisms leading to deterioration and spoilage of the food. By drying foods it is possible to lower the percent ERH. As the percent ERH is lowered, the growth of microorganisms slows or ceases since the water is no longer available for their growth. This occurs at an ERH of 60 percent. Microorganisms cannot survive below 60 percent ERH level. Therefore, all foods must be dried to below 60 percent ERH and kept in a dry environment to be shelf stable from microbial growth.


Fruits and vegetables contain certain enzymes. Enzymes in the food cause color and flavor changes. Some of them may become more extensive when food surfaces are cut and exposed to air. Although drying foods to about 60 percent ERH prevents microbial growth and drying method slows down the action of enzymes, drying does not inactivate enzymes. Certain chemical reactions and changes caused by enzymes continue during drying and storage and it will result in spoilage and deterioration of the product unless the enzyme activity is retarded or stopped. 

There are many variations in recommendations for treatment before drying. Pretreatment such as antioxidant coating, blanching, and/or sulfering may be recommended.

Real Cooking

Did You Know?
Vacuum-packing stores food in a vacuum environment, usually in an air-tight bag or bottle. The vacuum environment strips bacteria of oxygen needed for survival, slowing spoiling. Vacuum-packing is commonly used for storing nuts to reduce loss of flavor from oxidation.
* * *
One of the oldest methods of food preservation is by drying, which reduces water activity sufficiently to prevent or delay bacterial growth. Drying also reduces weight, making food more portable. Most types of meat can be dried; a good example is beef jerky. Many fruits can also be dried; for example, the process is often applied to apples, pears, bananas, mangoes, papaya, apricot, and coconut. Zante currants, sultanas and raisins are all forms of dried grapes. Drying is also the normal means of preservation for cereal grains such as wheat, maize, oats, barley, rice, millet and rye.
* * *
Meat can be preserved by jugging, the process of stewing the meat (commonly game or fish) in a covered earthenware jug or casserole. The animal to be jugged is usually cut into pieces, placed into a tightly-sealed jug with brine or gravy, and stewed. Red wine and/or the animal's own blood is sometimes added to the cooking liquid. Jugging was a popular method of preserving meat up until the middle of the 20th century.