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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.
Enzymes
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.
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Did You Know?
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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.
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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.
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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.
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