Chapter 18
Protection of Foods by Drying
The preservation of foods by drying is based on the fact that microorganisms and enzymes need
water in order to be active. In preserving foods by this method, one seeks to lower the moisture content
to a point where the activities of food-spoilage and food-poisoning microorganisms are inhibited.
Dried, desiccated, or low-moisture (LM) foods are those that generally do not contain more than 25%
moisture and have a water activity (a w ) between 0.00 and 0.60. These are the traditional dried foods.
Freeze-dried foods are also in this category. Another category of shelf-stable foods are those that contain
between 15% and 50% moisture and an a w between 0.60 and 0.85. These are the intermediate-moisture
(IM) foods. Some of the microbiological aspects of IM and LM foods are dealt with in this chapter.
PREPARATION AND DRYING OF LOW-MOISTURE FOODS
The earliest uses of food desiccation consisted of exposing fresh foods to sunlight until drying had
been achieved. Through this method of drying, which is referred to as sun drying, certain foods may
be successfully preserved if the temperature and relative humidity (RH) allow. Fruits such as grapes,
prunes, figs, and apricots may be dried by this method, which requires a large amount of space for large
quantities of the product. The drying methods of greatest commercial importance consist of spray,
drum, evaporation, and freeze-drying.
Preparatory to drying, foods are handled in much the same manner as for freezing, with a few
exceptions. In the drying of fruits such as prunes, alkali dipping is employed by immersing the fruits
into hot lye solutions of between 0.1% and 1.5%. This is especially true when sun drying is employed.
Light-colored fruits and certain vegetables are treated with SO 2 so that levels of between 1,000 and
3,000 ppm may be absorbed. The latter treatment helps to maintain color, conserve certain vitamins,
prevent storage changes, and reduce the microbial load. After drying, fruits are usually heat pasteurized
at 150–185
◦ F (65.6–85
◦ C) for 30–70 minutes.
Similar to the freezing preparation of vegetable foods, blanching or scalding is a vital step prior to
dehydration. This may be achieved by immersion from 1 to 8 minutes, depending on the product. The
primary function of this step is to destroy enzymes that may become active and bring about undesirable
changes in the finished product. Leafy vegetables generally require less time than peas, beans, or
carrots. For drying, temperatures of 140–145
◦ F (60–62.8
◦ C) have been found to be safe for many
vegetables. The moisture content of vegetables should be reduced below 4% in order to have satisfactory
storage life and quality. Many vegetables may be made more stable if given a treatment with SO 2 or
a sulfite. The drying of vegetables is usually achieved by use of tunnel-, belt-, or cabinet-type driers.
443
Protection of Foods by Drying
The preservation of foods by drying is based on the fact that microorganisms and enzymes need
water in order to be active. In preserving foods by this method, one seeks to lower the moisture content
to a point where the activities of food-spoilage and food-poisoning microorganisms are inhibited.
Dried, desiccated, or low-moisture (LM) foods are those that generally do not contain more than 25%
moisture and have a water activity (a w ) between 0.00 and 0.60. These are the traditional dried foods.
Freeze-dried foods are also in this category. Another category of shelf-stable foods are those that contain
between 15% and 50% moisture and an a w between 0.60 and 0.85. These are the intermediate-moisture
(IM) foods. Some of the microbiological aspects of IM and LM foods are dealt with in this chapter.
PREPARATION AND DRYING OF LOW-MOISTURE FOODS
The earliest uses of food desiccation consisted of exposing fresh foods to sunlight until drying had
been achieved. Through this method of drying, which is referred to as sun drying, certain foods may
be successfully preserved if the temperature and relative humidity (RH) allow. Fruits such as grapes,
prunes, figs, and apricots may be dried by this method, which requires a large amount of space for large
quantities of the product. The drying methods of greatest commercial importance consist of spray,
drum, evaporation, and freeze-drying.
Preparatory to drying, foods are handled in much the same manner as for freezing, with a few
exceptions. In the drying of fruits such as prunes, alkali dipping is employed by immersing the fruits
into hot lye solutions of between 0.1% and 1.5%. This is especially true when sun drying is employed.
Light-colored fruits and certain vegetables are treated with SO 2 so that levels of between 1,000 and
3,000 ppm may be absorbed. The latter treatment helps to maintain color, conserve certain vitamins,
prevent storage changes, and reduce the microbial load. After drying, fruits are usually heat pasteurized
at 150–185
◦ F (65.6–85
◦ C) for 30–70 minutes.
Similar to the freezing preparation of vegetable foods, blanching or scalding is a vital step prior to
dehydration. This may be achieved by immersion from 1 to 8 minutes, depending on the product. The
primary function of this step is to destroy enzymes that may become active and bring about undesirable
changes in the finished product. Leafy vegetables generally require less time than peas, beans, or
carrots. For drying, temperatures of 140–145
◦ F (60–62.8
◦ C) have been found to be safe for many
vegetables. The moisture content of vegetables should be reduced below 4% in order to have satisfactory
storage life and quality. Many vegetables may be made more stable if given a treatment with SO 2 or
a sulfite. The drying of vegetables is usually achieved by use of tunnel-, belt-, or cabinet-type driers.
443