444 Modern Food Microbiology
Meat is usually cooked before being dehydrated. The final moisture content after drying should be
approximately 4% for beef and pork.
Milk is dried as either whole milk or nonfat skim milk. The dehydration may be accomplished by
either the drum or spray method. The removal of about 60% water from whole milk results in the
production of evaporated milk, which has about 11.5% lactose in solution. Sweetened condensed milk
is produced by the addition of sucrose or glucose before evaporation, so that the total average content
of all sugar is about 54%, or over 64% in solution. The stability of sweetened condensed milk is due in
part to the fact that the sugars tie up some of the water and make it unavailable for microbial growth.
Eggs may be dried as whole egg powder, yolks, or egg white. Reducing the glucose content prior
to drying increases dehydration stability. Spray drying is the method most commonly employed.
In freeze drying (lyophilization, cryophilization), actual freezing is preceded by the blanching of
vegetables and the precooking of meats. The rate at which a food material freezes or thaws is influenced
by the following factors:
12
1. the temperature differential between the product and the cooling or heating medium;
2. the means of transferring heat energy to, from, and within the product (conduction, convection,
radiation);
3. the type, size, and shape of the package;
4. the size, shape, and thermal properties of the product.
Rapid freezing has been shown to produce products that are more acceptable than slow freezing.
Rapid freezing allows for the formation of small ice crystals and, consequently, causes less mechanical
damage to the food structure. Upon thawing, fast-frozen foods take up more water and, in general,
display characteristics more like the fresh product than the slow-frozen foods. After freezing, the
water in the form of ice is removed by sublimation. This process is achieved by various means of
heating plus vacuum. The water content of protein foods can be placed into two groups: freezable
and unfreezable. Unfreezable (bound) water has been defined as that which remains unfrozen below
−30
◦ C. The removal of freezable water takes place during the first phases of drying, and this phase
of drying may account for the removal of anywhere from 40% to 95% of the total moisture. The
last water to be removed is generally bound water, some of which may be removed throughout the
drying process. Unless heat treatment is given prior to freeze drying, freeze-dried foods retain their
enzymes. In studies on freeze-dried meats, it has been shown that 40–80% of the enzyme activity is
not destroyed and may be retained after 16 months of storage at −20
◦ C.
24 The final product moisture
level in freeze-dried foods may be about 2–8% or have an a w of 0.10–0.25.
36
Freeze drying is generally preferred to high-temperature vacuum drying. Among the disadvantages
of the latter compared to the former are the following:
17
1. pronounced shrinkage of solids;
2. migration of dissolved constituents to the surface when drying solids;
3. extensive denaturation of proteins;
4. case hardening: the formation of a relatively hard, impervious layer at the surface of a solid,
caused by one or more of the first three changes, that slows the rates of both dehydration and
reconstitution;
5. formation of hard, impervious solids when drying liquid solution;
6. undesirable chemical reactions in heat-sensitive materials;
7. excessive loss of desirable volatile constituents;
8. difficulty of rehydration as a result of one or more of the other changes.
Meat is usually cooked before being dehydrated. The final moisture content after drying should be
approximately 4% for beef and pork.
Milk is dried as either whole milk or nonfat skim milk. The dehydration may be accomplished by
either the drum or spray method. The removal of about 60% water from whole milk results in the
production of evaporated milk, which has about 11.5% lactose in solution. Sweetened condensed milk
is produced by the addition of sucrose or glucose before evaporation, so that the total average content
of all sugar is about 54%, or over 64% in solution. The stability of sweetened condensed milk is due in
part to the fact that the sugars tie up some of the water and make it unavailable for microbial growth.
Eggs may be dried as whole egg powder, yolks, or egg white. Reducing the glucose content prior
to drying increases dehydration stability. Spray drying is the method most commonly employed.
In freeze drying (lyophilization, cryophilization), actual freezing is preceded by the blanching of
vegetables and the precooking of meats. The rate at which a food material freezes or thaws is influenced
by the following factors:
12
1. the temperature differential between the product and the cooling or heating medium;
2. the means of transferring heat energy to, from, and within the product (conduction, convection,
radiation);
3. the type, size, and shape of the package;
4. the size, shape, and thermal properties of the product.
Rapid freezing has been shown to produce products that are more acceptable than slow freezing.
Rapid freezing allows for the formation of small ice crystals and, consequently, causes less mechanical
damage to the food structure. Upon thawing, fast-frozen foods take up more water and, in general,
display characteristics more like the fresh product than the slow-frozen foods. After freezing, the
water in the form of ice is removed by sublimation. This process is achieved by various means of
heating plus vacuum. The water content of protein foods can be placed into two groups: freezable
and unfreezable. Unfreezable (bound) water has been defined as that which remains unfrozen below
−30
◦ C. The removal of freezable water takes place during the first phases of drying, and this phase
of drying may account for the removal of anywhere from 40% to 95% of the total moisture. The
last water to be removed is generally bound water, some of which may be removed throughout the
drying process. Unless heat treatment is given prior to freeze drying, freeze-dried foods retain their
enzymes. In studies on freeze-dried meats, it has been shown that 40–80% of the enzyme activity is
not destroyed and may be retained after 16 months of storage at −20
◦ C.
24 The final product moisture
level in freeze-dried foods may be about 2–8% or have an a w of 0.10–0.25.
36
Freeze drying is generally preferred to high-temperature vacuum drying. Among the disadvantages
of the latter compared to the former are the following:
17
1. pronounced shrinkage of solids;
2. migration of dissolved constituents to the surface when drying solids;
3. extensive denaturation of proteins;
4. case hardening: the formation of a relatively hard, impervious layer at the surface of a solid,
caused by one or more of the first three changes, that slows the rates of both dehydration and
reconstitution;
5. formation of hard, impervious solids when drying liquid solution;
6. undesirable chemical reactions in heat-sensitive materials;
7. excessive loss of desirable volatile constituents;
8. difficulty of rehydration as a result of one or more of the other changes.