d. Excessive Moisture. Another form of physical change that causes food
deterioration is excessive moisture.
(1) Foods that take in moisture. The gross changes in foods from excessive
moisture are part of everyday experience. Dried, dehydrated, and freeze-dried foods
are especially susceptible to this form of deterioration. These types of food are very
hygroscopic (readily taking up and retaining moisture); if not properly packaged, the
product will become lumpy or caked if excessive moisture is present. This condition can
possibly lead to other forms of deterioration, such as bacterial growth and chemical
reactions such as oxidation.
(2) Effect of surface moisture. Moisture need not be present throughout the
food to exert major effects. Surface moisture resulting from slight changes in relative
humidity can be a major cause of lumping and caking, as well as surface defects such
as mottling, crystallization, and stickiness. The slightest amount of condensation on the
surface of the food can become a virtual pool for the multiplication of bacteria or the
growth of mold.
(3) Condensation from the food product. This condensation need not come
from the outside. In a moisture-proof package, food materials such as fruits and
vegetables can give off moisture from respiration and transpiration. This moisture is
then trapped within the package and can support the growth of microorganisms.
e. Mechanical Damage. The fifth form of physical change is mechanical
(1) Entering point for microorganisms. When an item receives mechanical
damage, not only is the appearance of the item affected but the damaged food tissue
also becomes more susceptible to other forms of deterioration. Mechanically damaged
foods are more susceptible to invasion by microorganisms, for the damaged area
serves as a port of entry.
(2) Starting point for enzyme activity. The cell walls of foods are also
destroyed by mechanical abrasion, and the inherent enzymes in the food product are
liberated from the cells. Once liberated, the enzymes begin the process of deterioration
or, more specifically, autolysis. The changes noted would be a softening in the texture,
development of off-colors, and development of off-flavors.
f. Light. Light, another form of physical change that causes food deterioration,
can cause fading of color in many food items. Some vitamins are destroyed by light,
notably riboflavin, vitamin A, and vitamin C. Milk in bottles exposed to the sun develops
"sunlight" flavor due to light induced fat oxidation and changes in the protein.