a. Action of Molds in Apple Decay. If an apple from a tree falls to the ground,
it is bruised in the fall. The microorganisms of the soil are present in billions per
spoonful and enter the torn cells caused by the bruise. As the apple juice is slightly
acid, most bacteria cannot grow very well in it. But the molds and yeasts can. The
molds force their way from one cell to another, building their roadway in new body
tissue as they go. They use the sugar from the apple juice to yield energy to build more
new tissue. In a few days, they can do an almost complete job of destroying the
structure of the apple. Although the apple has but a trace of protein, it is sufficient for
b. The Decay Process in Apple. While the molds have been feasting, yeasts
have not been mere bystanders. Almost at once, they dominate the bruise of the apple
and ferment the sugars to alcohol. Fruit flies are attracted to this alcohol, and they carry
bacteria. Carried to the fermenting tissue, the vinegar bacteria develop and burn up the
alcohol. Thus the apple is soon gone, used to make new cells of molds, yeasts, and
bacteria. The cellulose that so far remains unchanged is washed by the rain into the
surface of the soil. The apple seeds are now free and can possibly propagate to form,
in time, a new tree.
c. The Process in the Soil. The soil holds the cellulose and, in a few months,
cellulose is moved slowly down into the earth where there is little air. Here live a
number of important bacteria. Because they don't need air, these bacteria burn up the
cellulose and leave acid. This acid is now used by other bacteria, burning it to carbon
dioxide and water. The cycle is complete.
d. The Decay Process in Meat. With meat, we find that the process of
decomposition begins soon after the animal dies. Once dead, the tissue is no longer
able to resist the action of microorganisms. If the carcass is not protected, it will be
reduced eventually to carbon dioxide, ammonia, hydrogen sulfide, and water in much
the same manner as was the apple, but by different organisms.
e. Optimum Growth Conditions. These microorganisms are present in the air,
in soil, in dust, in water, and we carry them ourselves. All they usually need to start the
process is a source of energy; that is, a few drops of juice containing a trace of sugar. If
the food is moist and the organisms have humid growing conditions, the invasion is on.
FOOD SPOILAGE--ENZYME ACTION
The action of enzymes, occurring naturally in the cells of food, is also a leading
cause in deterioration of food after it has been killed or harvested. Enzymes will
naturally diffuse the cells and will chemically combine with other organic compounds or
act as catalysts for chemical reactions that will lead to the deterioration of the product
(self-deterioration). Complex oxidation-reduction reactions, with enzymes playing
primary roles, can lead to such things as rancidity of fats in meats, milk, and other dairy
products and putrefaction of meats and browning reactions in meats and vegetables.
The process of enzyme action can be either increased or retarded, depending on how