g. Organoleptic. Organoleptic (sensory evaluation) relates to or involves the
use of the sense organs in the subjective testing (flavor, odor, appearance) of food
Microorganisms are living things so small that they can be seen individually only
with the aid of a microscope. They are widely distributed in nature and are responsible
for many physical and chemical changes of importance to plants, animals, and humans.
a. Pathogenic and Nonpathogenic. Most students understand that not all
microorganisms are harmful or pathogenic (disease producing), but they do not fully
appreciate the fact that microorganisms make possible the continuing existence of
plants and animals on our earth. In addition, many different kinds of microorganisms
are used by industry to manufacture products of great value to a man such as
antibiotics (penicillin) and various food products, including cheese, bread, and wine.
However, the activities of nonpathogenic microorganisms are not always desirable.
Food products may be spoiled, fabrics and fibers may be rotted, and fermentation
processes may be upset by nonpathogenic but undesirable microorganisms.
b. More Research Needed. We are interested in microorganisms because of
their disease producing potential and because they are capable of causing both good
and bad physical and chemical changes in food, clothing, and the general environment.
We are also interested in ways and means of controlling undesirable organisms and
utilizing efficiently the activities of those that are beneficial. A study of the activities and
means of controlling microorganisms must be based upon the knowledge of their
nature, life, and actions.
c. Culturing Microorganisms. Microorganisms can be cultured (grown) for
study in a specially prepared medium. Various activities or procedures have been
developed in order to grow and isolate certain species of microorganisms.
Microorganisms need nutrients in order to live and grow. The food source used in
laboratories for growing microorganisms is called a culture medium. This medium
includes an organic carbon source, a nitrogen source, inorganic minerals, and any other
nutrients needed. The required nutrients are usually specific nutrients based on the
physiological needs of the particular species of organism being cultured; i.e., certain
amino acids, purines, pyrimidines, and vitamins. For many bacteria, a single compound
(such as amino acid) may serve as an energy source, carbon source, and nitrogen
source. Others require a separate compound for each.
(1) Liquid or solidified culturing media. These culturing media must be
moist since nutrients can only enter the microorganisms by virtue of diffusion or
osmosis. The culturing material can be either liquid media or solidified media. Liquid
culture media are referred to as nutrient broth and are made and kept in test tubes,
while solidified media are made and kept in either test tubes or petri dishes. Agar is
added to nutrient broth to solidify the media. Agar is a polysaccharide extract obtained