Microorganisms Common to Food
A major challenge for the sanitarian is to
protect the production area and other
involved locations against microbes that can
reduce the wholesomeness of food. Microor-
ganisms can contaminate and affect food,
with dangerous consequences to consumers.
The microorganisms most common to food
are bacteriaandfungi. The fungi, which are
less common than bacteria, consist of two
major microorganisms: molds (which are
multicellular) and yeasts (which are usually
unicellular). Bacteria, which usually grow at
the expense of fungi, are unicellular. Viruses,
although transmitted more from person to
person than via food, should also be men-
tioned because they may contaminate food as
a consequence of poor employee hygiene.
Molds
Molds are multicellular microorganisms
(eukaryotic cells) with mycelial (filamentous)
morphology. They consist of tubular cells,
ranging from 30 to 100 μm in diameter,
calledhyphae, which form a macroscopic
mass called a mycelium. Molds are charac-
terized by their display of a variety of colors
and are generally recognized by their
mildewy or fuzzy, cottonlike appearance.
They can develop numerous tiny spores that
are found in the air and can be spread by air
currents. These can produce new mold
growth if they are transferred to a location
that has conditions conducive to germina-
tion. Molds generally withstand greater vari-
ations in pH than do bacteria and yeasts and
can frequently tolerate greater temperature
variations. Although molds thrive best at or
near a pH of 7.0, a range from 2.0 to 8.0 can
be tolerated, though an acid-to-neutral pH is
preferred. Molds are thriftier at ambient
temperature than in a colder environment,
even though growth can occur below 0 ̊C.
Although they prefer a minimum water
activity (Aw) of approximately 0.90, growth
of a few osmiophilic molds can and does
occur at a level as low as 0.60. (Water activ-
ity is explained later in this chapter.) At an
Awof 0.90 or higher, bacteria and yeasts
grow more effectively and normally utilize
available nutrients for growth at the expense
of molds. When the Awgoes below 0.90,
molds are more likely to grow. Foods such as
pastries, cheeses, and nuts that are low in
moisture content are more likely to spoil
from mold growth.
Molds have been considered beneficial
and troublesome, ubiquitous microorgan-
isms. They often work in combination with
yeasts and bacteria to produce numerous
indigenous fermented foods and are involved
in industrial processes to produce organic
acids and enzymes. Molds are a major con-
tributor to food product recalls. Most do not
cause health hazards, but some produce
mycotoxins that are toxic, carcinogenic,
mutagenic, or teratogenic to humans and
animals.
Molds spread because they may be air-
borne. These fungi cause various degrees of
visible deterioration and decomposition of
foods. Their growth is identifiable through
rot spots, scabs, slime, cottony mycelium, or
colored sporulating mold. Molds may pro-
duce abnormal flavors and odors due to fer-
mentative, lipolytic, and proteolytic changes
caused by enzymatic reactions with carbohy-
drates, fats, and proteins in foods.
Molds have an absolute requirement for
oxygen and are inhibited by high levels of
carbon dioxide (5% to 8%). Their diversity is
evident through the ability to function as
oxygen scavengers and to grow at very low
levels of oxygen and even in vacuum pack-
ages. Some halophilic molds can tolerate a
salt concentration of over 20%.
Because molds are difficult to control, food
processors have encountered spoilage prob-
lems cauesd by these microorganisms. In the