Individual mold cells have rigid walls surrounding the protoplasm. The cells are often
cylindrical, and they vary greatly in size. Molds can be seen with the naked eye when
they grow in sufficient quantities. Molds multiply by spore formation. They are usually
found growing on solid substances such as wood, paper, cloth, leather, meat, fruits,
vegetables, and many other substances. Molds have definite colors such as white,
black, and green. They can grow in foods containing high percentages of sugar and
salt. Molds are capable of growing in a pH range of 2.0 to 8.5. Molds can grow at a
temperature as low as 15F (-9.45C), and a moisture content as low as 6 percent.
They produce large numbers of spores that are light in weight and are easily
transported by air currents. Molds are responsible for a wide variety of losses in food
products, but they are also desirable in several industrial processes (penicillin
production) and are responsible for the appearance and flavor of cheeses such as
Roquefort and Bleu.
1-10. YEASTS
Yeasts (figure 1-4) are unicellular organisms that are rather large when compared to
bacteria. Like the molds, the yeasts belong to the division Eumycetes. Yeast cells are
spherical, elliptical, or cylindrical. The protoplasm of a yeast cell is enclosed by a cell
wall and cytoplasmic membrane. It contains a nucleus but there are no flagella present
for locomotion. They reproduce by budding and have the ability to ferment sugars and
to produce alcohol and carbon dioxide. They can grow in higher concentrations of salt
and sugar than bacteria are able to tolerate. They prefer an acid medium of pH 5.5 to
6.5. Yeasts need more moisture (15 percent) than molds but require less moisture than
bacteria. Yeasts are capable of growing at lower temperatures than most species of
bacteria. Generally speaking, yeasts occupy a position intermediate in range between
molds and bacteria relative to their tolerance for extremes of sugar, salt, acid, moisture,
and temperature. Yeasts are important in bread making and the manufacture of
alcohol.
Figure 1-4. Yeasts.
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