this in diameter. (A micron is one-thousandth of a millimeter (0.001 mm) or about
0.00004 inch.) All bacteria can penetrate the smallest of openings, and many can pass
through the natural pores of an egg shell once the natural bloom of the shell is worn or
washed away.
b. Molds. Molds are larger than bacteria and yeast and more complex in
structure. They grow by a network of hair-like fibers called mycelia and send up fruiting
bodies that produce mold spores referred to as conidia. The blackness of bread mold
and the blue-colored veins of blue cheese are due to the conidia, while beneath the
fruiting heads, the hair-like mycelia anchor the mold to the food. The mycelia are a
micron or so in thickness and, like bacteria, can penetrate the smallest opening; or in
the case of weakened skin or shell can digest the skin and make their own route of
penetration.
c. Yeasts. Yeasts are somewhat larger than bacteria, of the order of 20
microns in individual cell length and about half this size in diameter. However, yeasts
are smaller than molds. Most yeasts are spherical or ellipsoidal in shape. Most yeast
cultures are cream, tan, or gray. However, some are yellow, pink, red, green, or brown.
Yeasts are associated with nearly all types of food products. Foods such as fresh
vegetables, meat, poultry, and cheese often contain yeasts, but in these foods, bacteria
outgrow the yeasts. When bacterial inhibitors are added, yeasts can dominate. Some
yeasts are found in foods such as honey, molasses, sugar, and fruit. Salt-tolerant
yeasts grow as films on brine food and on salted food and ham.
1-7.
ACTION OF FOOD ENZYMES
A second major cause of food deterioration is the actions of food enzymes.
Enzymes are organic catalysts which are produced by the cells of animals, plants, or
bacteria. We know that microorganisms possess enzymes which produce fermentation,
rancidity, and putrefaction of foods. Likewise, uninfected food plants and animals have
their own enzyme complement, the activity of which largely survives harvest and
slaughter. Cereal grains recovered after 60 years of storage have been found to still
possess the properties of respiration, germination, and growth--all enzyme controlled
functions. Unless these enzymes are inactivated by heat, chemicals, or some other
means, they continue to catalyze chemical reactions within foods. Some of these
reactions, if not allowed to go too far, are highly desirable, for example, continued
ripening of tomatoes after they are picked and natural tenderizing of beef on aging. But
ripening and tenderizing beyond an optimum point become food deterioration. The
weakened tissues are subject to microbial infections and the deterioration reaches the
point of rotting. This can happen in the field, commissary, and home refrigerator, given
sufficient time.
MD0723
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