Fundamental Food Microbiology, Third Edition - Fuad Fathir

NORMAL MICROBIOLOGICAL QUALITY OF FOODS AND ITS SIGNIFICANCE 47
ter). 1,2,7 Fish and shellfish are harvested from natural sources and aquacultures. In
general, they are rich in protein and nonprotein nitrogenous compounds; their fat
content varies with type and season. Except for mollusks, they are very low in
carbohydrates; mollusks contain about 3% glycogen.
The microbial population in these products varies greatly with the pollution level
and temperature of the water. Bacteria from many groups, as well as viruses, parasites,
and protozoa, can be present in the raw materials. Muscles of fish and shellfish
are sterile, but scales, gills, and intestines harbor microorganisms. Finfish and crustaceans
can have 10 3–8 bacterial cells/g. During feeding, mollusks filter large volumes
of water and can thus concentrate bacteria and viruses. Products harvested from
marine environments can have halophilic vibrios as well as Pseudomonas, Alteromonas,
Flavobacterium, Enterococcus, Micrococcus, coliforms, and pathogens such
as Vib. parahaemolyticus, Vib. vulnificus, and Clo. botulinum type E. Freshwater fish
generally have Pseudomonas, Flavobacterium, Enterococcus, Micrococcus, Bacillus,
and coliforms. Fish and shellfish harvested from water polluted with human and
animal waste can contain Salmonella, Shigella, Clo. perfringens, Vib. cholerae, and
hepatitis A and Norwalk-like viruses. They can also contain opportunistic pathogens
such as Aeromonas hydrophila and Plesiomonas shigelloides. The harvesting of
seafoods, especially shellfish, is controlled by regulatory agencies in the U.S. Water
with high coliform populations is closed to harvest.
Following harvest, microorganisms can grow rapidly in fish and crustaceans
because of high A w and high pH of the tissue and availability of large amounts of
nonprotein nitrogenous compounds. As many of the bacterial species are psychrotrophs,
they can grow at refrigerated temperature. Pathogens can remain viable for a
long time during storage. Microbial loads are greatly reduced during their subsequent
heat processing to produce different products.
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VI. VEGETABLES, FRUITS, AND NUTS
Vegetables include edible plant components such as leaves, stalks, roots, tubers,
bulbs, and flowers. In general, they are relatively high in carbohydrates, with pH
values of 5.0 to 7.0. Thus, different types of bacteria, yeasts, and molds can grow
if other conditions are favorable. 1,2,8 Fruits are high in carbohydrates, and have a pH
of 4.5 or below because of the presence of organic acids, and some also have
antimicrobial essential oils. Nuts can be from the ground (peanuts) or from trees
(pecans) and have protective shells and low A w (0.7). They are converted to nutmeats
for further use or to products such as peanut butter.
Microorganisms in vegetables can come from several sources, such as soil, water,
air, animals, insects, birds, or equipment, and vary with the types of vegetables. A
leafy vegetable has more microorganisms from the air, whereas a tuber has more
from the soil. Microbial levels and types in these products also vary greatly, depending
on environmental conditions and conditions of farming and harvesting. Generally,
vegetables have 10 3–5 microorganisms/cm 2 or 10 4–7 /g. Some of the predominant
bacterial types are lactic acid bacteria, Corynebacterium, Enterobacter, Proteus,
Pseudomonas, Micrococcus, Enterococcus, and sporeformers. They also have