Fundamental Food Microbiology, Third Edition - Fuad Fathir

MICROBIAL METABOLISM OF FOOD COMPONENTS 83
used for electron transfer by different groups of microorganisms important in food
can be illustrated by the following scheme:
\
Organic compounds Inorganic compounds (NO3, SO4) 1 � 5 2 � 6 3 4
Substrates Æ (2H:NAD/FD) Æ cytochrome(s) Æ cyt.a Æ O2 Ø 7
H2O2 or O2 Aerobes (some Bacillus spp., Pseudomonas, molds, and yeasts): 1,2,3,4¥
Anaerobes
Cyt. independent, e.g., some Clostridium spp.: 1, 5
Cyt. dependent, e.g., Desulfotomaculum spp.: 1, 2, 6
Facultative anaerobes
Cyt. independent, e.g., lactic acid bacteria, some yeasts
Aerobic: 1, 7
Anaerobic: 1, 5
Cyt. dependent, e.g., Enterobacteriaceae group
Aerobic: 1, 2, 3, 4, or 1, 7
Anaerobic: 1, 5 or 1, 2, 6
The energy-generating metabolic pathways also produce (from the substrates) many
metabolic products that the microbial cells either use for the synthesis of cellular
components or release into the environment. The nature of these metabolites differs
greatly and depends on the nature of the substrates, the type of microorganisms with
respect to their aerobiosis nature, and the oxygen availability (more correctly, redox
potential) of the environment. The metabolisms and growth of microorganisms in
food are important for several reasons. Microbial spoilage of foods with the loss of
acceptance qualities (e.g., flavor, texture, color, and appearance) is directly related
to microbial growth and metabolism. Toxin production in food by food-poisoning
microorganisms also results from their growth in a food. Many microbial metabolites
are also important for their ability to produce desirable characteristics in fermented
foods, such as texture, flavor, and long shelf life. Microbial metabolic products are
also used in foods for processing (enzymes), preservation (bacteriocins and acids),
and improving texture (dextran) and flavor (diacetyl).
Among the food components, microbial metabolisms of carbohydrates, proteins,
and lipids are of major importance. Some of these metabolic pathways are briefly
presented in this chapter. Foods, depending on the type, can contain many types of
carbohydrates, proteins, and lipids. This has been discussed briefly previously.
Depending on the type and source, foods also differ greatly in the amounts of the
three groups of nutrients. Plant foods are, in general, rich in carbohydrates, although
some (e.g., nuts, lentils, and beans) are also rich in protein and some others (e.g.,
oilseeds) are rich in lipids. Foods of animal origin are rich in proteins and lipids,
whereas some (e.g., meat and fish) are low in carbohydrates; others, such as milk,
organ meats (liver), and mollusks (oysters), are rich in proteins as well as carbohydrates.
Fabricated or formulated foods can have all the nutrients in sufficient