Fundamental Food Microbiology, Third Edition - Fuad Fathir

MICROBIAL METABOLISM OF FOOD COMPONENTS 89
Metabolism of food carbohydrates by microorganisms is undesirable when it is
associated with spoilage. On the other hand, fermentation of carbohydrates is desirable
in food bioprocessing and production of metabolites for use in foods (such as
lactate and diacetyl). Several end products are also used to identify microorganisms;
for example, 2,3-butanediol production by Enterobacter spp. helps to differentiate
them from the nonproducer Esc. coli strains (Voges Proskauer test). The microbial
ability to metabolize different polysaccharides, disaccharides, and monosaccharides
is also used to identify unknown isolates.
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IV. METABOLISM OF FOOD PROTEINS
Proteinaceous compounds present in foods include different types of simple proteins
(e.g., albumin, globulin, zein, keratin, and collagen), conjugated proteins (e.g., myoglobin,
hemoglobin, and casein), and peptides containing two or more amino acids.
Amino acids, urea, creatinine, trimethyl amine, and others form the nonprotein
nitrogenous (NPN) group. In general, microorganisms can transport amino acids and
small peptides (ca. 8 to 10 amino acids long) in the cells. Proteins and large peptides
in a food are hydrolyzed to amino acids and small peptides by microbial extracellular
proteinases and peptidases. Species from genera Alcaligenes, Bacillus, Clostridium,
Enterococcus, Enterobacter, Flavobacterium, Klebsiella, Lactococcus, Micrococcus,
Pseudomonas, and Serratia are among those capable of producing extracellular
proteinases and peptidases. Small peptides are transported in the cell and converted
to amino acids before being metabolized further. 1,4,5
A. Aerobic Respiration (Decay)
Many aerobic and facultative anaerobic bacteria can oxidize amino acids and use
them as their sole source of carbon, nitrogen, and energy. L-Amino acids generally
undergo either oxidative deamination or transamination to produce respective keto
acids, which are then utilized through different pathways. Several amino acids can
also be oxidized in different pathways by many bacterial species. Some examples
are conversion of L-threonine to acetaldehyde and glycine, L-tryptophan to anthranilic
acid, L-lysine to glutaric acid, L-valine to ketoisovalerate, L-leucine to ketoisocaproate,
L-arginine to citrulline, and L-histidine to urocanic acid.
B. Fermentation (Putrefaction)
Degradation of L-amino acids by anaerobic and facultative anaerobic bacteria is
carried out either with single amino acids or two amino acids in pairs. Metabolism
of single amino acids is carried out through different types of deamination (producing
the C-skeletone and NH 3), decarboxylation (producing amines and CO 2), and hydrolysis
(producing the C-skeletone, CO 2, NH 3, and H 2). The C-skeletones (fatty acids,
a-keto acids, unsaturated acids) are then used to supply energy and other metabolic
products. The metabolism of amino acids in pairs involves simultaneous oxidation–reduction
reactions between suitable pairs in which one acts as the hydrogen