86 ◾ Fundamental Food Microbiology
CO 2 ). The 4C succinate is then, through several steps, converted to oxaloacetate for reuse. During
these reactions, reducing compounds are generated that, in turn, enter the electron transport
system, thereby generating 2H + and 2e – . The terminal cytochrome, cytochrome oxidase (cyt.a),
releases the electron for its acceptance by oxygen. If the cyt.a transfers only two pairs of electrons
to molecular oxygen, the end product is H 2 O; if one pair of electrons is transferred, the product is
H 2 O 2 , which is subsequently hydrolyzed by microbial catalase or peroxidase to H 2 O and O 2 . Each
pyruvate can potentially generate 15 ATP molecules.
Synthesis of Polymers
Leuconostoc mesenteroides cells growing on sucrose hydrolyze the molecules and predominantly
metabolize fructose for energy production. Glucose molecules are polymerized to form dextran (a
polymer of glucose). Polymers are also formed from carbohydrates by some Lactococcus lactis and
Lactobacillus strains, Alcaligenes faecalis strains, and Xanthomonas spp. Some of these polymers are
useful as food stabilizers and to give viscosity in some fermented foods; they can also cause quality
loss in some foods.
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.
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. 8–10 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
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.
CO 2 ). The 4C succinate is then, through several steps, converted to oxaloacetate for reuse. During
these reactions, reducing compounds are generated that, in turn, enter the electron transport
system, thereby generating 2H + and 2e – . The terminal cytochrome, cytochrome oxidase (cyt.a),
releases the electron for its acceptance by oxygen. If the cyt.a transfers only two pairs of electrons
to molecular oxygen, the end product is H 2 O; if one pair of electrons is transferred, the product is
H 2 O 2 , which is subsequently hydrolyzed by microbial catalase or peroxidase to H 2 O and O 2 . Each
pyruvate can potentially generate 15 ATP molecules.
Synthesis of Polymers
Leuconostoc mesenteroides cells growing on sucrose hydrolyze the molecules and predominantly
metabolize fructose for energy production. Glucose molecules are polymerized to form dextran (a
polymer of glucose). Polymers are also formed from carbohydrates by some Lactococcus lactis and
Lactobacillus strains, Alcaligenes faecalis strains, and Xanthomonas spp. Some of these polymers are
useful as food stabilizers and to give viscosity in some fermented foods; they can also cause quality
loss in some foods.
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.
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. 8–10 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
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.