Fundamental Food Microbiology, Third Edition - Fuad Fathir

70 FUNDAMENTAL FOOD MICROBIOLOGY
defect), and some can be used in food production (such as dextrans as stabilizers).
Carbohydrate metabolism profiles are extensively used in the laboratory for the
biochemical identification of unknown microorganisms isolated from foods.
2. Proteins in Foods
The major proteinaceous components in foods are simple proteins, conjugated proteins,
peptides, and nonprotein nitrogenous (NPN) compounds (amino acids, urea,
ammonia, creatinine, trimethylamine). Proteins and peptides are polymers of different
amino acids without or with other organic (e.g., a carbohydrate) or inorganic
(e.g., iron) components and contain ca. 15 to 18% nitrogen. Simple food proteins
are polymers of amino acids, such as albumins (in egg), globulins (in milk), glutelins
(gluten in cereal), prolamins (zein in grains), and albuminoids (collagen in muscle).
They differ greatly in their solubility, which determines the ability of microorganisms
to utilize a specific protein. Many microorganisms can hydrolyze albumin, which is
soluble in water. In contrast, collagens, which are insoluble in water or weak salt
and acid solutions, are hydrolyzed only by a few microorganisms. As compared with
simple proteins, conjugated proteins of food on hydrolysis produce metals (metalloproteins
such as hemoglobin and myoglobin), carbohydrates (glycoproteins such
as mucin), phosphate (phosphoproteins such as casein), and lipids (lipoproteins such
as some in liver). Proteins are present in higher quantities in foods of animal origin
than in foods of plant origin. But plant foods, such as nuts and legumes, are rich in
proteins. Proteins as ingredients can also be added to foods.
Microorganisms differ greatly in their ability to metabolize food proteins. Most
transport amino acids and small peptides in the cells; small peptides are then
hydrolyzed to amino acids inside the cells, such as in some Lactococcus spp.
Microorganisms also produce extracellular proteinases and peptidases to hydrolyze
large proteins and peptides to small peptides and amino acids before they can be
transported inside the cells. Soluble proteins are more susceptible to this hydrolytic
action than are the insoluble proteins. Hydrolysis of food proteins can be either
undesirable (texture loss in meat) or desirable (flavor in cheese). Microorganisms
can also metabolize different NPN compounds found in foods.
Amino acids inside microbial cells are metabolized via different pathways to
synthesize cellular components, energy, and various by-products. Many of these byproducts
can be undesirable (e.g., NH 3 and H 2S production causes spoilage of food,
and toxins and biological amines cause health hazards) or desirable (e.g., some sulfur
compounds give cheddar cheese flavor). Production of specific metabolic products
is used for the laboratory identification of microbial isolates from food. An example
of this is the ability of Escherichia coli to produce indole from tryptophan, which
is used to differentiate this species from non-indole-producing related species (e.g.,
Enterobacter spp.).
3. Lipids in Foods
Lipids in foods include compounds that can be extracted by organic solvents, some
of which are free fatty acids, glycerides, phospholipids, waxes, and sterols. Lipids are