Factors Influencing Microbial Growth in Food ◾ 63
Food carbohydrates are metabolized by microorganisms principally to supply energy through
several metabolic pathways. Some of the metabolic products can be used to synthesize cellular
components of microorganisms (e.g., to produce amino acids by amination of some keto acids).
Microorganisms also produce metabolic byproducts associated with food spoilage (CO 2 to cause
gas defect) or food bioprocessing (lactic acid in fermented foods). Some are also metabolized to
produce organic acids, such as lactic, acetic, propionic, and butyric acids, which have an antagonistic effect on the growth and survival of many bacteria. Some of these metabolic pathways are
discussed in Chapters 8 and 12. Microorganisms can also polymerize some monosaccharides to
produce complex carbohydrates, such as dextrans, capsular materials, and cell walls (or outer
membranes in Gram-negative bacteria). Some of these carbohydrates from pathogens may cause
health hazards (the Gram-negative lipopolysaccharide is endotoxin and causes septicemia), some
may cause food spoilage (such as slime defect), and some can be used in food production (such as
dextrans as stabilizers). Carbohydrate and amino acid metabolism profiles are extensively used in
the laboratory for the biochemical identification of unknown microorganisms isolated from foods.
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 approximately
15%–18% nitrogen. Simple food proteins are polymers of amino acids, such as albumins (in eggs),
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), phosphates (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. 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 production of off-flavor) 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 byproducts. Many of these byproducts can be undesirable
(e.g., NH 3 and H 2 S production causes spoilage of food, and toxins and biological amines (such as
histamine) 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
Food carbohydrates are metabolized by microorganisms principally to supply energy through
several metabolic pathways. Some of the metabolic products can be used to synthesize cellular
components of microorganisms (e.g., to produce amino acids by amination of some keto acids).
Microorganisms also produce metabolic byproducts associated with food spoilage (CO 2 to cause
gas defect) or food bioprocessing (lactic acid in fermented foods). Some are also metabolized to
produce organic acids, such as lactic, acetic, propionic, and butyric acids, which have an antagonistic effect on the growth and survival of many bacteria. Some of these metabolic pathways are
discussed in Chapters 8 and 12. Microorganisms can also polymerize some monosaccharides to
produce complex carbohydrates, such as dextrans, capsular materials, and cell walls (or outer
membranes in Gram-negative bacteria). Some of these carbohydrates from pathogens may cause
health hazards (the Gram-negative lipopolysaccharide is endotoxin and causes septicemia), some
may cause food spoilage (such as slime defect), and some can be used in food production (such as
dextrans as stabilizers). Carbohydrate and amino acid metabolism profiles are extensively used in
the laboratory for the biochemical identification of unknown microorganisms isolated from foods.
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 approximately
15%–18% nitrogen. Simple food proteins are polymers of amino acids, such as albumins (in eggs),
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), phosphates (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. 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 production of off-flavor) 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 byproducts. Many of these byproducts can be undesirable
(e.g., NH 3 and H 2 S production causes spoilage of food, and toxins and biological amines (such as
histamine) 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