Microbial Metabolism of Food Components ◾ 81
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, the microbial metabolism 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 quantities to support microbial growth. In general, microorganisms preferentially metabolize carbohydrates as an energy source over proteins and lipids. Thus,
microorganisms growing in a food rich in metabolizable carbohydrates utilize carbohydrates, but
in a food low in metabolizable carbohydrates and rich in metabolizable proteins, they metabolize
proteins (after metabolizing the carbohydrates). In a food rich in both carbohydrates and proteins,
microorganisms usually utilize the carbohydrates first, then produce acids and reduce the pH.
Subsequent microbial degradation of proteins can be prevented at low pH, causing nondegradation of proteins or a protein-sparing effect. In the formulation of processed meat products, added
carbohydrates can provide this benefit.
Metabolism of Food Carbohydrates
Food carbohydrates comprise a large group of chemical compounds that include monosaccharides
(tetroses, pentoses, and hexoses), disaccharides, oligosaccharides, and polysaccharides. Although
carbohydrates are the most preferred source of energy production, microorganisms differ greatly
in their ability to degrade individual carbohydrates. 1–5 Carbohydrates that are degraded at the cellular level as monosaccharides, disaccharides, and trisaccharides can be transported inside the cell
and hydrolyzed to monosaccharide units before further degradation. Polysaccharides are broken
down to monosaccharides and disaccharides by extracellular microbial enzymes (e.g., α-amylase)
secreted in the environment before they can be transported and metabolized.
Degradation of Polysaccharides
Molds, some Bacillus spp. and Clostridium spp., and several other bacterial species can degrade
starch, glycogen, cellulose, pectin, and other polysaccharides by extracellular enzymes. The monosaccharides and disaccharides are then transported in the cell and metabolized. Breakdown of
these polysaccharides, especially pectins and cellulose, in fruits and vegetables by microorganisms
can affect the texture and reduce the acceptance quality of the products.
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, the microbial metabolism 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 quantities to support microbial growth. In general, microorganisms preferentially metabolize carbohydrates as an energy source over proteins and lipids. Thus,
microorganisms growing in a food rich in metabolizable carbohydrates utilize carbohydrates, but
in a food low in metabolizable carbohydrates and rich in metabolizable proteins, they metabolize
proteins (after metabolizing the carbohydrates). In a food rich in both carbohydrates and proteins,
microorganisms usually utilize the carbohydrates first, then produce acids and reduce the pH.
Subsequent microbial degradation of proteins can be prevented at low pH, causing nondegradation of proteins or a protein-sparing effect. In the formulation of processed meat products, added
carbohydrates can provide this benefit.
Metabolism of Food Carbohydrates
Food carbohydrates comprise a large group of chemical compounds that include monosaccharides
(tetroses, pentoses, and hexoses), disaccharides, oligosaccharides, and polysaccharides. Although
carbohydrates are the most preferred source of energy production, microorganisms differ greatly
in their ability to degrade individual carbohydrates. 1–5 Carbohydrates that are degraded at the cellular level as monosaccharides, disaccharides, and trisaccharides can be transported inside the cell
and hydrolyzed to monosaccharide units before further degradation. Polysaccharides are broken
down to monosaccharides and disaccharides by extracellular microbial enzymes (e.g., α-amylase)
secreted in the environment before they can be transported and metabolized.
Degradation of Polysaccharides
Molds, some Bacillus spp. and Clostridium spp., and several other bacterial species can degrade
starch, glycogen, cellulose, pectin, and other polysaccharides by extracellular enzymes. The monosaccharides and disaccharides are then transported in the cell and metabolized. Breakdown of
these polysaccharides, especially pectins and cellulose, in fruits and vegetables by microorganisms
can affect the texture and reduce the acceptance quality of the products.