64 ◾ Fundamental Food Microbiology
tryptophan, which is used to differentiate this species from non-indole-producing related species
(e.g., Enterobacter spp.).
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 relatively higher in
foods of animal origin than in foods of plant origin although nuts, oil seeds, coconuts, olives,
and avocados have high amounts of lipids. Fabricated or prepared foods can also vary greatly in
lipid content. Cholesterols are present in foods of animal origin or foods containing ingredients
from animal sources. Lipids are, in general, less preferred substrates for the microbial synthesis
of energy and cellular materials. Many microorganisms can produce extracellular lipases that can
hydrolyze glycerides to fatty acids and glycerol. Fatty acids can be transported in cells and used
for energy synthesis, whereas glycerol can be metabolized separately. Some microorganisms also
produce extracellular lipid oxidases, which can oxidize unsaturated fatty acids to produce different
aldehydes and ketones. In general, molds are more capable of producing these enzymes. However,
certain bacterial genera, such as Pseudomonas, Achromobacter, and Alcaligenes, can produce these
enzymes. Lysis of dead microbial cells in foods causes release of intracellular lipases and oxidases,
which then can carry out these reactions. In many foods, the action of these enzymes is associated with spoilage (such as rancidity), whereas in other foods, the enzymes are credited for desirable flavors (such as in mold-ripened cheeses). Some beneficial intestinal microorganisms, such
as Lactobacillus acidophilus strains, can metabolize cholesterol and are believed to be capable of
reducing serum cholesterol levels in humans.
Minerals and Vitamins in Foods
Microorganisms need several elements in small amounts, such as phosphorous, calcium, magnesium, iron, sulfur, manganese, and potassium. Most foods have these elements in sufficient
amounts. Many microorganisms can synthesize B vitamins, and foods also contain most B
vitamins.
In general, most foods contain different carbohydrates, proteins, lipids, minerals, and vitamins
in sufficient amounts to supply necessary nutrients for the growth of molds, yeasts, and bacteria, especially Gram-negative bacteria normally present in foods. Some foods may have limited
amounts of one or a few nutrients for rapid growth of some Gram-positive bacteria, especially
some fastidious Lactobacillus species. When their growth is desired, some carbohydrates, essential
amino acids, and B vitamins may be added to a food. It is not possible or practical to control
microbial growth in a food by restricting nutrients.
Growth Factors and Inhibitors in Food
Foods can also have some factors that either stimulate growth or adversely affect growth of microorganisms. 3 The exact nature of growth factors is not known, but they are naturally present in some
foods. An example is the growth factors in tomatoes that stimulate growth of some Lactobacillus
species. These factors can be added to raw materials during food bioprocessing or to media to isolate some fastidious bacteria from foods.
Foods also contain many chemicals, either naturally or added, that adversely affect microbial
growth. Some of the natural inhibitors are lysozyme in egg, agglutinin in milk, and eugenol in
tryptophan, which is used to differentiate this species from non-indole-producing related species
(e.g., Enterobacter spp.).
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 relatively higher in
foods of animal origin than in foods of plant origin although nuts, oil seeds, coconuts, olives,
and avocados have high amounts of lipids. Fabricated or prepared foods can also vary greatly in
lipid content. Cholesterols are present in foods of animal origin or foods containing ingredients
from animal sources. Lipids are, in general, less preferred substrates for the microbial synthesis
of energy and cellular materials. Many microorganisms can produce extracellular lipases that can
hydrolyze glycerides to fatty acids and glycerol. Fatty acids can be transported in cells and used
for energy synthesis, whereas glycerol can be metabolized separately. Some microorganisms also
produce extracellular lipid oxidases, which can oxidize unsaturated fatty acids to produce different
aldehydes and ketones. In general, molds are more capable of producing these enzymes. However,
certain bacterial genera, such as Pseudomonas, Achromobacter, and Alcaligenes, can produce these
enzymes. Lysis of dead microbial cells in foods causes release of intracellular lipases and oxidases,
which then can carry out these reactions. In many foods, the action of these enzymes is associated with spoilage (such as rancidity), whereas in other foods, the enzymes are credited for desirable flavors (such as in mold-ripened cheeses). Some beneficial intestinal microorganisms, such
as Lactobacillus acidophilus strains, can metabolize cholesterol and are believed to be capable of
reducing serum cholesterol levels in humans.
Minerals and Vitamins in Foods
Microorganisms need several elements in small amounts, such as phosphorous, calcium, magnesium, iron, sulfur, manganese, and potassium. Most foods have these elements in sufficient
amounts. Many microorganisms can synthesize B vitamins, and foods also contain most B
vitamins.
In general, most foods contain different carbohydrates, proteins, lipids, minerals, and vitamins
in sufficient amounts to supply necessary nutrients for the growth of molds, yeasts, and bacteria, especially Gram-negative bacteria normally present in foods. Some foods may have limited
amounts of one or a few nutrients for rapid growth of some Gram-positive bacteria, especially
some fastidious Lactobacillus species. When their growth is desired, some carbohydrates, essential
amino acids, and B vitamins may be added to a food. It is not possible or practical to control
microbial growth in a food by restricting nutrients.
Growth Factors and Inhibitors in Food
Foods can also have some factors that either stimulate growth or adversely affect growth of microorganisms. 3 The exact nature of growth factors is not known, but they are naturally present in some
foods. An example is the growth factors in tomatoes that stimulate growth of some Lactobacillus
species. These factors can be added to raw materials during food bioprocessing or to media to isolate some fastidious bacteria from foods.
Foods also contain many chemicals, either naturally or added, that adversely affect microbial
growth. Some of the natural inhibitors are lysozyme in egg, agglutinin in milk, and eugenol in