62 ◾ Fundamental Food Microbiology
are rich in carbohydrates but can be poor sources of proteins, minerals, and some vitamins. Some
foods, such as milk, and many prepared foods have all five nutrient groups in sufficient amounts
for microbial growth.
Microorganisms normally present in food vary greatly in their nutrient requirements with
bacteria requiring the most, followed by yeasts and molds. Microorganisms also differ greatly in
their ability to utilize large and complex carbohydrates (e.g., starch and cellulose), large proteins
(e.g., casein in milk), and lipids. Microorganisms are capable of using these molecules do so by
producing specific extracellular enzymes (or exoenzymes) and hydrolyzing the complex molecules
to simpler forms outside before transporting them inside the cell. Molds are the most capable of
doing this. However, this provides an opportunity for a species to grow in a mixed population even
when it is incapable of metabolizing the complex molecules. Microbial cells, following death and
lysis, release intracellular enzymes that can also catalyze breakdown of complex food nutrients to
simpler forms, which can then be utilized by other microorganisms.
Carbohydrates in Foods
Major carbohydrates present in different foods, either naturally or added as ingredients, can be
grouped on the basis of chemical nature as follows:
Monosaccharides
Hexoses: glucose, fructose, mannose, galactose
Pentoses: xylose, arabinose, ribose, ribulose, xylulose
Disaccharides
Lactose (galactose + glucose)
Sucrose (fructose + glucose)
Maltose (glucose + glucose)
Oligosaccharides
Raffinose (glucose + fructose + galactose)
Stachyose (glucose + fructose + galactose + galactose)
Polysaccharides
Starch (glucose units)
Glycogen (glucose units)
Cellulose (glucose units)
Inulin (fructose units)
Hemicellulose (xylose, galactose, mannose units)
Dextrans (α-1,6 glucose polymer)
Pectins
Gums and mucilages
Lactose is found only in milk and thus can be present in foods made from or with milk and milk
products. Glycogen is present in animal tissues, especially in liver. Pentoses, most oligosaccharides, and polysaccharides are naturally present in foods of plant origin.
All microorganisms normally found in food metabolize glucose, but their ability to utilize
other carbohydrates differs considerably. This is because of the inability of some microorganisms
to transport the specific monosaccharides and disaccharides inside the cells and the inability to
hydrolyze polysaccharides outside the cells. Molds are the most capable of using polysaccharides.
are rich in carbohydrates but can be poor sources of proteins, minerals, and some vitamins. Some
foods, such as milk, and many prepared foods have all five nutrient groups in sufficient amounts
for microbial growth.
Microorganisms normally present in food vary greatly in their nutrient requirements with
bacteria requiring the most, followed by yeasts and molds. Microorganisms also differ greatly in
their ability to utilize large and complex carbohydrates (e.g., starch and cellulose), large proteins
(e.g., casein in milk), and lipids. Microorganisms are capable of using these molecules do so by
producing specific extracellular enzymes (or exoenzymes) and hydrolyzing the complex molecules
to simpler forms outside before transporting them inside the cell. Molds are the most capable of
doing this. However, this provides an opportunity for a species to grow in a mixed population even
when it is incapable of metabolizing the complex molecules. Microbial cells, following death and
lysis, release intracellular enzymes that can also catalyze breakdown of complex food nutrients to
simpler forms, which can then be utilized by other microorganisms.
Carbohydrates in Foods
Major carbohydrates present in different foods, either naturally or added as ingredients, can be
grouped on the basis of chemical nature as follows:
Monosaccharides
Hexoses: glucose, fructose, mannose, galactose
Pentoses: xylose, arabinose, ribose, ribulose, xylulose
Disaccharides
Lactose (galactose + glucose)
Sucrose (fructose + glucose)
Maltose (glucose + glucose)
Oligosaccharides
Raffinose (glucose + fructose + galactose)
Stachyose (glucose + fructose + galactose + galactose)
Polysaccharides
Starch (glucose units)
Glycogen (glucose units)
Cellulose (glucose units)
Inulin (fructose units)
Hemicellulose (xylose, galactose, mannose units)
Dextrans (α-1,6 glucose polymer)
Pectins
Gums and mucilages
Lactose is found only in milk and thus can be present in foods made from or with milk and milk
products. Glycogen is present in animal tissues, especially in liver. Pentoses, most oligosaccharides, and polysaccharides are naturally present in foods of plant origin.
All microorganisms normally found in food metabolize glucose, but their ability to utilize
other carbohydrates differs considerably. This is because of the inability of some microorganisms
to transport the specific monosaccharides and disaccharides inside the cells and the inability to
hydrolyze polysaccharides outside the cells. Molds are the most capable of using polysaccharides.