88 ◾ Fundamental Food Microbiology
foodborne pathogens while growing in foods is important because of its ability to produce proteins
that are toxins. They include thermostable toxins of Staphylococcus aureus, thermolabile toxins of
Clostridium botulinum, and toxins produced by some bacteria associated with foodborne infections
(such as Shiga toxin). The ability of some microbial species to synthesize essential amino acids (such
as l-lysine), antibacterial peptides (such as nisin and pediocin), and enzymes (such as amylases and
proteinases) in relatively large amounts has been used for beneficial purposes in foods.
Metabolism of Food Lipids
The main lipids in food are monoglycerides, diglycerides, and triglycerides; free saturated and
unsaturated fatty acids; phospholipids; sterols; and waxes, with the glycerides being the major lipids.
Microorganisms have a low preference for metabolizing lipids. Being hydrophobic, lipids are difficult to degrade when present in a large mass. In emulsion, they can be metabolized by the microorganisms at the oil-water interphase. Glycerides are hydrolyzed by extracellular lipases to release
glycerol and fatty acids. The fatty acids then can be transported inside the cells and metabolized by
β-oxidation to initially generate acetyl~CoA units before being utilized further. Fatty acids, if produced at a rapid rate, accumulate in the food. Unsaturated fatty acids can be oxidized by microbial
oxidases to initially produce hydroperoxides and then carbonyl compounds (aldehydes and ketones).
Some of the microorganisms that are important in food and release lipases (hydrolytic
enzymes) are found in the following genera: Alcaligenes, Enterobacter, Flavobacterium, Micrococcus,
Pseudomonas, Serratia, Staphylococcus, Aspergillus, Geotrichum, and Penicillium. Oxidative enzymes
are produced mainly by the molds. Both groups of enzymes are associated with food spoilage, but
oxidative enzymes are also important for desirable flavor in mold-ripened cheeses.
Conclusion
Microbial growth in food is accomplished through the metabolism of food nutrients, principally
the metabolizable carbohydrates, proteins, and lipids. Many metabolic pathways can be used,
which are determined by the nature and concentration of a nutrient, the microbial type, and the
oxidation-reduction potential of the food environment. Many types of end products are produced
during metabolism of the nutrients, which, depending on the chemical nature, are associated with
food spoilage, food poisoning, or production of fermented food. Some are also used to improve
texture and flavor of foods. The metabolic end products are also useful to identify unknown
microbial isolates from a food. As long as the physical and nutritional environments are maintained, bacterial cells continue to multiply, generating energy and cellular materials through the
metabolic processes. However, a change in the environment can cause some species to shut down
the cell multiplication cycle and trigger the sporulation cycle, in which a cell is differentiated as
an endospore. This survival strategy under unfavorable conditions and its importance in food
microbiology are discussed in Chapter 9.
QUESTIONS
1. Discuss the major differences among aerobic respiration, anaerobic respiration, and fermentation of food nutrients by microorganisms.
2. Discuss how aerobes, anaerobes, and facultative anaerobes differ from each other in their
ability to transfer electrons by different acceptors.
foodborne pathogens while growing in foods is important because of its ability to produce proteins
that are toxins. They include thermostable toxins of Staphylococcus aureus, thermolabile toxins of
Clostridium botulinum, and toxins produced by some bacteria associated with foodborne infections
(such as Shiga toxin). The ability of some microbial species to synthesize essential amino acids (such
as l-lysine), antibacterial peptides (such as nisin and pediocin), and enzymes (such as amylases and
proteinases) in relatively large amounts has been used for beneficial purposes in foods.
Metabolism of Food Lipids
The main lipids in food are monoglycerides, diglycerides, and triglycerides; free saturated and
unsaturated fatty acids; phospholipids; sterols; and waxes, with the glycerides being the major lipids.
Microorganisms have a low preference for metabolizing lipids. Being hydrophobic, lipids are difficult to degrade when present in a large mass. In emulsion, they can be metabolized by the microorganisms at the oil-water interphase. Glycerides are hydrolyzed by extracellular lipases to release
glycerol and fatty acids. The fatty acids then can be transported inside the cells and metabolized by
β-oxidation to initially generate acetyl~CoA units before being utilized further. Fatty acids, if produced at a rapid rate, accumulate in the food. Unsaturated fatty acids can be oxidized by microbial
oxidases to initially produce hydroperoxides and then carbonyl compounds (aldehydes and ketones).
Some of the microorganisms that are important in food and release lipases (hydrolytic
enzymes) are found in the following genera: Alcaligenes, Enterobacter, Flavobacterium, Micrococcus,
Pseudomonas, Serratia, Staphylococcus, Aspergillus, Geotrichum, and Penicillium. Oxidative enzymes
are produced mainly by the molds. Both groups of enzymes are associated with food spoilage, but
oxidative enzymes are also important for desirable flavor in mold-ripened cheeses.
Conclusion
Microbial growth in food is accomplished through the metabolism of food nutrients, principally
the metabolizable carbohydrates, proteins, and lipids. Many metabolic pathways can be used,
which are determined by the nature and concentration of a nutrient, the microbial type, and the
oxidation-reduction potential of the food environment. Many types of end products are produced
during metabolism of the nutrients, which, depending on the chemical nature, are associated with
food spoilage, food poisoning, or production of fermented food. Some are also used to improve
texture and flavor of foods. The metabolic end products are also useful to identify unknown
microbial isolates from a food. As long as the physical and nutritional environments are maintained, bacterial cells continue to multiply, generating energy and cellular materials through the
metabolic processes. However, a change in the environment can cause some species to shut down
the cell multiplication cycle and trigger the sporulation cycle, in which a cell is differentiated as
an endospore. This survival strategy under unfavorable conditions and its importance in food
microbiology are discussed in Chapter 9.
QUESTIONS
1. Discuss the major differences among aerobic respiration, anaerobic respiration, and fermentation of food nutrients by microorganisms.
2. Discuss how aerobes, anaerobes, and facultative anaerobes differ from each other in their
ability to transfer electrons by different acceptors.