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Chapter 22
Food Spoilage by
Microbial enzymes
introduction
The metabolism of food nutrients, principally carbohydrates, nitrogenous compounds, and lipids,
by spoilage bacteria enables the cells to increase in number as well as produce metabolites that can
adversely reduce the acceptance quality of a food. A food is considered spoiled when the changes
are detectable and the microbial population has reached approximately 10 7–9 /mL/g or/cm 2 . The
changes are brought about by the catalytic actions of a large number of microbial enzymes. Most
microbial enzymes are intracellular and act on the nutrients that can be transported inside the cells
through several transport mechanisms. A bacterial cell contains many enzymes, many of which
are intracellular, and some are extracellular. Intracellular enzymes are involved in the metabolism of small nutrient molecules of food that are transported inside the cells. Many intracellular enzymes can also act on intracellular large molecules, such as endonucleases, mucopeptidase,
lipases, and proteinases. Extracellular enzymes, after synthesis, either remain bound to the cell
surface or are excreted into the food environment. Many of the latter group can hydrolyze large
nutrient molecules of food (e.g., polysaccharides, proteins, and lipids) to small molecules before
they are transported into the cells (see Chapters 8 and 12).
Most foods have some amounts of low-molecular-weight, metabolizable carbohydrates (monosaccharides and disaccharides and their derivatives, such as glucose-6-phosphate), nitrogenous
compounds (small peptides, amino acids, nucleosides, nucleotides, urea, creatinine, and trimethylamine oxide), free fatty acids, and some organic acids (lactic, citric, and malic acids). Many spoilage microorganisms, particularly spoilage bacteria, are able to utilize the low-molecular-weight
food components to reach a population of 10 7–9 /g/mL or/cm 2 food and cause detectable food
spoilage. Thus, the supply of extra nutrients from the hydrolysis of the macromolecules of foods
by bacterial extracellular enzymes is not necessary for the onset of spoilage of many foods. In
fact, studies with Pseudomonas spp. and Bacillus spp. show that in the presence of low-molecularweight nitrogenous compounds, the synthesis of extracellular proteinases is repressed. When the
supply of the small molecules is used up, the mechanism is derepressed, leading to the synthesis
Chapter 22
Food Spoilage by
Microbial enzymes
introduction
The metabolism of food nutrients, principally carbohydrates, nitrogenous compounds, and lipids,
by spoilage bacteria enables the cells to increase in number as well as produce metabolites that can
adversely reduce the acceptance quality of a food. A food is considered spoiled when the changes
are detectable and the microbial population has reached approximately 10 7–9 /mL/g or/cm 2 . The
changes are brought about by the catalytic actions of a large number of microbial enzymes. Most
microbial enzymes are intracellular and act on the nutrients that can be transported inside the cells
through several transport mechanisms. A bacterial cell contains many enzymes, many of which
are intracellular, and some are extracellular. Intracellular enzymes are involved in the metabolism of small nutrient molecules of food that are transported inside the cells. Many intracellular enzymes can also act on intracellular large molecules, such as endonucleases, mucopeptidase,
lipases, and proteinases. Extracellular enzymes, after synthesis, either remain bound to the cell
surface or are excreted into the food environment. Many of the latter group can hydrolyze large
nutrient molecules of food (e.g., polysaccharides, proteins, and lipids) to small molecules before
they are transported into the cells (see Chapters 8 and 12).
Most foods have some amounts of low-molecular-weight, metabolizable carbohydrates (monosaccharides and disaccharides and their derivatives, such as glucose-6-phosphate), nitrogenous
compounds (small peptides, amino acids, nucleosides, nucleotides, urea, creatinine, and trimethylamine oxide), free fatty acids, and some organic acids (lactic, citric, and malic acids). Many spoilage microorganisms, particularly spoilage bacteria, are able to utilize the low-molecular-weight
food components to reach a population of 10 7–9 /g/mL or/cm 2 food and cause detectable food
spoilage. Thus, the supply of extra nutrients from the hydrolysis of the macromolecules of foods
by bacterial extracellular enzymes is not necessary for the onset of spoilage of many foods. In
fact, studies with Pseudomonas spp. and Bacillus spp. show that in the presence of low-molecularweight nitrogenous compounds, the synthesis of extracellular proteinases is repressed. When the
supply of the small molecules is used up, the mechanism is derepressed, leading to the synthesis