288 ◾ Fundamental Food Microbiology
and excretion of extracellular proteinases. The proteinases hydrolyze the large protein molecules of
food to produce small peptides and amino acids for transport and endogenous metabolism in the
cells, which, in turn, intensify the spoilage. In general, microbial food spoilage from the metabolism of low-molecular-weight nutrients occurs at the early stage of microbial growth; spoilage from
the breakdown of macromolecules by extracellular enzymes appears late in the sequence of events.
After microbial cells die normally or are killed by nonthermal treatments so that the intracellular and extracellular enzymes are not inactivated or destroyed, the enzymes can cause food
spoilage even in the absence of viable cells or growth of microorganisms. Many microbial cells in
a food that are subjected to freezing (then thawing), drying (then rehydration), modified atmosphere or vacuum packaging, refrigeration, high hydrostatic pressure, an electric pulse field, or
high-intensity light, or are exposed to some preservatives, may die and undergo autolysis to release
intracellular enzymes. If these foods are stored for a long time under conditions that favor catalysis
of one or more intracellular and extracellular enzymes, they can undergo spoilage. Ripening (not
spoilage) of cheddar cheese (at low temperatures and low A W ) is a good example; wherein following death, bacterial intracellular enzymes are involved in the breakdown of milk nutrients, not to
cause spoilage but to impart desirable product characteristics. In most foods, the initial microbial
population is generally low and spoilage by microbial enzymes in the absence of growth may not
be of practical significance. However, if a product is heavily contaminated with a high initial
microbial load (in the absence of growth) and then subjected to a nonthermal treatment that kills
microorganisms but does not inactivate enzymes, spoilage of the food by microbial enzymes can
occur (Chapter 21). In thermally processed foods, several heat-stable enzymes of the microorganisms retain their activity even after the producer cells are killed. During subsequent storage of
the food under favorable conditions, these enzymes can break down the food nutrients to cause
spoilage. Among the heat-stable enzymes, some extracellular proteinases, lipases, and phospholipases of several psychrotrophic bacteria found in food cause spoilage of thermally processed dairy
products. 1–3
Characteristics of Heat-Stable enzymes
of Psychrotrophic Bacteria
Raw milk, between production and pasteurization in commercial operations, is usually stored for
1–2 weeks at refrigerated temperatures (<7°C). Psychrotrophic bacteria in the raw milk, coming
from water, equipment, and the environment, can multiply during storage. Depending on initial
numbers, bacterial species and strains, temperature and time of storage, and the extent of temperature abuse, the population can reach the level of producing sufficient amounts of extracellular
heat-stable enzymes. Even though raw milk may not be spoiled, the heat-stable enzymes produced
can cause spoilage of heat-treated dairy products manufactured from this raw milk. Heating, such
as pasteurization and ultrahigh temperature treatments (UHT), kills psychrotrophic bacteria but
does not inactivate heat-stable enzymes.
In raw milk, heat-stable proteinases, lipases, and, to some extent, phospholipases produced
by several psychrotrophic Gram-negative bacteria are considered to be of major economic importance because of the spoilage potential of the products. 4,5 Species from genera Pseudomonas
(Pseudomonas fluorescens, Pse. fragi), Aeromonas, Flavobacterium, Shewanella (Alteromonas),
Serratia, and Acinetobacter produce heat-stable extracellular proteinases, and Pseudomonas,
Alcaligenes, Shewanella, Acinetobacter, and Serratia produce heat-stable lipases. Pseudomonas spp.
also produce heat-stable phospholipases. Many species from these genera are normally present in
and excretion of extracellular proteinases. The proteinases hydrolyze the large protein molecules of
food to produce small peptides and amino acids for transport and endogenous metabolism in the
cells, which, in turn, intensify the spoilage. In general, microbial food spoilage from the metabolism of low-molecular-weight nutrients occurs at the early stage of microbial growth; spoilage from
the breakdown of macromolecules by extracellular enzymes appears late in the sequence of events.
After microbial cells die normally or are killed by nonthermal treatments so that the intracellular and extracellular enzymes are not inactivated or destroyed, the enzymes can cause food
spoilage even in the absence of viable cells or growth of microorganisms. Many microbial cells in
a food that are subjected to freezing (then thawing), drying (then rehydration), modified atmosphere or vacuum packaging, refrigeration, high hydrostatic pressure, an electric pulse field, or
high-intensity light, or are exposed to some preservatives, may die and undergo autolysis to release
intracellular enzymes. If these foods are stored for a long time under conditions that favor catalysis
of one or more intracellular and extracellular enzymes, they can undergo spoilage. Ripening (not
spoilage) of cheddar cheese (at low temperatures and low A W ) is a good example; wherein following death, bacterial intracellular enzymes are involved in the breakdown of milk nutrients, not to
cause spoilage but to impart desirable product characteristics. In most foods, the initial microbial
population is generally low and spoilage by microbial enzymes in the absence of growth may not
be of practical significance. However, if a product is heavily contaminated with a high initial
microbial load (in the absence of growth) and then subjected to a nonthermal treatment that kills
microorganisms but does not inactivate enzymes, spoilage of the food by microbial enzymes can
occur (Chapter 21). In thermally processed foods, several heat-stable enzymes of the microorganisms retain their activity even after the producer cells are killed. During subsequent storage of
the food under favorable conditions, these enzymes can break down the food nutrients to cause
spoilage. Among the heat-stable enzymes, some extracellular proteinases, lipases, and phospholipases of several psychrotrophic bacteria found in food cause spoilage of thermally processed dairy
products. 1–3
Characteristics of Heat-Stable enzymes
of Psychrotrophic Bacteria
Raw milk, between production and pasteurization in commercial operations, is usually stored for
1–2 weeks at refrigerated temperatures (<7°C). Psychrotrophic bacteria in the raw milk, coming
from water, equipment, and the environment, can multiply during storage. Depending on initial
numbers, bacterial species and strains, temperature and time of storage, and the extent of temperature abuse, the population can reach the level of producing sufficient amounts of extracellular
heat-stable enzymes. Even though raw milk may not be spoiled, the heat-stable enzymes produced
can cause spoilage of heat-treated dairy products manufactured from this raw milk. Heating, such
as pasteurization and ultrahigh temperature treatments (UHT), kills psychrotrophic bacteria but
does not inactivate heat-stable enzymes.
In raw milk, heat-stable proteinases, lipases, and, to some extent, phospholipases produced
by several psychrotrophic Gram-negative bacteria are considered to be of major economic importance because of the spoilage potential of the products. 4,5 Species from genera Pseudomonas
(Pseudomonas fluorescens, Pse. fragi), Aeromonas, Flavobacterium, Shewanella (Alteromonas),
Serratia, and Acinetobacter produce heat-stable extracellular proteinases, and Pseudomonas,
Alcaligenes, Shewanella, Acinetobacter, and Serratia produce heat-stable lipases. Pseudomonas spp.
also produce heat-stable phospholipases. Many species from these genera are normally present in