Food Spoilage by Microbial Enzymes ◾ 291
in cream. A cream that has undergone lipolysis foams excessively and takes a longer time to churn
during processing of butter. The butter prepared from such cream is susceptible to rancidity more
quickly.
Butter containing residual heat-stable bacterial lipases undergoes rapid lipid hydrolysis even
during storage at –10°C. Lipases that preferentially release short-chain fatty acids (C4–C8), with
and without long-chain fatty acids, cause the most off-flavor in butter.
Milk Powder
The activity of heat-stable bacterial proteinases and lipases present in raw and pasteurized milk
is not denatured during the manufacture of spray-dried milk powder. The low A W prevents these
enzymes from degrading the proteins and lipids in dry milk. Powdered whole milk, nonfat milk,
and whey are used as ingredients in a wide variety of foods that can have high amounts of proteins
and lipids and are expected to have a long shelf life. Bakery products, ice cream, desserts, processed
meat products, chocolate, cheese products, and condensed dairy products are some of the foods
in which dry milks are used. These products can develop off-flavor and texture defects from the
action of heat-stable proteinases and lipases during storage.
Spoilage of Foods by Microbial enzymes at Low temperatures
Limited studies have shown that bacterial extracellular proteinases are not directly associated with
development of off-flavor in fish and meat. The initial off-flavors associated with degradation of
nitrogenous compounds develop from the bacterial metabolism of nonprotein nitrogenous (NPN)
compounds present in these products. However, the bacterial extracellular proteinases once produced are able to act on tissue proteins and cause texture defects (such as slime formation). This
probably occurs when the NPN compounds have been used up and the population has reached
approximately 10 8 /g or/cm 2 of the products. Protein hydrolysis by bacterial proteinases favors the
development of a higher degree of putrefactive changes. Some proteinases of Pse. fragi reduce oxymyoglobin and discolor meat. The enzymes probably hydrolyze the globin part of the polypeptide
chain of the myoglobin, which alters the reactivity of the heme group, leading to discoloration
of meat. Muscle lipids can also be hydrolyzed by bacterial lipases, causing flavor defects in meat
and fish. However, because lipids are susceptible to rancidity caused by lipases of the flesh and by
autooxidation, the contribution of bacterial lipases is probably very little.
The heat stability of the bacterial proteinases and lipases associated with flavor and texture
defects in raw meat and fish is not known. However, these enzymes are produced by the same
psychrotrophic bacteria, namely, Pseudomonas, Aeromonas, and similar species that are known to
produce extracellular heat-stable enzymes. 8,9 It can be assumed that some of these enzymes are
heat stable. It will be important to determine whether these enzymes are potentially able to cause
spoilage of low-heat-processed (pasteurized) meat products, some of which are expected to have a
shelf life of 100 days.
In recent years, some specialty raw chicken meat products that were stored at refrigerated temperatures (–1°C to 0°C for 28 days) or at frozen temperatures (–20°C up to 90 days) were reported
to develop distinct flavor defects before the expiration dates. Microbiological analysis of the freshly
prepared products and the products with flavor defects during storage led to the suspicion that
bacterial enzymes active at low temperatures might be associated with the flavor defects. The
enzymes could be the exoenzymes produced by the bacteria in a product while it was still fresh,
in cream. A cream that has undergone lipolysis foams excessively and takes a longer time to churn
during processing of butter. The butter prepared from such cream is susceptible to rancidity more
quickly.
Butter containing residual heat-stable bacterial lipases undergoes rapid lipid hydrolysis even
during storage at –10°C. Lipases that preferentially release short-chain fatty acids (C4–C8), with
and without long-chain fatty acids, cause the most off-flavor in butter.
Milk Powder
The activity of heat-stable bacterial proteinases and lipases present in raw and pasteurized milk
is not denatured during the manufacture of spray-dried milk powder. The low A W prevents these
enzymes from degrading the proteins and lipids in dry milk. Powdered whole milk, nonfat milk,
and whey are used as ingredients in a wide variety of foods that can have high amounts of proteins
and lipids and are expected to have a long shelf life. Bakery products, ice cream, desserts, processed
meat products, chocolate, cheese products, and condensed dairy products are some of the foods
in which dry milks are used. These products can develop off-flavor and texture defects from the
action of heat-stable proteinases and lipases during storage.
Spoilage of Foods by Microbial enzymes at Low temperatures
Limited studies have shown that bacterial extracellular proteinases are not directly associated with
development of off-flavor in fish and meat. The initial off-flavors associated with degradation of
nitrogenous compounds develop from the bacterial metabolism of nonprotein nitrogenous (NPN)
compounds present in these products. However, the bacterial extracellular proteinases once produced are able to act on tissue proteins and cause texture defects (such as slime formation). This
probably occurs when the NPN compounds have been used up and the population has reached
approximately 10 8 /g or/cm 2 of the products. Protein hydrolysis by bacterial proteinases favors the
development of a higher degree of putrefactive changes. Some proteinases of Pse. fragi reduce oxymyoglobin and discolor meat. The enzymes probably hydrolyze the globin part of the polypeptide
chain of the myoglobin, which alters the reactivity of the heme group, leading to discoloration
of meat. Muscle lipids can also be hydrolyzed by bacterial lipases, causing flavor defects in meat
and fish. However, because lipids are susceptible to rancidity caused by lipases of the flesh and by
autooxidation, the contribution of bacterial lipases is probably very little.
The heat stability of the bacterial proteinases and lipases associated with flavor and texture
defects in raw meat and fish is not known. However, these enzymes are produced by the same
psychrotrophic bacteria, namely, Pseudomonas, Aeromonas, and similar species that are known to
produce extracellular heat-stable enzymes. 8,9 It can be assumed that some of these enzymes are
heat stable. It will be important to determine whether these enzymes are potentially able to cause
spoilage of low-heat-processed (pasteurized) meat products, some of which are expected to have a
shelf life of 100 days.
In recent years, some specialty raw chicken meat products that were stored at refrigerated temperatures (–1°C to 0°C for 28 days) or at frozen temperatures (–20°C up to 90 days) were reported
to develop distinct flavor defects before the expiration dates. Microbiological analysis of the freshly
prepared products and the products with flavor defects during storage led to the suspicion that
bacterial enzymes active at low temperatures might be associated with the flavor defects. The
enzymes could be the exoenzymes produced by the bacteria in a product while it was still fresh,