290 ◾ Fundamental Food Microbiology
under normal short-term refrigerated storage, these defects may not be enough to detect. In addition, psychrotrophic bacteria contaminating milk after pasteurization can also multiply during
storage and cause spoilage, especially when the milk is either stored for a long time or temperature
abused. In that event, it is difficult to differentiate the role of bacterial growth and heat-stable
enzymes in spoilage.
Ultrahigh Temperature (UHT)-Treated Milk Products
UHT-treated milks, heated at 140°C–150°C for 1 to 5 seconds, are considered commercially
sterile products with a shelf life of three months at 20°C. Spoilage of these products during storage at 20°C has been observed in the form of bitter flavor, sediments, and gel formation, resulting
from the action of heat-stable proteinases, and rancid flavor from the action of heat-stable lipases.
Generally, the changes produced by proteinases are more predominant than those associated with
lipases. However, in the presence of heat-stable phospholipases, the lipolysis caused by lipases can
be detected. It is speculated that phospholipases degrade the phospholipids in the membrane of fat
globules and increase the susceptibility of fat to lipases. 7,9
The time of spoilage of UHT-treated milk by heat-stable proteinases is dependent on the numbers and strains of Pseudomonas species growing in the raw milk. In a controlled study, UHTtreated milk was prepared from raw milk inoculated with a Pse. fluorescens strain and grown to 8 ×
10 5 to 5 × 10 7 cells/mL. During storage at 20°C, the product with 5 × 10 7 cells/mL gelled in 10–14
days; the product with 8 × 10 6 cells/mL gelled in 8–10 weeks; and the product with 8 × 10 5 cells/mL
did not gel in 20 weeks but had sediments. Raw milk whose caseins have been highly degraded by
proteolytic enzymes may even be unstable to high heat treatment and thus cannot be used for the
manufacture of UHT-treated milk.
Cheeses
Proteolytic activity by the extracellular proteinases of psychrotrophic bacteria in raw milk was
reported to reduce cheese yield and increase the levels of nitrogenous compounds in whey.
Depending on the proteolysis of caseins, the loss in cheese yield can be as high as 5%. The loss
was directly related to the storage time of the raw milk and psychrotrophic counts. In addition,
the heat-stable proteolytic enzymes were associated with increased proteolysis of cheeses (especially
soft cheeses, such as cottage cheese), lower flavor quality, and higher texture problems in cheddar
cheese. Lipases have also been implicated in the development of off-flavor in cheese.
Cultured Dairy Products
Buttermilk and yogurt, made from milk that has had substantial growth of psychrotrophic bacteria before heat treatment, generally have poor texture and rapidly develop off-flavor during storage,
even though they have low pH and are stored at approximately 10°C. These defects are attributed
to bacterial heat-stable proteinases.
Cream and Butter
Cream and butter are more susceptible to spoilage by heat-stable lipases than by proteinases.
Extracellular lipases of psychrotrophic bacteria preferentially partition with the cream phase of
milk, increasing their concentrations in cream. Lipases are responsible for off-flavor development
under normal short-term refrigerated storage, these defects may not be enough to detect. In addition, psychrotrophic bacteria contaminating milk after pasteurization can also multiply during
storage and cause spoilage, especially when the milk is either stored for a long time or temperature
abused. In that event, it is difficult to differentiate the role of bacterial growth and heat-stable
enzymes in spoilage.
Ultrahigh Temperature (UHT)-Treated Milk Products
UHT-treated milks, heated at 140°C–150°C for 1 to 5 seconds, are considered commercially
sterile products with a shelf life of three months at 20°C. Spoilage of these products during storage at 20°C has been observed in the form of bitter flavor, sediments, and gel formation, resulting
from the action of heat-stable proteinases, and rancid flavor from the action of heat-stable lipases.
Generally, the changes produced by proteinases are more predominant than those associated with
lipases. However, in the presence of heat-stable phospholipases, the lipolysis caused by lipases can
be detected. It is speculated that phospholipases degrade the phospholipids in the membrane of fat
globules and increase the susceptibility of fat to lipases. 7,9
The time of spoilage of UHT-treated milk by heat-stable proteinases is dependent on the numbers and strains of Pseudomonas species growing in the raw milk. In a controlled study, UHTtreated milk was prepared from raw milk inoculated with a Pse. fluorescens strain and grown to 8 ×
10 5 to 5 × 10 7 cells/mL. During storage at 20°C, the product with 5 × 10 7 cells/mL gelled in 10–14
days; the product with 8 × 10 6 cells/mL gelled in 8–10 weeks; and the product with 8 × 10 5 cells/mL
did not gel in 20 weeks but had sediments. Raw milk whose caseins have been highly degraded by
proteolytic enzymes may even be unstable to high heat treatment and thus cannot be used for the
manufacture of UHT-treated milk.
Cheeses
Proteolytic activity by the extracellular proteinases of psychrotrophic bacteria in raw milk was
reported to reduce cheese yield and increase the levels of nitrogenous compounds in whey.
Depending on the proteolysis of caseins, the loss in cheese yield can be as high as 5%. The loss
was directly related to the storage time of the raw milk and psychrotrophic counts. In addition,
the heat-stable proteolytic enzymes were associated with increased proteolysis of cheeses (especially
soft cheeses, such as cottage cheese), lower flavor quality, and higher texture problems in cheddar
cheese. Lipases have also been implicated in the development of off-flavor in cheese.
Cultured Dairy Products
Buttermilk and yogurt, made from milk that has had substantial growth of psychrotrophic bacteria before heat treatment, generally have poor texture and rapidly develop off-flavor during storage,
even though they have low pH and are stored at approximately 10°C. These defects are attributed
to bacterial heat-stable proteinases.
Cream and Butter
Cream and butter are more susceptible to spoilage by heat-stable lipases than by proteinases.
Extracellular lipases of psychrotrophic bacteria preferentially partition with the cream phase of
milk, increasing their concentrations in cream. Lipases are responsible for off-flavor development