Fundamental Food Microbiology, Third Edition - Fuad Fathir

308 FUNDAMENTAL FOOD MICROBIOLOGY
or strains initially degrade k-casein. With time, they can also degrade other casein
fractions. Proteinases from psychrotrophic species or strains of Flavobacterium,
Aeromonas, and Serratia also showed initial differences in the degradation of b- and
k-caseins. a-Casein is degraded last by all strains. 1–4
Lipases of psychrotrophic bacteria differ in their specificity toward lipids. A
lipase from a strain of Pse. fragi specifically hydrolyzed fatty acids from positions
1 and 3 of the triglycerides, whereas another lipase from a second strain hydrolyzed
only at position 1. Similar specificities have been observed with extracellular lipases
from Pse. fluorescens. Some lipases of psychrotrophic bacteria equally hydrolyze
all three fatty acids.
Among the phospholipases produced by psychrotrophic Gram-negative bacteria,
phospholipase C from Pseudomonas spp. has relatively high heat stability. It is not
destroyed by pasteurization. Phospholipase C from Pse. fluorescens is more active
against phosphatidylethanolamine than other phospholipids. 1–4
III. SPOILAGE OF FOODS WITH HEAT-STABLE MICROBIAL ENZYMES
The presence of heat-stable extracellular enzymes of psychrotrophic bacteria in raw
milk can cause spoilage of dairy products made from it. In addition, when these
dairy products are used as ingredients to make other food products, the action of
heat-stable enzymes can also reduce their acceptance qualities. Several examples
are used here to emphasize the spoilage potential of these enzymes. 1,2,5–7
A. Pasteurized Milk
Heat-stable proteinases and lipases of psychrotrophic bacteria are not inactivated by
pasteurization and can cause proteolysis of casein and lipolysis of milk lipids to
produce flavor defects. However, 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.
B. Ultrahigh Temperature (UHT)-Treated Milk Products
UHT-treated milks, heated at 140 to 150�C for 1 to 5 s, are considered commercially
sterile products with a shelf life of 3 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, due to 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.