Fundamental Food Microbiology, Third Edition - Fuad Fathir

310 FUNDAMENTAL FOOD MICROBIOLOGY
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.
IV. SPOILAGE OF FOODS BY MICROBIAL ENZYMES AT LOW
TEMPERATURE
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
to ca. 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
Pseudomonas fragi reduce oxymyoglobin and discolor meat. The enzymes probably
hydrolyze the globin part or the peptide 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. 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-heatprocessed
(pasteurized) meat products, some of which are expected to have a shelf
life of 100 d.
In recent years, some specialty raw chicken meat products, that were stored at
refrigerated temperature (–1 to 0�C for 28 d) or at frozen temperature (–20�C up to
90 d), 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,
which acted on specific food components during long storage and produced the
flavor components in detectable amounts. At low temperatures, bacterial cells can
also die, lyse, and release intracellular enzymes. Some of these enzymes can be
active at low temperature to produce a defect that can be detected during long storage.
It is likely that in the future, commercial production of many such value-added