284 ◾ Fundamental Food Microbiology
the meat was thawed (see Chapter 22). To overcome the problem, it will be important to adopt
good sanitary practices and proper temperature control during processing and before freezing the
product.
Gas and Slime Development in Vacuum- Packaged
Smoked Salmon Products
Processors have reported different types of spoilage in vacuum-packaged refrigerated smoked
salmon hot dog and deli-type products. The hot dog packages had large amounts of gas and very
thick, slimy, yellow-whitish purge. The deli-type packages had very little gas but large volumes
of cloudy purge. Examination revealed that H 2 S was absent in the gas, and pH of the purge was
approximately 4.8 for both types of samples. Phase-contrast microscopy of the purge revealed the
presence of coccoid to lenticular cells in small chains in the hot dogs but different-sized rods in
the deli samples. Microbiological enumeration of the purge revealed the presence of lactic acid
bacterial CFUs more than 10 10 /mL. Biochemical analysis revealed the predominant coccoid and
lenticular isolates in hot dogs to be Leu. mesenteroides ssp. dextranicum and the predominant rodshaped isolates in deli-type products to be Lab. sake. Both bacterial isolates were killed at 71°C in
five minutes (the products were processed at 72°C, internal temperature) but were able to multiply
at 4°C.
It was suggested that both lactic acid bacterial strains were present in the packaged products
as post-heat treatment contaminants. The ingredients had both glucose and brown sugar in fairly
high concentrations, which facilitate both psychrotrophic facultative anaerobic strains to multiply at refrigerated storage temperatures. The heterofermentative Leuconostoc species produced gas
and dextran (from sugar). Both reduced the original product pH of 5.6 to 4.8, causing the loss of
bound water to produce purge in the bags. The recommendations are to replace the sugar in the
formulation, use proper sanitation to reduce post-heat contamination load, maintain the storage
temperature below 4°C, and reduce temperature abuse.
Conclusion
The examples in this chapter describe how products are contaminated with different types of
spoilage bacteria, many as post-heat contaminants. Complicated machineries, handling of large
volumes of products, and a desire for a low initial microbial load by good sanitation have probably
selected out those bacteria that can establish in the facilities and contaminate the products. In
the absence of competition and being psychrotrophic and anaerobic or facultative anaerobic, even
from a low initial contamination level, the bacteria can multiply and reach high levels during long
storage to cause spoilage. In the absence of effective preservatives as well as because of temperature
abuse during storage, transport, and display, they can grow more rapidly and cause rapid spoilage
of the products. In some situations, poor sanitation (contamination with Gram-negatives) of the
facilities was observed. Table 21.3 summarizes some general characteristics of predominant spoilage bacteria in vacuum-packaged refrigerated processed foods. It is also important to recognize
that because processed products are stored at refrigerated or frozen temperatures for a long time
(60–100 days or more), some enzymes of the contaminating microorganisms can cause spoilage
even in the absence of live cells. This is discussed in Chapter 22.
the meat was thawed (see Chapter 22). To overcome the problem, it will be important to adopt
good sanitary practices and proper temperature control during processing and before freezing the
product.
Gas and Slime Development in Vacuum- Packaged
Smoked Salmon Products
Processors have reported different types of spoilage in vacuum-packaged refrigerated smoked
salmon hot dog and deli-type products. The hot dog packages had large amounts of gas and very
thick, slimy, yellow-whitish purge. The deli-type packages had very little gas but large volumes
of cloudy purge. Examination revealed that H 2 S was absent in the gas, and pH of the purge was
approximately 4.8 for both types of samples. Phase-contrast microscopy of the purge revealed the
presence of coccoid to lenticular cells in small chains in the hot dogs but different-sized rods in
the deli samples. Microbiological enumeration of the purge revealed the presence of lactic acid
bacterial CFUs more than 10 10 /mL. Biochemical analysis revealed the predominant coccoid and
lenticular isolates in hot dogs to be Leu. mesenteroides ssp. dextranicum and the predominant rodshaped isolates in deli-type products to be Lab. sake. Both bacterial isolates were killed at 71°C in
five minutes (the products were processed at 72°C, internal temperature) but were able to multiply
at 4°C.
It was suggested that both lactic acid bacterial strains were present in the packaged products
as post-heat treatment contaminants. The ingredients had both glucose and brown sugar in fairly
high concentrations, which facilitate both psychrotrophic facultative anaerobic strains to multiply at refrigerated storage temperatures. The heterofermentative Leuconostoc species produced gas
and dextran (from sugar). Both reduced the original product pH of 5.6 to 4.8, causing the loss of
bound water to produce purge in the bags. The recommendations are to replace the sugar in the
formulation, use proper sanitation to reduce post-heat contamination load, maintain the storage
temperature below 4°C, and reduce temperature abuse.
Conclusion
The examples in this chapter describe how products are contaminated with different types of
spoilage bacteria, many as post-heat contaminants. Complicated machineries, handling of large
volumes of products, and a desire for a low initial microbial load by good sanitation have probably
selected out those bacteria that can establish in the facilities and contaminate the products. In
the absence of competition and being psychrotrophic and anaerobic or facultative anaerobic, even
from a low initial contamination level, the bacteria can multiply and reach high levels during long
storage to cause spoilage. In the absence of effective preservatives as well as because of temperature
abuse during storage, transport, and display, they can grow more rapidly and cause rapid spoilage
of the products. In some situations, poor sanitation (contamination with Gram-negatives) of the
facilities was observed. Table 21.3 summarizes some general characteristics of predominant spoilage bacteria in vacuum-packaged refrigerated processed foods. It is also important to recognize
that because processed products are stored at refrigerated or frozen temperatures for a long time
(60–100 days or more), some enzymes of the contaminating microorganisms can cause spoilage
even in the absence of live cells. This is discussed in Chapter 22.