292 ◾ Fundamental Food Microbiology
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 temperatures 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 specialty products with a long shelf life will increase. It will be important to
recognize the problem and determine the cause to overcome such product defects. These aspects
are discussed with actual incidents in Chapter 21.
Conclusion
Enzymes of microorganisms either secreted outside or released from inside following lysis of the
cells can cause spoilage of specific foods even in the absence of live cells or cell growth. Some of
these enzymes can be resistant to physical and chemical treatments given to food and can be active
at the storage conditions of the food. As our likings for the refrigerated foods increase and the
foods are stored for a longer time, problems associated with these enzymes will increase. There is
a need to determine the presence of live cells as well as their enzymes to predict the shelf life of
foods. In Chapter 23, some of the methods currently used to determine shelf life as well as some
new approaches are discussed.
QUESTIONS
1. Discuss the role of intracellular enzymes of spoilage bacteria in food spoilage.
2. Describe the role of extracellular enzymes of spoilage bacteria in food spoilage.
3. Briefly discuss the production and characteristics of heat-stable enzymes by psychrotrophic
Gram-negative bacteria. List three bacterial genera that produce heat-stable proteinases and
lipases.
4. Briefly describe the problems associated with heat-stable microbial enzymes in UHT milk,
cottage cheese, cheddar cheese, and cream.
5. Explain possible spoilage problems from using nonfat dry milk and cream made from a
batch of raw milk in which a Pse. flourescens grew to 5 × 10 6 cells/mL; nonfat dry milk is used
to make yogurt and cream to make sour cream.
6. What are the possibilities for heat-stable enzymes of spoilage bacteria to cause spoilage
of a low-heat-processed, vacuum-packaged, refrigerated meat product with a shelf life of
15 weeks?
7. Nowadays, many foods are stored under refrigerated or frozen temperatures (≤–20°C).
Discuss possible spoilage problems in these foods because of microbial enzymes in the
absence of microbial growth.
References
1. Cousin, M.A., Presence and activity of psychrotrophic microorganisms in milk and dairy products: A
review, J. Food Prot., 45, 172–207, 1982.
2. Ghaly, A.E., Dave, D., Budge, S., and Brooks, M.S., Fish spoilage mechanisms and preservation techniques: Review, Am. J. Appl. Sci., 7, 859–877, 2010.
3. Singh, J.R., Saxena, S., and Gupta, R., Microbial pectinolytic enzymes: A review, Process Biochem., 40,
2931–2944, 2005.
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 temperatures 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 specialty products with a long shelf life will increase. It will be important to
recognize the problem and determine the cause to overcome such product defects. These aspects
are discussed with actual incidents in Chapter 21.
Conclusion
Enzymes of microorganisms either secreted outside or released from inside following lysis of the
cells can cause spoilage of specific foods even in the absence of live cells or cell growth. Some of
these enzymes can be resistant to physical and chemical treatments given to food and can be active
at the storage conditions of the food. As our likings for the refrigerated foods increase and the
foods are stored for a longer time, problems associated with these enzymes will increase. There is
a need to determine the presence of live cells as well as their enzymes to predict the shelf life of
foods. In Chapter 23, some of the methods currently used to determine shelf life as well as some
new approaches are discussed.
QUESTIONS
1. Discuss the role of intracellular enzymes of spoilage bacteria in food spoilage.
2. Describe the role of extracellular enzymes of spoilage bacteria in food spoilage.
3. Briefly discuss the production and characteristics of heat-stable enzymes by psychrotrophic
Gram-negative bacteria. List three bacterial genera that produce heat-stable proteinases and
lipases.
4. Briefly describe the problems associated with heat-stable microbial enzymes in UHT milk,
cottage cheese, cheddar cheese, and cream.
5. Explain possible spoilage problems from using nonfat dry milk and cream made from a
batch of raw milk in which a Pse. flourescens grew to 5 × 10 6 cells/mL; nonfat dry milk is used
to make yogurt and cream to make sour cream.
6. What are the possibilities for heat-stable enzymes of spoilage bacteria to cause spoilage
of a low-heat-processed, vacuum-packaged, refrigerated meat product with a shelf life of
15 weeks?
7. Nowadays, many foods are stored under refrigerated or frozen temperatures (≤–20°C).
Discuss possible spoilage problems in these foods because of microbial enzymes in the
absence of microbial growth.
References
1. Cousin, M.A., Presence and activity of psychrotrophic microorganisms in milk and dairy products: A
review, J. Food Prot., 45, 172–207, 1982.
2. Ghaly, A.E., Dave, D., Budge, S., and Brooks, M.S., Fish spoilage mechanisms and preservation techniques: Review, Am. J. Appl. Sci., 7, 859–877, 2010.
3. Singh, J.R., Saxena, S., and Gupta, R., Microbial pectinolytic enzymes: A review, Process Biochem., 40,
2931–2944, 2005.