474 ◾ Fundamental Food Microbiology
QUESTIONS
1. List the microbiological objectives of reducing A W in foods.
2. Discuss the mechanisms by which low A W produces an antimicrobial effect.
3. Define sorption isotherm. Describe the hysteresis loop and discuss how there can be problems in selecting the correct A W in a food from the hysteresis loop.
4. Discuss the influences of different solutes in lowering the A W value of a food and controlling
microbial growth. (Use NaCl, sucrose, and glycerol as examples.)
5. Briefly describe the interaction of A W with other intrinsic and extrinsic factors in controlling
microorganisms in a food.
6. List how different microorganisms are controlled in a food as the A W value drops from 0.99
to 0.60. Discuss the minimal A W for growth, toxin production, sporulation, and germination by a bacterial species (Clo. botulinum).
7. Briefly explain the microbiological concerns of naturally dried foods, freeze-dried foods,
smoked foods, rehydrated foods, and IMFs.
8. Briefly discuss how in the rural areas in many developing countries lower A W along with
lower pH and a suitable preservative can be used to extend the shelf life of some perishable
foods (use one example) at room temperature (see Chapter 40).
References
1. Grant, W.D., Life at low water activity, Phil. Trans. R. Soc. Lond. B, 359, 1249–1267, 2004.
2. Wijnker, J.J., Koop, G., and Lipman, L.J.A., Antimicrobial properties of salt (NaCl) used for the preservation of natural casings, Food Microbiol., 23, 657–662, 2006.
3. Sperber, W.H., Influence of water activity on foodborne bacteria: A review, J. Food Prot., 46, 142–150,
1983.
4. Sagar, V.R. and Suresh Kumar, P., Recent advances in drying and dehydration of fruits and vegetables:
A review, J Food Sci Technol, 47, 15–26, 2010.
5. Chirife, J. and María Del Buera, P., Water activity, glass transition and microbial stability in concentrated/semimoist food systems, J. Food Sci., 59, 921–927, 1994.
6. Slade, L., Levine, H., and Reid, D.S., Beyond water activity: Recent advances based on an alternative
approach to the assessment of food quality and safety, Crit. Rev. Food Sci. Nutr., 30, 115–360, 1991.
7. Lewicki, P.P., Design of hot air drying for better foods, Trends Food Sci Technol, 17, 153–163, 2006.
8. Claussen, I.C., Ustad, T.S., Strommen, I., and Walde, P.M., Atmospheric freeze drying: A review,
Drying Technol., 25, 947–957, 2007.
9. Cooper, M., Douglas, G., and Perchonok, M., Developing the NASA food system for long-duration
missions, J. Food Sci., 76, R40–R48, 2011.
10. Rao, M.S., Chander, R., and Sharma, A., Development of shelf-stable intermediate-moisture meat
products using active edible chitosan coating and irradiation, J. Food Sci., 70, m325–m331, 2005.
11. Erickson, L.E., Recent developments in intermediate moisture foods, J. Food Prot., 45, 484–491, 1982.
QUESTIONS
1. List the microbiological objectives of reducing A W in foods.
2. Discuss the mechanisms by which low A W produces an antimicrobial effect.
3. Define sorption isotherm. Describe the hysteresis loop and discuss how there can be problems in selecting the correct A W in a food from the hysteresis loop.
4. Discuss the influences of different solutes in lowering the A W value of a food and controlling
microbial growth. (Use NaCl, sucrose, and glycerol as examples.)
5. Briefly describe the interaction of A W with other intrinsic and extrinsic factors in controlling
microorganisms in a food.
6. List how different microorganisms are controlled in a food as the A W value drops from 0.99
to 0.60. Discuss the minimal A W for growth, toxin production, sporulation, and germination by a bacterial species (Clo. botulinum).
7. Briefly explain the microbiological concerns of naturally dried foods, freeze-dried foods,
smoked foods, rehydrated foods, and IMFs.
8. Briefly discuss how in the rural areas in many developing countries lower A W along with
lower pH and a suitable preservative can be used to extend the shelf life of some perishable
foods (use one example) at room temperature (see Chapter 40).
References
1. Grant, W.D., Life at low water activity, Phil. Trans. R. Soc. Lond. B, 359, 1249–1267, 2004.
2. Wijnker, J.J., Koop, G., and Lipman, L.J.A., Antimicrobial properties of salt (NaCl) used for the preservation of natural casings, Food Microbiol., 23, 657–662, 2006.
3. Sperber, W.H., Influence of water activity on foodborne bacteria: A review, J. Food Prot., 46, 142–150,
1983.
4. Sagar, V.R. and Suresh Kumar, P., Recent advances in drying and dehydration of fruits and vegetables:
A review, J Food Sci Technol, 47, 15–26, 2010.
5. Chirife, J. and María Del Buera, P., Water activity, glass transition and microbial stability in concentrated/semimoist food systems, J. Food Sci., 59, 921–927, 1994.
6. Slade, L., Levine, H., and Reid, D.S., Beyond water activity: Recent advances based on an alternative
approach to the assessment of food quality and safety, Crit. Rev. Food Sci. Nutr., 30, 115–360, 1991.
7. Lewicki, P.P., Design of hot air drying for better foods, Trends Food Sci Technol, 17, 153–163, 2006.
8. Claussen, I.C., Ustad, T.S., Strommen, I., and Walde, P.M., Atmospheric freeze drying: A review,
Drying Technol., 25, 947–957, 2007.
9. Cooper, M., Douglas, G., and Perchonok, M., Developing the NASA food system for long-duration
missions, J. Food Sci., 76, R40–R48, 2011.
10. Rao, M.S., Chander, R., and Sharma, A., Development of shelf-stable intermediate-moisture meat
products using active edible chitosan coating and irradiation, J. Food Sci., 70, m325–m331, 2005.
11. Erickson, L.E., Recent developments in intermediate moisture foods, J. Food Prot., 45, 484–491, 1982.