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CHAPTER 10 ANSWERS TO STUDY QUESTIONS
1. The acidity/alkalinity of the medium in which a microorganism exists affects its ability to
survive. A medium, or food, that has a pH much above or below 7 results in denaturation of
membranes and transport enzymes. In acid environments, the cell expends a lot of energy in the
form of ATP, driving protons to the outside in order to maintain a neutral pH inside (the
cytoplasm). Once the cell is depleted of ATP, internal pH decreases, resulting in enzyme
denaturation/loss of function. This leads to cell death due to an inability of the cell to carry on key
enzyme-mediated life-sustaining metabolic functions.
2. Poising effect is the resistance of a medium or food to change in terms of its oxidationreduction potential. In other words, it is the resistance to lose or gain electrons. Oxidation can be
thought of as the addition of oxygen or the loss of electrons, while reduction is the loss of oxygen
or the gain of electrons. As an example, foods that contain reducing sugars (monosaccharides in
fruits and vegetables) tend to gain electrons. The pH of foods can play a role in the poising effect –
the higher the pH, the more the food will resist oxidation, and the lower the pH, the more the food
resists reduction.
3. There are 12 genera of lactic acid bacteria, including Lactococcus, Lactobacillus, Leuconostoc,
Pediococcus, and Streptococcus. These organisms produce lactic acid from the fermentation of
hexoses such as glucose. They are typically found in plant material, although they are ubiquitous
in other environments too (fruits, vegetables, meats).
4. Formation of these compounds requires a decarboxylase enzyme (lysine decarboxylase or
ornithine decarboxylase) which removes a carboxyl group from lysine and ornithine to form
cadaverine and putrescine.
5. Molds profuce hyphae during growth, covering the surface of a food such that it becomes
visible to the naked eye, and which changes food color and texture. Mold enzymes can hydrolyze
food components, resulting in the production of odors and flavors. Examples are black rot of citrus
fruits (pectin breakdown caused by Aspergillus), and the spoilage of butter contaminated with
Cladosporium.
6. As the organisms grow, they utilize the oxygen in the medium, causing it to become reduced. In addition,
as the medium becomes anaerobic, microorganisms continue to grow at the same rate as before, suggesting
that the organisms are facultative anaerobes.
7. Due to the lack of carbohydrates, rennet coagulation is used to lower the pH, instead of microorganisms.
8. b.
9. c.
10. d.
11. b.
12. a.
13. a.
14. c.
15. b.
16. b.
17. a.
18. a.
19. a.
20. a.
21. b.
22. b.
23. b.
24. b.
25. b.
26. a.