Chapter 30 Food Microbiology

Question 1

Intrinsic factors are?

Answer 1

the natural characterisitics of a foo that influenece tge rate of microbial growth.

Question 2

Examples of Intrinsic factors are?

Answer 2

• water availability
• ph (acidity)
• nutrients
• biological barriers
• antimicrobial chemicals

Question 3

Extrinisc factors are?

Answer 3

the enviormental conditions that influence the rate of microbial growth

Question 4

Examples of extrinsic factors?

Answer 4

• storage temperature

- atmosphere ( the presence or absence of O2 affects the type of microbial population able to grow in food)

Question 5

the 2 genera associated with chicken

Question 6

Why is Staphylococcus aureus more likely to be found in high numbers on ham than on fresh meat?

Answer 6

It does not compete well with most microbes found on foods, but the salt inhibits
most of those organisms, allowing Staphylococcus aureus to grow with little competition.

Question 7

Which is important to refrigerate: fresh stew or bread? Why?

Answer 7

Fresh stew. Bread has a low aw, so relatively few organisms can grow on it, and those that do grow quite slowly. Stew supports the rapid growth of many kinds of bacteria, including pathogens

Question 8

Why would the cooking process create anaerobic conditions?

Answer 8

The heat drives off the O2 (dissolved gases are less soluble at higher temperatures).

Question 9

Describe how the metabolism of the lactic acid bacteria differs from that of most other microorganisms that can grow aerobically

Answer 9

Most organisms that grow aerobically use aerobic respiration; lactic acid bacteria only ferment (aerotolerant anaerobes or obligate fermenters).

Question 10

How does the use of starter cultures improve the safety of fermented meat products?

Answer 10

Starter cultures increase the number of microbes that create acidic end products
(lactic acid); thereby inhibiting the pathogens that otherwise might grow.

Question 11

How could cottage cheese be produced without bacteria?

Answer 11

Adding an acidulant (an acid like citric acid) that causes the milk to coagulate.

Question 12

What characteristics of Pseudomonas species allow them to spoil such a wide variety of foods?

Answer 12

Certain Pseudomonas species can degrade a wide variety of compounds and
therefore can grow on many types of foods. In addition, some species are psychrotrophs, so they can grow at refrigeration temperatures.

Question 13

Why do fungi most commonly spoil breads and fruits?

Answer 13

Fungi grow more slowly than bacteria so they are normally out-competed. However, if bacteria are inhibited by low aw or low pH, the fungi can multiply to high numbers.

Question 14

How does boiling a home-canned food immediately prior to serving it prevent botulism?

Answer 14

Botulinum toxin is heat-labile, so boiling the food immediately before consumption
destroys any toxin (denaturation) that might have been produced.

Question 15

Which foodborne pathogen can cause hemolytic uremic syndrome (HUS)?

Answer 15

E. coli O157:H7

Question 16

Why would a large number of competing microorganisms in a food sample result in
a lack of sensitivity of culture methods for detecting pathogens?

Answer 16

It makes it more difficult to isolate the pathogen; a bit like finding a needle in a
haystack. Selective media, however, can make the process a bit simpler.

Question 17

Why are the process temperatures for canning low-acid foods higher then ones
for acidic foods?

Answer 17

Clostridium botulinum can grow in low acid foods (green beans), so those processes are designed to destroy the heat-stable endospores of that organisms.
Clostridium botulinum does not grow in high acid (low pH) foods.

Question 18

Why are nitrates added to cured meats?

Answer 18

Nitrates inhibit the growth of Clostridium botulinum (the nitrates also help meat
retain its pink color).

Question 19

Microorganisms are often grouped according to their optimum growth temperatures.
Which groups are most likely to spoil refrigerated foods?

Answer 19

Psychrotrophs and psychrophiles