Microbial behaviour in food

Question 1

Describe the phases of bacterial growth

Answer 1

Lag phase - short period of adjustment to the environment
Logarithmic growth phase - growth begins and accelerates to a phase of rapid constant exponential growth
Stationary phase - depletion of nutrients and accumulation of toxic metabolic products causes cell division and cell death to be balanced
Death phase - population decreased due to death of cells, death>growth

Question 2

What is the D-value

Answer 2

The decimal reduction time - the time to destroy 90% of the population at a given temperature (minutes)

Question 3

The smaller the D value the more..?

Answer 3

Heat sensitive

Question 4

What is the Z value?

Answer 4

The number of degrees required for the thermal death time curve (D-value) to decrease by one log cycle (10 times less e.g. 100 to 10)

Question 5

What are some intrinsic factors that affect microbial growth in food?

Answer 5

Nutrients
pH
Redox potential
Water activity

Question 6

What is redox potential?

Answer 6

The tendency of a medium to accept or donate electrons to oxidise or reduce

Question 7

Do obligate aerobes need a high or low redox potential?

Answer 7

High

Question 8

What is meant by water activity?

Answer 8

Defined as the ratio of the water vapour pressure of a food to that of pure water at the same temperature

Question 9

What are some methods of reducing water activity?

Answer 9

Drying
Freezing
Adding solutes
Altering the microstructure of a food

Question 10

Define these types of organisms:
Halophilic
Xerophilic
Osmophilic

Answer 10

1. grows in high salt conc
2. can grow in dry food
3. can grow in the high conc of ionised substances (sugars)

Question 11

What are extrinsic factors?

Answer 11

Storage factors

Question 12

Give some examples of extrinsic factors

Answer 12

Relative humidity
Temperature
Gaseous atmosphere
Freezing

Question 13

Define the temperature ranges of the following:

• thermophiles
• mesophiles
• psychrotrophs
• psychrophiles

Answer 13

40-90 degrees
5-47 degrees
-5 to 20 degrees
-5 to 35 degrees

Question 14

Give a bacterial example of a psychrotroph

Answer 14

Listeria monocytogenes - chilled foods

Yersinia enterocolitica - chilled foods

Question 15

Give some bacterial examples of mesophiles

Answer 15

Staphylococcus aureus
Escherichia coli
Salmonella
- food stored at ambient temps

Question 16

Which bacteria is a thermophile?

Answer 16

Clostridium botulinum - canned foods

Question 17

What are implicit properties?

Answer 17

Properties and interactions of micro-organisms

Question 18

Give some implicit property examples

Answer 18

Specific growth rate
Mutualism - growth of one bacteria helps another
Antagonism - e.g. lactic fermentation restricts bacterial growth

Question 19

What is the bacterial stress response?

Answer 19

Bacterial feel a change in the environment

- response can alter bacterial behaviour for survival

Question 20

Describe the bacterial stress response in salmonella

Answer 20

• fast growing, max growth rate can be increased
• toxigenic/virulent: more likely to cause foodborne disease
• resistant to the factor that originally caused injury