Microbial behaviour in food

Question 1

What microorganisms affect food?

Answer 1

Spoilage microorganisms
Food poisoning micoorganisms
Probiotics and fermentation agents

Question 2

Spoilage microorganisms can be good or bad. Give an example of each

Answer 2

Good = penicillium, aspergillus
Bad = clostridium, bacillus

Question 3

What are the 4 phases of bacterial growth and what happens in each?

Answer 3

Lag phase - adjust to environment
Exponential phase - rapid growth
Stationary phase - depletion of nutrients and accumulation of toxic metabolic products
Death phase - cells die

Question 4

What is the D-value?

Answer 4

Decimal reduction time

Time required to destroy 90% of the population at a given temperature

Question 5

What is the Z-value?

Answer 5

Temperature required for D=value to decrease by 10 times (1 log)

Question 6

What intrinsic factors affect microbial growth of food?

Answer 6

Nutrients
pH 
Redox potential 
Water activity 
Antimicrobial factors

Question 7

How does sugar content affect fungal growth?

Answer 7

Increased sugar content = increased fungal growth

Question 8

How does pH affect microbial growth in food?

Answer 8

Affects membrane transport of nutrients
Affects ATP synthesis
Affects enzyme stability

Question 9

What is redox potential?

Answer 9

The tendency of a medium to accept or donate electrons

Question 10

What redox potential do obligate aerobes need compared to obligate anaerobes?

Answer 10

Obligate aerobes = high

Obligate anaerobes = low

Question 11

What is water activity?

Answer 11

Ratio of water vapour pressure of a food compared to pure water

Question 12

What affects water activity?

Answer 12

Relative humidity of atmosphere

Question 13

How can water activity in food be decreased?

Answer 13

Drying
Freezing
Adding NaCl

Question 14

What does halophilic mean? What is opposite to this?

Answer 14

Grow in high salt concentrations

Osmophilic

Question 15

What is a food that can grow in dry food?

Answer 15

Xerophilic

Question 16

Give examples of antimicrobial factors that affect food growth

Answer 16

Egg white proteins

Milk proteins

Question 17

Extrinsic factors are the environment the food is stored in. What extrinsic factors affect microbial food growth?

Answer 17

Relatie humidity
Gaseous atmosphere
Temperature

Question 18

What colour mould does penicillium form?

Answer 18

Green/blue

Question 19

What bacteria causes white spots (on meat) and black spots (chilled meat)?

Answer 19

White spots = Sporotrichum carnis

Black spots = Cladosporium herbarum

Question 20

What gases are the bases of modified atmosphere packing? Why?

Answer 20

CO2, N2

Produce carbonic acid with water - decreases pH - decreases surface contamination

Question 21

What are the different types of aerobes bacteria can be?

Answer 21

Obligate anaerobe - only grow without O2
Microaerophilic - reduced O2
Facultative anaerobe - grow with or without O2
Aerobic - grow in O2

Question 22

How does temperature affect food at high and low temperatures

Answer 22

Denatures proteins (enzymes, membranes) at high temperatures
Decreased nutrient availability at low temperatures

Question 23

What temperatures do thermophiles like? What about mesophiles? Psychotrophs? Psychrophiles?

Answer 23

Thermophiles = 40+
Mesophiles = -5-40
Psychotrophs = -5-35
Psychrophiles = -5-20

Question 24

What temperature is considered safe from most food growth?

Answer 24

-12

Question 25

Why should bacteria be rapidly frozen, not slowly?

Answer 25

Rapid freezing - water crystallises inside cell and kills it

Slow freezing - more likely to survive

Question 26

What are implicit factors of food growth?

Answer 26

Factors that affect properties and interactions of microorganisms
E.g. specific growth rate, mutualism, antagonism

Question 27

What is mutualism?

Answer 27

The growth of one organism helps another

Question 28

What is antagonism?

Answer 28

Lactic fermentation restricts bacterial growth of pathogens

Question 29

What is the bacterial stress response?

Answer 29

Change in environment causes stress response
Bacteria alters behaviour to survive
E.g. high (sublethal temperature) may change cell membrane lipids

Question 30

How do stress response genes help bacteria?

Answer 30

Faster growth
After - resistant to factor that cause the injury
Can adapt to other adverse factors