Microbial behaviour in food

Question 1

List different kind of microorganism affecting food- good or bad

Answer 1

“spoilage microorganisms”
Usually not associated with Food poisoning

– Pseudomonas, Brochotrix, Proteus, Clostridia and Bacilli…
– Penicillium, Aspergillus …

Question 2

List food poinsoning microorganisms

Answer 2

lead to infection/ intoxication

– Bacterial – Salmonella, Campylobacter spp., Listeria, E. coli….
– Fungi
– Viruses
– Toxins

Question 3

Bacteria growth ….. how many phases?

Answer 3

4 distinctive stages

Lag
Logarithmic growth
Stationary
death

Question 4

Describe each phase of bacterial growth

Answer 4

lag phase - (short) period of adjustment to environment. Depends on many diff circumstances. Can vary in length quite considerably
Ø logarithmic growth phase - growth begins and accelerates to a phase of rapid, constant exponential growth
Ø stationary phase - depletion of nutrients and accumulation of toxic metabolic products growth is slowed to a point where cell division and cell death are in balance. The only known species not reached stationary phase are humans
Ø death phase - population decreases due to death of cells because of lack of nutrients and so on

Question 5

What is the D value in terms of bacterial death?

Answer 5

Time required to destroy 90% of the population at a given temp (one log cycle)

Question 6

What is the Z value in terms of bacteria death?

Answer 6

– Represents temperature, derived from D value

– the number of degrees required for the thermal death time curve (D-value) to decrease by one log cycle (10 times)

Question 7

What factors affect microbial growth?

Answer 7

1. Intrinsic factors: internal, physical chemical properties of food
2. Extrinsic - storage environment
3. Implicit- properties and interactions of microorganisms itself (growth rate, mutualism, antagonism, physiological status, strain diversity, adaptaion)

Question 8

List 5 intrinsic factors that affect microbial growth in food

Answer 8

§	Nutrients for bacteria to grown
§	pH and buffering capacity of foodstuff
§	Redox potential related to atmosphere present in food
§	Water activity
§	Presence of antimicrobial factors

Question 9

Briefly discuss how nutrient content affects microbial growth in food

Answer 9

An INTRINSIC factor affecting microbial growth in food:

§ Are the available nutrients suitable for growth?
§ Which micro-organisms are favoured?
§ High sugar content (fruit addition in yogurt) increases risk of fungal growth
§ Different microorganisms have different enzymes allowing them to grow on different media
§ Knowledge of the composition of the food will help predict the micro-organisms to control
§ E.g. jam – never bacteria growth as v jam, no food stuff, high sugar content that bacteria hate. But moulds can grow as they like sugars

Question 10

Briefly discuss how ph and buffering foodstuff affects microbial growth in food

Answer 10

An intrinsic factor

pH is the negative logarithm of the hydrogen ion capacity

§ Affects transmembrane transport of nutrients, ATP synthesis
§ Affects the stability of enzymes and consequently the growth and metabolism of micro-organism growth

§ Different micro-organisms like different pH (bac>yeast>fungi)

§ Different food commodities have different pH

§	Muscle: 5.6
§	Fish: 6.2-6.5
§	Milk: 6.4-6.6
§	Egg white: 9.2
§	Raw food stuff such as meat or mikl after processing have pH neutral but as matures and biochemical processed occur, become more acidic
§	Egg white v alkali

Question 11

Briefly discuss how Redox potential Eh affects microbial growth in food

Answer 11

§ To do with atmosphere and presence of O2
§ “The tendency of a chemical species to accept or donate electrons to oxidise or to reduce is termed
§ Different food commodities have different redox potential

Question 12

Briefly discuss how Water activity affects microbial growth in food

Answer 12

Intrinsic

(aw) is defined as the ratio of the water vapour pressure of a food (p) to that of pure water (p0) at the same temperature: aw = p / p0”

Different food commodities have different water activity (0-1 scale):
0 = absolute dry matter, doesn’t exist naturally in environment 1 = distilled water

Question 13

What methods reduce water activity

Answer 13

– Drying
– Freezing – microorganisms can’t utilise it anymore
– Addition of solutes (NaCl, sugars) to food
– Altering the microstructure of a food (for example churning, butter is a water-in-fat emulsion prepared from cream, a fat-in-water emulsion)

Question 14

Different categories within water activity

Answer 14

– Halophilic microorganisms – can grow in higher salt concentrations (NaCl)
– Osmophilic – can grow in the high concentration of ionized substances (i.e. sugars)
– Xerophilic – can grow in dry food e.g. staphylococcus aureas

Question 15

Briefly discuss how Presence of Antimicrobial factors affects microbial growth in food

Answer 15

§ Egg-white proteins (i.e. ovotransferrin that kill bacteria)
§ Milk proteins (i.e. Lactoferrin, Lysozyme)
§ Egg shell and cuticle
§ Nitrites – most common food preservative (added to cured meat products to stabilize red colour and inhibit Cl. botulinum)
§ Woodsmoke compounds

Question 16

List extrinsic factors

Answer 16

Environmental:
§ Relative humidity
§ Temperature
§ Gaseous atmosphere (and related redox potential)

Question 17

How does the extrinsic factor relative humidity affect microbial growth in food

Answer 17

§ Affects water activity

§ Affects surface of food

Question 18

How can surface of food be affected by the relative humidity?

Answer 18

§ White spots by Sporotrichum carnis, which affects only the surface of the meat
§ Black spots by Cladosporium herbarum, often on imported chilled carcases. Usually accompanied by decomposition
§ Penicillium gives green-blueish mould

Question 19

How does the extrinsic factor gaseous atmosphere affect microbial growth in food

Answer 19

§ Oxygen concentration affects redox potential
§ CO2, and N2 are used as the basis of modified atmosphere packing – leads to reduction of deoxy myoglobin  keeps meat look fresher (red)
§ Affects pH: produces carbonic acid with water, reduces surface contamination
§ Acts as a weak organic acid penetrating membranes, affecting nutrient transport and enzyme availability

Question 20

How are microorganisms classified with regards to growth in gaseous atmosphere

Answer 20

§ aerobic (grow in the presence of O2);
§ facultatively anaerobic (grow in the presence and absence of O2);
§ strictly anaerobic (grow only in the absence of O2); or
§ micro-aerophilic (grow preferentially in atmospheres with reduced O2 tension).

Question 21

NAmes for microorganisms that grow in certain temps:

Answer 21

§ Thermophiles: 40 to 90oC, (55-75oC optimum)
§ Mesophiles: 5 to 47oC, (30-40oC optimum)
§ Psychrophiles: -5 to 20oC, (12-15oC optimum)
§ Psychrotrophs: -5 to 35oC, (25-30oC optimum

Question 22

Which are the most important class of temperature for food preservation?

Answer 22

§ Mesophiles and psychrotrophs the most important in food preservation

Question 23

What is the bacteria stress response?

Answer 23

• Bacteria “feel” a change in the environment
• The stress response can alter bacterial behaviour = survival:
– High temp.(sub-lethal heat)  response change in cell’s membrane lipids
– Simple attachment can induce response

• ~ 30% Salmonella genome has ‘stress-response’ genes, making bacteria more:
– fast-growing; max growth rate can be increased even if the lag phase is lengthened;
– toxigenic/virulent, and therefore more likely to cause food-borne disease;
– resistant to the factor that initially caused injury; and
– able to adapt to other adverse factors subsequently imposed along the food chain.

Question 24

Facts about freezing and microbial growth

Answer 24

• Some fungi and yeasts can grow at -10oC
• But, food stored at -12oC or at lower temperatures is safe from microbiological growth
• Slow freezing - better survival of bacteria
• Rapid freezing – water inside bacterial cell crystallises leading to cell death