Factors affecting microbial growth in foods

Question 1

List the factors which affect the growth and survival of microorganisms in food.

Answer 1

1. Intrinsic effects
2. Extrinsic effects
3. Processing effects

Question 2

What are intrinsic parameters?

Answer 2

The characteristics of plant and animal tissues affects the growth of microorganisms.

These are inherent in the food.

Intrinsic parameters have a major effect on the types of microorganisms that are capable of growing on or in the food.

Question 3

List the intrinsic factors that affect microbes in food.

Answer 3

1. Biological structures
2. Nutrient content
3. Moisture content
4. Oxidation - reduction potential
5. pH and organic acids
6. Antimicrobial constituents

Question 4

How does the biological structures affect the microorganisms present in food?

Answer 4

• development of effective physical barriers against invasion of microorganisms
• The natural covering of some foods provides excellent protection against the entry of microorganisms
• The inner part of whole healthy tissues are essentially sterile.

Question 5

What factors increases the opportunity for the biological structures of plants and animals to be contaminated?

Answer 5

• After harvest: the opportunity for penetration of organisms increases particularly with the removal of the protective skin
• Process such as peeling, skinning and chopping expose tissue and effectively increase the distribution of contaminating microorganisms.

Question 6

What is required for microorganisms to grow in food?

Answer 6

1. Water
2. Source of energy (complex and simple sugars, alcohols, fats & amino acids)
3. Source of nitrogen (heterotrophs - amino acids; other nitogenous compounds used are nucleotides, peptides, proteins, urea)
4. Vitamins and growth factors (B vitamins)
5. Minerals (Ca, Mg, Fe, S, Mn, K)

Question 7

Nutrient requirements are dependent upon what?

Answer 7

The genetic makeup of the organism and its ability to synthesize components de novo (from scratch)

Question 8

What influences the type of microorganisms that will grow and the products they produce?

Answer 8

The chemical compositon of food

Question 9

What is the most important intrinsic factors affecting foods?

Answer 9

pH

Question 10

Between strong acids and weak acids, weak acids are considered more important, why is that?

Answer 10

• Weak acids are more common in nature than strong acids, for example, citric acid in citrus, benzoic acid in cranberries and lactic acid
• Strong acids dissociate completely while weak acids remain in equilibrium with the dissociated and undissociated forms.
• Weak acids have a greater effect on microbial cells because undissociated acids, being lipophilic, can cross the cell membrane and once inside they become dissociated. This doesn’t kill the microorganisms , instead inhibit growth, causing very extended lag phases.
• Acidification f cell contents results in a breakdown of membrane transport and essential biochemical pathways which lead to inhibition of growth and eventually death

Question 11

What effect does adverse pH have on cells?

Answer 11

Adverse pH makes cells more sensitive to toxic agents and environmental conditions.

Question 12

What happens when pH is decreased?

Answer 12

Decreasing pH decreases the rate of growth and survival during storage, heat or drying.

Question 13

How does pH affect the following microorganisms:
a) Yeast and moulds
b) Bacteria

Answer 13

a) Yeast and moulds are able to grow at lower pH than bacteria
b) Gram neg, bacteria are more sensitive to low pH than gram pos. bacteria

Question 14

Define water activity.

Answer 14

Water activity is the measure of the availability of water for biological functions and relates to water present in a food in the free form

Question 15

How does water activity affect microorganisms?

Answer 15

• the availability of water in food greatly affects the ability of microorganisms to grow
• Microorganisms will not grow below 0.60 aw
• Bacteria are more sensitive to decreasing water activity than yeasts.
• Most bacterial pathogens are inhibited below 0.90 aw
• Gram negative bacteria require higher water activity than gram positive species.

Question 16

What is one way the water activity of a food can be reduced?

Answer 16

By increasing the concentration of solutes in the aqueous phase of the food by:
1. Drying
2. Freezing
3. Addition of solutes

Question 17

What are the effects of lowering water activity (aw) below optimum?

Answer 17

1. Increase the length of the lag phase
2. Decrease growth rate/increased generation time
3. Decrease in size of final population

Question 18

What is redox potential (oxidation - reduction potential) of a substrate?

Answer 18

The redox potential of a substrate is the ease with which the substrate is able to gain or lose electrons

Question 19

What happens when a substance is highly oxidized?

Answer 19

The electrical potential will be more positive.

Question 20

What happens when a substance is highly reduced?

Answer 20

the more negative its potential.

Question 21

What factors determine the redox potential of food?

Answer 21

1. pH
2. Oxygen tension surrounding the food
3. Access of the food to atmosphere
4. Poising capacity (resistance to change - analogous to buffer capacity)

Question 22

Fill in the blanks:
a) _______ require ________ Eh values (______)
b) ________ require _______ Eh values (_______)
c) _________ grows best under __________

Answer 22

a) Aerobes; positive; oxidised
b) Anaerobes; negative; reduced
c) Microaerophiles; slightly reduced conditions

Question 23

What’s the relationship between redox potential and yeasts and moulds in food?

Answer 23

• Most yeast and mould are aerobic, a few are facultative

Question 24

Explain antimicrobial constituents in food.

Answer 24

• Many foods contain natural constituents which have antimicrobial properties
• Many herbs and spices contain essential oils with antimicrobial properties

Question 25

What are some examples of food with antimicrobial constituents?

Answer 25

• Eggs has several natural inhibitory agents (lysozyme, ovamucoid, conalbumin, ovoflavoprotein, avidin)
• Milk has several antimicrobial systems

Question 26

What are extrinsic parameters?

Answer 26

The properties of the storage environment of the food.

Question 27

Name the extrinsic parameters affecting microbial growth in foods.

Answer 27

1. Temperature
2. Relative humidity
3. Presence and concentration of gases in the environment
4. Presence and activities of other microoganisms (Implicit effects)

Question 28

Extrinsic parameters affect what?

Answer 28

1. Affects the food and the microorganisms associated with it
2. Influence the extent and rate of microbial growth

Question 29

What is the most important extrinsic parameter?

Answer 29

Temperature

Question 30

Discuss the relationship between yeasts and molds and temperature.

Answer 30

• Molds grow over a wider temperature range than bacteria
• Yeast can grow within psychrotrophic and mesophilic but not thermophilic range

Question 31

How does temperature affect microorganisms in/on food?

Answer 31

• Most bacteria do not grow in hot or cold temperatures
• Colds foods must be kept 5 C or colder; Hot foods must kept at 60 C or hotter

Question 32

What is the Danger Zone and what are the effects on food?

Answer 32

• The range of temperature between 5 C - 60 C
• When potentially hazardous foods are left in the Danger Zone, bacteria can grow fast or make people sick

Question 33

How long should food be kept at room temperature?

Answer 33

2 hours

Question 34

When is relative humidity important?

Answer 34

• when maintaining the water activity (aw) and controlling surface microbial growth

Question 35

How does gases in the environment affect microbes in food?

Answer 35

The mixture of gases surrounding a food will affect the redox potential of the surface of the food and therefore affects the growth of microorganisms

Question 36

What are implicit effects?

Answer 36

This is related to how the different microorganisms interact in the food environment.

Question 37

What are the types of implicit effects?

Answer 37

1. Antagonism
2. Synergism
3. Syntrophism

Question 38

Define the following:
1) Synergism
2) Syntrophism

Answer 38

1. Synergism refers to when both organisms benefit from the relationship but the association is not obligatory
2. Syntrophism refers to nutrients essential for the proliferation of one organism in is produced by another.

Question 39

What is meant by processing effects?

Answer 39

This is the influence on the type of organism present as a result of industrial or culinary treatment

Question 40

Though most modes of processing results in the elimination of pathogens, what else can happen?

Answer 40

Increased colonization due to contamination and proliferation.

Question 41

What are interactive effects?

Answer 41

Refers to how the intrinsic and extrinsic parameters in food interact.

Question 42

How do these factors affect each other (interactive effects)?

Answer 42

When any parameter falls out of the optimum for a given organism, that organism will become more sensitive to changes in all other parameters

Question 43

What are the most important factors affecting microbial growth in foods?

Answer 43

1. Temperature
2. pH
3. Water activity