Factors affecting microbial growth in foods

Question 1

The factors which affect the growth and survival of microorganisms in foods are divided into?

Answer 1

• Intrinsic effects
• Extrinsic effects ( such as Implicit effects)
• Processing effects

Question 2

True or False. Plants and animals that serve as food sources have all evolved mechanisms of defence against the invasion and proliferation of microorganisms.

Answer 2

True

Question 3

What are intrinsic parameters?

Answer 3

those factors that are inherent in the food.

Question 4

List the intrinsic parameters that affect microbial growth in foods.

Answer 4

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

Question 5

How does the biological structure of foods affect the types of microbes that grow?

Answer 5

• plants and animals have developed effective physical barriers against the invasion of microorganisms such as:
  1. The natural covering of some foods provides excellent protection (eg, the testa of seeds, shells of nuts, hides of animals)
  2. The inner part of whole healthy tissues are sterile
• the process of peeling, skinning and chopping exposing tissue increases the contamination of microorganisms

Question 6

How does the nutrient content of foods affect the types of microbes that grow?

Answer 6

• chemical composition of food influences the type of microorganisms that grow and the products produced
• Nutrient requirements are dependent
  upon the genetic makeup of the
  organism and its ability to
  synthesize component de novo
  (from scratch)

Question 7

What are the types of nutrients in food that are most likely used by microbes? Give examples of each type.

Answer 7

1. Sources of energy (Complex and simple sugars, alcohol, fats and amino acids)
2. Sources of Nitrogen ( amino acids, nucleotides, peptides, proteins and urea)
3. Vitamins ( B vitamins)
4. Minerals (several elements in small amounts - Ca, Mg, Fe, S, Mn, K)
5. Growth factors

Question 8

How does the pH of foods affect the types of microbes that grow?

Answer 8

• pH is the most important intrinsic factor affecting foods
• weak acids are more common in nature than strong acids
• weak acids don’t dissociate completely
• weak acids have a greater effect on microbial cells because undissociated acids are lipophilic so they are able to cross the cell membrane and once inside they become dissociated
• leads to acidification which leads to inhibition of growth and eventual death of the microbes.

Question 9

Foods can be categorized based on their pH in what categories?

Answer 9

• High acid (<3.7)
• Acid (3.7-4.6)
• Medium acid (4.6 - 5.3)
• Low acid (>5.3)

Question 10

Adverse pH makes cells what?

Answer 10

more sensitive to toxic agents and
environmental conditions

Question 11

What happens when pH decreases?

Answer 11

the rate of growth and survival of microbes decreases during storage, heating or drying

Question 12

How does pH affect the different food microbes?

Answer 12

• yeast and moulds are able to grow at lower pH than bacteria
• Gram -ve bacteria are more sensitive to low pH than gram +ve

Question 13

How does the water activity of foods affect the types of microbes that grow?

Answer 13

• The availability of water in food greatly affects the ability of microorganisms to grow.
• The aW of most food is above 0.98
  -The aW of food can be reduced by increasing the concentration of solutes in the aqueous phase of the food by:
  Drying, Freezing, and Addition of
  solutes
• Microorganisms will not grow below 0.60 aW
• In general, bacteria are more sensitive to decreasing aW than yeast
• Most bacterial pathogens are inhibited below 0.90 aW
• Gram-negative bacteria require higher aW than Gram-positive species

Question 14

What effects do lowering aW below optimum have on microbes?

Answer 14

– increase the length of the lag phase, WATER ACTIVITY
– decrease growth rate/increased generation time
– decrease in size of the final population
- range of aw permitting growth is decreased as more environmental conditions
become unfavourable.
- When both incubation temp & pH where
made unfavourable the Minimum aw Got
even higher

Question 15

How does the oxidation-reduction potential of food affect the types of microbes that grow?

Answer 15

• Positive redox potential is related to high O2
  levels
• Aerobes require positive Eh values (oxidised)
• Anaerobes require negative Eh values (reduced)
• Microaerophiles grow best under slightly reduced condition

Question 16

The redox potential of a food is determined by?

Answer 16

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

Question 17

How does oxidation-reduction potential affect the different food microbes?

Answer 17

• Most yeast and mould in foods are aerobic, and a few are facultative
• Most fresh plant and animal food have low, well-poised redox potential in their interior.
• The exterior of chopped-up products has a higher redox potential

Question 18

How do antimicrobial constituents affect the different food microbes?

Answer 18

• Many foods contain natural constituents
  which have antimicrobial properties
• Many herbs and spices contain essential
  oils with antimicrobial properties.
• Egg white has several natural inhibitory
  agents
  lysozyme - weakens the gram + bacteria,
  ovomucoid - enzyme inhibitor,
  conalbumin - chelates metals,
  ovoflavoprotein - sequesters riboflavin
  avidin - binds biotin
• Milk has several antimicrobial systems

Question 19

_____ _____ can be considered nature’s preservation techniques.

Answer 19

Intrinsic parameters

Question 20

What are extrinsic parameters?

Answer 20

those properties of the storage environment that influence the growth of microbes in food.
- affect both the food and the microorganisms
- influences the extent and rate of microbial growth

Question 21

Some examples of extrinsic parameters are:

Answer 21

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

Question 22

How does temperature affect the different food microbes?

Answer 22

• Storage temperature is the most important parameter affecting the shelf life of highly perishable foods
• Moulds grow over a wider temperature range
  than bacteria.
• Yeast can grow within the psychotropic and
  mesophilic ranges but not in the thermophilic
  range
• to keep foods safe: 1. cold foods - 5 degrees Celsius
  2. hot foods - 60 degrees Celsius

Question 23

The range of temperature between 5 degrees and 60 degrees celsius is known as?

Answer 23

Danger Zone

Question 24

What happens when food temperatures enter the Danger Zone?

Answer 24

• bacteria can grow fast or make poisons that can make people sick.

Question 25

How does relative humidity affect the different food microbes?

Answer 25

• The relative humidity (RH) of the
  the environment surrounding the food
  during storage is important in
  maintaining the aW and controlling
  surface microbial growth
• RH is most important when it
  differs significantly from the aW
  of the food

Question 26

How do gases in the environment affect the different food microbes?

Answer 26

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

Question 27

What are the implicit effects that affect food microbes?

Answer 27

1. Antagonism
2. Synergism
3. Syntrophism

Question 28

What are the processing effects that affect food microbes?

Answer 28

1. Industrial or culinary treatment influences the type of organism present
2. Most modes of processing results in the elimination of pathogens but this can also lead to increased colonization due to contamination and proliferation,

Question 29

How do the intrinsic and extrinsic parameters interact?

Answer 29

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

Temperature, pH and water activity are
generally, the most important factors affecting
microbial growth in foods.