Factors affecting microbial growth in foods Flashcards
List the factors which affect the growth and survival of microorganisms in food.
- Intrinsic effects
- Extrinsic effects
- Processing effects
What are intrinsic parameters?
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.
List the intrinsic factors that affect microbes in food.
- Biological structures
- Nutrient content
- Moisture content
- Oxidation - reduction potential
- pH and organic acids
- Antimicrobial constituents
How does the biological structures affect the microorganisms present in food?
- 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.
What factors increases the opportunity for the biological structures of plants and animals to be contaminated?
- 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.
What is required for microorganisms to grow in food?
- Water
- Source of energy (complex and simple sugars, alcohols, fats & amino acids)
- Source of nitrogen (heterotrophs - amino acids; other nitogenous compounds used are nucleotides, peptides, proteins, urea)
- Vitamins and growth factors (B vitamins)
- Minerals (Ca, Mg, Fe, S, Mn, K)
Nutrient requirements are dependent upon what?
The genetic makeup of the organism and its ability to synthesize components de novo (from scratch)
What influences the type of microorganisms that will grow and the products they produce?
The chemical compositon of food
What is the most important intrinsic factors affecting foods?
pH
Between strong acids and weak acids, weak acids are considered more important, why is that?
- 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
What effect does adverse pH have on cells?
Adverse pH makes cells more sensitive to toxic agents and environmental conditions.
What happens when pH is decreased?
Decreasing pH decreases the rate of growth and survival during storage, heat or drying.
How does pH affect the following microorganisms:
a) Yeast and moulds
b) Bacteria
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
Define water activity.
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
How does water activity affect microorganisms?
- 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.
What is one way the water activity of a food can be reduced?
By increasing the concentration of solutes in the aqueous phase of the food by:
1. Drying
2. Freezing
3. Addition of solutes
What are the effects of lowering water activity (aw) below optimum?
- Increase the length of the lag phase
- Decrease growth rate/increased generation time
- Decrease in size of final population
What is redox potential (oxidation - reduction potential) of a substrate?
The redox potential of a substrate is the ease with which the substrate is able to gain or lose electrons
What happens when a substance is highly oxidized?
The electrical potential will be more positive.
What happens when a substance is highly reduced?
the more negative its potential.
What factors determine the redox potential of food?
- pH
- Oxygen tension surrounding the food
- Access of the food to atmosphere
- Poising capacity (resistance to change - analogous to buffer capacity)
Fill in the blanks:
a) _______ require ________ Eh values (______)
b) ________ require _______ Eh values (_______)
c) _________ grows best under __________
a) Aerobes; positive; oxidised
b) Anaerobes; negative; reduced
c) Microaerophiles; slightly reduced conditions
What’s the relationship between redox potential and yeasts and moulds in food?
- Most yeast and mould are aerobic, a few are facultative
Explain antimicrobial constituents in food.
- Many foods contain natural constituents which have antimicrobial properties
- Many herbs and spices contain essential oils with antimicrobial properties
What are some examples of food with antimicrobial constituents?
- Eggs has several natural inhibitory agents (lysozyme, ovamucoid, conalbumin, ovoflavoprotein, avidin)
- Milk has several antimicrobial systems
What are extrinsic parameters?
The properties of the storage environment of the food.
Name the extrinsic parameters affecting microbial growth in foods.
- Temperature
- Relative humidity
- Presence and concentration of gases in the environment
- Presence and activities of other microoganisms (Implicit effects)
Extrinsic parameters affect what?
- Affects the food and the microorganisms associated with it
- Influence the extent and rate of microbial growth
What is the most important extrinsic parameter?
Temperature
Discuss the relationship between yeasts and molds and temperature.
- Molds grow over a wider temperature range than bacteria
- Yeast can grow within psychrotrophic and mesophilic but not thermophilic range
How does temperature affect microorganisms in/on food?
- 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
What is the Danger Zone and what are the effects on food?
- 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
How long should food be kept at room temperature?
2 hours
When is relative humidity important?
- when maintaining the water activity (aw) and controlling surface microbial growth
How does gases in the environment affect microbes in food?
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
What are implicit effects?
This is related to how the different microorganisms interact in the food environment.
What are the types of implicit effects?
- Antagonism
- Synergism
- Syntrophism
Define the following:
1) Synergism
2) Syntrophism
- Synergism refers to when both organisms benefit from the relationship but the association is not obligatory
- Syntrophism refers to nutrients essential for the proliferation of one organism in is produced by another.
What is meant by processing effects?
This is the influence on the type of organism present as a result of industrial or culinary treatment
Though most modes of processing results in the elimination of pathogens, what else can happen?
Increased colonization due to contamination and proliferation.
What are interactive effects?
Refers to how the intrinsic and extrinsic parameters in food interact.
How do these factors affect each other (interactive effects)?
When any parameter falls out of the optimum for a given organism, that organism will become more sensitive to changes in all other parameters
What are the most important factors affecting microbial growth in foods?
- Temperature
- pH
- Water activity
