Factors affecting microbial growth in foods Flashcards
The factors which affect the growth and survival of microorganisms in foods are divided into?
- Intrinsic effects
- Extrinsic effects ( such as Implicit effects)
- Processing effects
True or False. Plants and animals that serve as food sources have all evolved mechanisms of defence against the invasion and proliferation of microorganisms.
True
What are intrinsic parameters?
those factors that are inherent in the food.
List the intrinsic parameters that affect microbial growth in foods.
- Biological structures
- Nutrient content
- Moisture content
- Oxidation-reduction potential
- pH and organic acids
- Antimicrobial constituents
How does the biological structure of foods affect the types of microbes that grow?
- 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
How does the nutrient content of foods affect the types of microbes that grow?
- 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)
What are the types of nutrients in food that are most likely used by microbes? Give examples of each type.
- Sources of energy (Complex and simple sugars, alcohol, fats and amino acids)
- Sources of Nitrogen ( amino acids, nucleotides, peptides, proteins and urea)
- Vitamins ( B vitamins)
- Minerals (several elements in small amounts - Ca, Mg, Fe, S, Mn, K)
- Growth factors
How does the pH of foods affect the types of microbes that grow?
- 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.
Foods can be categorized based on their pH in what categories?
- High acid (<3.7)
- Acid (3.7-4.6)
- Medium acid (4.6 - 5.3)
- Low acid (>5.3)
Adverse pH makes cells what?
more sensitive to toxic agents and
environmental conditions
What happens when pH decreases?
the rate of growth and survival of microbes decreases during storage, heating or drying
How does pH affect the different food microbes?
- yeast and moulds are able to grow at lower pH than bacteria
- Gram -ve bacteria are more sensitive to low pH than gram +ve
How does the water activity of foods affect the types of microbes that grow?
- 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
What effects do lowering aW below optimum have on microbes?
– 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
How does the oxidation-reduction potential of food affect the types of microbes that grow?
- 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
The redox potential of a food is determined by?
- pH
- Oxygen tension surrounding the food
- Access of food to the atmosphere
- Poising capacity ( resistance to change - analogous to buffer capacity)
How does oxidation-reduction potential affect the different food microbes?
- 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
How do antimicrobial constituents affect the different food microbes?
- 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
_____ _____ can be considered nature’s preservation techniques.
Intrinsic parameters
What are extrinsic parameters?
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
Some examples of extrinsic parameters are:
- Temperature of storage
- Relative humidity of the environment
- Presence and concentration of gases in the environment
- Presence and activities of other microorganisms (Implicit effects)
How does temperature affect the different food microbes?
- 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
The range of temperature between 5 degrees and 60 degrees celsius is known as?
Danger Zone
What happens when food temperatures enter the Danger Zone?
- bacteria can grow fast or make poisons that can make people sick.
How does relative humidity affect the different food microbes?
- 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
How do gases in the environment affect the different food microbes?
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
What are the implicit effects that affect food microbes?
- Antagonism
- Synergism
- Syntrophism
What are the processing effects that affect food microbes?
- Industrial or culinary treatment influences the type of organism present
- Most modes of processing results in the elimination of pathogens but this can also lead to increased colonization due to contamination and proliferation,
How do the intrinsic and extrinsic parameters interact?
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.
