Microbial behaviour in food Flashcards
What microorganisms affect food?
Spoilage microorganisms
Food poisoning micoorganisms
Probiotics and fermentation agents
Spoilage microorganisms can be good or bad. Give an example of each
Good = penicillium, aspergillus Bad = clostridium, bacillus
What are the 4 phases of bacterial growth and what happens in each?
Lag phase - adjust to environment
Exponential phase - rapid growth
Stationary phase - depletion of nutrients and accumulation of toxic metabolic products
Death phase - cells die
What is the D-value?
Decimal reduction time
Time required to destroy 90% of the population at a given temperature
What is the Z-value?
Temperature required for D=value to decrease by 10 times (1 log)
What intrinsic factors affect microbial growth of food?
Nutrients pH Redox potential Water activity Antimicrobial factors
How does sugar content affect fungal growth?
Increased sugar content = increased fungal growth
How does pH affect microbial growth in food?
Affects membrane transport of nutrients
Affects ATP synthesis
Affects enzyme stability
What is redox potential?
The tendency of a medium to accept or donate electrons
What redox potential do obligate aerobes need compared to obligate anaerobes?
Obligate aerobes = high
Obligate anaerobes = low
What is water activity?
Ratio of water vapour pressure of a food compared to pure water
What affects water activity?
Relative humidity of atmosphere
How can water activity in food be decreased?
Drying
Freezing
Adding NaCl
What does halophilic mean? What is opposite to this?
Grow in high salt concentrations
Osmophilic
What is a food that can grow in dry food?
Xerophilic
Give examples of antimicrobial factors that affect food growth
Egg white proteins
Milk proteins
Extrinsic factors are the environment the food is stored in. What extrinsic factors affect microbial food growth?
Relatie humidity
Gaseous atmosphere
Temperature
What colour mould does penicillium form?
Green/blue
What bacteria causes white spots (on meat) and black spots (chilled meat)?
White spots = Sporotrichum carnis
Black spots = Cladosporium herbarum
What gases are the bases of modified atmosphere packing? Why?
CO2, N2
Produce carbonic acid with water - decreases pH - decreases surface contamination
What are the different types of aerobes bacteria can be?
Obligate anaerobe - only grow without O2
Microaerophilic - reduced O2
Facultative anaerobe - grow with or without O2
Aerobic - grow in O2
How does temperature affect food at high and low temperatures
Denatures proteins (enzymes, membranes) at high temperatures Decreased nutrient availability at low temperatures
What temperatures do thermophiles like? What about mesophiles? Psychotrophs? Psychrophiles?
Thermophiles = 40+ Mesophiles = -5-40 Psychotrophs = -5-35 Psychrophiles = -5-20
What temperature is considered safe from most food growth?
-12
Why should bacteria be rapidly frozen, not slowly?
Rapid freezing - water crystallises inside cell and kills it
Slow freezing - more likely to survive
What are implicit factors of food growth?
Factors that affect properties and interactions of microorganisms
E.g. specific growth rate, mutualism, antagonism
What is mutualism?
The growth of one organism helps another
What is antagonism?
Lactic fermentation restricts bacterial growth of pathogens
What is the bacterial stress response?
Change in environment causes stress response
Bacteria alters behaviour to survive
E.g. high (sublethal temperature) may change cell membrane lipids
How do stress response genes help bacteria?
Faster growth
After - resistant to factor that cause the injury
Can adapt to other adverse factors
