Food microbiology Flashcards
spoilage microorganisms
pseudomonas, brochotrix, proteus, clostridia and bacilli
peniccium, aspergillus
food poisoning miscroorganisms
salmonella campbylobater spp. listeria e.coli fungi viruses toxins
phases of bacterial growth
lag pahse (adjustment to envrionment) logarithmic phase (accelerated rapid constant exponential growth) stationary phase (depletion of nutrients, accumulation of toxic metabolic products- cell division and cell death are in balance) death phase- population decreases due to death of cells
D-value
the decimal reduction time
time required to destroy 90% of the population (one log cycle) at given temperature
Z-value
the number of degrees required for the thermal death time curve (d-value) to decrease by one log cycle (10 times)
spores and vegetative cells
spores take longer and higher temperatures to kill
clostridium botulinum spores to decrease from and to
10 to the 12, to 10 to the 0 ( 1 spore)
times D value by 12 to get number of mins
add on Z value
3 types of factors that affect microbial growth
intrinsic (physiochemical properties of food)
extrinsic (storage environment)
implicit (properties and interactions of microorganisms)
intrinsic factors
nutrients pH and buffering capacity redox potential water activity presence of antimicrobial factors
nutrient content
high sugar= fungal
pH and buffering
pH= -ve logarithm of the H ions capacity affects stability of enxymes and growth and metabolism bacteria- 6-8 yeasts- 4.5-6 filamentous fungi- 4.5-4
muscle- 5.6
fish- 6.2-6.5
milk- 6.4-6.6
egg white- 9.2
most bacteria- 4.5-9
aciduric bacteria- 3-9
alkali tolerant- 5-11
moulds and yeast- 2-11
redox potential
the tendancy of a medium to accept or donate electrons to oxidise or reduce
obligate aerobes need high redox potential
obligate anaerobes grwo in low or negative redox
water activity
aw- ratio of water vapour pressure of a food to that of pure water (p0) at the same temp
aw= p/p0
may be affected by the relative humidity of the atmosphere
how to reduce aw
drying, freezing, addition of solutes (naCl, sugars)
alterating the microstructure of a food (butter is a water in fat emulsion prepared from cream which is a fat in water emulsion)
halophilic
osmophilic
xerophilic
can grow in higher salt conc.
can grow in the high conc. of ionised substances (ie sugars)
can grow in dry foods
presence of antimicrobial factors
egg-white proteins milk porteins egg shell and cuticle nitrites (added to cured meat to stabalise red colur and inhibit cl.botulinum) woodsmoke compounds
extrinsic factors
relative humidity
temeprature
gaseous atmosphere (related to redox potential)
relative humidity
affects water acitivity affects surface of food: white spots- surface of meat black spots- usually accompanied by decompostiion penicillium- green-blueish mould
gaseous atmosphere
O2 conc. affects redox potential
CO2 and N2 basis for modified atmopshere packing
affects pH- carbonic acid with water- reduces surface contamination
acts as a weak organic acid penetratig membranes- affecting nutrient transport and membrane availability
aerobic
facultatively anaerobic
strictly anaerobic
micro-aerophilolic
grow in presence of O2
grow in presence and absence of O2
grow preferentially in atmospheres with reduced O2 tension
temerpature
membrane integrity is affected in high temperatures
nutrient availability is affected at low temps
thermophiles mesophiles psychrophiles psychotrophs which ones are most important in food preservation
40-90 degrees (55-75 op) 5-47 (30-40 op) -5-20 (12-15 op) -5- 35 (25-30 op) mesophies and psycotrophs
safe range of temps
less than 5 or more than 60
which food borne pathogens can grow at low temperatures
listeria monocytogenes
botulinum
freeezing
slow and rapid
some fungi can grow at -10
food stored at -12 or lower is safe from microbial growth
slow freezing- better survival of bacetria
rapid freexxing- water inside bacterial cell crystalises leading to cell death
implicit factors
specific growth rate- bacteria grow faster than moulds in fresh meat
mutulism- growth of one bacteria helps another
antagonism– lactic fermentation restriccts bacterial growth of pathogens
physiological status, strain diversity and adpatation
what is the bacterial stress response?
feel a change in the environment
the stress response can alter behaviour= bacterial survival- high temp sub lethal - response change in cell’s membrane lipids
makes bacteria more fast growng, toxigenic/virulent, resistance to factor that initially caused injury, able to adapt to other adverse factors subsequently imposed along food chain
