Food microbiology

Question 1

spoilage microorganisms

Answer 1

pseudomonas, brochotrix, proteus, clostridia and bacilli

peniccium, aspergillus

Question 2

food poisoning miscroorganisms

Answer 2

salmonella
campbylobater spp.
listeria
e.coli
fungi
viruses
toxins

Question 3

phases of bacterial growth

Answer 3

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

Question 4

D-value

Answer 4

the decimal reduction time

time required to destroy 90% of the population (one log cycle) at given temperature

Question 5

Z-value

Answer 5

the number of degrees required for the thermal death time curve (d-value) to decrease by one log cycle (10 times)

Question 6

spores and vegetative cells

Answer 6

spores take longer and higher temperatures to kill

Question 7

clostridium botulinum spores to decrease from and to

Answer 7

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

Question 8

3 types of factors that affect microbial growth

Answer 8

intrinsic (physiochemical properties of food)
extrinsic (storage environment)
implicit (properties and interactions of microorganisms)

Question 9

intrinsic factors

Answer 9

nutrients
pH and buffering capacity
redox potential
water activity
presence of antimicrobial factors

Question 10

nutrient content

Answer 10

high sugar= fungal

Question 11

pH and buffering

Answer 11

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

Question 12

redox potential

Answer 12

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

Question 13

water activity

Answer 13

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

Question 14

how to reduce aw

Answer 14

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)

Question 15

halophilic
osmophilic
xerophilic

Answer 15

can grow in higher salt conc.
can grow in the high conc. of ionised substances (ie sugars)
can grow in dry foods

Question 16

presence of antimicrobial factors

Answer 16

egg-white proteins
milk porteins
egg shell and cuticle
nitrites (added to cured meat to stabalise red colur and inhibit cl.botulinum)
woodsmoke compounds

Question 17

extrinsic factors

Answer 17

relative humidity
temeprature
gaseous atmosphere (related to redox potential)

Question 18

relative humidity

Answer 18

affects water acitivity
affects surface of food:
white spots- surface of meat
black spots- usually accompanied by decompostiion
penicillium- green-blueish mould

Question 19

gaseous atmosphere

Answer 19

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

Question 20

aerobic
facultatively anaerobic
strictly anaerobic
micro-aerophilolic

Answer 20

grow in presence of O2
grow in presence and absence of O2
grow preferentially in atmospheres with reduced O2 tension

Question 21

temerpature

Answer 21

membrane integrity is affected in high temperatures

nutrient availability is affected at low temps

Question 22

thermophiles
mesophiles
psychrophiles
psychotrophs
which ones are most important in food preservation

Answer 22

40-90 degrees (55-75 op)
5-47 (30-40 op)
-5-20 (12-15 op)
-5- 35 (25-30 op)
mesophies and psycotrophs

Question 23

safe range of temps

Answer 23

less than 5 or more than 60

Question 24

which food borne pathogens can grow at low temperatures

Answer 24

listeria monocytogenes

botulinum

Question 25

freeezing

slow and rapid

Answer 25

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

Question 26

implicit factors

Answer 26

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

Question 27

what is the bacterial stress response?

Answer 27

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