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Physiology of Microbes > Control of microbial growth: Food Preservation L21 > Flashcards

Control of microbial growth: Food Preservation L21 Flashcards

1
Q

what is the aim of food preservation

A

prolong time for which a food remains wholesome & safe for consumption

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2
Q

what is the main cause of food spoilage

A

growth & activity of microorganisms

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3
Q

what % of microbes associated with food are pathogens

A

2%

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4
Q

what % are food spoilage organisms

A

98%

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5
Q

are all food contaminated

A

food products are all naturally contaminated

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6
Q

what are the methods of food preservation based on

A

knowledge of physiology

growth parameters of microbes

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7
Q

what food pressure is there

A

food chain pressure

spoilage - food goes to landfill

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8
Q

what naturally happens to food

A

spoil naturally (loss colour, oxidise) accelerated by fact microbes are present

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9
Q

what is the purpose of microbes

A

allow them to get food out of their environment

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10
Q

what is the geometric growth

A

1 generation to go from ‘good’ to ‘bad’

numbers leap up quickly

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11
Q

what is the tipping point

A

Number of microbes that can tolerate (no problem) goes through exponential growth (doubling) quickly go from good level to bad level
Going from number of microbes can cope with to number of microbes that can cause disease
can be food spoilage or transition from non-hazardous from non-hazardous to hazardous (FI or FP)

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12
Q

what does rapid multiplication require for permissive growth conditions

A 
ambient temperatures
correct nutrients 
correct O2 levels 
correct pH
correct moisture/water activity
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13
Q

what are the six methods of food preservation

A 
high temp
low temp
dehydration
chemical
irradiation
controlled atmosphere packing
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14
Q

what is cidal

A

lethal effect, irreversible, microbe is dead

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15
Q

what is static

A

inhibitory effect (reversible) stopping microbes growing, stop microbes going to tipping point

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16
Q

what determines the choice of preservative method

A
  • resistance of species present to challenge

- Microbial load - numbers of microorganisms present in food

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17
Q

what must considered - resistance of species present to challenge in which preservative methods

A

vegetative cells vs spores
Gram-positive vs Gram-negative – the structure, they have different things on surface, will react differently
species adapted to extremes – e.g. doesn’t mind low pH as it can adapt

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18
Q

what determines the D value

A

microbial numbers

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19
Q

what can be used to kill microbes in D value

A

Chemical, heat irradiation to kill

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20
Q

what is needed to kill spores

A

Spores much more resistant, need something far more robust

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21
Q

what are food preservation processes designed to do

A

reduce microbial load (D-values)

prevent microbial growth (including spore germination)

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22
Q

what is important to consider to prevent recontamination

A

Packaging of food after treatment important to prevent recontamination

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23
Q

what reaction do high temperatures do

A

proteins & enzymes denatured = irreversible damage

cidal reaction

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24
Q

how do enzymes aid preservation

A

inactivation of secreted enzymes

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25
Q

how can enzymes be bad for food preservation

A

prolonged high temps CHANGE PROPERTIES OF FOOD
Therefore a compromise is needed
effect taste of food

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26
Q

how can temperature be used as a food preservation

A

Pasteurisation

kills most pathogens (mesophiles) & reduces microbial load; food stored at low temp

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27
Q

what is high temperature food preservation designed to do

A

kill two bacteria responsible for major human health threats

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28
Q

what are the bacteria that pasteurisation kill

A 
Mycobacterium tuberculosis
Coxiella burnetii (Q fever)
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29
Q

what does pasteurisation do

A

reduces microbial load; inactivates lipases

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30
Q

what is HTST

A

high temp short time

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31
Q

what happens in HTST

A

72°C for min 15 s

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32
Q

what is LTLT

A

Low temperature long time

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33
Q

what happens in LTLT

A

63°C for 30 min

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34
Q

what is UHT

A

Ultra heat treatment

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35
Q

what happens in UHT

A

141°C for 3-4 s

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36
Q

what does the rate of kill in D value depend on

A

how far above the maximum specific growth rate (µmax value) for the test organism

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37
Q

what does the rate of kill depend on

A

heat capacity of the “heating menstruum”

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38
Q

which type of food takes longer to reach threshold

A

dense foods

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39
Q

what is canning in food preservation

A

designed to kill spores of most heat resistant pathogen

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40
Q

what pathogen is canning designed to kill

A

Clostridium botulinum; 50 °C

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41
Q

how does canning prevent the growth of pathogens

A

Anaerobic conditions prevents growth of aerobes

low pH prevents outgrowth of C. botulinum

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42
Q

what will make the canning process easier

A

start with low pH as have to lower it

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43
Q

what do the parameters used for canning depend on

A

pH of the food and the initial microbial load (easier if start with lower pH)

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44
Q

what is needed in canning for potatoes

A

For non-acidic product (e.g. potatoes) 12D cook required

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45
Q

what is assumed in the 12D cook for canning

A

batch of cans is contaminated with 1 spore of C. botulinum per can
Sterilization conditions calculated that predict that only 1 spore would survive if 10^12 cans of product were treated
i.e. 12-fold decimal reduction (down to 10^-12)

46
Q

how is it calculated how long food is needed to be cooked to remove microbes for canning

A

number of D x number of minutes

This calculation used to assume the elimination of C. botulinum under practical conditions

47
Q

what happens to microbial load at low temperature

A

growth inhibited is a reversible process

48
Q

what temperature is chilling

A

0-15 degrees C

49
Q

what effect does chilling have

A

will inhibit growth but not kill, inhibits spoilage

Enzyme rates slow so growth slows

50
Q

what temperature is refrigeration

A

0-7 degrees C

51
Q

what effect does refrigeration have

A

Microbes membranes of mesophiles gel, transport stops and growth is inhibited

52
Q

what is the problem with using low temperature to kill microbes

A

psychrotrophs can maintain growth

e.g. Listeria can adapt below standard minimum temp for mesophiles

53
Q

what temperature is freezing

A

-18 degrees C

54
Q

what happens when freezing occurs

A

water unavailable for metabolism or transport

55
Q

what can freezing lead to in vegetative cells

A

drop in microbial load of vegetative cells

Water turn to ice get ice crystal formation in cytoplasm damages vegetative cell structure

56
Q

what effect does freezing have on viruses

A

they are stable

57
Q

what effect does freezing have on spores

A

little effect on spores are biologically dehydrated

58
Q

why does freezing have little effect on viruses and spores

A

Neither contain significant levels of water

59
Q

what is water activity

A

the water ‘available’ to react with new substances, or allow growth of microorganisms

60
Q

what is Aw

A

water activity

61
Q

how can Aw be calculated

A

partial pressure of water in sample / partial pressure of pure water

62
Q

what is the Aw for pure water

A

1

63
Q

what is the effect on Aw for any solute added

A

reduces Aw

64
Q

what happens if have pure water and heat it up

A

all water will go into gas phase

65
Q

what do cells require to grow

A

minimum water content to grow

66
Q

what effects the movement of water in cell

A

Concentration of solutes affects level of water in the cytoplasm movement of water

67
Q

what is used in the lab to prevent water in the cytoplasm moving

A

isotonic buffers such as MRD used to resuspend cells

68
Q

which water activity level keeps food preserved longer

A

most bacteria wont grow in low water activities

69
Q

what occurs in dehydration process and effect

A

curing with salt (adds more solute to sample and draws water out), smoking & addition of sugar all act to reduce Aw

70
Q

what Aw do most bacteria need for growth

A

0.9 or above

71
Q

what bacteria can grow below 0.9 Aw

A

staph aureus can grow at Aw = 0.86

72
Q

what can grow below Aw 0.65

A

Viruses NOT affected
Spore stability NOT affected
Fungi can produce toxins when growing on dry foods

73
Q

what does dehydration do to cells

A

cells are not irreversibly damaged

but growth is inhibited

74
Q

what is the negative effect of dehydration

A

often microbes are stressing, will try to maximise the amount of food they can get out of the system
If things go wrong can be worse than if they were normal – not dehydrated

75
Q

what effect can chemical preservatives have to preserve food

A

reducing pH of foods (includes adding organic acids)
by addition of chemicals or fermentation
few pathogens grow below pH 4.2

76
Q

what are organic acids like

A

lipophilic

77
Q

what is lipophilic

A

will dissolve into membranes as membranes are lipids

78
Q

what effect does lipophilic organic acids have

A
  1. Lower the external pH
  2. Acidify the cytoplasm after passing through the membrane (will dissociate when reach cytoplasm, and acidifiy the cell)
  3. Also can have Biochemical effects on cell at levels which do not reduce pH
79
Q

what is the reaction when organic acids acidify the cytoplasm

A

WAH ⇋ WA- H+

External) (Internal

80
Q

what are other commonly used chemical preservatives

A

SO2 and sulphites

nitrite and nitrates

81
Q

what do sulphites and SO2 act as

A

reducing agents

82
Q

what is the effect that sulphite and SO2 have

A

Gases diffuse into cells most efficiently at low pH (uncharged state)

83
Q

what are sulphites and SO2 good at

A

inhibiting yeasts & moulds

84
Q

what are sulphites and SO2 used for

A

fruit drinks, wine & sausages

85
Q

what is the problem with using SO2

A

some people are sensitive

86
Q

what does nitrates and nitrites act as

A

reducing agents

87
Q

what happens to undissociated nitrous acid and nitrous oxide

A

taken up by cells

88
Q

what does nitrate and nitrite work best in

A

acid foods

89
Q

what are nitrates and nitrites commonly used in

A

cured meats as also help to maintain colour & flavour

90
Q

are spices antimicrobial

A

at high concentrations

not usually high enough levels in food, as only used for flavouring

91
Q

what is the problem with using woodsmoke as a chemical preservatives

A

contains antimicrobial chemicals

aldehydes, phenols & methanol are flavour compounds that have an antimicrobial effect, but they can also be toxic

92
Q

what is used in irradiation

A

UV radiation is non-ionising but is bactericidal

Energy absorbed by proteins and nucleic acids

93
Q

why is UV good at killing microbes

A

resonance effect

94
Q

why is UV irradiation limited

A

UV irradiation has poor penetration so limited to surface sterilisation

95
Q

what are UV lamps used for

A

to sterilize workspaces and equipment used in microbiology laboratories/food preparation areas as well as health care facilities

96
Q

what is a big problem with using irradiation as sterilisation

A

Microorganisms easily shielded from UV light in fissures, cracks and shaded areas

97
Q

what can irradiation be used for

A

reduce mould and bacterial contamination during food production, packaging, storage or transportation

98
Q

what does irradiation improve

A

product quality, shelf life andreduces health risks

99
Q

where is irradiation used out of the lab

A

surface and air disinfection used in a wide range of food industries
bakeries, dairies, hatcheries and egg packing centres,
meat processing plants,cold stores
producers of fruit, vegetables, herbs and spices

100
Q

what is MAP

A

Modified atmosphere packaging

101
Q

how is fresh food sealed in non-air atmospheres, effect

A

various combinations of CO2, N2 & O2 used

microbial growth inhibited

102
Q

what is MAP often combined with

A

low temperature storage to increase shelf life of fresh foods

103
Q

which type of bacteria are more readily suppressed

A

Gram -ve spoilage organisms more readily suppressed than Gram +ve

104
Q

what is filtration

A

physical removal (exclusion) of microbial cells from a liquid

105
Q

what happens in filtration process

A

Solutions or gases are passed through a filter of sufficient pore diameter (generally 0.22 µm) to remove the smallest known bacterial cells

106
Q

what is the problem with filtration

A

won’t remove all viruses

will still spoil

107
Q

when is filtration used

A

Commonly used for sterilization of liquids which would be denatured by heat
e.g. antibiotics, injectable drugs, amino acids, vitamins, etc
also used for some liquid foods e.g. milk

108
Q

what is hurdle technology

A

Combining a number of sub-lethal injuries can produce an effective preservative measure without changing the taste/texture properties of the food

109
Q

why did hurdle technology come about

A

Public drive towards minimally processed foods requires milder preservation techniques

110
Q

what is the problem with hurdle technology

A

cells may adapt to sub-lethal injury and induce cross-protection
Stress responses – if do challenge cells with pH are instantly more resistant to salt
If given acid challenge first it was able to tolerate a second stress

Physiology of Microbes (21 decks)

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