Control of microbial growth: Food Preservation L21 Flashcards
(110 cards)
1
Q
what is the aim of food preservation
A
prolong time for which a food remains wholesome & safe for consumption
2
Q
what is the main cause of food spoilage
A
growth & activity of microorganisms
3
Q
what % of microbes associated with food are pathogens
A
2%
4
Q
what % are food spoilage organisms
A
98%
5
Q
are all food contaminated
A
food products are all naturally contaminated
6
Q
what are the methods of food preservation based on
A
knowledge of physiology
growth parameters of microbes
7
Q
what food pressure is there
A
food chain pressure
spoilage - food goes to landfill
8
Q
what naturally happens to food
A
spoil naturally (loss colour, oxidise) accelerated by fact microbes are present
9
Q
what is the purpose of microbes
A
allow them to get food out of their environment
10
Q
what is the geometric growth
A
1 generation to go from ‘good’ to ‘bad’
numbers leap up quickly
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)
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
13
Q
what are the six methods of food preservation
A
high temp low temp dehydration chemical irradiation controlled atmosphere packing
14
Q
what is cidal
A
lethal effect, irreversible, microbe is dead
15
Q
what is static
A
inhibitory effect (reversible) stopping microbes growing, stop microbes going to tipping point
16
Q
what determines the choice of preservative method
A
- resistance of species present to challenge
- Microbial load - numbers of microorganisms present in food
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
18
Q
what determines the D value
A
microbial numbers
19
Q
what can be used to kill microbes in D value
A
Chemical, heat irradiation to kill
20
Q
what is needed to kill spores
A
Spores much more resistant, need something far more robust
21
Q
what are food preservation processes designed to do
A
reduce microbial load (D-values)
prevent microbial growth (including spore germination)
22
Q
what is important to consider to prevent recontamination
A
Packaging of food after treatment important to prevent recontamination
23
Q
what reaction do high temperatures do
A
proteins & enzymes denatured = irreversible damage
cidal reaction
24
Q
how do enzymes aid preservation
A
inactivation of secreted enzymes
25
how can enzymes be bad for food preservation
prolonged high temps CHANGE PROPERTIES OF FOOD
Therefore a compromise is needed
effect taste of food
26
how can temperature be used as a food preservation
Pasteurisation
| kills most pathogens (mesophiles) & reduces microbial load; food stored at low temp
27
what is high temperature food preservation designed to do
kill two bacteria responsible for major human health threats
28
what are the bacteria that pasteurisation kill
```
Mycobacterium tuberculosis
Coxiella burnetii (Q fever)
```
29
what does pasteurisation do
reduces microbial load; inactivates lipases
30
what is HTST
high temp short time
31
what happens in HTST
72°C for min 15 s
32
what is LTLT
Low temperature long time
33
what happens in LTLT
63°C for 30 min
34
what is UHT
Ultra heat treatment
35
what happens in UHT
141°C for 3-4 s
36
what does the rate of kill in D value depend on
how far above the maximum specific growth rate (µmax value) for the test organism
37
what does the rate of kill depend on
heat capacity of the “heating menstruum”
38
which type of food takes longer to reach threshold
dense foods
39
what is canning in food preservation
designed to kill spores of most heat resistant pathogen
40
what pathogen is canning designed to kill
Clostridium botulinum; 50 °C
41
how does canning prevent the growth of pathogens
Anaerobic conditions prevents growth of aerobes
| low pH prevents outgrowth of C. botulinum
42
what will make the canning process easier
start with low pH as have to lower it
43
what do the parameters used for canning depend on
pH of the food and the initial microbial load (easier if start with lower pH)
44
what is needed in canning for potatoes
For non-acidic product (e.g. potatoes) 12D cook required
45
what is assumed in the 12D cook for canning
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
how is it calculated how long food is needed to be cooked to remove microbes for canning
number of D x number of minutes
| This calculation used to assume the elimination of C. botulinum under practical conditions
47
what happens to microbial load at low temperature
growth inhibited is a reversible process
48
what temperature is chilling
0-15 degrees C
49
what effect does chilling have
will inhibit growth but not kill, inhibits spoilage
| Enzyme rates slow so growth slows
50
what temperature is refrigeration
0-7 degrees C
51
what effect does refrigeration have
Microbes membranes of mesophiles gel, transport stops and growth is inhibited
52
what is the problem with using low temperature to kill microbes
psychrotrophs can maintain growth
| e.g. Listeria can adapt below standard minimum temp for mesophiles
53
what temperature is freezing
-18 degrees C
54
what happens when freezing occurs
water unavailable for metabolism or transport
55
what can freezing lead to in vegetative cells
drop in microbial load of vegetative cells
| Water turn to ice get ice crystal formation in cytoplasm damages vegetative cell structure
56
what effect does freezing have on viruses
they are stable
57
what effect does freezing have on spores
little effect on spores are biologically dehydrated
58
why does freezing have little effect on viruses and spores
Neither contain significant levels of water
59
what is water activity
the water ‘available’ to react with new substances, or allow growth of microorganisms
60
what is Aw
water activity
61
how can Aw be calculated
partial pressure of water in sample / partial pressure of pure water
62
what is the Aw for pure water
1
63
what is the effect on Aw for any solute added
reduces Aw
64
what happens if have pure water and heat it up
all water will go into gas phase
65
what do cells require to grow
minimum water content to grow
66
what effects the movement of water in cell
Concentration of solutes affects level of water in the cytoplasm movement of water
67
what is used in the lab to prevent water in the cytoplasm moving
isotonic buffers such as MRD used to resuspend cells
68
which water activity level keeps food preserved longer
most bacteria wont grow in low water activities
69
what occurs in dehydration process and effect
curing with salt (adds more solute to sample and draws water out), smoking & addition of sugar all act to reduce Aw
70
what Aw do most bacteria need for growth
0.9 or above
71
what bacteria can grow below 0.9 Aw
staph aureus can grow at Aw = 0.86
72
what can grow below Aw 0.65
Viruses NOT affected
Spore stability NOT affected
Fungi can produce toxins when growing on dry foods
73
what does dehydration do to cells
cells are not irreversibly damaged
| but growth is inhibited
74
what is the negative effect of dehydration
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
what effect can chemical preservatives have to preserve food
reducing pH of foods (includes adding organic acids)
by addition of chemicals or fermentation
few pathogens grow below pH 4.2
76
what are organic acids like
lipophilic
77
what is lipophilic
will dissolve into membranes as membranes are lipids
78
what effect does lipophilic organic acids have
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
what is the reaction when organic acids acidify the cytoplasm
WAH ⇋ WA- H+
| External) (Internal
80
what are other commonly used chemical preservatives
SO2 and sulphites
| nitrite and nitrates
81
what do sulphites and SO2 act as
reducing agents
82
what is the effect that sulphite and SO2 have
Gases diffuse into cells most efficiently at low pH (uncharged state)
83
what are sulphites and SO2 good at
inhibiting yeasts & moulds
84
what are sulphites and SO2 used for
fruit drinks, wine & sausages
85
what is the problem with using SO2
some people are sensitive
86
what does nitrates and nitrites act as
reducing agents
87
what happens to undissociated nitrous acid and nitrous oxide
taken up by cells
88
what does nitrate and nitrite work best in
acid foods
89
what are nitrates and nitrites commonly used in
cured meats as also help to maintain colour & flavour
90
are spices antimicrobial
at high concentrations
| not usually high enough levels in food, as only used for flavouring
91
what is the problem with using woodsmoke as a chemical preservatives
contains antimicrobial chemicals
| aldehydes, phenols & methanol are flavour compounds that have an antimicrobial effect, but they can also be toxic
92
what is used in irradiation
UV radiation is non-ionising but is bactericidal
| Energy absorbed by proteins and nucleic acids
93
why is UV good at killing microbes
resonance effect
94
why is UV irradiation limited
UV irradiation has poor penetration so limited to surface sterilisation
95
what are UV lamps used for
to sterilize workspaces and equipment used in microbiology laboratories/food preparation areas as well as health care facilities
96
what is a big problem with using irradiation as sterilisation
Microorganisms easily shielded from UV light in fissures, cracks and shaded areas
97
what can irradiation be used for
reduce mould and bacterial contamination during food production, packaging, storage or transportation
98
what does irradiation improve
product quality, shelf life and reduces health risks
99
where is irradiation used out of the lab
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
what is MAP
Modified atmosphere packaging
101
how is fresh food sealed in non-air atmospheres, effect
various combinations of CO2, N2 & O2 used
| microbial growth inhibited
102
what is MAP often combined with
low temperature storage to increase shelf life of fresh foods
103
which type of bacteria are more readily suppressed
Gram -ve spoilage organisms more readily suppressed than Gram +ve
104
what is filtration
physical removal (exclusion) of microbial cells from a liquid
105
what happens in filtration process
Solutions or gases are passed through a filter of sufficient pore diameter (generally 0.22 µm) to remove the smallest known bacterial cells
106
what is the problem with filtration
won't remove all viruses
| will still spoil
107
when is filtration used
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
what is hurdle technology
Combining a number of sub-lethal injuries can produce an effective preservative measure without changing the taste/texture properties of the food
109
why did hurdle technology come about
Public drive towards minimally processed foods requires milder preservation techniques
110
what is the problem with hurdle technology
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
