Control of microbial growth: Food Preservation L21 Flashcards
what is the aim of food preservation
prolong time for which a food remains wholesome & safe for consumption
what is the main cause of food spoilage
growth & activity of microorganisms
what % of microbes associated with food are pathogens
2%
what % are food spoilage organisms
98%
are all food contaminated
food products are all naturally contaminated
what are the methods of food preservation based on
knowledge of physiology
growth parameters of microbes
what food pressure is there
food chain pressure
spoilage - food goes to landfill
what naturally happens to food
spoil naturally (loss colour, oxidise) accelerated by fact microbes are present
what is the purpose of microbes
allow them to get food out of their environment
what is the geometric growth
1 generation to go from ‘good’ to ‘bad’
numbers leap up quickly
what is the tipping point
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)
what does rapid multiplication require for permissive growth conditions
ambient temperatures correct nutrients correct O2 levels correct pH correct moisture/water activity
what are the six methods of food preservation
high temp low temp dehydration chemical irradiation controlled atmosphere packing
what is cidal
lethal effect, irreversible, microbe is dead
what is static
inhibitory effect (reversible) stopping microbes growing, stop microbes going to tipping point
what determines the choice of preservative method
- resistance of species present to challenge
- Microbial load - numbers of microorganisms present in food
what must considered - resistance of species present to challenge in which preservative methods
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
what determines the D value
microbial numbers
what can be used to kill microbes in D value
Chemical, heat irradiation to kill
what is needed to kill spores
Spores much more resistant, need something far more robust
what are food preservation processes designed to do
reduce microbial load (D-values)
prevent microbial growth (including spore germination)
what is important to consider to prevent recontamination
Packaging of food after treatment important to prevent recontamination
what reaction do high temperatures do
proteins & enzymes denatured = irreversible damage
cidal reaction
how do enzymes aid preservation
inactivation of secreted enzymes
how can enzymes be bad for food preservation
prolonged high temps CHANGE PROPERTIES OF FOOD
Therefore a compromise is needed
effect taste of food
how can temperature be used as a food preservation
Pasteurisation
kills most pathogens (mesophiles) & reduces microbial load; food stored at low temp
what is high temperature food preservation designed to do
kill two bacteria responsible for major human health threats
what are the bacteria that pasteurisation kill
Mycobacterium tuberculosis Coxiella burnetii (Q fever)
what does pasteurisation do
reduces microbial load; inactivates lipases
what is HTST
high temp short time
what happens in HTST
72°C for min 15 s
what is LTLT
Low temperature long time
what happens in LTLT
63°C for 30 min
what is UHT
Ultra heat treatment
what happens in UHT
141°C for 3-4 s
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
what does the rate of kill depend on
heat capacity of the “heating menstruum”
which type of food takes longer to reach threshold
dense foods
what is canning in food preservation
designed to kill spores of most heat resistant pathogen
what pathogen is canning designed to kill
Clostridium botulinum; 50 °C
how does canning prevent the growth of pathogens
Anaerobic conditions prevents growth of aerobes
low pH prevents outgrowth of C. botulinum
what will make the canning process easier
start with low pH as have to lower it
what do the parameters used for canning depend on
pH of the food and the initial microbial load (easier if start with lower pH)
what is needed in canning for potatoes
For non-acidic product (e.g. potatoes) 12D cook required
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)
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
what happens to microbial load at low temperature
growth inhibited is a reversible process
what temperature is chilling
0-15 degrees C
what effect does chilling have
will inhibit growth but not kill, inhibits spoilage
Enzyme rates slow so growth slows
what temperature is refrigeration
0-7 degrees C
what effect does refrigeration have
Microbes membranes of mesophiles gel, transport stops and growth is inhibited
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
what temperature is freezing
-18 degrees C
what happens when freezing occurs
water unavailable for metabolism or transport
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
what effect does freezing have on viruses
they are stable
what effect does freezing have on spores
little effect on spores are biologically dehydrated
why does freezing have little effect on viruses and spores
Neither contain significant levels of water
what is water activity
the water ‘available’ to react with new substances, or allow growth of microorganisms
what is Aw
water activity
how can Aw be calculated
partial pressure of water in sample / partial pressure of pure water
what is the Aw for pure water
1
what is the effect on Aw for any solute added
reduces Aw
what happens if have pure water and heat it up
all water will go into gas phase
what do cells require to grow
minimum water content to grow
what effects the movement of water in cell
Concentration of solutes affects level of water in the cytoplasm movement of water
what is used in the lab to prevent water in the cytoplasm moving
isotonic buffers such as MRD used to resuspend cells
which water activity level keeps food preserved longer
most bacteria wont grow in low water activities
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
what Aw do most bacteria need for growth
0.9 or above
what bacteria can grow below 0.9 Aw
staph aureus can grow at Aw = 0.86
what can grow below Aw 0.65
Viruses NOT affected
Spore stability NOT affected
Fungi can produce toxins when growing on dry foods
what does dehydration do to cells
cells are not irreversibly damaged
but growth is inhibited
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
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
what are organic acids like
lipophilic
what is lipophilic
will dissolve into membranes as membranes are lipids
what effect does lipophilic organic acids have
- Lower the external pH
- Acidify the cytoplasm after passing through the membrane (will dissociate when reach cytoplasm, and acidifiy the cell)
- Also can have Biochemical effects on cell at levels which do not reduce pH
what is the reaction when organic acids acidify the cytoplasm
WAH ⇋ WA- H+
External) (Internal
what are other commonly used chemical preservatives
SO2 and sulphites
nitrite and nitrates
what do sulphites and SO2 act as
reducing agents
what is the effect that sulphite and SO2 have
Gases diffuse into cells most efficiently at low pH (uncharged state)
what are sulphites and SO2 good at
inhibiting yeasts & moulds
what are sulphites and SO2 used for
fruit drinks, wine & sausages
what is the problem with using SO2
some people are sensitive
what does nitrates and nitrites act as
reducing agents
what happens to undissociated nitrous acid and nitrous oxide
taken up by cells
what does nitrate and nitrite work best in
acid foods
what are nitrates and nitrites commonly used in
cured meats as also help to maintain colour & flavour
are spices antimicrobial
at high concentrations
not usually high enough levels in food, as only used for flavouring
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
what is used in irradiation
UV radiation is non-ionising but is bactericidal
Energy absorbed by proteins and nucleic acids
why is UV good at killing microbes
resonance effect
why is UV irradiation limited
UV irradiation has poor penetration so limited to surface sterilisation
what are UV lamps used for
to sterilize workspaces and equipment used in microbiology laboratories/food preparation areas as well as health care facilities
what is a big problem with using irradiation as sterilisation
Microorganisms easily shielded from UV light in fissures, cracks and shaded areas
what can irradiation be used for
reduce mould and bacterial contamination during food production, packaging, storage or transportation
what does irradiation improve
product quality, shelf life andreduces health risks
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
what is MAP
Modified atmosphere packaging
how is fresh food sealed in non-air atmospheres, effect
various combinations of CO2, N2 & O2 used
microbial growth inhibited
what is MAP often combined with
low temperature storage to increase shelf life of fresh foods
which type of bacteria are more readily suppressed
Gram -ve spoilage organisms more readily suppressed than Gram +ve
what is filtration
physical removal (exclusion) of microbial cells from a liquid
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
what is the problem with filtration
won’t remove all viruses
will still spoil
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
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
why did hurdle technology come about
Public drive towards minimally processed foods requires milder preservation techniques
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
