Food Storage and Preservation
StorageStorage and Preservationand Preservation
Principles of Preservation Methods of Preservation
Drying, curing & smoking Fermentation Pasteurisation & Sterilisation Chilling and Freezing
Principles of preservationPrinciples of preservation
Preservation of foods has a long history There are many traditional methods as well as newer
ones All methods depend on manipulation of one or more
of Temperature
pH
Water activity (Aw)
Drying, and SmokingDrying, and Smoking
These methods all involve reducing Aw Water is removed by heating The temperature should be
Above 63°C (ie above the danger zone) but Not so high as to cook the food
Smoking involves drying the food in an atmosphere of wood smoke
Smoking of itself is insufficient to preserve the food Compounds in the smoke have Bacteriocidal and Anti-
oxidant properties This is an example of “hurdle technology”
CuringCuring
Curing involves treating the food with salts This has an osmotic effect, drawing water out of the
food Thus, there is a reduction in Aw
Salts used include sodium chloride and nitrites Nitrites inhibit Clostridium botulinum
Nitrosamines formed during curing are suspected carcinogens
A balance of risk between the beneficial and negative effects of nitrites needs to be identified
However, current evidence suggests curing with nitrite is not a significant source of nitrosamines
FermentationFermentation
Fermentation involves encouraging selected micro-organisms to grow on the food
Many fermentation processes involve lactobacilli These produce lactic acid which reduces the pH below
about 4.5 Below about 4.5, few bacteria will grow
Thus most food poisoning organisms are inhibited
Many traditional sausages involve a combination of curing and lactobacillus fermentation
Another example of hurdle technology
Pasteurisation and Pasteurisation and SterilisationSterilisation
Pasteurisation and sterilisation kill micro-organisms by heating
Pasteurisation involves heating below 100°C and kills vegetative organisms
Sterilisation involves heating above 100°C and kills both vegetative organisms and microbial spores.
PasteurisationPasteurisation
Pasteurisation aims to kill vegetative bacteria while having a minimal impact on food quality
Typical pasteurisation conditions are 62.8°C – 65.6°C for 30 min. or
71.7°C for 15 sec
Then cool rapidly below 10°C for storage Cooking also effectively pasteurises food
Official advice is
Heat to a core temperature of 70°C for 2 min.
However heating to a core temperature of 75°C will achieve the same effect
SterilisationSterilisation Sterilisation is important in canned food products The food is placed in cans and heated to a temperature
typically in the range 115°C – 120°C The degree of sterilisation is determined by the Fo
value This is a measure of the equivalent time at 121°C The Fo value is chosen to minimise the risk of there
being clostridium botulinum in the food.
Decimal reduction timeDecimal reduction time
Microbial death is an exponential process
A graph of log N vs. time is a straight line
The time taken to reduce the number of viable organisms by one log cycle is called the Decimal reduction time, D
Log N
Time
One log cycle
D-value
z-valuez-value
The D-value is temperature dependent
The relationship with temperature is exponential
The increase in temperature required to reduce the D-value by one log cycle is called the z-value
A knowledge of D-value and z-value together allow us to calculate the sterilisation time
Log D
Temp
One log cycle
z-value
FF00 Value Value
In canning, there is a risk of contamination by C. botulinum
The consequences of this are very serious - 50% fatality rate,
To achieve this, a reduction of 1012 is specified called a 12D reduction
Food subjected to a 12D reduction is referred to as commercially sterile
There is no absolute guarantee of sterility
FF00 Value Value
The D-value for c. botulinum is 0.2 min at 121ºC i.e. D121 = 0.2 min
A 12D reduction means we must sterilise for at least 12 x 0.2 = 2.4 minutes at 121ºC. This is the F0 value
The F0 value is the total sterilisation time at 121ºC Although a 12D reduction is the minimum specified for C.
botulinum, F0 values achieved are often greater
This allows for a margin of safety and for other factors
FF00 Value Value
In practice sterilisation is not always carried out at 121ºC
Sterilisation of cans is typically carried out at about 115ºC
This means a longer sterilisation time since the D-value at 115ºC is about 4 x longer than that at 121ºC
To achieve the same degree of sterilisation at 115 as 2.4 min at 121 requires a time of about 9.6 min
In both cases, a F0 value of 2.4 has been achieved.
Examples of FExamples of F00 values values
¶
Product¶ Fo·values¶Babyfoods¶ 3-5¶Meats·in·gravy¶ 12-15¶Sliced·meat·in·gravy¶ 10¶Meat·pies¶ 10¶Sausages·in·fat¶ 4-6¶Frankfurters·in·brine¶ 3-4¶Curried·meats·and·vegetables¶ 8-12¶Poultry·and·Game,·whole·in·brine ¶ 15-18¶Chicken·fillets·in·jelly¶ 6-10¶"Sterile"·ham¶ 3-4¶Petfoods¶ 15-18¶¶
Low temperature storageLow temperature storage
Low temperature storage involves both Refrigeration: storage at 0°C – 7°C Freezing: storage below 0°C Both processes slow growth but do not kill micro-
organisms
ChillingChilling
Chilling involves cooling food to between 0°C and 7°C. Chilling allows storage for 5 – 7 days When chilling food it is important to achieve rapid
cooling of the surface where the bulk of bacterial contamination occurs
Interior cooling should then take place as rapidly as possible. With meat particular conditions apply
EU regulations require carcasses to be chilled below 7°C throughout
The interior of a carcass, if properly handled should be sterile. Chilling to 7°C throughout a carcass may take up to 48 hours. Chilling too rapidly may damage food quality
FreezingFreezing Freezing permits long term storage of food Mammoths have been preserved in permafrost for over
10 000 years Freezing will kill some, but not all vegetative organisms
Spores are generally resistant to freezing Freezing also slows chemical and enzymic processes
e.g. Oxidative rancidity of fat is inhibited Useful storage times at -18°C are typically
Red meat: 6 – 12 months Poultry: 3 months Fruit & Vegetables: 3 – 6 months Fish: 6 months
FreezingFreezing
Rate of freezing has an impact on food quality Slow freezing causes more damage to food structure
But fewer micro-organisms survive slow freezing
Slower freezing results in larger ice crystals forming leading to
Physical damage to food structure
Reduced water holding capacity
In the case of meat, darker colour.
In general, it is best to freeze rapidly
IrradiationIrradiation
Exposing food to irradiation (X-rays, -rays) will preserve the food
Vegetative organsims but not spores are killed
Advantages Effective pasteurisation of the food
Large pieces of food can be processed
Disadvantages Some loss of vitamins
Potential production of off flavours
Potential production of some carcinogens
Public acceptability
StorageStorage
Why Store? Ensure availability Cope with fluctuations Take advantage of bulk purpose Year round supply of seasonal items.
Storage facilitiesStorage facilities
Fit for purpose (dry store, chill, frozen etc.) Separate types of food
Raw, cooked Protect from contamination/infestation Weatherproof Keep out light Easy to clean Transport
Access Condition of vehicles
Stock controlStock control
Product life Rotation (FIFO) Labelling Disposal of waste
Concluding commentsConcluding comments
A variety of methods are available to allow food to be safely stored for extended periods
Many of these have a long history Many storage and preservation methods have an effect
on food quality There is no such thing as absolute safety Although safety should be a primary consideration,
there is need for a balance between safety and quality