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