Food Storage and Preservation

Storage and Preservation  Principles of Preservation  Methods of Preservation  Drying, curing & smoking  Fermentation  Pasteurisation & Sterilisation  Chilling and Freezing

Principles 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 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”

Curing  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

Fermentation  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 Sterilisation  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.

Pasteurisation  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

Sterilisation  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 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-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

F 0 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 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

F 0 Value  The D-value for c. botulinum is 0.2 min at 121ºC  i.e. D 121 = 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 F 0 value  The F 0 value is the total sterilisation time at 121ºC  Although a 12D reduction is the minimum specified for C. botulinum, F 0 values achieved are often greater  This allows for a margin of safety and for other factors

F 0 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 F 0 value of 2.4 has been achieved.

Examples of F 0 values

Low 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

Chilling  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

Freezing  Freezing permits long term storage of food  Mammoths have been preserved in permafrost for over 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

Freezing  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

Irradiation  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

Storage  Why Store?  Ensure availability  Cope with fluctuations  Take advantage of bulk purpose  Year round supply of seasonal items.

Storage 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 control  Product life  Rotation (FIFO)  Labelling  Disposal of waste

Concluding 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