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 10 12 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.
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 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
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