Food Storage and Preservation

1. Food Storage and Preservation

2. Storageand Preservation • Principles of Preservation • Methods of Preservation • Drying, curing & smoking • Fermentation • Pasteurisation & Sterilisation • Chilling and Freezing

3. 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)

4. 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”

5. 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

6. 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

7. 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.

8. 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

9. 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.

10. Log N One log cycle Time D-value 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

11. Log D One log cycle Temp z-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

12. F0 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

13. F0 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

14. F0 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.

15. Examples of F0 values

16. 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

17. 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

18. 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

19. 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

20. Irradiation • Exposing food to irradiation (X-rays, g-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

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

22. 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

23. Stock control • Product life • Rotation (FIFO) • Labelling • Disposal of waste

24. 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