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Food Technology - Preservation


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Slides explaining the different methods of food preservation. Informative for students studying AS or A2 Food Technology. A summary of preservative methods and short exam questions at the end.

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Food Technology - Preservation

  2. 2. WHY DO WE NEED TO PRESERVE FOOD?  So that surplus foods from good harvests can be stored and then used in times of shortage.  So that we can enjoy seasonal fruits and vegetables all year round.  To enable it to be transported long distances without it decaying.  To allow it to be stored in the home for long periods without going off.
  3. 3. CANNING  Uses heat sterilisation to destroy micro-organisms that cause food spoilage. The cans are heated which drives out any air before it is sealed with a double seam to prevent leakage or the re-entry of bacteria. Foods preserved by this method have a long shelf life and can be stored at ambient temperatures.  The severity of the heat treatment can be lessened if the food contains acid, salt or sugar. The most important pH is 4.5, as below this the very dangerous organism Clostridium botulinum is inhibited.  Cans are cooled in chlorinated water to prevent any possible contamination in cans which may have a slight defect. This could infect the food and cause an outbreak of food poisoning some time later.
  4. 4. BOTTLING  Toughened glass is used because it can be taken to high temperatures for heat treatment methods such as bottling fruit and vegetables. It is easy to sterilise glass.  The bottle (or jar) can be filled either with raw foods and then heated slowly to boil the foods, destroying bacteria and spores. Air is expelled through a specially designed lid, and as the jar cools down, a vacuum is formed. The sealed jar prevents the re-entry of bacteria. As there is no oxygen present, bacteria are unable to multiply.  Bottling can also be used for high temperature methods such as jamming, pickling and chutney production. Glass is non-reactive and therefore suitable for use with acidic mixtures. Bottled food has a very long shelf life at ambient temperatures.
  5. 5. PASTEURISATION/ UHT (ULTRA HEAT TREATMENT)  Food is heated to temperatures in excess of 100C to ensure that spores are destroyed.  In the case of milk the product is heated to not less than 132.2C and is packaged under aseptic conditions. It is similar to pasteurised milk in terms of nutritive value and the heat is so quick the colour and flavour changes are not so obvious as in sterilised milk.  The storage life is vastly longer however, for all products that are heated there will be some loss of colour, flavour and texture to a greater or lesser extent. Heat sensitive vitamins e.g. thiamine, riboflavin, niacin and vitamin C will be affected.  There’s a reduction of 10-20% of amino acid lysine in high protein foods. Fruit juices may be fortified with additional vitamin C.  Packages are hermetically sealed. The packaging materials used are robust and prevent entry of air unless opened or pierced.
  6. 6. STERILISATION  This process uses a temperature in excess of 100C in order to destroy nearly all micro-organisms present in a food. This is important as some micro-organisms can form spores which have the ability to survive at high temperatures.  If the correct temperature is not reached there is the possibility that the spores will germinate and grow and food poisoning could result. Some organisms can survive the sterilisation process if not processed for enough time or a high enough temperature, e.g. Clostridium botulinum.  The product is packed in air-tight containers either before or after heat treatment. If packaging follows heating, the containers must be sterilised before use and filled under aseptic conditions.  Sterilising enables milk to be kept for 2-3 weeks unopened, but results in a burnt, caramelised flavour and browning.
  7. 7. FREEZE-DRYING  Not to be confused with AFD (Accelerated freeze-drying)  Rapid drying systems cause the outer edges and corners of food to become dried out and rigid thus fix the shape of the food pieces early in the process.  Water is removed from the centre of the food to produce a light honeycomb product, which readily rehydrates when added to water (slow drying allows the product to shrink further and produce a dense dried food, which is difficult to rehydrate).  As the food is porous the food is excellent for rehydration e.g. coffee, meat, fruit and vegetables.  The water-soluble vitamins may be lost from freeze-dried foods. Some odours are lost as a result of this process.  Out of all the drying processes, this is the most effective way of maintaining the original characteristics of a product.  This is a more costly process, but where colour, texture, flavour, shape and reconstitutability properties are of paramount importance (e.g. cuppa soups, snack pots, coffee), this process gives the most organoleptic results.
  8. 8. SMOKING  Process primarily used for meat and fish in combination with salt to add flavour and delay spoilage.  The degree of preservation is minimal with this process, because high levels of salt and smoke make the food less palatable and not so good for health reasons.  Therefore, in order to preserve smoked products, additional methods such as chilling and vacuum packing are used.  Smoking is either hot or cold. In cold smoking the product remains uncooked. The temperature doesn’t exceed 30C.  Most products are then cooked, except salmon, which is eaten raw. The organoleptic qualities are considered desirable.  In hot smoking the temperature is allowed to rise so that the food cooks. Smoked foods are often coloured yellowy as a result of the process.  They are partially dried and salty, so the texture and flavour are distinctive. This is a positive organoleptic selling point for these products.
  9. 9. SMOKING CONT... Wood Smoking:  It contains a number of substances that have anti- microbal activity e.g. formaldehyde and higher aldehydes, phenols and methanol, all of which are highly inhibitory to micro-organisms.
  10. 10. DRYING  Micro-organisms cannot live, grow and reproduce without moisture.  It is the removal of moisture by warmth or high temperature. This is an effective method of preservation.  Foods can be dried using commercial methods e.g. spray drying, freeze-drying, AFD, tunnel drying and sunlight e.g. tomatoes and fruits or by oven drying e.g. herbs, coffee and vegetables.  Dried foods have a long shelf life and can be stored at ambient temperatures.
  11. 11. VACUUM PACKING  Foods are prepared and then packed into a pouch or foil wrapper and the air is sucked out, forming a vacuum inside the pack.  Removing air takes away oxygen and moisture which bacteria require in order to survive and multiply.  Some vacuum packs can be stored at ambient temperatures e.g. coffee and have a medium shelf life. Others stored at chill temperatures e.g. smoked fish, cheese and cured meats. These have a short shelf life.  Organoleptic qualities are often well maintained using this method.
  12. 12. MAP (MODIFIED ATMOSPHERE PACKAGING)  Also known as ‘Controlled Atmosphere Packaging’  Preserves food in sealed gas flushed packs  In the packs oxygen level can be lowered or the carbon dioxide level or nitrogen levels increased  Carbon dioxide retards the growth of bacteria. Oxygen helps retain the colour of the food e.g. meat stays red and nitrogen is used to reduce the rate of oxidation. The ratio of these gasses depends upon the food being packaged.  Food is prepared and placed in the container.  Container is flushed with selected gas for the food type and is hermetically sealed.  MAP is often carried out in conjunction with chilling in the case of chilled meals or prepared salads.
  13. 13. MAP CONT...  Once the packaging is opened the food has a normal shelf life and must be stored accordingly.  The packaging used must not be reactive and sufficiently strong to resist damage through piercing or splitting.  The most commonly used materials are plastics, which may be ovenable, with a film cover.
  14. 14. IRRADIATION  This method of preservation is where ionising radiations are applied to foods to kill bacteria.  There is said to be no risk of residual radioactivity and the process should not be confused with radioactive contamination or radioactivity.  It is expensive to carry out and as yet is only permitted in the UK for use with some vegetables, spices and poultry products.  It extends shelf life by delaying ripening (bananas), inhibiting sprouting (potatoes and onions) and killing moulds (strawberries).  It reduces significantly the numbers of pathogenic parasites in meat.  No visible changes to irradiated food other than reduction of undesirable factors above.  At permitted doses of irradiation, the organoleptic properties of most foods are unaffected.
  15. 15. SUMMARY OF PRESERVATION METHODS Preservation Method Description Irradiation Micro-organisms are killed as food is bombarded with ionising radiation. All irradiated food must be labelled. Modified Atmosphere Packaging (MAP) Foods are packed in a combination of CO2, N and O2 gas instead of air – this delays decay. Additives Synthetic or natural substances added to food in small quantities to lengthen shelf life. Chilling A short term method of preserving food between 1 and 5C that slows rate of decay. Smoking Traditionally meat and fish were hung over wood fires. This changes colour and flavour of foods.
  16. 16. Preservation Method Description Sugar Preservatives When fruit is boiled with sugar the sugar stops the micro-organisms multiplying. Fermentation Alcohol or acids are used as preservatives – beer, wine and soy sauce. Salting Coating food in salt or placing it in a salt solution reduces the water content and micro-organism growth is retarded. Bottling/Pickling Food is packed into glass jars and covered in acidic, alcoholic, salt or sugar solutions. Spray Drying A fine spray of liquid is injected into hot chambers. The water evaporates leaving the solid part of the product. Fluidised Bed Drying Warm air is blown upwards, directly underneath the food causing it to flow and remain separate.
  17. 17. Preservation Method Description Canning Food is sterilised and placed in air tight containers. The process is: -Washing and preparation -Blanching -Filling the cans -Sealing the cans -Heating a retort (large pressure cooker) -Cooling -Labelling Roller Drying A liquid or paste form of the food is uniformly spread over rollers or drums which rotate slowly. The water evaporates and the dried product is scraped off. Sun Drying Slow and only practical in hot, dry climates. Immersion Freezing Refrigerants are directly sprayed onto food.
  18. 18. Preservation Method Description Plate Freezing Food is prepared and packed between flat hollow metal plates that press tightly on the food. Blast Freezing Blasting a stream of cold air (-40C) over food. Fluidised Bed Freezing The food floats on jets of cold air. Scraped Heat Exchange Food is scraped off a cold surface. This method reduces ice-crystal formation producing a smooth end product. Cryogenic Freezing Liquid Nitrogen (-196C) or CO2 (- 78C) is sprayed onto small food items. Batch Pasteurisation The product is held at 62-63C for 30- 35 minutes.
  19. 19. Preservation Method Description High Temperature/ Short Time (HTST) Pasteurisation The product is heated 72C for 15 seconds. Sterilisation A severe form of heat processing that destroys most micro-organisms. The flavour and appearance of a food product may change. Ultra Heat Treatment (UHT) Food is heated to 130-150C for 1-3 seconds. The food is kept moving so fewer changes occur.
  20. 20. SHORT EXAM QUESTIONS  Why does dehydration stop the growth of micro- organisms?  Describe Roller Drying  Write a sentence to describe the following methods of freezing: Plate freezing, Scraped heat exchange, Blast freezing  What are the differences between UHT and Sterilisation?  What is MAP?