Preservation and Shelf-life

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Microorganisms

Yeast -

  • fermentation - sugar ---> alcohol + carbon dioxide 
  • bread, alcohol 
  • conditions: 
    • warm
    • moist
    • no oxygen 

Moulds - 

  • produce specific flavours and textures 
  • cheese
  • spoilage:
    • thread-like filaments 
    • reproduce by producing spores 
    • harmful - produce myotoxins 
  • conditions:
    • moist
    • 20-30°C
    • acid, alkaline, salt or sugar foods 
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Microorganisms 2

Bacteria - 

  • used in food manufacture 
  • cheese, yogurt 
  • spoilage - 
    • often undetected - looks, taste and smells the same 
    • pathogenic - cause food poisoning - e.g. E-coli 
  • conditions - 
    • warmth - optimum 37 ° C
      • danger zone 5 ° C - 65 ° C
    • moisture 
    • food
    • oxygen - not all require oxygen 
    • neutral pH 
    • time - reproduce rapidly 
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Enzymes

Proteins that speed up chemical reactions 

Functions:

  • bread and brewing - fermentation 
  • cheese - ripening stage 

Spoilage:

  • can cause 'browning' 
  • Reduced by: 
    • high temperatures
    • acidic conditions 
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High-Risk Foods

  • Raw fish 
  • Dairy products
  • Cooked meat and poultry
  • Shellfish and seafood
  • Gravies, sauces, stocks, soups, and stews 
  • Egg [roducts 
  • Cooked rice
  • Protein-based baby foods 
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Low-Risk Foods

  • high acid content foods 
    • pickled, chutney, fruit juice
  • high sugar content foods 
    • marmalades, jams, fruit packed syrup 
  • sugar based confectionary 
    • sweets, icing 
  • unprocessed raw vegetables 
    • potatoes, carrots 
  • edible oils and fats 
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Storage of Food

Fridge:

  • avoid opening the door regularly 
  • avoid putting in hot food - raises temperature - steam - condenses - raises temperature of other foods - bacterial growth 
  • cover food

Freezer:

  • freezer 'star' rating - temperature range
  • thawing and refreezing - structure is damaged - loss of colour, flavour, texture, nutrition - bacteria grow quickly in thawed foods 
  • foods with a large proportion of water - delicate structure - ice crystals damage cell structure - collapse - don't freeze well 
  • freezer burn - greyish white marks when food has been packaged badly - dehydrates - safe to eat - loss of flavour, colour and texture 
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Preservation - chemicals

Vinegar - 

  • acid - pH 3.5 
  • bacteria cannot survive below 4.5 
  • pickled onions, cabbage, eggs 

Salt - 

  • ham, bacon, fish 
  • brine solution - tuna, vegatables
  • reduced moisture content - osmosis 

Sugar - 

  • high concentrations - absorbs water - stops bacteria growing 
  • jams, marmalade, jellies 
  • coating candies, crystallised fruit 
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Preservation - modern methods

Additives - 

  • destroy bacteria - prevent them reproducing 

Modified Atmosphere Packaging (MAP) 

  • altering gas - prevents bacteria - longer shelf life 
  • controlled atmosphere packaging (CAP) 
  • oxygen, nitrogen and carbon dioxide 
  • process: 
    • package fresh foods in peak conditions 
    • once opened - has normal shelf life
    • replacing air - by 'gas flashing' 
    • sealing the plastic bag or lid to food tray by hermetic sealing process 
  • can see the product 
  • chilled meats, vegetables - lettuce, fruits 
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Preservation - modern methods 2

Vacuum packed - 

  • remove air - prevents bacteria 
  • once opened - normal shelf life 
  • anaerobic conditions 
  • maintains colour and texture 
  • bacon, smoked fish 
  • Coffee - Accelerated Freeze Dried (AFD) - vacuum packed - doesn't lose taste 
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