• Question: How does different methods of preserving food help its shelf life?

    Asked by livvilivvilivliv to Anil, Blanka, Cees, Emma, Mike on 26 Jun 2012.
    • Photo: Blanka Sengerova

      Blanka Sengerova answered on 26 Jun 2012:

      Amongst other things, food preservation includes:

      – cold storage – slows the reactions down that degrade the food (bacteria and fungi work more slowly)
      – drying – water is removed to prevent microbial activity as microbes need water to work in (salting does a similar process as salt removes water by osmosis)
      – vacuum storage – removes oxygen which is required for microbial activity
      – smoking – one of the oldest methods of preservation, but not sure why it works, maybe because it kills bacteria?
      – pickling – putting the food into a liquid that is edible but can kill bacteria such as vinegar

      And a number of others. Does this help for a start? I guess Anil might be a better source of information…

    • Photo: Emma Trantham

      Emma Trantham answered on 26 Jun 2012:

      This is one of the things I had to learn at vet school along with some other food hygiene stuff so I have trawled back through my memory and come up with a few other methods that I can think of:

      -sugar – we put fruits in sugar syrup to preserve them. Because there is more free water in the fruit than the syrup, water is drawn out of the fruit and so the amount of water available to bacteria is reduced. Most bacteria/molds need water to survive so by reducing the amount of water to a certain level we can stop almost all bacterial/mold growth

      – salt curing – you may come across salt-cured fish if you have ever been abroad but in this country you are most likely to come across salt-cured ham. Like with the sugar, the salt draws water out of the food and makes it so that there is less water available to any bacteria/molds.

      -canning – here food is put into the can, the can is sealed and then it is heated to very high temperatures. The high temperature kills any harmful organisms and then because the can is already sealed no new ones can get back in.

      -artificial preservatives – there are also some man-made preservatives that can be put into food. I think these work by absorbing oxygen (oxygen is needed for the oxidation of food – oxidation of food=spoilage)

      I bet there are still some more though…

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