In this series of ten blogs, various parts of the GCSE Biology, GCSE Science, IGCSE Biology specifications will be explored through the context of food. As well as assisting students revising for their GCSE and IGCSE exams, the blogs also provide an every-day context for science which all readers should find accessible, interesting and useful.
Toast and jam is one of my favourite breakfasts; strawberry is my very favourite and I use it on scones, to make puddings and even on digestive biscuits for a midnight snack. Have you ever wondered how a jar of jam keeps for so long without going bad? The secret is in the sugar.
Making jam is easy: there are three ingredients – fruit, a little water and lots of sugar. Boiling them up for long enough causes the pectin, naturally present in the fruit, to set and also ensures the jam is sterile. No microorganism could survive 105oC.
Every time you open your jar of jam to spread it on your toast, you expose the surface of the jam to the ‘microbial rain’ which exists in the air. Bacteria and mould-causing fungi waft around in the air constantly and some will inevitably land on your favourite strawberry preserve.
You would think that, given all the sugar and a moist surface, the top of jam is the ideal place for microorganisms to flourish. However, jams are not called preserves for nothing. The surface of jam is a very concentrated sugar solution. That is, lots of sugar dissolved in not very much water. The cytoplasm of a microorganism is a much more dilute solution (lots of water compared to sugars and other substances dissolved within it).
The cell membrane of a microorganism is a partially permeable membrane. In this case, water is able to diffuse across it, but sugar, being too large a molecule, is unable to cross. Water will cross a microorganism’s cell membrane from the area where there is lots of water (inside the microbe) to the area where there is less (the jam). This is called osmosis. So much water leaves the microorganism that it quickly loses too much cytoplasm and dies, leaving your lovely jam unharmed.
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