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Stop The Rot

Two apple wedges with faces carved into them. The left apple is red and mouldy with a sad face. The right apple is black and withered with a smiley face.

How did people keep food from going off before they had fridges? Discover how a little salt can keep your food edible for months (if you don't mind salty food).

Australian Curriculum links

Science > Biological Sciences > Year 3 > ACSSU044

You'll need

  • 1 apple
  • 1 pop stick (from a craft shop or ice block)
  • 1 knife
  • 1/2 cup salt
  • 1/2 cup bicarbonate of soda
  • 2 plastic cups
  • 1 mixing bowl
  • paper
  • pencil

Try this

  1. Cut the apple into quarters.
  2. Carve a face into one of the apple quarters using a pop stick. You can make any face you like.
  3. Weigh the apple face and record its weight
  4. Place the apple face into a dry plastic cup.
  5. Add 1/2 a cup of salt and 1/2 a cup of bicarbonate of soda into the mixing bowl and mix well to create a salt mixture.
  6. Pour some of the salt mixture over the apple face so that it is completely covered. Tap the sides of the cup to make sure that the salt mixture fills all the little air pockets. Fill the cup with the rest of the salt mixture to completely cover the apple.
  7. Store in a warm, dry place, away from direct sunlight.
  8. Carve a face into another of the apple quarters and record its weight. Place it in the other plastic cup but do not add salt mixture. Store alongside the other apple.
  9. Wash your hands
  10. Discard the remaining apple quarters (or have a friend repeat steps 1 to 8 with the remaining apple quarters).
  11. After a week, remove the apple quarters from their containers. Gently brush off any excess salt mixture (do not wash with water).
  12. Weigh each apple quarter. Record the new weights.
  13. Compare the appearance of the salted apple with the unsalted apple. Compare the weights of the salted apple with the unsalted apple.
  14. Optional: Return the apple quarters to their original plastic cups.  Ensure the salted apple quarter is completely covered by the salt mixture.  Store the cups in their original locations for another week.  Repeat steps 11 to 13.  Discard the cups, salt mixture and apple quarters. Wash your hands.

Further investigation

  • Find out how much water you can draw from the apples. Put them back in the salt mix and re-weigh them every week. How long does it take to drain all the water out?Calculate the weight of water that has been lost from each apple to find out how much has been absorbed by the salt.
  • Try using different types of fruit or food. Which foods preserve well? Which foods continue to rot over time? How watery or salty are each of these foods?

What's happening?

When food is left untreated, bacteria and mould begin to grow on it over time. As the bacteria and mould feed on the food, they produce toxic chemicals that can make the food unsuitable to eat. This process is known as 'rotting'. In this experiment the unsalted apple rots but doesn't lose weight. The salted apple doesn’t rot but does lose weight.

Mould spreads by sending tiny spores into the air which can travel very large distances. If the mould spores settle on a surface they will only grow into mould if the conditions are right. Mould and bacteria require moist, nutrient-rich, low salt environments. Thus the unsalted apple is a very suitable environment for bacteria and mould to feed and grow.  The bacteria and mould feed on the unsalted apple, causing it to rot. That’s why the unsalted apple appears black and squishy in some parts and perhaps smells slightly.

The salted apple, on the other hand, doesn’t appear damaged by rotting and has less mould growing on it. This apple’s environment doesn’t have enough water and is too salty for the bacteria and mould to feed and grow. The water in the salted apple is absorbed by the salt mixture. Due to the loss of water (as well as reactions between the natural chemicals in the apple and the oxygen in the air) the salted apple appears brown and shrivelled.

As most of the water has been removed from the salted apple, it should weigh less than it did a week earlier. The apple left unsalted should weigh almost the same as it did initially, since no water will have been removed.

Salt and bicarbonate of soda are both hygroscopic. This means that they can absorb water from their surroundings, either by touch or through the air. When food is placed in a salt mixture, the salt draws water out of the food, causing it to dry out. Similarly, hygroscopic salts will also draw moisture out of the cells of bacteria, killing them before they can try to grow and spread.  This further helps to preserve and prolong the life of the food.