When you use the sterilizer, white or brown spots may appear on the heating element in the base. This is limescale that is building up. The harder the water, the faster scale builds up. 
Limescale deposits can slow down the sterilization cycle and even impair its effectiveness.

You should descale the sterilizer as follows:

1. Always unplug the appliance and wait until it has cooled down completely before descaling.
2. Pour 80 ml/2.7 oz white vinegar (5% acetic acid) and 20 ml/0.7 oz water into the water reservoir.
3. Leave the solution in the water reservoir until the scale has dissolved.
4. Empty the water reservoir and rinse the heating plate with clean water. Empty, and then wipe the plate with a damp cloth. Note: Do not rinse under the faucet!

Repeat this process as often as necessary, at least every 1-2 weeks. In severe cases, citric acid or common descaling agents can be used to remove limescale.