How to thicken syrup

Syrup, a mixture of sugar and water, has many uses. Flavoured syrups are a feature of all kinds of dishes and a vital ingredient in some cocktails. Syrup is ideal for preserving soft fruit, such as plums. In theory, you just simmer a sugar-water mix on the hob until you get the perfect syrupy thickness. In reality, it can be tricky to get the consistency of your syrup just right. If you're running out of patience with a runny syrup, or you've simmered too long and ended up with caramel instead, these tips should help.

Get the right mix

Measure 1 cup caster sugar and 1 cup water into a small saucepan. These quantities are just right for a syrup runny enough to pour over ice-cream. If you want a thicker, more treacly syrup, use 2 cups caster sugar to 1 cup water.

Place the saucepan on the hob over a moderate heat. Stir your mixture gently with a wooden spoon, to help the sugar dissolve. Allow the mixture to come to a gentle boil.

Watch your mixture closely. It will start off white and cloudy, but in about five minutes, it should gradually become completely clear. Once this happens, your syrup should be the right consistency, not too thick and not too runny. If it doesn't quite turn out like that, go on to the next step.

Turn down the heat and let your syrup mix just simmer if it is still too runny. Don't be tempted to boil it hard, as the mixture's temperature is critical. You simply need to drive off the excess water in the mix. Stir gently, so the mixture does not burn. Be patient, as it may take some time to get rid of the extra water. Eventually, your syrup will thicken, just the way you want it.

Testing tips

Put a sugar thermometer into your mixture, to check that it is not getting too hot. Let the thermometer sit in the mixture for around a minute to be sure of your reading. The ideal temperature range is between 110 to 112C / 230 to 235F.

Take a little of your mixture on the end of a teaspoon, if you don't have a sugar thermometer. Drip the syrup mixture into a tumbler of cold water. If the drop forms a long thread, your mixture is the right temperature for syrup. If the drop forms into a ball, then the heat has got too high.

Turn down the heat if your test shows the mixture's temperature is too high. You need a relatively low temperature for syrup, otherwise you'll end up with caramel or toffee instead.


Drop a vanilla pod into the mix as your syrup cooks to flavour it. Or try a little lemon or orange zest.

If you've heated your mixture beyond the syrup-forming point, don't despair. Consider whacking up the heat until your mixture browns and darkens and make it into caramel instead.


The sugar-water mixture gets very hot and retains the heat for a long time after it comes off the hob. Avoid getting splashes on your skin to prevent any burns.

Things You'll Need

  • cup
  • caster sugar
  • water
  • small saucepan
  • wooden spoon
  • sugar thermometer (optional)
  • teaspoon
  • tumbler
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About the Author

British writer Martin Malcolm specializes in children's nonfiction. His books include "A Giant in Ancient Egypt" and "Poetry By Numbers." His schoolkids' campaign for the Red Cross won the 2008 Charity Award. A qualified teacher, he has written for the BBC and MTV. He holds a Master of Arts in English from the University of London.