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How is rock candy made?
Rock candy is not the same thing as British seaside rock. It is a cluster of big sugar crystals, grown slowly on a string or stick from a very strong syrup. Here is how it works, and why weaker syrups grow nothing at all.
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Rock candy, also called rock sugar or crystal sugar, is a sweet made of relatively large sugar crystals. Unlike most sweets, it is not cooked into shape. It is grown.
How it is made
- Make a strong syrup. Sugar is dissolved in water. Heating the water first lets far more sugar dissolve, and that produces larger crystals.
- Give the crystals a start. A string, a stick or plain granulated sugar gives the dissolved sugar a surface on which crystals can begin to form.
- Wait. As the syrup stands, sugar comes out of the solution and builds up as crystals. They form after six to seven days. In one test, the first crystals showed after about a day, but it took at least five days to grow candy worth eating.
- Colour, if you like. Food colouring can be added to the syrup to make coloured rock candy.
The science
More than twice as much sugar dissolves in boiling water as in water at room temperature. So when a strong, hot syrup cools, it holds more sugar than it normally could. It is supersaturated, and the extra sugar is ready to come out of the solution. It just needs a starting point. A seed crystal, or anything that disturbs the syrup, sets off crystallisation, and the sugar builds up in large, crunchy crystals, each new layer added around the last a little like the rings of an onion.
Fondant starts from the same kind of supersaturated syrup, but it is stirred hard once it has cooled, so it forms a great many tiny crystals instead of a few big ones: see how is fondant made?
Why the syrup has to be so strong
Bethany Brookshire of Science News Explores tested this in 2020. A syrup of three parts sugar to one part water grew an average of 10.5 g of visible candy on each string. Syrups of equal parts sugar and water, or one part sugar to three of water, grew essentially nothing after five days, and some of the cups went mouldy.
An old way to eat sugar
Islamic writers in the first half of the 9th century described making candy sugar by growing crystals from cooling supersaturated sugar solutions. The English words sugar candy date from the late 13th century, from Old French çucre candi, and go back through Arabic qandi to Persian qand, meaning cane sugar. Rock sugar is still widely used. In East Frisia a lump called Kandis goes in the bottom of the teacup; in Iran, rock candy called nabat is taken with tea, and saffron is the most popular flavour; and in India, mishri is eaten as a sweet or used to sweeten milk or tea.
Not British rock
The American term rock candy should not be confused with British rock, the opaque boiled sugar stick sold at the seaside, which is hard at first and turns chewy in the mouth: see how is seaside rock made? and why is rock called rock? For more on the word candy, see what is the difference between sweets and candy?
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Questions and answers
How is rock candy made?
Sugar is dissolved in hot water until the syrup holds far more sugar than cold water could, which is called a supersaturated solution. A string or stick is hung in the syrup, and over about a week the extra sugar comes out of the solution as large crystals on its surface.
Why do you need so much sugar to make rock candy?
Crystals only grow when the syrup holds more sugar than it can keep dissolved as it cools. In one test by Science News Explores, a syrup of three parts sugar to one part water grew about 10.5 g of candy per string, while weaker syrups grew essentially nothing in five days, and some went mouldy.
Is rock candy the same as seaside rock?
No. American rock candy is large, natural sugar crystals. British rock is a boiled sugar stick that is amorphous and opaque, hard at first and then chewy in the mouth.
Sources
- Rock candy, Wikipedia
- Rock candy science 2: no such thing as too much sugar, Science News Explores
- Fondant, Wikipedia
- Crystallization, Wikipedia
Keep reading
- How it is madeHow is fondant made?Fondant is sugar and water boiled to the soft ball stage, cooled undisturbed, then beaten hard so many tiny sugar crystals form at once, giving a smooth cream.
- How it is madeHow is seaside rock made?Seaside rock is boiled sugar, pulled until it turns white, wrapped in a coloured casing and rolled out by hand into long thin sticks.
- Questions people askWhy is rock called rock?Nobody recorded who first called it rock, but hard sugar sweets have been called rock since at least 1723. What the records show, and what they do not.
- Questions people askWhat is the difference between sweets and candy?None, really. Sweets is the British and Irish word, candy the American and Canadian one, and lollies the Australian. Where the words came from.
- How it is madeHow are boiled sweets made?Boiled sweets are sugar syrup boiled to around 160°C, flavoured, coloured and shaped while warm, then set into a glassy solid about 98 per cent sugar.
- How it is madeHow is sugar made from sugar cane?Cane is crushed at a mill to make raw sugar, which is shipped to a refinery to be washed, cleaned, decolourised, crystallised again under vacuum and dried.