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Why does candy floss go hard?
Fresh candy floss is light and fluffy. Leave it out for a while and it shrinks, goes sticky and ends up as hard, crunchy lumps of sugar. A food science study measured exactly how fast that happens. Here is why, and how to keep it fluffy for longer.
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Candy floss is made by melting sugar and spinning it through tiny holes, so that it sets in the air as fine threads. The sugar cools so fast that it never forms crystals: it sets as an amorphous, glassy solid. That glassy sugar is the reason candy floss is so light, and also the reason it does not last: see how is candy floss made?
Glassy sugar pulls in water
Glassy sugar is hygroscopic: it takes up water from the air around it. Food scientists Theodore and Peter Labuza, in a 2004 study published in the Journal of Food Processing and Preservation, found that as the humidity rises, candy floss absorbs water and turns rubbery.
Once that happens, two things follow. The fine threads lose their stiffness and collapse, so the cloud shrinks and goes sticky. Then the sugar crystallises, and the collapsed floss sets into hard, crunchy lumps.
How fast it happens
The Labuzas stored candy floss at 25°C in air of different humidities and watched what happened:
- 0 to 11% humidity, very dry air: it stayed stable for more than 12 months.
- 33% humidity: it collapsed and crystallised within 3 days.
- 45%, 54% and 75% humidity: it collapsed and crystallised in less than a day.
In other words, candy floss only keeps its shape in very dry air. Leave it open on a damp day and it will not last long.
Keeping it fluffy
The answer is to keep air, and the water in it, away from the floss. Candy floss is sometimes sold ready packed in a plastic bag, which keeps the air out until you open it. Keep it sealed until you are ready to eat it, and eat it soon after opening. Low moisture sweets in general last best in dry, covered storage: see how long do sweets last?
Related science
The same glassy threads that go hard in damp air dissolve almost instantly in water: see why does candy floss dissolve in water?
Citable formats
For journalists, researchers, AI assistants and content creators. Pick the format you need:
Free to cite, quote, and reuse with attribution to sweeties.net.
Questions and answers
Why does candy floss get hard and crunchy?
Because its glassy sugar takes up water from the air. The moisture makes the threads rubbery so they collapse, and then the sugar crystallises, turning the fluffy cloud into hard, crunchy lumps.
How long does candy floss stay fluffy?
It depends on the air. In a 2004 study at 25°C, candy floss kept in very dry air, at up to 11% humidity, stayed stable for over a year, but at 33% humidity it collapsed and crystallised within 3 days, and at 45% humidity or more, within a day.
How do you keep candy floss soft?
Keep it sealed away from the air until you eat it, and eat it soon after opening, especially on a damp day, because the water in the air is what makes it collapse.
Sources
- Influence of temperature and relative humidity on the physical states of cotton candy, T. P. Labuza and P. S. Labuza, 2004, Journal of Food Processing and Preservation, via University of Minnesota
- Cotton candy, Wikipedia
- Confectionery, Wikipedia
Keep reading
- Sweet tips and how toWhy does candy floss dissolve in water?Candy floss is sugar spun into very fine, glassy threads with a huge surface area. That is why a whole cloud of it vanishes in water almost at once.
- How it is madeHow is candy floss made?Candy floss is sugar melted in a spinning head and flung through tiny holes, where it cools into fine threads. The machine was invented in 1897, by a dentist.
- The people behind the sweetsThe dentist who invented candy flossCandy floss was invented by a dentist. In 1897 William Morrison of Nashville and sweet maker John C. Wharton built a machine that spun sugar into fairy floss.
- Questions people askIs candy floss vegan?Usually, yes. Candy floss is spun sugar with a little flavouring and colour. Check the colour, as carmine comes from insects, and where the sugar is from.
- Sweet tips and how toWhy does seaside rock go soft?Rock goes soft and sticky because sugar pulls moisture out of the air. Here is the science, and how to keep a stick of rock hard for longer.
- Questions people askHow long do sweets last?Sweets last a long time because they hold so little water, but each kind ages differently. Jellies harden, boiled sweets go sticky and chocolate can bloom.