A good boiled sweet is really just sugar, water and glucose syrup pushed to the edge of caramelisation and then cooled fast enough to trap it in a glass-like state rather than letting it crystallise. Getting that balance right, and deciding whether to pull the batch or leave it alone, is what separates a clear barley sugar from a striped, opaque humbug.
Boiling to hard crack
A batch starts as a simple syrup of sugar, water and glucose syrup (or invert sugar), boiled in steam-jacketed pans until it reaches roughly 150°C, known in confectionery as the hard crack stage. At this point nearly all the water has driven off and a drop of the syrup dropped into cold water will set instantly into a hard, brittle thread rather than a soft ball. The glucose syrup is doing important work here too: it interferes with sucrose molecules lining up into crystals, which is what lets the finished sweet set as a smooth, glassy solid instead of a sugary lump.
The slab, the pull, and why some sweets turn opaque
Once cooked, the molten sugar is poured onto a cooling slab, traditionally water-cooled steel or marble, where colours and flavourings are worked in while it's still pliable. From here the batches split into two families:
- Unpulled sweets — like barley sugar or clear fruit drops, are simply cut and shaped while still translucent, cooling into a hard, glass-clear amorphous solid, the same physical state as actual glass
- Pulled sweets — like humbugs, bullseyes and mint imperials, are stretched, folded and stretched again on a pulling hook or pulling machine. This works thousands of microscopic air bubbles into the sugar mass, which scatter light and turn what was a clear amber slab into a satiny, opaque, often lighter-coloured sweet. Pulling also aerates the sugar slightly, giving pulled sweets their slightly softer bite compared to a clear boiled sweet
Striped sweets like humbugs get their look by pulling a white (aerated) rope and a coloured, unpulled rope together before they're twisted and cut, which is why the white stripe is always a touch more matte than the coloured one.
Shaping and why they sometimes go cloudy later
The pulled or unpulled rope is fed through a batch roller and then a die roller, which pinches it into individual sweets at high speed before they're cooled on trays or conveyor and wrapped, ideally within minutes, since exposed sugar starts drawing moisture from the air immediately. Two different faults can show up after that: because boiled sweets are hygroscopic, humidity in storage or a poorly sealed wrapper leaves them feeling tacky and looking dull. Separately, if a batch didn't have enough glucose syrup to block recrystallisation, or cooled too slowly, the sucrose can start forming tiny crystals throughout the sweet, a fault called graining, which turns a once-clear sweet cloudy and gritty from the inside. It's one reason traditional retro sweets recipes are so particular about their glucose ratios.
Frequently asked questions
What temperature are boiled sweets cooked to?
Boiled sweets are typically cooked to the hard crack stage, around 150°C, at which point almost all the water has boiled off and the sugar syrup will set glass-hard as it cools.
Why are some boiled sweets clear and others opaque?
Clarity comes down to whether the sweet is pulled. Sweets like barley sugar are left unpulled and cool into a glass-like amorphous solid, while sweets like humbugs and bullseyes are stretched and folded, a process called pulling, which beats in thousands of tiny air bubbles that scatter light and turn the mixture opaque and satiny.
Why do boiled sweets sometimes turn cloudy or sugary in the wrapper?
Two separate things cause this. Boiled sweets are hygroscopic, so if they absorb moisture from the air the surface turns tacky and dull. Separately, if there isn't enough glucose syrup or invert sugar in the batch to stop the sucrose recrystallising, the sweet can grain, turning gritty and cloudy from the inside out.