Why Does the Same Syrup Become Fudge or Toffee? A Few Degrees Apart

A syrup of sugar and water boils at a temperature determined by its concentration, and as water evaporates the concentration rises and so does the boiling point. Measuring the temperature therefore measures the concentration indirectly and very precisely, which is far easier than measuring water content directly. A syrup boiling at one hundred and twelve degrees contains a specific proportion of water, and one at one hundred and fifty contains far less. Since what a sugar preparation sets into depends almost entirely on how much water remains, the thermometer reading determines the outcome, and a few degrees separates one confection from another entirely.

The traditional scale runs from wet to dry and each stage has a use:

• •Thread, around 110 to 112 degrees, for syrups and some icings
• •Soft ball, around 112 to 116, for fudge, fondant and buttercream
• •Firm ball, around 118 to 120, for caramels
• •Hard ball, around 121 to 130, for nougat and marshmallow
• •Soft crack, around 132 to 143, for taffy and some toffees
• •Hard crack, around 146 to 154, for brittles, lollipops and pulled sugar

The names come from a test used before thermometers were available and still used as a check. A small quantity of the boiling syrup is dropped into cold water and then handled, and what it does identifies the stage directly. At the earliest stage it forms a thread when pulled between the fingers. At soft ball it forms a ball that flattens when removed from the water. At firm and hard ball the ball holds its shape progressively better. At soft crack it forms threads that bend, and at hard crack threads that snap. The test works because cooling the sample to room temperature shows exactly the consistency the finished sweet will have, which is what the cook actually wants to know.

Reading a temperature accurately is less straightforward than it appears and several errors are common. Water boils below one hundred degrees at altitude, and since every stage is defined relative to that boiling point, every target temperature must be reduced accordingly, by roughly half a degree for each hundred and fifty metres above sea level. A thermometer bulb touching the base of the pan reads the metal rather than the syrup. A bulb not immersed deeply enough reads the vapour. Cheap instruments are frequently several degrees out and should be checked in boiling water before use. And the syrup near the surface is cooler than the syrup at the base, so the reading depends on where the probe sits.

Temperature sets the water content and a second variable sets the texture, namely whether the sugar crystallises and how finely. Fudge and fondant are deliberately crystallised, and the difference between smooth and gritty is entirely a matter of crystal size, which is controlled by cooling the syrup undisturbed to a specific temperature before beating it, since beating a hot syrup produces few large crystals and beating a cooled one produces many tiny ones. Caramels and toffees are prevented from crystallising at all, by adding glucose syrup, an acid or butter, which interfere with the orderly packing that crystals require. A stray sugar crystal on the side of the pan can seed the whole batch, which is why the sides are washed down during boiling.

Boiling point rises as a syrup concentrates, so the thermometer reading measures how much water remains and therefore what the result sets into, with a few degrees separating fudge from toffee. The traditional names come from dropping a sample into cold water and handling it. Whether the sugar is allowed to crystallise, and how finely, is a second control operating independently of temperature.