The science behind candy
Boiling sugar forms the basis of all sweet (candy) making. The sugar is dissolved in other liquids, usually either water or milk, and boiled until a certain temperature is reached. The addition of sugar increases the boiling point of the liquid. As the water evaporates the solution becomes supersaturated, and the boiling temperature continues to increase beyond 100°C. This temperature is crucial - in general, the higher the temperature, the harder the candy.
The high temperature also causes caramellisation, a physical change in the structure of the sugar, and this affects the taste and colour of the product.
There are basically two types of boiled sugar candy, crystalline (creamy) and non-crystalline (amorphous). The creamy types are opaque and include fudges and fondants; amorphous types include toffee, caramel, brittle and butterscotch.
The degree of crystallisation depends on the treatment of the boiled mixture after the required temperature has been reached. It is important that the supersaturated sugar solution does not form large crystals as it cools or the finished product is coarse and grainy. You are aiming for very small crystals to give the right 'feel' - the smaller the crystals the smoother the product. Creamy candies are cooled to 104°C and then beaten, kneaded or worked with a spatula causing sudden and rapid crystallisation with very tiny crystals.
Great care must be taken to ensure that any sugar crystals that form on the edge of the pan and utensils during cooking do not fall into the solution and seed it. This can be achieved by brushing the crystals with a wet brush as soon as they form, to redissolve them, and by washing any equipment such as sugar thermometers between uses. The boiling liquid must never be stirred once the sugar has dissolved.
Many recipes include butter, evaporated milk, honey, vinegar, cream of tartar or corn syrup. These convert some of the sugar to invert sugar which interferes with crystal growth. This can be of great benefit, but if too much is used the product may never set or will tend to soften quickly.
Stages of boiling
As stated above the actual boiling temperature is critical, and is best tested with a sugar-boiling thermometer. If you do not have one of these the best way to test the readiness is to drop small amounts, perhaps a half teaspoonful, into very cold water and judge it by feel.
Smooth: 102° - 104°C
At this stage the sugar is a syrup which clings slightly to the finger.
Soft ball: 113° - 118°C
Used for fondants and fudges. A soft ball forms under water which flattens on removal.
Firm or hard ball: 118° - 130°C
For caramels, marshmallows and nougat. A firm ball forms which holds its shape but can still be moulded.
Soft crack: 132° - 143°C
For toffees. When dropped into cold water the syrup forms threads which are hard but not brittle.
Hard crack: 149° - 154°C
For hard toffee and rock. The syrup forms brittle threads when dropped into water.
Caramel: above 154°C
For praline and caramel. The syrup is brittle as above and becomes a rich golden brown.
Good Housekeeping Cookery Book