Crystallization of sugar

When making a caramel or candies we want a smooth texture and not a grainy one. Large sugar crystals will cause a grainy texture in confectionary products. How can we prevent the over-crystallization of the sugar and thereby prevent unwanted grainy textures?

Dissolving sugar in a liquid results in a sugar syrup. Most of the time the dissolution of the sugar is promoted by heating the mixture. When the solution starts to boil, water evaporates and the solution becomes more concentrated. Crystals start to form only when the sugar solution reaches a certain concentration. This state is called a supersaturated solution. Now the solution is in a state where not all of the sugar can be solubilised. So the sugar goes out of solution and forms crystals. The saturation point (the maximum concentration of sugar in water without crystal formation) is influenced by the temperature of the solution. It is possible to cool an unsaturated solution to a state of supersaturation. And vice versa, it is possible to heat a supersaturated mixture to an unsaturated solution. Far less sugar can be dissolved in cold water than in warm water, this is also the reason why we heat the mixture to obtain a fast and complete dissolution of the sugar. When boiling the solution, water will evaporate and the sugars will get more concentrated. It is possible to supersaturate the boiling solution, this can be observed when crystals form in the solution while boiling. In most cases this is unwanted. Better results will be obtained when the boiling solution is kept unsaturated and subsequently cooled down to a supersaturated state.

If you want to make a candy, like fudge, you need a creamy texture. This creamy texture comes from the formation of tiny sugar crystals. So when preparing fudge, you need to steer the crystallisation towards the formation of tiny crystals and avoid the growth of these crystals, because big crystals result in a grainy texture.

Agitation, stirring or beating supersaturated syrup incorporates air and promotes the formation and growth of sugar crystals due to the rapid movement of the molecules. If the solution is allowed to cool before it is beaten, only tiny crystals forms, which are not allowed to grow. This results in a creamy mass. So when making candy, like fudge, you should allow cooling the sugar syrup to 38°C before beating it, otherwise you will end up with crystals that are too large, resulting in a grainy texture. The beating must also continue until the formation of the tiny crystals is complete, otherwise the excess molecules, the sugar that is still in solution, will migrate to already formed crystals and increase their size. These large crystals will again produce a grainy product.

supersaturated sugar solution

supersaturated solution beaten above 38°C

supersaturated solution beaten below 38°C

Impurities in the sugar syrup may also result in the formation large sugar crystals. Impurities promote premature crystal formation, which will grow to big unwanted crystals. Some products can be added prevent the formation and growth of crystals. These products such as cream, butter, egg white, … are called interfering agents. The agents coat the crystals and prevent the growth of large crystals. Boiling the sugar syrup to the exact temperature is also very important, complete solution of the sugar is very important. Undisolved crystals can also grow out to big crystals, even at room temperature. That is what occurs when we make a “meringue italienne” with bad cooked sugar syrup. The present crystals will grow and result in a grainy texture.

Some sugars as glucose do not form large crystals that easily. By adding some glucose to the sugar for the preparation of sugar syrup or caramel, the glucose will prevent the growing of large crystals.

Sugar, sucrose, is formed from one molecule of glucose and one molecule of fructose. By adding some lemon juice or cream of tartar to the sugar for the preparation of sugar syrup or caramel, we speed up the break-up of the sucrose in glucose and fructose and so decrease the formation of large crystals.