Glucose syrup is used not to sweeten, but to prevent sugar from recrystallizing. This thick syrup made from starch makes it harder for sucrose molecules to form an orderly structure. That is why it gives candy, ice cream, and ganache a smooth texture. It is less sweet than granulated sugar, and because it attracts moisture, it keeps products soft for longer.
On its own, sucrose forms an orderly structure as it cools, and the syrup turns grainy. Glucose syrup contains glucose and maltose molecules of different sizes; those differently sized molecules fit between the sucrose molecules and stop the crystals from growing. The result is a syrup that stays fluid and glossy even after cooling. Lemon juice does the same thing on a smaller scale, but glucose syrup gives a more reliable result because its effect does not depend on acidity or boiling time. The package lists different grades of thickness; a medium-thick one is enough for home cooking.
Glucose syrup is useful where crystallization causes the most problems. In hard candy, it helps the syrup stay clear like glass. In ice cream, it binds some of the water and reduces the size of ice crystals. It adds gloss to ganache and sauces and also delays the mixture from turning grainy days later. Most syrup-soaked desserts do not need it because lemon is already used in them. In other words, glucose syrup is not an ingredient in every recipe; it is useful in recipes where crystals are a problem. A small amount is also used in jam to delay sugar from forming crystals at the bottom of the jar.
The amount varies by recipe, and glucose syrup usually replaces only part of the sugar rather than all of it. Since it is less sweet than granulated sugar, replacing sugar one-for-one makes a dessert bland. Weighing it is the most accurate method because syrup sticks to the spoon and container. Dipping the spoon in hot water first or wiping it with a thin layer of oil helps the syrup flow off cleanly; otherwise, some of the measured amount stays on the spoon. If pouring it directly onto the scale, tare the container and measure it in one pour.
If you cannot find glucose syrup, there are a few alternatives, but none is exactly the same. Corn syrup is the closest substitute and works the same way. Honey prevents crystallization but also adds its own flavor and color. Invert sugar can also be used. Granulated sugar itself will never do this job; substituting sugar removes the reason for adding the syrup, and the mixture will turn grainy again. Grape molasses can also be used, but its dark color makes it unsuitable for light-colored desserts.
Storage is easy. In a tightly closed jar in a cool, dry cupboard, glucose syrup keeps for a long time without spoiling; it does not need refrigeration, and the cold can make it less fluid and harder to work with. Wipe the rim of the jar after each use, because syrup left around the edge dries and sticks the lid shut. Do not discard syrup that appears to have hardened; set the jar in a container of warm water and leave it for a few minutes to make it fluid again. If water drips into the jar, the syrup will turn sour over time, so always use a dry spoon.
A Note for Readers
This content is for informational purposes only and is not a substitute for medical advice. Consult your doctor before making any decision about your health.