Glucose syrup
Glucose syrup is a viscous clear mass produced by hydrolysing starch, usually corn or wheat. Pastry does not use it for sweetness — it is less sweet than sucrose — but for control over crystallisation.
Its principal function: glucose syrup molecules obstruct sucrose molecules from arranging into a crystal lattice. Hence its use in caramel to stop it graining, in ice cream to prevent large ice crystals, and in ganache and glaze for smoothness and shine.
Its second property is hygroscopicity. The syrup holds moisture, so products stay soft longer and stale more slowly.
Syrups are graded by DE, the dextrose equivalent, which reflects how far the starch has been broken down. Higher DE means sweeter and thinner; lower DE means more viscous with stronger anti-crystallising action. Pastry work most often uses DE 38–42.