What is malt syrup?

A thick, dark, sweet syrup made by sprouting barley grains, then mashing, cooking, and concentrating the resulting sugary liquid. Two forms: diastatic (still contains active enzymes that break down starches, used to boost yeast activity in long-fermented doughs) and non-diastatic (cooked higher to deactivate enzymes — used purely for sweetness and color). In bread baking, malt syrup is mostly associated with NY-style bagels: it's added to both the dough (sweetness, deeper crust color) and the boiling water (gloss, signature flavor). King Arthur and Hodgson Mill are the most common US brands. Honey works as a flavor substitute in a pinch but loses the malty depth; brown sugar gives sweetness without the characteristic flavor.