Maltose
A simple sugar (a disaccharide of two glucose units) formed from the breakdown of starch, found in sprouted grains, beer and certain syrups.
Maltose is a disaccharide made of two glucose molecules. It appears when starch is broken down (by amylase enzymes), notably during the germination of grains (“malt”) and during digestion.
A fresh angle (the sugar of malt and beer): maltose is at the heart of making beer and whisky. Germinating barley (malting) activates the enzymes that turn starch into maltose, which yeasts then ferment into alcohol. Maltose is also the sugar in “malt syrup” used in baking and patisserie.
Its high glycaemic index: made of two glucose units, maltose has a very high GI (~105, even above pure glucose by some measures), so it raises blood sugar quickly. Maltose syrup (common in Asian products and some processed foods) is therefore a sugar with a strong glycaemic impact.
A “health” trap: some products “without sucrose” or “with malt syrup” give an impression of naturalness, but maltose is still a high-GI sugar. “Barley malt syrup” sounds healthier than “sugar”, but the glycaemic effect is comparable, or even higher.
In digestion: the enzyme maltase splits maltose into absorbable glucose. This is a normal step in the digestion of all starchy foods (bread, pasta, potatoes), where starch is broken down into maltose and then glucose.
Sources: barley malt, malt syrup, beer, some breakfast cereals, sweet potatoes (a little), sweet Asian products.