The mystery of Glucose Syrups

John Percival wasn't wrong when he said that "nothing is so puzzling or so annoying as the use of the term Golding" but I bet he'd have had a few words to say about the term "glucose syrup" too. You might think it's straight forward: glucose syrup, a syrup of glucose, but in fact you'd be wrong. 

As the Handbook of Brewing puts it:

"glucose syrups used in brewing are in fact solutions of a large range of sugars and will contain, in varying proportions depending upon the method of manufacture, dextrose [glucose], maltose, maltotriose, maltotetraose, and larger dextrins."

"Glucose" syrups are in fact hydrolysed (i.e. broken down) starch solutions. Starch can be thought of as being made up of large chains of glucose units and depending on where the chain is broken a range of molecules will be produced: a single unit broken off gives glucose, units of two are maltose, three maltotriose, four maltotetraose and longer molecules we usually call dextrins. 

Here's  a table showing the composition of different glucose syrups made using acids and/or enzymes to hydrolyse the starch and comparing the sugar spectrum to wort:

So the amount of glucose in a glucose syrup is in fact highly variable. When buying a glucose syrup you need to look carefully at what its composition is, the one I use mostly at work is actually high in dextrins and has very low fermentability.