Package size 500gm and 1kg
Lactose is a milk sugar (makes up about 2 -8% of milk solids) and is not fermentable by brewing yeast. It will therefore add sweetness, some body and calories to beer.
More commonly used in darker heavy beers and stouts.
It can be added at any stage in the brewing process though more commonly to the boiling wort.
Lactose can be used up to 5-10% of your fermentables.