The present invention relates to a process for applying optical brightening agent (OBA) to a sheet of paper or paperboard substrate. The process comprises applying the composition comprising a cooked starch and a powdered optical brightener to at least one surface of a paper or paperboard substrate at the size press in a paper or paperboard manufacturing process to form a sized paper or paperboard substrate; and drying the sized paper or paperboard substrate to form a dried sized paper or paperboard substrate.