Dear George

You can use both low fat milk and or 0.5% Caseine for detection of phosoproteins on the western blots depending on detection method. However, using milk at blocking reagent you should monitor pH because during overnight incubation with primary antibodies the lactic acid present in milk lowers the pH in both PBS and TBS based buffers. For more info see the links under

http://www.pall.com/main/Laboratory/Literature-Library-Details.page?id=48923

http://www.thermoscientific.com/ecomm/servlet/productscatalog_11152__92506_-1_4

http://www.piercenet.com/browse.cfm?fldID=5A414328-5056-8A76-4EA5-51555CF0015D&utm_source=thermoscientific&utm_medium=node&utm_term=blocking-buffers-for-protein-methods&utm_campaign=value-prop-punchouts

http://wolfson.huji.ac.il/purification/PDF/PhosphoProtein/PhosTag_PAGE_SDS.pdf

Good luck

I think casein is better than milk. You can block your PVDF membranes for any long time in casein but can't in milk.

I tried both of them. I prepared 5% milk powder in PBS and 0.5% casein in PBS. As a result, casein was better and also background was less

Do not use casein or milk (which contains casein) for phosphopeptide detection because casein is a phosphoprotein and therefore the only thing you will get is a huge background... BSA or protein-free blocking agents (i.e. 1% PVP40 + 0.05% Tween20) are to be used. Also I recommend TBS buffer instead of PBS in this case.