This article mentions metals and oil, but it applies to food commodities too. The main reason sugar went up ahead of other food commodities is because sugar is the easiest one to store.

"If institutional investors are forced to gain commodities exposure in the physical market, they will create direct demand and push up prices. Regulation will have caused the very result it is trying to prevent.

Big investors are more likely to try to buy futures on foreign futures exchanges in London, Dubai or Shanghai, bankers say.

Facing a limit on holdings in paper futures contracts, bankers say they have received inquiries from pension funds and other big investors about the practicality of warehousing.

Critics say investors could respond by bailing out of futures and hoarding actual commodities, an ugly prospect in the event of a global shortage. "


http://www.gata.org/node/7869