China Investment Corporation's purchase of an $856m stake in Noble Group, the commodities trading company, is the clearest indication yet that Beijing wants to secure agricultural commodities supplies after last year's food crisis.

The investment in a sector aligned so closely with the government's strategic objectives marks a shift for the sovereign wealth fund, which is interested increasingly in deals that are linked directly to the perceived national interest.

When CIC was established in late 2007, officials assured the international community repeatedly that it was a purely financial investor whose main purpose was to earn higher returns on the country's foreign exchange reserves, the bulk of which are invested in low-yielding US government securities.