Their invention could be used to make a sparkling substitute for paint, banknotes that are hard to counterfeit and chemical sensors that can act as visible sell-by dates.

Food is another expenditure that has proved vulnerable to hard times, with the rural poor turning increasingly to "food auctions," which offer items that may be past their sell-by dates.

In January, Parkers Farm, Inc. of Coon Rapids Minnesota issued a total recall on all of their products regardless of "sell by" dates because of a potential listeria contamination.