The lunchbox, which was 20cm in diameter, was first discovered in 2012 but tests on its contents have only recently been completed, with the findings published this week. Researchers used carbon dating to confirm the age of the box, and initially believed it contained porridge. They extracted biomolecules from a residue inside the box, and used a technique known as gas chromatography to search for traces of milk.
However, instead of milk they found molecules known as alkylresorcinols, which can still be found today in whole grains such as barley, rye and wheat.
Andre Colonese, an archaeologist at the University of York, said: "There are very few biomarkers for plants, and they are usually poorly preserved on historical finds ... you can imagine the relevance of this study as we have now a new tool for tracking early culinary use of cereal grains.
"It really is very exciting."
"The molecular marker for grain also helps us explore the beginnings of agriculture," added Jessica Hendy, from the Max Planck Institute.
She said: "It will enable us to gather information on when and how wheat spread throughout Europe."