The results indicated that the δ13C and δ15N values in raw beef were not significantly different compared with processed beef, but the δ2H values were significantly higher in processed beef than in raw beef.
In a taste test at the time, some school children favored burgers with higher amounts of processed beef.
Samples of the processed beef obtained by The Times revealed lower levels of alkalinity, suggesting less ammonia was used.
The federal school lunch program used an estimated 5.5 million pounds of the processed beef last year alone.
Some processed beef products, like many sausages and hot dogs, are produced using machines that scour a cow carcass for all available meat.
In 60 minutes, the plant produced a batch of 26,880 pounds of processed beef that tested positive for E. coli.
Creagh, who said she would not personally eat any processed beef products currently on sale, told BBC Radio 4's Today programme: "We've had 10m beefburgers withdrawn.
But Brown told the FSA board that contamination of horse and pork had also been found at "very low" levels in processed beef.
Unprocessed beef and chicken induced low amounts of bioluminescence, whereas processed beef and chicken induced values above the cut-off limit established.
In general, boiling, frying, and roasting had no significant effect on δ13C and δ15N values, but the δ2H value of processed beef increased.
Europe has been transfixed after horsemeat was found in processed beef in Britain, Ireland and the Netherlands; French, Polish and Romanian suppliers were implicated.
