Undercooking makes germs strong.

Food safety experts always advocate cooking meat carefully--especially ground-meat products--so that even the core reaches germ-killing temperatures. But a new federal study demonstrates that precooking servings to sublethal temperatures before the final cooking actually makes germ killing more difficult.

Food microbiologist Vijay K. Juneja of the Agriculture Department's Eastern Regional Research Center in Wyndmoor, Pa., studied ground beef inoculated with Escherichia coli O157:H7. This bacterium infects a large proportion of cattle entering U.S. slaughterhouses (SN: 3/25/00, p. 199). If it survives to the dinner table, it can trigger lethal hemorrhagic food poisoning (SN: 7/22/00, p. 53).

Using deliberately tainted meat, Juneja made hamburgers and fried them in a skillet until the patties reached an internal temperature of 68 [degrees] C (155 [degrees] F), which took roughly 8 minutes. This killed the E. coli--unless the burgers had been preheated for 15 to 30 minutes at 46 [degrees] C, which simulates conditions found in warming trays, slow-cook pots, or malfunctioning commercial cookers, Juneja notes. Then, it took 12 minutes to kill those bacteria. This extra heating led to the hotter meat temperatures required to finally kill off the microbes.

Sublethal warming "causes the bacteria to synthesize heatshock proteins that make them more resistant to heat or any other stress," Juneja explains. In effect, he says, "it makes them more hardy."

The good news is that E. coli's increased thermal tolerance is transitory. Juneja's data show that the bacteria in thermally mishandled hamburger retained their heat resistance for a full day if stored at room temperatures, but for just 14 hours if stored at typical refrigeration temperatures.