Is a dry chemical powder from a fire extinguisher safe if mix in food?

No, The typical dry chem extinguisher has ammonium phosphate, potassium bicarbonate and sodium bicarbonate in it. While the sodium bicarb is nothing more than baking soda the the ammonium bicarb is harmful if swallowed. Potassium bicarb if ingested can upset the electrolyte balance in your body. So in short, don't eat the dry chem contaminated food. Recently the grocery store i was shopping at had a small electrical fire in the poultry section, an employee used a dry chem ext on the fire and they store had to throw out all food that had been in contact with the dust per the health dept. they ended up throwing out 4 1/2 full shopping carts of chicken ( all sealed in plastic) the health dept would not let them rinse and return the sealed packages.