Should you throw moldy bread in the trash, or just trim around the green spot? Can Sunday’s leftovers be Friday’s meal?

Scientists have developed methods to detect spoilage, but until these are available on a mass scale, food science and safety experts have some tips.

First — slimy, stinky, spotty or chunky changes in food don’t mean very much in terms of safety. It may not taste good, that doesn’t mean it’s going to make you sick. That’s because there’s a difference between what food scientists call spoilage bacteria and pathogens.

Spoilage bacteria form into slimy films on lunch meat, soggy edges on vegetables or stinky chicken. But the pathogens that do make you sick are odorless, colorless and invisible.

Since consumers can’t count on looks or smell, instead use the rule of four: no more than four days at 40 degrees Fahrenheit or 4 degrees centigrade. (Freezing fresh food at zero degree Fahrenheit will keep it safe indefinitely.) Temperature can slow or stop bacterial growth of either the pathogens or the spoilage variety. Forty degrees Fahrenheit buys people three days for safety with raw chicken and ground beef, three days with cuts of beef and lamb, and four days for leftovers.

Allowing anything to go above the cold 40 degrees along the way from store to frying pan can make the difference between illness and safety — and about 25 percent of refrigerators in the U.S. are kept at too high a temperature for safety. Be sure to check yours.