We’ve all done it. You take an old carton of milk out of the fridge and give it a quick sniff to see if it’s sour or still good. Heck, in my house, I’m the go-to smell-tester because I have a nose like a hound. I’m continually being asked to smell miscellaneous leftovers to see if they are still good to eat. But, while smelling foods will tell us whether they smell rank and may taste awful, it does not tell us whether they are safe. A food that smells perfectly fine can still make you sick.
The organisms that cause foods to decay, producing that putrid smell we associate with rotten meat, for example, are not the same organisms that give us food poisoning. When your milk smells sour or rotten, it is because there are bacteria growing in it that are breaking down the proteins. In the old days, the bacteria would have more likely been fermenting bacteria that produce lactic acid. Some of these same bacteria are used intentionally to make yogurt or buttermilk as well as other fermented products. They ferment the lactose sugar in milk and produce lactic acid as a by-product. The lactic acid is what causes most of the sour flavor. But your modern pasteurized milk will more likely contain psychrotrophic bacteria that thrive in colder conditions.
If It Looks and Smells Fine…It May Not Be
Some of the spoilage organisms, like psychrotrophic bacteria that grow on meat in the refrigerator, may make the meat look green or black. The bacteria themselves, though, may not harm you if you eat the food. Other putrifying organisms produce odors and slime and other gross effects. These organisms themselves may not infect you, but they can produce toxins in the food that can make you ill. Since we avoid eating foods that look and smell rotten, this is rarely a problem.
However, the pathogens that can directly infect us, like E. coli, Salmonella, or Listeria, do not cause decay. They give off no smell and we cannot detect their presence visually. Some of these bacteria multiply and divide inside our bodies, making us sick. Others produce dangerous enterotoxins. These are pathogens like Salmonella, E. coli, Staphylococcus aureus, and Listeria monocytogenes. They all may have a particular way of making us sick, but we cannot see or smell them when they are present.
It is true, of course, that when food looks spoiled and smells rotten, it is much more likely to have a large amount of pathogenic bacteria in it. However, food that does not look or smell bad at all can still be contaminated.
This is why you should not keep leftovers in the refrigerator, in general, for more than 3 to 4 days without freezing them or throwing them away. They may not seem spoiled by that time, but if they have been contaminated with harmful pathogens, these organisms will have had time to grow and multiply, making them much more likely to make you sick.
On the other hand, modern packaging has thrown a further wrench into the usefullness of the small test. You might open a cry-vac container of meat and find that it smells a little off, even though it’s still perfectly safe to eat. So,
