Food spoilage is caused by tiny invisible organisms called bacteria. Bacteria are everywhere we go, and most of them don't do us any harm. In fact, some of them are good for us.
What Do Bacteria Like?
As living organisms go, bacteria are pretty boring. For one thing, they can't move. The only time they go anywhere is when someone moves them. (See: What is Cross-Contamination?) Otherwise, they stay right where they are.
If they're lucky, they get to eat, and if they're really lucky, they get to reproduce.
They do this by splitting into two identical selves. And then each one of them splits, and so on and so on. Some manage to do this two or three times an hour.
Unfortunately, the longer it goes on, the more spoiled our food becomes, because that's what they're living on — our food. Particularly foods that are high in protein, like meats, poultry, fish, eggs and dairy products.
To be sure, some of them will go for low-protein foods like fruits and vegetables, but those ones are a lot slower. Which is why an apple left on your kitchen counter for a couple of days would still be safe to eat, while a steak clearly would not.
Spoiled Food Vs. Hazardous Food
It's important to note that spoiled food isn't necessarily dangerous food. For one thing, most people won't eat food that smells bad, looks slimy or whatever.
And you can't get food poisoning from something you didn't eat.
Moreover, the microorganisms that cause ordinary food spoilage aren't necessarily harmful to us. In fact, centuries before refrigerators, the earliest sauces and seasonings were used to mask the "off" tastes and smells of food that had begun to spoil.
This continues to be true in parts of the world where people don't have home refrigeration units (which, interestingly enough, includes most people alive on the planet today).
The bacteria we're concerned with from a food safety standpoint are the so-called "pathogens" that cause food poisoning. And these pathogens, like salmonella or E. coli, don't produce any smells, off-tastes or changes in the food's appearance — a slimy surface, for instance, or some sort of discoloration.
So how do we control these nasties? One way would be to starve them out. As noted above, bacteria need food to survive. Get rid of the food, and your bacteria problem disappears. Unfortunately, though, without food, the field of culinary arts has very little to offer.
So we'll assume food is part of the equation. Bacteria still have several other, quite specific, requirements, each of which can be controlled to some extent. There are six of them, in fact. Along with food, we'll also assume the existence of oxygen. Unless you're a practitioner of the art of garde manger, preparing something like duck confit, oxygen comes with the territory.
That leaves four additional factors that we can control:
- pH Level (Acidity)
There's a saying in foodservice: "Keep cold foods cold, and keep hot foods hot."
Keeping cold foods cold means storing them at temperatures between 40°F, which is where normal refrigeration kicks in, down to about 0°F, which is where you'd want your freezer to be. Bacteria still multiply at cold temperatures, they just do it a lot more slowly.
At freezing temperatures, bacterial growth slows to nearly nil.
Freezing doesn't kill them, though — all it does is make them cold. Once you thaw that food, watch out! Any bacteria that were there before freezing will just warm up and start multiplying again — with a vengeance.
Food Temperature Danger Zone
You see, bacteria thrive between 41°F and 140°F, a range of temperatures that's known as the Temperature Danger Zone. Perhaps not surprisingly, it's the same temperature range that humans thrive in.
Not only that, but our bodies' natural temperature of 98.6°F is so right smack in the middle of that Danger Zone, it's not even funny. Bacteria can't wait to get inside of us. Once they make it to our intestines, it's like a bacteria Mardi Gras.
To minimize this danger, perishable food shouldn't be allowed to spend more than an hour in the Food Temperature Danger Zone — cumulatively. Any longer than that and it should either be cooked or thrown away.
Here's a table showing the key temperatures of the Temperature Danger Zone.
Keeping Hot Foods Hot
Keeping hot foods hot presents other challenges. Bacterial growth slows down once again at temperatures hotter than 140°F, so hot foods that are being served on a buffet, for example, must be kept hotter than that at all times.
Keep in mind that 140°F doesn't kill bacteria — it only stops them from multiplying.
If you actually want to kill bacteria, you've got to heat them up to at least 165°F. The same rule applies to cooked food that should happen to drop below 140°F — you get an hour, total. After that, you either need to heat it up to 165°F again or throw it away. And by the way, you can only reheat it once. If it drops below 140°F a second time, you need to toss it.
Time: It Waits for No One!
Time works hand in hand with temperature in encouraging the growth of bacteria. Let's say you buy a package of uncooked chicken breasts. Maybe it's in your shopping cart for 15 minutes while you shop, then it's in your car for another 15 minutes while you drive home. So before you even get that chicken home, bacteria have had a full 30 minutes to run rampant.
Then later they might spend another 15 minutes on your counter while you prep them, bringing the cumulative total to 45 minutes already. As you can see, you really don't have much wiggle room.
Like all living organisms, bacteria need water to survive. Foods high in moisture like meats, poultry, seafoods and dairy products, as well as fruits and vegetables, are prime breeding ground for harmful bacteria. Low-moisture foods, including dried grains and legumes such as rice or beans, will typically keep for a very long time without spoiling or harboring bacteria.
Another aspect of the moisture factor is that through a process called osmosis, sugar and salt actually suck the moisture out of bacteria, effectively killing them by dehydration. As a result, a high salt and/or sugar content will tend to preserve foods — which is why salt and sugar are used in brining and curing of meats.
pH level (Acidity)
pH is a measure of how acidic something is, and it runs on a scale of 0 to 14. Anything lower than 7 is considered acid and anything higher than 7 is considered base or alkaline. A value of 7 would be considered neutral. Ordinary water, for example, has a pH of 7.
As it turns out, bacteria can't stand anything too acidic or too alkaline. For bacteria to thrive, the pH environment needs to be neutral.
Well, guess what foods fall into that category? Yep — animal based products like seafood, meat, poultry, eggs and milk.
By contrast, most vegetables and pasta have a very high pH when uncooked, but turn neutral — hence, more hazardous — when cooked. Highly acidic foods such as citrus, tomatoes, apples, vinegar, berries and so on, are relatively unattractive to bacteria from a pH standpoint. They'll grow, it just takes a lot longer.
(That's why you don't have to keep ketchup in the fridge. You don't do that, do you?)
It may seem like there are a lot of ways to control the growth of bacteria in our food — and technically, it's true. But we can't control time. It keeps ticking away no matter what.
And while we can change the moisture and acidity levels of foods, relying on that method alone would mean eating a lot more chicken jerky and pickled eggs. For that reason, temperature really is the most crucial element in controlling the spread of food-borne illness.
Here's a table showing the key temperatures of the Temperature Danger Zone.