At the end of my freshman year of college I got stuck with the task of cleaning out a refrigerator I’d shared with three other students equally ill-equipped to handle the dizzying freedom of a parent-free kitchen. Much of the food within Moldy bread, photo by Henry Mühlpfordthad been hanging out for the better part of the academic year and was considerably altered by the experience. Once-white rice had turned a strange shade of pink. Brown sauces had become green. A lone cucumber in the produce drawer had attempted to escape the plastic bag holding it by transforming into a liquid. It was, in a word, gross.

Storing perishable foods at cold temperatures may slow their demise, but eventually spoilage still sets in. What makes some foods perishable is their ability to provide food and shelter for various species bacteria and fungi. While you’re flipping through recipe books to plan your next meal, these microorganisms are already eating the ingredients. They’re quite tiny, of course, so at first you don’t notice that you’re sharing your food. But as the microbes multiply, their presence becomes more visible.

Food spoilage is generally defined as changes in color, smell, taste and texture that render a product “unacceptable to the consumer” (i.e., thoroughly disgusting). Some of these changes are simply the appearance of colonies. That is, the microbial population has grown large enough that it can be seen without a microscope. The fuzzy green stuff forming on that old loaf of bread, for instance, is likely a colony of mold. And the layer of slime coating the nearby cold cuts may be a buildup of yeast or bacterial cells.

Other changes are results of microbial eating habits. To free usable nutrients, some microbes use enzymes to cut through the cell walls of plants, turning your crisp vegetables in a mushy mess. Lactic acid bacteria, notorious sourers of dairy products, metabolize the sugars in milk, releasing lactic acid in the process. (Not ideal if you were hoping for something to put in your coffee, but these same bacteria can also be useful when their fermentation process is employed for making products like yogurt and sour cream.)

The nutritional content of the food will affect which microbes can thrive there and thus what kind of spoilage you can expect to see (and smell). Foods composed mainly of carbohydrates have more to offer to fungi than to bacteria. The results aren’t very visually appetizing, but the stink factor is low. Not so for meats and dairy products. Bacterial breakdown of their fats and proteins can release chemical stenches well beyond what a box of baking soda can tackle.

Despite its assault on the senses, most spoiled food encountered when you tidy the fridge is not a threat to human health. The pathogenic microbes that cause food poisoning (Salmonella, and certain strains of E. coli for example) may be able to survive the perils of our digestive tracts, but they don’t replicate well at proper refrigeration temperatures of 0-5°C. (One important exception to this is Listeria monocytogenes, the bacterial species responsible for the food-borne disease Listeriosis.) Spoilage microorganisms that can multiply in the chill of refrigeration are a pretty harmless lot. In theory, you could just shave the mold and rinse the slime from your microbe-addled foodstuffs and carry on making dinner without any ill effects.* Though you certainly wouldn’t want to try the same thing with food that had spoiled at room temperature, which could harbor pathogenic microbes alongside the humble spoilage organisms. And sadly, pathogenic microbes are not polite enough to alert us to their presence by making food look or taste inedible. Contaminated items are often perfectly delicious, which can make tracing the source of individual food poisoning cases a challenge.

The real problem with food spoilage microbes, then, isn’t illness. It’s food waste. By some estimates, 20 percent of food worldwide is thrown away due to spoilage. That’s a shame. You should go put those sagging carrots into a soup before it’s too late.

* Note: Before attempting this you might want to ask yourself how sure you are of the temperature inside your refrigerator (mine doesn’t even have a thermometer) and how confident you are that the food was stored properly before you purchased it.

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