Why Does Food Go Off?
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
Food goes off mostly because microbes grow in it, and partly because chemical reactions change it even when nothing is growing. Every preservation method humans have invented is an attack on one of these two processes, usually by removing something the microbes need.
The microbes at work
Bacteria, yeasts and moulds are present almost everywhere, and given the right conditions they multiply rapidly. As they feed on the food they produce waste products that cause off smells, sliminess and sour tastes.
Bacterial growth is exponential, which is why food can seem fine and then be clearly spoiled a few hours later. A single bacterium dividing every twenty minutes becomes millions within a working day at room temperature.
What microbes need
Microbial growth requires a specific set of conditions, and preservation works by removing at least one:
- •Water, in a form that is available rather than bound or frozen
- •Nutrients, particularly sugars and proteins
- •A suitable temperature, with the danger zone roughly 5 to 60 degrees Celsius
- •A tolerable pH, since most spoilage organisms dislike strong acid
- •Often oxygen, though some bacteria thrive without it
The chemical side
Not all spoilage is microbial. Fats react with oxygen and turn rancid, producing the stale smell of old nuts or oil, and this happens even in dry foods where nothing can grow.
Enzymes naturally present in food also keep working after harvest, which is why fruit ripens and then goes soft, and why cut apples brown. Blanching vegetables before freezing exists specifically to deactivate these enzymes, since freezing stops microbes but not enzyme activity.
What preservation methods do
Every traditional method targets one of the requirements. Drying and salting remove available water. Freezing locks water into ice so microbes cannot use it and slows everything chemically.
Pickling adds acid below the tolerance of most spoilage organisms. Canning heats food to kill microbes then seals it against recontamination. Smoking adds antimicrobial compounds and dries the surface. Vacuum packing removes oxygen. Modern refrigeration simply slows everything down without stopping it, which is why the fridge buys days rather than months.
Spoiled versus unsafe
An important distinction. Spoilage organisms make food unpleasant but are usually not dangerous. Pathogens, the organisms that cause food poisoning, often produce no smell, taste or visible change at all.
This is why the smell test is unreliable for safety. Food that smells fine can still be hazardous, and food that smells bad is usually just unappetising. It is also why date labels distinguish between best before, which is about quality, and use by, which is about safety and should be respected.
Understanding date labels
Date labels cause enormous confusion and a substantial amount of avoidable food waste. The two main types mean quite different things.
Use by is a safety date, applied to foods where microbial growth poses a genuine risk, such as fresh meat, fish and ready meals. It should be respected. Best before is a quality date, indicating when the manufacturer expects the food to be at its best, and applies to things like dried pasta, tinned goods and biscuits. Food past its best before is generally safe though possibly less good, and judgement rather than the label applies.
The takeaway
Food goes off because microbes multiply and because fats and enzymes react chemically, preservation works by removing water, warmth, oxygen or neutral pH, and the smell test tells you about spoilage rather than about safety.