What makes foods go bad?

Food spoilage is a natural process that causes foods to deteriorate in quality over time. Understanding what makes foods go bad can help us store foods properly and reduce food waste. This article will examine the most common factors that cause foods to spoil, including microbial growth, environmental exposure, enzymatic reactions, and chemical changes.

What are the most common causes of food spoilage?

The four main causes of food spoilage are:

  • Microbial growth – bacteria, molds, and yeasts
  • Environmental exposure – air, light, humidity
  • Enzymatic reactions – enzymes in the food
  • Chemical changes – oxidation, pH changes, protein breakdown

Understanding each of these factors can help us prevent or slow down the spoilage process.

How does microbial growth cause food spoilage?

Microorganisms like bacteria, molds, and yeasts are the most common causes of food spoilage. They are present everywhere in the environment, so keeping them out of food is impossible. When microbes contaminate food, they grow and multiply, causing the food to deteriorate through changes in taste, texture, appearance, and smell.

Different microbes prefer different environments and types of food. For example, mold grows well on bread, fruits, and cheese at room temperature. Yeast grows well on sugary foods. Bacteria prefer protein-rich foods like meat, fish, eggs, and dairy. Controlling food storage conditions like temperature, moisture, acidity, oxygen levels, and salt or sugar content can slow the growth of microbes.

Common food spoilage microorganisms

  • Molds – grow on bread, fruits, vegetables, jams, nuts, and fermented foods
  • Yeasts – ferment sugars and starches in foods like honey, syrups, and fruit juices
  • Bacteria – prefer protein-rich foods like meat, fish, dairy, and eggs

How does environmental exposure cause food spoilage?

Exposure to environmental factors like air, light, and improper storage temperatures accelerates the deterioration of foods. Fats turn rancid when exposed to oxygen in the air. Pigments that give color to fruits and vegetables fade with exposure to light. High temperatures speed up chemical reactions and stimulate microbial growth. Low moisture environments can make foods stale or shriveled.

Proper food storage controls environmental exposure by sealing out excess air in packages, keeping foods away from light, and regulating temperature and humidity. For example, vacuum sealing meats prevents contact with oxygen to avoid rancidity. Canned foods are protected from air and light exposure. Refrigerating perishable foods slows chemical reactions and microbe growth.

Effects of environmental factors on food spoilage

  • Oxygen – causes oxidation, fats turn rancid
  • Light – degrades pigments, vitamins, fats, and flavor
  • Heat – speeds up chemical reactions, microbial growth
  • Moisture loss – makes foods dry, stale, or shriveled

How do enzymes cause food spoilage?

Enzymes are naturally present in raw foods and cause chemical changes during ripening and over time. Enzymatic reactions continue after harvest and are responsible for changes in texture, color, flavor, and nutrient levels. For example, proteolytic enzymes degrade proteins causing meats to become tender. Amylase enzymes convert starch into sugars, causing vegetables to become sweeter during storage. Enzymatic browning reactions cause cut apples and potatoes to turn brown when exposed to air.

Processing methods like blanching, pasteurization, and canning inactivate enzymes by using high heat. Refrigeration also slows enzymatic reactions. Modified atmosphere packaging that replaces air with inert gases reduces enzyme activity. Controlled use of enzymes is also important in food processing for fermenting products like cheese, wine and beer.

Examples of enzymatic reactions in foods

  • Browning – in cut fruits, vegetables, shrimps
  • Softening – makes fruits and vegetables mushy
  • Off-flavors – bitterness, rancidity due to amino acid/fat breakdown
  • Color fading – loss of green color in vegetables

How do chemical changes cause food spoilage?

Natural chemical changes take place during food storage and handling that lower food quality and nutritional value. Fatty acids in fats and oils get oxidized when exposed to air, heat, and light, causing rancid odors and flavors. Vitamin C content declines rapidly after harvest. Sugars and proteins react with each other causing browning and texture changes. Acidic foods can raise the pH during fermentation. Oxygen exposure causes pigment degradation, giving faded color.

Controlling temperature, light exposure, moisture, oxygen levels and using food additives helps slow chemical deterioration. For example, antioxidants prevent oxidation of fats, oils, and pigments. Acidulants maintain acidity in canned foods. Humectants retain moisture and softness. Modified atmosphere packaging with nitrogen gas prevents oxidative chemical reactions.

Examples of chemical reactions affecting food quality

  • Rancidity – oxidation of fats into unpleasant fatty acids
  • Vitamin loss – degradation of water-soluble vitamins like C and B
  • Browning – sugars and proteins reacting to form melanoidins
  • pH change – increasing pH due to microbial growth
  • Color fading – pigment degradation by light, oxygen, metals

What are the signs of food spoilage?

Recognizing when food spoilage has occurred helps prevent consuming deteriorated, potentially unsafe food. Here are the most common visible and sensory signs that foods are past their prime:

  • Mold – fuzzy spots or cotton-like growth on surface
  • Discoloration – unnatural colors or faded pigments
  • Texture change – sliminess, mushiness, dryness, toughness
  • Unpleasant odors – sour, rancid, alcoholic, rotten smell
  • Off flavors – bitter, sour, metallic, acidic taste
  • Gas bubbles – inside packaging due to CO2 from microbial growth

How can proper food storage prevent spoilage?

Proper food storage is the best way to maximize freshness and shelf life by slowing the natural causes of food deterioration. Recommended storage methods for common fridge and pantry staples include:

Refrigerator Storage Tips

Food Storage Tips
Raw meats, poultry, fish Store in coldest part of fridge in sealed containers 2-4 days max
Dairy products Keep in original packaging until expiry date, use within 1 week of opening
Eggs Store in original carton on internal shelf, use within 3-5 weeks
Fruits, vegetables Store in high humidity drawers away from ethylene producing produce
Cooked foods, leftovers Cover tightly and place on top shelf, use within 3-4 days

Pantry Storage Tips

Food Storage Tips
Canned foods Store in cool, dry place, use within 1 year
Dry goods Keep in sealed containers in cool, dry place, check expiry dates
Breads, baked goods Store at room temperature in sealed bag or container for 3-5 days
Potatoes, onions Keep in cool, ventilated area away from light, use within 1 month
Oils, nuts Refrigerate after opening, use within 3-6 months

How can preservatives help prevent food spoilage?

Food preservatives and additives help extend the shelf life of foods by inhibiting the natural causes of spoilage. Common preservative types and their functions include:

  • Antimicrobials – sorbates, benzoates, and propionates stop microbial growth
  • Antioxidants – BHA, BHT, vitamin E prevent oxidation and rancidity
  • Acidifiers – citric acid, vinegar maintain pH and taste
  • Chelators – EDTA prevents oxidation and color changes
  • Gases – carbon dioxide, nitrogen slow microbial growth by replacing air
  • Salts and sugars – reduce available moisture to inhibit microbial growth

While chemical preservatives extend shelf life, they don’t necessarily make foods safer or more nutritious. Some consumers prefer to avoid foods with additives. Proper refrigeration, freezing, canning, and drying can preserve many foods naturally without added preservatives.


Understanding the science behind food spoilage allows us to implement the best storage, handling, and preservation techniques. The main factors that degrade foods over time are microbial growth, environmental exposure, enzymatic reactions, and chemical changes. Using proper temperatures, moisture, light exposure, and packaging helps slow natural deterioration and keep foods fresh and safe for longer.

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