Signs of Food Spoilage

This lesson covers the signs and causes of food spoilage as well as the beneficial uses of microorganisms in food production, as required by the AQA GCSE Food Preparation and Nutrition specification (8585), section 3.4. You need to understand how enzymic action, mould growth and yeast action cause food to deteriorate, and also how these same processes are harnessed beneficially in food manufacturing.

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What Is Food Spoilage?

Food spoilage is the process by which food deteriorates and becomes unfit for consumption. Unlike food poisoning (which may produce no visible changes), spoiled food usually shows obvious signs — changes in appearance, texture, taste or smell.

Food spoilage is caused by three main factors:

1. Enzymic action (enzymes within the food)
2. Microbial action (bacteria, yeasts and moulds)
3. Chemical reactions (e.g., oxidation, rancidity)

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Enzymic Action

Enzymes are biological catalysts — proteins that speed up chemical reactions. All living things contain enzymes, and they continue to work in food after it has been harvested, picked or slaughtered.

Ripening

Enzymes cause fruit to ripen after picking:

Stage	What Happens
Unripe	Fruit is firm, green, sour; starch has not yet converted to sugar
Ripe	Enzymes convert starch to sugar; fruit softens, changes colour, develops sweet flavour
Overripe	Enzymes continue working; fruit becomes mushy, brown, develops off-flavours
Decayed	Cell walls break down; fruit collapses, attracts moulds and bacteria

flowchart LR
    A["Unripe<br/>Firm, green,<br/>starchy"] --> B["Ripe<br/>Soft, coloured,<br/>sweet"]
    B --> C["Overripe<br/>Mushy, browning,<br/>off-flavours"]
    C --> D["Decayed<br/>Collapsed,<br/>mouldy"]

    style A fill:#27ae60,color:#fff
    style B fill:#f1c40f,color:#000
    style C fill:#e67e22,color:#fff
    style D fill:#8b4513,color:#fff

Enzymic Browning

When fruits and vegetables are cut, bruised or damaged, enzymes react with oxygen in the air to cause enzymic browning. This is the brown discolouration you see on:

• Cut apples, pears and bananas
• Sliced avocado and potatoes
• Bruised peaches and nectarines

The enzyme responsible is polyphenol oxidase. Enzymic browning can be slowed or prevented by:

Method	How It Works
Adding lemon juice (citric acid)	Acid lowers pH, reducing enzyme activity
Adding vitamin C (ascorbic acid)	Acts as an antioxidant, preventing the reaction with oxygen
Immersing in water	Removes contact with oxygen
Blanching	Brief heating (e.g., in boiling water for 1–2 minutes) denatures the enzymes
Refrigeration	Low temperatures slow enzyme activity
Covering with cling film	Reduces contact with oxygen

Exam Tip: Enzymic browning is a common topic. You should be able to explain why it occurs (enzymes react with oxygen) and give at least three methods to prevent it, explaining the science behind each method.

Autolysis

After an animal is slaughtered, enzymes naturally present in the meat begin to break down the muscle tissue in a process called autolysis. In controlled conditions, this is used deliberately in meat ageing (also called "hanging" or "dry ageing"):

• Enzymes tenderise the meat by breaking down connective tissue
• Flavour develops and intensifies
• Meat is aged in controlled conditions (temperature, humidity, airflow)
• Beef may be aged for 21–35 days

If not controlled, however, autolysis causes meat to become slimy, discoloured and develop off-smells.

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Mould Growth

As covered in Lesson 1, moulds are multicellular fungi that produce visible fuzzy growths on food. Signs of mould spoilage include:

Sign	Description
Visible fuzzy growth	Green, white, grey, blue or black patches
Musty or earthy smell	Off-putting odour
Soft, sunken areas	Underneath the mould, the food may be soft and discoloured
Discolouration	Colour changes in the food surface
Changed texture	Food becomes soft and mushy

Foods Commonly Affected by Mould

• Bread and baked goods
• Soft fruits (strawberries, raspberries, peaches)
• Cheese (especially soft cheese)
• Jams and preserves (if not sealed properly)
• Nuts (can produce dangerous aflatoxins)

Safety Considerations

• Soft foods (bread, fruit, soft cheese): discard the entire item — mould hyphae penetrate deep into soft foods, and mycotoxins may have spread throughout
• Hard foods (hard cheese, firm vegetables like carrots): you can cut at least 2.5 cm around and below the mould — the dense structure prevents deep penetration
• Never sniff mouldy food — inhaling mould spores can cause respiratory problems

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Yeast Action

Yeasts cause food spoilage through unwanted fermentation. Signs include:

Sign	Description
Fizzing or bubbling	Carbon dioxide production from fermentation
Alcoholic smell	Ethanol production
Sour or off-taste	Changed flavour from fermentation products
Cloudy appearance	In liquids such as fruit juice
Swollen packaging	CO₂ gas builds up inside sealed containers

Foods Commonly Affected by Yeast Spoilage

• Fruit juices and soft drinks
• Jams and preserves (especially if sugar content is low)
• Bread dough (if left too long, over-fermentation occurs)
• Fruits with damaged skin
• Yoghurt

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Bacterial Spoilage

Bacteria cause food to deteriorate through various visible and detectable changes:

Sign	Description
Slime	Slimy coating on meat, fish or poultry
Off-smells	Sour, putrid or ammonia-like odours
Discolouration	Green, grey or brown patches on meat; yellowing of fish
Gas production	Bloated or swollen packaging
Changed texture	Food becomes soft, mushy or sticky
Sour taste	Milk going sour (lactic acid bacteria)

Exam Tip: Remember the key difference: spoilage bacteria cause visible changes (you can see, smell or taste that food has gone off), while pathogenic bacteria do not — contaminated food looks, smells and tastes normal.

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Beneficial Uses of Microorganisms

Not all microbial activity is harmful. Many foods rely on controlled microbial action for their production. The AQA specification requires you to know about beneficial uses of bacteria, yeasts and moulds.

Beneficial Uses of Bacteria

Product	Bacteria Involved	Process
Yoghurt	Lactobacillus, Streptococcus thermophilus	Bacteria ferment lactose (milk sugar) to produce lactic acid, which thickens the milk and gives yoghurt its tangy flavour
Cheese	Various lactic acid bacteria	Bacteria acidify milk, causing casein protein to coagulate; further bacterial action develops flavour during ageing
Sauerkraut	Lactobacillus species	Cabbage is fermented; bacteria produce lactic acid which preserves the cabbage and gives a sour taste
Sourdough bread	Wild Lactobacillus species	Bacteria in the sourdough starter produce lactic acid, giving the bread its characteristic sour flavour

Beneficial Uses of Yeasts

Product	Yeast Involved	Process
Bread	Saccharomyces cerevisiae (baker's yeast)	Yeast ferments sugars in dough, producing CO₂ (makes dough rise) and alcohol (evaporates during baking)
Beer	Saccharomyces cerevisiae	Yeast ferments sugars from malted barley, producing alcohol and CO₂ (carbonation)
Wine	Saccharomyces cerevisiae	Yeast ferments sugars in grape juice, producing alcohol
Marmite/Vegemite	Yeast extract	By-product of brewing; concentrated yeast extract rich in B vitamins

Beneficial Uses of Moulds

Product	Mould Involved	Process
Blue cheese (Stilton, Roquefort, Gorgonzola)	Penicillium roqueforti	Mould grows through the cheese creating blue-green veins; produces strong, distinctive flavour
White-rind cheese (Brie, Camembert)	Penicillium camemberti	Mould grows as a white coating on the outside; enzymes soften the cheese from outside in
Quorn (mycoprotein)	Fusarium venenatum	Mould is grown in fermenters, producing a high-protein meat alternative
Soy sauce	Aspergillus oryzae	Mould ferments soya beans and wheat to produce soy sauce
Tempeh	Rhizopus oligosporus	Mould binds cooked soya beans into a firm cake; traditional Indonesian protein source

mindmap
  root((Beneficial<br/>Microorganisms))
    Bacteria
      Yoghurt
      Cheese
      Sauerkraut
      Sourdough
    Yeasts
      Bread
      Beer
      Wine
      Marmite
    Moulds
      Blue cheese
      Brie & Camembert
      Quorn
      Soy sauce
      Tempeh

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Preservation Methods: Linking to Food Spoilage

Understanding how food spoils helps explain why different preservation methods work: