1. Mode of spoilage
FST 313 INTRODUCTION TO FOOD SCIENCE AND TECHNOLOGY
NAWA HASSAN LECTURER FST KCAMS
2. SPOILAGE
Food spoilage is the deterioration of food so that it loses its original quality,
nutritional value, flavor, or safety. Spoiled food may look, smell, or taste
unpleasant, and in some cases, it can also cause illness if consumed.
Understanding spoilage is key in food science because:
• It helps prevent foodborne illness
It reduces food waste
It extend shelf life
It extends consumer trust
3. Types of Spoilage:
• Sensory changes: appearance, odor, texture, taste
• Chemical changes: oxidation, fermentation
• Biological contamination: microbial growth, pest infestation
The major agents that cause spoilage include:
1. Enzymes
2. Microorganisms (Bacteria, Yeasts, Molds)
3. Insects
4. Birds
5. Physical Factors (heat, light, oxygen, humidity, mechanical damage)
4. Enzymes
Enzymes are proteins naturally present in food that speed up chemical reactions. After a plant
or animal is harvested or slaughtered, enzymes remain active and begin breaking down food
components like proteins, fats, and carbohydrates.
How Enzymes Cause Spoilage:
Enzymatic browning: This occurs in cut fruits (like apples, bananas, or potatoes) due to the
enzyme polyphenol oxidase (PPO) reacting with oxygen. It forms brown-colored pigments
(melanin).
Lipase activity: Enzymes like lipase break down fats into free fatty acids, which can cause
rancidity—an unpleasant, off-putting odor and flavor (especially in butter, milk, and oil-rich
products).
Protease activity: These enzymes break down proteins in meat or fish. Over time, this leads to
the release of ammonia or sulfur compounds, causing off-odors and slimy texture.
5. Control of Enzymatic Spoilage:
• Blanching: Brief heat treatment inactivates enzymes in fruits/vegetables
before freezing.
• Low temperature: Refrigeration slows enzyme activity.
• pH modification: Adding acid (like lemon juice) inhibits enzymes.
• Oxygen control: Vacuum packaging or antioxidants reduce oxygen, slowing
browning.
6. Microorganisms (Bacteria, Yeasts, Molds)
What Are Microorganisms?
These are microscopic living organisms that can grow on or in food, using it as a nutrient
source. While some are beneficial (like those used in yogurt or cheese), many cause spoilage or
foodborne illnesses.
Categories:
a. Bacteria:
• Some bacteria break down proteins and fats, releasing foul-smelling gases.
• Examples:
• Pseudomonas spp. grow in refrigerated meat and fish, causing sour odor and slime.
• Lactic acid bacteria sour milk by fermenting lactose into lactic acid.
• Clostridium botulinum causes deadly botulism in improperly canned foods.
7. b. Yeasts:
• Yeasts ferment sugars in food, producing alcohol and gas.
• Cause spoilage in fruit juices, syrups, and sauces.
• Not harmful in most cases but change the taste and texture.
c. Molds:
• Filamentous fungi that appear as fuzzy patches on bread, fruits, cheese.
• Produce mycotoxins (e.g., aflatoxins), which are toxic and carcinogenic.
• Thrive in warm, humid conditions and on acidic foods.
8. Conditions Favoring Microbial Growth:
• Temperature: Most grow rapidly between 5°C and 60°C (the “danger zone”).
• Moisture: Water activity above 0.85 is ideal for bacteria.
• Oxygen: Aerobic microbes need oxygen; anaerobes grow without it.
• pH: Neutral to slightly acidic foods are more prone.
Control Methods:
• Refrigeration and freezing
• Heat treatments (pasteurization, sterilization)
• Drying and dehydration
• Preservatives (e.g., salt, sugar, vinegar, sorbates, nitrates)
• Proper packaging (vacuum, modified atmosphere)
9. Insects
Insects physically damage food and also act as carriers of spoilage microbes.
They can:
• Lay eggs in food (e.g., fruit fly larvae in overripe fruit)
• Bore holes in grains (weevils, beetles)
• Introduce bacteria or fungi through saliva, feces, or body parts
Common Pests:
• Weevils: Damage whole grains, rice, beans
• Fruit flies (Drosophila spp.): Attracted to sugary and fermenting fruits
• Moths and beetles: Found in flour, dried fruits, spices
10. PREVENTION
• Use of airtight packaging
• Store food in dry, clean environments
• Fumigation or pest control
• Maintain hygiene in storage areas
11. RODENTS
• Eat and contaminate food
• Spread dangerous diseases (e.g., Leptospira, Salmonella)
• Leave urine, feces, and hair
• Gnaw through packaging and electrical wires
How They Spoil Food:
• Damage stored grains, vegetables, packaged goods
• Contaminate more than they consume
Control Methods:
• Traps and baits
• Rodent-proof buildings
• Clean, sealed storage areas
• Regular inspection and sanitation
12. Birds
Birds cause:
• Physical damage to grains and fruits
• Contamination via droppings, feathers, or nesting materials
• Transmission of diseases, including fungal spores and bacteria
Common Examples:
• Birds feeding in open grain storage facilities
• Droppings on open-air market produce
Prevention:
• Bird-proof netting
• Use of scare devices (sound, visual)
• Covered storage facilities
13. Physical Factors
These are non-living conditions that contribute to spoilage.
a. Temperature
• High temperatures speed up enzyme activity and microbial growth.
• Cold temperatures can cause freezer burn or damage delicate products like
lettuce.
b. Oxygen
• Oxygen promotes:
• Lipid oxidation (rancid flavors in oils, nuts)
• Growth of aerobic microbes
• Vacuum packaging or nitrogen flushing is used to reduce oxygen.
14. c. Moisture and Humidity
• High humidity causes:
• Mold growth
• Sogginess in dry goods
• Dry foods stored in humid places absorb moisture and spoil.
d. Light
• Light breaks down vitamins (especially vitamin A and riboflavin)
• Causes photo-oxidation of fats, especially in milk
• Use of opaque containers and UV-resistant packaging helps.
e. Mechanical Damage
• Bruising fruits or cracking eggs provides an entry point for
microbes.
• Damaged cells release enzymes, speeding up spoilage.
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