Handout A

• Acceptance of food spoilage is dependent upon consumer expectation and is not always related to food safety
• Spoilage Mechanisms of Different Food Products

  1. Spoilage of Food Products

  2. Shelf-Life Indicators
• Spoilage Mechanisms of Gram-Positive, Spore-Forming Bacteria

  1. Sweet curdling (Renin coagulation without acidification)

  2. Flat sour spoilage (Acid production but no gas)

  3. Gas production and swelling of cans

  4. Sulfur stinker spoilage (Hydrogen sulfide)
• Spoilage Microorganisms

  1. Gram-negative bacteria

  2. Gram-positive, non-spore-forming bacteria

  3. Gram-positive, spore-forming bacteria

  4. Yeast and mold
• Spoilage Mechanisms of Gram-positive, Non-Spore-Forming Bacteria


  1. Production of a thick polysaccharide (slime or ‘rope’)

  2. Production of diacetyl, lactic acid, and acetic acid
• Gram-negative Bacteria Spoilage Mechanisms

  1. Production of heat-stable proteases and lipases (off-flavor)

  2. Production of colored pigments
• Food spoilage is a huge economical problem worldwide
• Spoiled and Unspoiled are subjective terms
• Gram-negative Bacteria


  1. e.g. Pseudomonas, Alteromonas, Shewanella putrefaciens, Aeromonas

  2. Spoilage of dairy products, red meat, fish, poultry and eggs during cold storage

  3. Foods with high water activity and neutral pH

  4. Foods that are stored without a modified atmosphere (i.e. normal levels of oxygen)
• Gram-positive, Non-Spore-Forming Bacteria

  1. e.g. LAB, Acetobacter, Pediococcus

  2. Spoilage of meats stored under MAP or vacuum packaging

  3. Spoilage of beer and wine
• Causes of Food Spoilage

  1. Activity of bacteria, yeasts, and molds

  2. Insect damage

  3. Physical injury (e.g. bruising, pressure, freezing, drying and radiation)

  4. Activity of indigenous enzymes (animal and plant tissues)

  5. Chemical changes
• Yeast and Molds

  More tolerant of low water activity
• Food Spoilage

  1. Involves any change that renders the food unacceptable for human consumption

  2. Off taste, odor, and color

  3. Gas and slime

  4. Visible growth of microorganisms
• Gram-Positive, Spore-Forming Bacteria


  1. e.g. Bacillus, Clostridium

  2. Heat-treated foods (e.g. Pasteurized milk, canned goods)
• Chemical changes in the muscles cause the musculature to lose extensibility and become stiff
• Handlers are a source of human pathogens contaminating the carcasses
• Slaughter time: Internal tissues of healthy animals are sterile
• Animal hide contains microbial deposits on dehaired carcasses and freshly cut surfaces
• Storage environment, including circulating air, is a significant source of organisms on the surfaces of all slaughtered animals
• Primary sources of microbes in fresh meats include stick knife, animal hide, gastrointestinal tract, rumen, lymph nodes, handlers, containers, and storage environment
• Spoilage mechanisms of Yeast and Molds

  • Pectinolytic enzymes (Soften the plant tissues)
  • Colored sporangia that visibly color the food
  • Rhizopus nigricans – Black spots
  • Penicillium, Aspergillus – Green spots
  • Neurospora sitophila – Red spots
  • Whiskers in meat
• Depleted oxygen supply (Reduction of O/R potential) leads to rigor mortis
• Stick knife is used for slitting the jugular vein, leading to bloodstream contaminants and microbial deposits in the carcass
• Rigor Mortis is one of the recognizable signs of death
• Spoilage of Food Products

  • Meat products
  • Seafoods
  • Dairy products
  • Egg
  • Vegetable and Fruit Products
  • Cereals, flour, and bakery products
• Gastrointestinal Tract, Rumen, and Lymph Nodes can contaminate the surface of freshly dressed carcasses
• Containers are a primary source of contaminants for ground or minced meats
• Factors contributing to the spoilage of meats

  • Abundance of all nutrients required for the growth of bacteria, yeasts, and molds
  • High water activity
  • Highly proteinaceous (strongly buffered)
  • Only a few bacterial types predominate in spoiled meats as opposed to fresh meat
  • e.g. Pseudomonas, LAB
• Spoilage of salted and dried fish is due to fungi
• Leg muscles of poultry are more likely to spoil faster than breast muscles
• Spoilage of cured meat is relatively insusceptible to spoilage by most bacteria but susceptible to mold
• Processed meats are cured, smoked, or cooked
• Fresh iced fish spoilage is caused by bacteria like Pseudomonas
• Molds are the primary agent of spoilage in antibiotic-treated poultry
• Spoilage of sausage, bacon, and bologna is characterized by sliminess on the outside of the casings
• Mollusks like oysters, clams, squid, and scallops with high carbohydrate content are susceptible to fermentative bacteria and yeast spoilage
• Milk is an ideal growth medium for bacteria with intrinsic flora from cow’s milk ducts and milking equipment like Pseudomonas, Micrococcus, Lactobacillus
• No difference in the spoilage of saltwater and freshwater fish
• Processed Meats

  • Sausage
  • Bacon
  • Bologna
  • Ham
• Growth of psychrophilic microorganisms causes milk spoilage