Module 1 - Topic 3 (LE 1, pt. 3)

• Food spoilage - involves any change that renders a product unacceptable for human consumption and it may result from a variety of causes (spoilage may be caused by a wide variety of reactions including some that are essentially physical, chemical, enzymic, and microbiological
• Chemical Reactions (Spoilage) in Food

  1. Oxidative rancidity - perception of the objectionable flavors and odors caused by the oxidation of the unsaturated fatty acid chains of lipids by atmospheric oxygen
  2. Oxidative and reductive discolouration 
  3. Nonenzymic browning - Maillard reaction 
  4. Destruction of Nutrients 
• Physical Reactions (Spoilage) in Food

  1. Mass transfer - movement of low Molecular Weight components
  2. Loss of crisp textures 
  3. Evaporative loss of taste 
  4. Freeze-induced structural damage
• Enzymic Reactions (Spoilage) in Food

  1. Lipolytic rancidity 
  2. Rancidity catalysed by lipoxygenases 
  3. Proteolysis
  4. Enzymic browning - ex: polyphenol oxidase (PPO) is the enzyme responsible for discolouration of banana (discoloration of banana is more of enzymic spoilage rather than chemical)
• Microbiological Reactions (Spoilage) in Food

  1. Growth or presence of infectious microorganisms 
  2. Growth of toxinogenic microorganisms 
  3. Growth of spoilage microorganisms
• —> Spoiled food is food that tastes and smells off 

  —> Spoiled foods are not poisonous 

  —> Spoilage is a quality, not a safety issue

  • There’s a distinct difference between food quality and food safety 
  • The loss of the desired quality is due to many changes, including undesirable growth of microorganisms
• —> there is a relationship between microbial growth and food quality 

  • Soured milk is unacceptable on its own can be used to make scones 
  • The overgrowth of pseudomonas app. Is undesirable in meat, yet desirable in hung game birds (game bird = any number of a species of wild birds which are hunted, killed, and eaten by humans)
  • Acetic acid production during wine storage is unacceptable, yet acetic acid production from wine is necessary for the production of vinegar 
• —> Food spoilage, again, involves any change that renders the food unacceptable for human consumption 

  • Insect damage
  • Physical injury due to bruising, pressure, freezing, drying, and radiation
  • Enzyme and nonenzmye-mediated chemical changes 
  • Activity of bacteria, yeasts, and moulds

  —> factors associated with food spoilage are associated with its intrinsic food properties 
• —> Food spoilage is also dependent on cross contamination during harvesting, slaughter, and processing in combination with temperature (food spoilage can occur anywhere from farm to fork)

  • The conditions during distribution and storage are particularly favourable for growth of certain microorganisms that could cause spoilage
• —> When we intentionally spoil milk to make cheese or yogurt, we add a known number of bacteria that eat sugars and convert them into known products with desirable characteristics (ex: lactobacilli = lactic acid) 
• • Cheese = desirable spoilage, curdled milk = undesirable spoilage (wherein the organisms are unknown, are usually a mixture of different bacteria that eat sugar and produce a mixture of products — some bacteria will eat sugar and produce lactic acid, some will produce alcohol, some will produce acetone, and when these different substances/metabolites mix together, they will not taste good generating undesirable characteristics)
• —> How did this spoilage take place? 

  • Packaging materials passed the Quality Assurance (QA) despite damage (small damage even a pinhole size can allow microorganisms in the product)
  • The finished product were not handled properly after release from the food plant during distribution, display, or vending which may result in the damage 
• Temperature effect on spoilage

  • Lower temperature delays the lag time or the lag phase and so the shelf life is longer 
  • Overall: Lower temperature delays the lag phase, while Higher temperature promotes the lag phase
• —> For fresh foods, the primary quality changes may be categorised as bacterial growth and metabolism resulting in possible pH changes and formation of toxic compounds, off odors, gas, and slime formation — this may be for fruits and vegetables 

  —> Oxidation of lipids and pigments in fat-containing foods resulting in off flavours, formation of compounds with adverse biological effects or discolouration — this may be for the meat and those food that contains fat
• —> Spoilage of pasteurised milk held at normal temperature is caused principally by thermoduric bacterium most specifically bacillus cereus — it proliferates during milk spoilage 

  • Thermoduric bacterium - bacteria that can survive varying extents of the pasteurization process — these bacteria are readily isolated from poorly cleaned dairy equipment and pipelines, although numbers of thermoduric bacteria are usually very small in refrigerates bulk milk tanks 
  • Thermostable proteases are protein-hydrolysing enzymes that are responsible for the bitterness in spoiled milk 
• —> Although there have been progress in the characterisation of the total microbial flora and metabolites during spoilage, not much has been established in terms of the relationship between food composition and specific spoilage organisms (SSOs) — SSOs cause undesirable chemical changes in foods 

  • Physically looking at and smelling food can tell us food is spoiled, but it would be harder to establish which specific spoilage organism caused this spoilage 
• • Among the total microbiota, only those with the ability coproduce metabolites in adequate quantities to cause alteration or changes in sensorial properties can be considered as the main cause of spoilage — these microorganisms are called SSO’s or Specific Spoilage Organisms (these SSO’s might belong to one microbial genus or species in contrast to the total microbiota because the total microbiota may consist of greater than one microbial group, genus, or species)
• As storage time is increased

  The number of SSO’s grow quickly under particular storage conditions
• As time progresses

  SSO’s become a larger fraction of the total microbiota
• SSO’s

  Produce metabolites or the chemical spoilage indices called CSI’s
• CSI’ cause organoleptic rejection
• Organoleptic rejection

  • Changes in flavours
  • Discolouration
  • Sliminess
• These changes render the food unacceptable for human consumption
• Concentration of the metabolite


  Determines the product shelf-life
• • foods are classified based on their susceptibility 
  • Nonperishable (dried, powdered foods such as flour and starches)
  • Semi-perishable (fresh cut products)
  • Perishables (raw meats, poultry, seafoods)
• Gram negative spoilage bacteria

  • Pseudomonas
  • Alteromonas
  • Shewanella putrefaciens
• Gram negative spoilage bacteria


  • Spoil dairy products, red meat, fish, poultry and eggs during cold storage (they are either psychrophiles or psychrotrophs)
• Bacteria responsible for vegetable spoilage

  • Erwinia carotovora
  • Pseudomonas
• Erwinia carotovora and Pseudomonas are responsible for ca. 35% of vegetable spoilage
• Microorganisms that can cause activation of thermoduric or heat-stable proteases and lipase in milk even after pasteurization


  • A variety of these microorganisms
• Chemical spoilage could be caused by proteases, but these proteases are activated by these bacteria
• It's still microbial spoilage because there is still spoilage bacteria that did not inactivate after pasteurization
• Examples of Gram-positive, non-spore formers:

  • Lactic acid bac teria, brocothrix thermosphacta (unaerobic bacteria) — they typically cause spoilage of meats stored under modified atmosphere packaging or vacuum packaging
  • Acetobacters and Pediococcus spp. Can produce “ropes” in beer 
  • Lactic Acid Bacteria can produce dactyl in beers and last acid wines
• Examples of Gram-positive, non-spore formers:

  • Bacillus cereus — grow in pasteurised milk at 5C and cause “sweet curdling” and “bitty cream”
  • B. Sterothermophilus causes “flat sour” spoilage of canned products
  • Cl. Saccharolyticum causes spoilage of canned products characterised by can swelling
  • Desulfotomaculum nigrificans produces H2S that causes the “sulfur stinker” spoilage of canned goods
• —> Spoilage yeast and moulds activate pectinolytic enzymes which soften the plant tissues causing rot in fruits 

  • as much as 30% of all fruit rots may be attributed to penicillium
  • Bread is spoiled by rhizomes nigrificans, penicillium, aspergillum, and neurospora sitophila
  • Saccharomyces and turolopsis spoil jams, syrups, and honeys.
• Food spoilage fungi

  Aspergillus niger — spices

  Byssochlamys fulva — cereals in airtight packs 

  Fusarium oxysporum — fruits

  Mucor spp. -- Meats

  Neosartorya fischeri — pasteurized foods

  Penicillium roqueforti — meats, eggs, and cheeses

  saccharomyces spp. — soft drinks, James, syrups, and honeys

  Trichoderma harzianum — margarines

  Zygosaccharomyces bailii — dressings
• Control fungi in food:

  • High temperature processing can denature spores
  • Weak acid preservatives like sorbic acid and antibiotics such as natamycin are used to control fungal growth in foods
  • Some fungus have however, become resistant to sorbic acids and some degrade the compound to the malodorous pentadiene (so what we do is hurdle technology) ex: combining sorbic acid with other food grade preservatives 
• —> Shelf life is the time between the production and packaging of the product and the point at which it becomes unacceptable to the consumer — it is therefore related to the total quality of the food and linked to production design, ingredient specification, manufacturing process, transportation, and storage (at retail and in the home) note: spoilage can occur anywhere from farm to fork

  —> Shelf life depend upon the kind of food — an dit is essential for the food manufacturer to identify the intrinsic and extrinsic parameters which limit the shelf life of a certain food
• —> shelf life can also be defined as the time during which the food product will remain safe, will be certain to retain desired sensory, (chemical, physical, and microbiological characteristics, and will comply with any label declaration or of nutritional data when stored under recommended conditions 
• Examples of shelf life of some foods:

  Bread - up to one week at ambient

  Sauces/dressings - 1-2 years at ambient

  Pickles - 2-3 years at ambient

  Chilled foods - up to 4 months at 0-8C

  Frozen foods - 12-18 months in freezer cabinets

  Canned foods - unlacquered (uncoated) cans = 12-18 months, lacquered can = 2-4 years