Read the summary and the most important questions on Food Microbiology

• Spoilage

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• Influences on growth rate?

  • Intrinsic factors (physico-chemical properties of food) 
  • Extrinsic factors (properties of food environment) 
  • Implicit factors (properties and interactions of micro-organisms) 
  • Processing (changes/food/environment/micro-organisms) 

• Improvement of food preservation?

  • By product formulation (increase or decrease nutrients) 
  • By processing (introduce or remove barriers) 

• pH range of foods from low to high?

  
  Citrus fruit < Fruit < Vegetable < Meat < Milk < Fish < Egg white. 

• Applications of pH in food preservation?

  • Fermentation 
  • Acidic preservative

• Pitfalls of pH in food preservation?

  • Yeast and/or mould dissimilate acid
  • Acid-tolerant bacteria can grow (proteolytic)
  • Pathogenic bacteria can grow (especially in low acidic foods) 

• Redox potential of aerobic organisms?

  • Most energy via oxidative phosphorylation with oxygen as terminal electron acceptor
  • Enzymes (superoxide dismutase (SOD) and catalase) disable toxic products of oxygen

• Redox potential of anaerobic organisms?

  • No SOD or catalase
  • No growth at high redox potentials (in presence of oxygen) 

• Effects on redox potential of foods?

  • Milling/grinding = higher access to oxygen = higher redox potential
  • Low pH = many H+ = higher redox potential 
  • Microbial growth = less oxygen = lower redox potential

• Water activity of micro-organisms?

  • Xerophilic = grow on dry foods (aw = 0.60) 
  • Osmophilic = grow in high concentrations of unionized compounds (sugar) (aw = 0.62) 
  • Halotolerant = grow in salted products, tolerate elevated levels of sodium chloride
  • Halophilic (obligate) = requires high levels of sodium chloride to grow

• Reduction of water activity?

  • Adding solutes (salt, sucrose) 
  • Removing water 
  • Combined application