Food Microbiology

• what are the four most important aspects to consider with food and microorganisms?

  • spoilage
  • preservation
  • food production
  • disease causing microorganisms
• food spoilage -> any change in appearance, smell, taste of a food product that makes it unacceptable to the consumer
• Water activity (aw) -> a measure of available water
• water can be immobilized/unavailable if sorbed to food surfaces or interacting molecules
• for growth most bacteria require aw greater (>) then 0.9, however, fungi need less water so a lower aw
• How is water activity (aw) calculated?

  aw = vapor pressure of the food itself (when in a completely undistirbed balance with the surrounding air / vapour pressure of distilled water under identical conditions
• What's the scale for water activity and types of foods?

  High - perishable, semiperishable, nonperishable - Low
• a food item is analogous to the flask of growth medium
• the exponential growth rate is effected by extrinsic and intrinsic factors
• extrinsic factors -> temperature, Oxygen, pH, aw
• intrinsic factors -> nutritive value, natural antimicrobial substances, makeup of foods microbial community
• extent of lag phase depends on microbial contaminants , their previous growth history
• the last few doublings in exponential phase are when spoilage damage becomes noticeable and is important to shelf life estimates
• food preservation is dependent on how long the lag phase is, thus if you want a long food preservation then you have to have a longer lag phase
• how can the log phase be extended?

  • storage in cold temperatures
  • Heat processing
• Cold temperature storage

  • fridge and freezer
  • microbial growth slows but does not stop
  • psychrotolerant microorganisms may spoil refrigerated foods
• Heat processing

  • cooking -> reduces microbial load
  • canning ->produces food that is stable without refrigeration
  • pasteurization reduces microbial load and kills pathogens (cold/hot sequences)
  • aseptic food processing
• Clostridium botulinum

  • anaerobe
  • spore former
  • does not noticeably spoil food or alter can
• What's the problem with honey?

  spores can be found in honey however they cant grow in the high sugar content. When diluted with water allows to grow in stomach but cant survive low pH. It is important not to feed to babies as they do not have an acidic stomach
• by increasing the acidity of food (pickling) by adding a acidulate it helps preserve food
• fermentation also aids in food preservation
• How can one reduce the water activity (aw) of food?

  • drying
  • lyophilization (freeze drying) -> camping rations
  • addition of sugar or salt -> jams, meats
• Irradiation in food safety

  • ionizing radiation
  • food does not become radioactive
  • causes reduction in microbial load
• MAP = modified atmosphere packaging
• Modified atmosphere packaging is a process whereby the oxygen is removed from the atmosphere and replaced with a controlled amount of carbon dioxide or nitrogen gas which eliminates oxidation reactions and reduces growth of food spoilage microbes
• What is hurdle technology?

  • using multiple levels of antimicrobial control in food
  • levels can be applied together
• What happens when hurdles are applies together synergistically?

  • give better overall protection
  • alter the properties of the food to a lesser extent