Factors Influencing Growth of Microorganisms in Food


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Factors Influencing Growth of Microorganisms in Food

  1. 1. Factors InfluencingGrowth ofMicroorganisms in Food
  2. 2. Factors Influencing Growth ofMicroorganisms in Food Food Microbiologists must have through understanding of the factors which influence microbial growth The ability of microorganisms to grow or multiply in a food is determined by the food environment as well as the environment where the food is stored.
  3. 3. • Many parameters can affect the growth and survival of microorganisms. These parameters are divided into: 1- INTRINSIC• Nutrient Content• pH and organic acids• Water Activity• Biological Structures
  4. 4.  2-EXTRINSIC• Relative humidity• Temperature• Gaseous Environment
  5. 5. Intrinsic Parameters Nutrient Content The chemical composition of a food influences the type of microorganisms that will grow and the products that they will produce during growth. They use organic compounds as energy and carbon sources. Food is rich in nutrients. Meat are rich source of vit. B and fruits are low but fruits are rich in ascorbic acid. Egg white contain biotin but also contains Avidin which ties up biotin making it unavailable to micro-organisms & eliminating possible spoilage organism those which must have biotin supply.
  6. 6.  pH and organic acids Yeast & molds are acid tolerant than bacteria. Some foods have low pH because of inherent acidity. For ex. Fermented product because of lactic acid during fermentation. Vegetable juices have low buffering power so decrease in pH with production of only small amount of acid by Lactic acid bacteria. Milk is high in protein & permits growth & acid production in LAB in the manufacture of fermented milk.
  7. 7.  Water Activity All microorganisms require water in an available form to grow and metabolize. Availability of water is measured by water activity (aw). aw= Vapor pressure of food substrate/ Vapor pressure of pure water at the same temperature. aw of pure water is 1.0. The aw of a food can be reduced by increasing the concentration of solutes in the aqueous phase of the food.
  8. 8.  Yeasts and molds can tolerate lower aw than bacteria. Gram-negative bacteria require higher a w than Gram-positive bacteria.Some of the aw valueo 0.98 & above – fresh meat, fish, fruits, vegetableso 0.93-0.98 - tomato paste, bread, cheeseo 0.85-0.93 – beef, condensed milko 0.60-0.85 - nuts, jam, jellieso Below 0.60 – chocolate, honey, potato chips, biscuits.
  9. 9.  Biological Structures Outer barriers against the invasion of microorganisms (E.g the skin of fruits and vegetables form a protective layer to invasion by microorganisms). Damages during harvesting pr processing (peeling, skinning, chopping) expose tissues and increase microbial loads throughout the product. Milk has no protective barrier. Ground meat spoils faster than whole meat cuts(grinding distributes surface microorganisms throughout). Eggs are usually sterile inside but heavily contaminated on the shell, crack in the shell allows microbes to enter.
  10. 10. Extrinsic Parameters Relative humidity When the food commodity have low water activity, they are stored in atm. of high relative humidity. Storage of fresh vegetables require control of relative humidity & if it too low than many of the vegetables will loose water & become flaccid. Temperature Microorganisms grow over a wide range of Temperatures. For ex. Psychrotrophs – grow well below 7 C, optimum at 20-30 C• Mesophiles – grow well 20-45 C, optimum at 30-40 C
  11. 11. • Thermophiles – grow well at and above 45 C, optimum at 55-65 C. Gaseous Environment Oxygen comprises 21% of earth & is the most important gas in contact with food.
  12. 12. Thanks