Factors Influencing Growth of Microorganisms in FoodPresentation Transcript
Factors InfluencingGrowth ofMicroorganisms in Food
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.
• 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
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.
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.
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.
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.
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.
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
• 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.