Food Microbiology 1 Unit 4 Microbial Growth Microbial

Food Microbiology 1 Unit 4 Microbial Growth

Microbial Growth • Bacteria are single-celled organisms • Bacteria multiply in a process called binary fission in which two cells arise from one • Growth Rate: is the change in cell number per unit time • The interval for one cell to become two is known as the generation time or doubling time • Quickest generation time is 9 mins • Bacteria typically 15 -30 mins • In extreme cases it can be one year

Example: Microbial Growth Hypothetical problem to appreciate how rapidly bacteria are capable of multiplying Facts: The weight of one Escherichia coli cell is 1 x 10 -12 g The generation time or doubling time is 20 mins The mass of the Earth is 1. 45 x 1021 Kg Starting at 9: 00 am with one Escherichia coli under conditions of unlimited nutrients how many E. coli would be present after 3 hours? 48 hours? How does this relate to the mass of the Earth?

Answer: 20 min/generation = 3 generations/hour 3 hours x 3 generations/hour = 9 generations 1 x 2 x 2 x 2= 29 = 512 cells After 48 hours: ( Use your calculators for this xy) 2. 2 x 1043 cells; weigh 2. 2 x 1028 Kg which is 1. 5 x 107 times the mass of planet earth!!!

Growth Curves

Lag Phase • Considered as the adjustment period when the organism adapts to new surroundings • No Microbial growth • Synthesize enzymes to adapt to the environment • Recovery from stress or injury • This period may be extended in unfavorable environments • In extreme cases the lag phase can last for weeks

Exponential (log phase) • Growth is stable • Growth rate is constant for a given bacteria under specified conditions • Catabolic processes generate energy • Anabolic processes build cell structures

Stationary Phase • Over time, essential nutrients become depleted or waste products build up to toxic levels so that logarithmic phase ceases and results in stationary phase • No net growth in stationary phase (cell ‘replacing’ but number not increasing • Cell functions such as energy metabolism may continue • Endospore-forming bacteria produce the endospore once the culture has entered the stationary phase

Death phase • Viable cell count decreases • Under certain circumstances cell death is accompanied by cell lysis

Factors Affecting Microbial Growth • Food Microbiologists must have through understanding of the factors which influence microbial growth • Many parameters can affect the growth and survival of microorganisms. These parameters are divided into: 1 - Intrinsic 2 - Extrinsic

Intrinsic Parameters • Are those inherent to the food These include: Ø Biological Structures Ø Nutrient Content Ø p. H and organic acids Ø Water Activity Ø Oxidation-reduction potential Ø Antimicrobial constituents • Intrinsic parameters have a major impact on the type of microorganisms that are capable of growing on or in a food

Extrinsic Parameters • Are those properties of the storage environment which affect both the food and the microorganisms associated with the food These include: Ø Temperature of storage Ø Relative humidity of environment Ø Presence and concentrations of gases in the environment Extrinsic parameters effect both the food and microorganisms and influence the extent and rate at which microorganisms will grow

Intrinsic Parameters 1 - Biological Structure • Outer barriers against the invasion of microorganisms (E. g the skin of fruits and vegetables form a protective layer to invasion by microorganisms) • Inner parts of whole healthy tissues are sterile • Damages during harvesting pr processing (peeling, skinning, chopping) expose tissues and increase microbial loads throughout the product

Biological Structure • 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

2 - 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 • All microorganisms important to food are chemoheterotrophs • They use organic compounds as energy and carbon sources • Food is rich in nutrients • Food provide microorganisms with water, a source of energy, carbon and nitrogen sources, and growth factors such as vitamins and minerals

3 - 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 • % moisture of foods is not the same as aw

Water Activity • The aw of a food can be reduced by increasing the concentration of solutes in the aqueous phase of the food • Accomplished by drying, freezing, or addition of solutes (sugar in jams) • All microorganisms have optimum and minimum aw requirements • The maximum limit is slightly less than 1. 0 (organisms cannot grow in absolutely pure water) • Yeasts and molds can tolerate lower aw than bacteria • Gram-negative bacteria require higher aw than Grampositive bacteria

• aw affects growth, toxin production, spore germination, and heat resistance of microorganisms • Microorganisms will not grow in foods with aw below 0. 60 • Bacterial pathogens are usually inhibited at aw less than 0. 9 except Staphylococcus aureus can grow at aw 0. 86 • Most fresh foods have very high aw (0. 98 -0. 99), processed foods range (pudding 0. 99; crackers 0. 3)

Minimum Water Activity that will support the growth of bacteria and yeasts and molds Microorganisms Spoilage Bacteria Spoilage Yeast Spoilage Molds Minimum aw 0. 90 -0. 91 0. 87 -0. 94 0. 70 -0. 80 Clostridium botulinum Salmonella spp Staphylococcus aureus 0. 90 -0. 98 0. 95 -0. 96 0. 86 -0. 92