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 10-12g

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 1x2x2x2x2x2x2x2x2x2= 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:

Biological Structures Nutrient Content pH and organic acids Water Activity Oxidation-reduction potential 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:

Temperature of storage Relative humidity of environment 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

• 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

3- Water Activity

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 Gram-positive 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 Minimum aw

Spoilage Bacteria 0.90-0.91Spoilage Yeast 0.87-0.94Spoilage Molds 0.70-0.80

Clostridium botulinum 0.90-0.98Salmonella spp 0.95-0.96Staphylococcus aureus 0.86-0.92