Chapter 6 – November 25, 2019
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Microbial growth
Physical requirements
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Temperature
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Minimum growth temperature
Optimum growth temperature
Maximum growth temperature
Psychrotrophs
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Grow between 0’C and 20-30‘C
Cause food spoilage
There are microorganisms that can
resist high levels of temperature
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Food preservation temperatures
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In the danger zone, rapid growth of
bacteria, some may produce toxics
In refrigerator temperature, may allow
slow growth pf spoilage bacteria, very
few pathogens
Food spoils fast
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The more rice, the faster it spoils
Mold likes dry places
PH
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Phosphorus - in DNA, RNA, ATP and
membranes
Trace elements
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Most bacteria grow between pH 6.5 and
7.5
Molds and yeast grow between pH 5
and 6
Acidophiles – grow in acidic
environment
Most of our food are mostly acidic
Osmotic Pressure
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Hypertonic environments, or an
increase in salt or sugar, cause
plasmolysis – shrinkage of cell’s
cytoplasm
Extreme or obligate halophiles (high
pressure bacteria) – require high
osmotic pressure
Facultative halophiles – tolerate high
osmotic pressure (doesn’t care)
Carbon
- Structural organic molecules, energy
source
- Chemoheterotrophs use organic
carbon sources
- Autotrophs use CO2
Nitrogen
- In amino acids and proteins
- Most bacteria decompose proteins
- Some bacteria use NH4+ or NO3- A few bacteria use N2 in nitrogen
fixation
Sulfur
- In amino acids and proteins
- Most bacteria decompose proteins
- Some bacteria use SO- or H2S
Inorganic elements required in small
amounts
Usually as enzyme cofactors
Not all oxygen is good for bacteria – Toxic
Oxygen
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Superoxide dismutase
Catalase
Peroxidase
Organic Growth Factors