Ultraviolet light inactivates microbes by
​ disrupting the structure of their DNA making them unable to replicate. 

physical growth factors and microbial growth

high temperatures are
primarily bactericidal:
 capable of killing bacteria 

review question:

what is the difference between bacteriostatic and bactericidal?

water activity is important because it affects the safety, quality, and texture of food. Water activity is used to control the growth of microorganisms like bacteria, yeasts, and mold. 
Water activity is used to control the stability of dehydrated and semi-moist foods. ​

1. regular/batch pasteurization:
    63°C for 30 min kills E. coli,
    M. tuberculosis & Salmonella 
2. HTST/flash
     72°C for 15 seconds
3. UHT
     140°C for 3 seconds can               serve to sterilize

 water activity in food:
fresh fruits/veggies: 0.99
fresh meat: 0.99
milk: 0.99
bacon: 0.97
bread: 0.95. 

pickles and jams have tons
of dissolved salt and sugar which hog all the water (the solvent) leaving less available water for microbes to use.

boiling

high moist heat:
permanently denatures DNA  and proteins
includes:

pasteurizing

plus...

temperature 

Most bacteria thrive with a higher water activity.
Certain fruits and vegetables with higher moisture content are more susceptible to bacterial spoilage.

high dry heat:
oxidizes organic material
can be used to sterilize surgical instruments
​requires longer times and temps: 2 hours and 180 degrees

15 lbs per inch *2

​121.5 degrees C

review question:

define psychrophile, mesophile, thermophile.
why are pathogens usually mesophilic?

water activity is measured on a scale of 0.0 to 1.0 with 1.0 = pure water. 
The lower the water activity, the drier the substance and the less likely it is for that substance to serve as a growth medium for microbes.  

autoclaving

                           in today's lab
  we will assess the growth of four different bacterial species            exposed to incubation under different temperatures.
 question:
    what is the optimal growth temperature for each of these bacterial species?
 hypothesis:
    your educated guess using this information..
   
  1. M. luteus is a mesophile
  2. S. marcescens is a psychrotolerant mesophile 
  3. E. coli is a mesophile
​  4. B. stearothermophilus is a thermophile

review question:

What was the purpose of the lab where we exposed 4 different bacterial strains to 4 different temperatures? 

moisture content

UV radiation

temperature

Higher water activity substances tend to support more microorganisms. bacteria usually require water activity values of at least 0.91 compared to fungi which need 0.6 or so.  Every microorganism has a limit of water activity below which it will not grow.

autoclave uses
moist heat sterilization:
steam generated under pressure kills microbes

controlling microbial growth

how do
1. temperature
2. moisture
3. UV light
​ affect microbial growth?

Water activity (aw) is a measurement of how much water in a substance is available for microorganisms to grow.

low temperatures are
primarily bacteriostatic:
 inhibiting growth/reproduction of bacteria without killing them
​by reducing enzyme activity .
can not be used for sterilization 

100 degrees C