1. Effecting factors of microorganism growth –osmotic pressure
Objective
Compare bacterial growth in different osmotic pressure reacted with sugar or salt
Principle
Bacterial growth needs an environment with a high water activity. Therefore, when sugar or salt is added into food, the water activity of the food will be decreased but osmotic pressure will be increased.
It is an oldest method to inhibit the bacterial growth in food Chapter 24

2. Materials and methods Bacteria: Media Escherichia coli,
Staphylococcus aureus
Media
tryptic soy agar (TSA)
Regular salt and sugar

3. Salt and sucrose Methods 0, 3, 6, 9% Each concentration has two plates
16 plates per group
Methods
Separate the plates to four sections
Streak each bacterial species onto each section
Incubate plate at 37℃
Observe growth one the next day
Compare different species and plates

4. Water activity of agar Aw1 Aw2 平均 0%TSA 0.987 0.986 3% salt +TSA 0.972
0.959
0.953
0.956
9% salt +TSA
0.937
3%sugar +TSA
0.984
0.98
0.982
6%sugar +TSA
0.975
0.978
9%sugar +TSA
0.977
0.976

6. Compare results between different bacterial species
Compare results between sugar and salt. Which one is more effective?

7. Effecting factors of microorganism growth –pH
Objective
Compare bacterial growth in different pH
bacteria:
Escherichia coli
Lactobacillus acidophilus
Medium
TSB at pH 4.0, 5.0, 7.0, 9.0, 10.0
Chapter 23

8. Difco™ Lactobacilli MRS Agar
Approximate Formula Per Liter
Proteose Peptone No g
Beef Extract g
Yeast Extract g
Dextrose g
Polysorbate g
Ammonium Citrate g
Sodium Acetate g
Magnesium Sulfate g
Manganese Sulfate g
Dipotassium Phosphate g

9. procedures Each pH has two tubes 10 MRS tubes per group
Inoculate E. coli and L. acidophilus into MRS tubes in the same range of pH

10. Observe growth in on the next day
Incubate at 37°C
Observe growth in on the next day
Compare different species and plates