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

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

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

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

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

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

Difco™ Lactobacilli MRS Agar Approximate Formula Per Liter Proteose Peptone No. 3 ........................................... 10.0 g Beef Extract ..............................................................10.0 g Yeast Extract .............................................................. 5.0 g Dextrose ...................................................................20.0 g Polysorbate 80 ........................................................... 1.0 g Ammonium Citrate .................................................... 2.0 g Sodium Acetate ......................................................... 5.0 g Magnesium Sulfate .................................................... 0.1 g Manganese Sulfate .................................................... 0.05 g Dipotassium Phosphate .............................................. 2.0 g

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 4.0-10.0

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