Microbial population growth can be studied using the colorimeter to measure transmittance or absorbance.
Yeast is easy to use and gives good results. We have found the doubling time for a culture of yeast grown at 30°C to be about 4.5 hours. The actual number of cells can be measured using a haemocytometer.
The results shown below were obtained using the following recipe at 35 °C with aeration.
Within the range 0.1 to 0.7 absorbance units the number of cells per cm3 can be approximated using this formula.
number of cells (millions per cm3) = 50 × (absorbance – 0.05)
The results shown below were obtained by growing a culture of E. coli B in nutrient broth at 25°C in a 4cm3 cuvette. The colorimeter containing the cuvette was connected to a computer then placed in an incubator to maintain the temperature. The absorbance of blue light was recorded every 10 minutes for 8 hours.