|MadSci Network: Microbiology|
I am sorry but this most likely will not work. First off, standard commercial yogurt does have active (viable) bacteria present (billions per gram ) but these organisms are often injured in the commercial product. The spectrometer 20 measures light scattering and the more turbid (greater number of bacteria) a solution the less light will pass.
However the spectrometer cannot distinguish between live and dead bacteria or even other particles which will be present in the yogurt. Once you heat your diluted solution and kill some of the bacteria the spectrophotometer will yield the same reading since the only difference will be that some of the bacteria will be dead but not gone.
I would suggest that you ask your teacher if you could grow yeast in the class room. You can purchase bakerís yeast at the local grocery store. Mix it with a little warm water and let it sit for a few hours or overnight. Next day you should have a health culture of yeast with approximately 10-100 million per milliliter. You could then dilute your culture, heat it at different temperatures and then plate it onto a growth medium (Potato Dextrose Agar pH 3.5) or (Potato Dextrose agar with antibiotics). No bacteria should grow on these media.
Good Luck !
Try the links in the MadSci Library for more information on Microbiology.