Table of Contents

• Section 1. How to Do a Viable Plate Count
• 1: Why we need to dilute
• 2: Setting the stage: Diluting Cold Press Coffee
• 3: The dilution factor
• 4: Overall dilution
• 5: Design a dilution!
• 6: How to scale back up
• 7: Count a plate!
• 8: Why plate more than one dilution?
• 9: Picking the plate count
• Section 2. Practice Problems
• 10: Putting it all together
• 11: More dilution problems
• 12: Quickies
• 13: Is he dead yet?
• 14: Review

 

How to scale back up

So far we've figured out how to make a dilution, which we can then plate and count. But by definition, we're counting only a fraction of what was originally there. How do we account for that? By scaling up.

"Scaling up" means starting from a sample and figuring out how many were in the original brew, or stock, or whatever was originally there.

For example, we can start with a cup of wimpy coffee and figure out how much caffeine was in the original brew. All we need to know is what the overall dilution factor was. In the case of wimpy coffee, it was 1/50, or 0.02.

So, when we count the caffeine molecules in a cup of wimpy coffee, we know we got 1/50th of what was in a cup of the original brew -- or in other words, there was 50 times as much in a cup of the original. This method is called multiplying by the inverse (of the dilution factor).

• If the dilution factor is in the form of a fraction,
  "flip" the fraction
  (i.e., 1/50 becomes multiply by 50/1).
• If the dilution factor is in decimal form,
  multiply by 1 over the decimal
  (i.e., 0.02 becomes multiply by 1/0.02).

Finally, notice that I'm telling you the total number of caffeine molecules ONE CUP of the original brew. If I don't know the actual amount of original brew, I also won't know how much caffeine there was total.

Back to the lab

Here are some problems, ranging from easy to a bit hard...