Science Buddies: "Ask an Expert"

Hello. My student is looking at performing one of the projects on your website. https://www.sciencebuddies.org/science- ... p015.shtml
But after looking at it, we are not sure if it has quite a bit of lurking variables. We are especially concerned about the exact measuring of increase of bacteria. Meat before the experiment is not treated with anything and may end up having a numerous amouts of unknown bacteria which in turn will have an influence on our results. How can we avoid this? Do we count the number of colonies before and then after the experiment? Thank you very much.

sciencefan,

If I understand correctly you will need to take a baseline measurement. For example the instructions say to "Cut, measure, and weigh 14 pieces of meat (approx. 5 cm × 5 cm × 2 cm, about 65 g each). Set aside two pieces of meat for a baseline bacterial count."

The more initial and final bacteria counts that are completed, may enhance your results. I do understand there are some extraneous variables involved since you are working with meat. I suggest that your student performs some statistical tests and acknowledge the uncertainty in his/her report.

If you have anymore questions let us know, another expert may be able to provide more information.

Thank you and good luck!
Travis Sigafoos

Hello, and thank you again.
We have been looking over the porject and my student noticed that one factor is a major lurking variable: how do we know for sure when the frozen piece of meat is fully defrosted? The meat on the counter, for example, may be there for 2 hours or more. The longer it stays threre the more bacteria will be growing. Do we continually poke the meat during the defrosting stage? Is there a way of making this part of the experiment more controlled? thanks.

Hi sciencefan,

One suggestion is to perhaps use a smaller cut of meat (ignoring what the procedure says; just make sure the sizes are equal among all the meat pieces). Since the meat is smaller, it will defrost faster and won't need sit out for a long time.

Hope that helps!

-Sam

sciencefan,
Another option would be to do a preliminary test where you determine how long it takes for the meat to dethaw to room temperature, then just use that same amount of time for all of your pieces of meat.

Matt Badtke