Two if my sisters' school science fair is coming up and I am helping both of them. For my youngest sister, I had a few questions about her project. She is planning on doing "How do food preservatives affect the growth of microorganisms"? First, I wanted to know if its good for her grade level. She is in 5th grade but I wanted her to do something a bit advanced then her level. Also, she was interested in doing this project when I explained the experiment to her and she also researched about it. Secondly, her question/problem could be "How do food preservatives affect the growth of microorganisms," correct? If this is her question, she will be testing whether high or low amount of food preservative affect the bacterial growth, will it increase or decrease growth, correct? I just wanted to confirm a few of these questions to make sure I understood the project problem before we actually jumped into doing the experiment. I hope I get a reply soon.
That sounds like a very interesting question! As long as your sister understands the concept of the project, I don't see why that's not appropriate for her grade level!
What kind of food preservatives are you going to be testing, and what microbes? How are you going to measure their growth? Why is this a relevant topic to study? What will be your main hypothesis? These are all questions to think of as you plan out your project.
Let us know if you have anymore questions as you are brainstorming and planning these experiments.
She will probably be testing salt. I was also considering using vinegar instead of salt. The microbes will be microbes in general, I don't think we will be testing for specific microbes since we are just testing how preservatives affect microbe growth. As the project says, we will just count the bacterial colonies and we will have separate agar plates for different amounts of food preservative solution. Each agar plate will be streaked with different amounts of food preservative concentrations and then incubated for the bacterial growth to occur.
However, something I don't understand about the experiment procedure is step 5. We have to take samples of the preservative on the 1st, 3rd, 5th, and 7th day and streak onto the agar plates, but what I don't understand is after we streak the 1st day, do we incubate the plates and then on the 3rd day streak and incubate, and so on...or do we streak the 1st day, leave it alone, streak the 3rd day, and so on and then incubate after streaking the 7th day?
This experiment will help find how and what amount of food preservative will prevent food spoilage. My sister's hypothesis is if you increase the amount of food preservative, then it will decrease the bacterial growth.
I have another question about the agar plates. Is it alright if we buy petri dishes and agar separate instead of buying ready-to-use agar dishes? I was planning on buying petri dishes and then making agar and pouring an equal amount into each dish to make agar dishes. Please reply as soon as possible. Someone please also help with my previous questions. Thank you.
Project Question: I'm registering because I'm interested in volunteering with the Ask an Expert program to help students with their science fair projects.
Yes, that is definitely a legitimate alternative to buying ready-to-use agar plates. Mostly, the agar (which comes in powdered form, I believe?) will come with instructions on how to prepare the agar before pouring it into the dishes--this procedure will likely will involve boiling (just a heads up). I hope this helps!
In regards to your question about the incubation, your first thought is correct. You would take a sample on Day 1, streak, and then incubate. Then, you take a sample on Day 3, streak, and then incubate, and so on with the other days.
Ok. Thank you all so much for the information. Although your questions answered most of my questions, the procedure to this project is really confusing. This is the link to the project just in case you need it and can't find it on science buddies:
I don't really understand steps 3-5. Why do we need two replicates for each condition as says in step 3. What do they mean in step 4 in general and as said before, I will be using salt as a preservative. Can you give an example to explain step 4?
Lastly for step 5, just to make sure, do we for example take condition #1 2.5% ( as shown on the table) streak it the 1st, 3rd, 5th, and 7th day and etc. with the rest of the conditions? If so, then why do they ask for 40 Petri dishes in the materials? Please also explain this step. Thank you.
To answer your first question, the point of doing two replicates for each condition in step 3 is to assess for the variability of your results. It'll help you see whether the results you get for each condition are consistent (for example, if for condition #1, your first replicate gave you 25 colonies and your second replicate gave you 23 colonies, that can tell you that there's not too much variability, but if your first replicate gave you 25 colonies and your second replicate gave you 50 colonies, then it either means that there is natural variability in how well bacteria grow in the same conditions or there is inconsistency in the way you performed your experimental procedures).
For step 4, if you are using salt, then you have nothing to worry about. You just need to follow their instructions on how much salt to add to the broth to give you the desired preservative concentration. For step 4, they are just saying that if the preservative you are using is not salt, you would need to adjust the amount of preservative you add accordingly to get it to the desired concentrations.
For step 5, according to my understanding, you just dip an inoculating loop or cotton applicator into the broth from each condition on the 1st, 3rd, 5th, and 7th day and streak them on agar according to the method they mentioned. Since you have 5 conditions (control and conditions #1-4), 2 replicates for each condition, and 4 days, that adds up to a total of 40 plates, which is what the materials state. Basically, you just keep the jars of broth going for 7 days, but on each of the days of interest, you dip a loop or cotton applicator into the broth of each condition and plate them on agar to assess the number of bacteria in the jar.
Hope that helped. Let us know if you have anymore questions. Other experts, if I explained anything incorrectly, please correct me.
Ok. Thank you so much. Your answers helped a lot. So basically, for each condition there will be 8 Petri dishes. I completely understand now. Just to make sure the broth should be warm or cooled before streaking and also kept in the fridge when not being used, correct? If I have anymore questions, I'll ask, but thank you for these responses.
That is a good question about what the temperature of the broth should be when you streak it. It depends on what you define as your "1st day". If your Day 1 is the day where you make the broth and put the preservatives in, then I would probably wait until the broth cools down to room temperature before I dip a loop into the broth and streak that onto an agar plate. This is because for all of your other days, your broth is going to be cooled so you want to keep that consistent. If your Day 1 is the day AFTER you make the broth and put the preservatives in, then your broth is going to be cooled and not warm anyway. While the broth is not being used, I would probably keep them out at room temperature instead of in the fridge because bacteria tend to not grow or grow very fast at low temperatures and you don't want that to be another factor that could be affecting your bacterial counts in each of your conditions.
