Science Buddies: "Ask an Expert"

Hi, I am doing project called "Sucrose & Glucose & Fructose, Oh My! Uncovering Hidden Sugar in Your Food", and it really confuses me a lot in the "Analysing Your Results" part. My science fair board is due in two days but because of the issue of the equation I can not even finish my results and conclusion. Please help me with the following questions, thank you so much!
It says:
a. Look at the graph you made in the "Testing Invertase Activity" section. At the linear time point, what was the glucose concentration? If you multiply the glucose concentration at the linear time point by the original sucrose concentration (10%), this will tell you what percentage of the total sucrose was converted to glucose at this time point. We will call this value "Percentage of sucrose converted." You can see this equation written out in Equation 1.
For example, if at the linear time point the 10% sucrose solution had a reading of 1% glucose, then this means that 10% of the total sucrose (10% multiplied by 1%) had been converted to glucose.
Original sucrose concentration × Glucose concentration at linear time point=Percentage of sucrose converted

So here is my first question: Is the example wrong that 10% multiplied by 1% should = 0.1 X 0.01 = 0.001=0.1%?

Then it says:
b. Look at the data you collected in the "Testing the Foods for Glucose Concentration Before and After Adding Invertase" section. To determine the original sucrose concentration of each of your food samples, use Equation 1 and divide the percentage of sucrose converted (which you just determined) by the glucose concentration at the linear time point.
For example, if at the linear time point a food had a 2.5% glucose reading, and we calculated that the percentage of sucrose converted is 20%, then we could calculate that the original sucrose concentration of this food was 8% (20% divided by 2.5%).

It means to calculate original sucrose concentration, original sucrose concentration = percentage of sucrose converted / glucose concentration at linear time point. I got my "percentage of sucrose converted" is 7.5%, the glucose concentration of honey at linear time point is 20%, which i got 7.5% divided by 20% =0.375% for original sucrose concentration. The glucose concentration at linear time point of salad dressing is 1.5%, and 7.5% divided by 1.5% = 5% for original sucrose concentration. It is even much higher than honey which doesn't make sense.
Depends on that equation, I found the higher glucose concentration at linear time point always has lower original sucrose concentration, like honey and syrup have the highest glucose concentration but the results showing that they have the lowest original sucrose concentration as 0.375% and 0,75%. It is very unusual.
I have talked about it with my math teacher and I really consider the equation is wrong.
would it be Glucose concentration at linear time point / original sucrose concentration = percentage of sucrose converted seem more accurate and reasonable?
Please help me with my problems and I really appreciate that!

Hello zhaoxin990614,

I’m assuming that you are working on the project described here:

https://www.sciencebuddies.org/science- ... lism.shtml

After reviewing the procedure section that you asked about it appears to me that equation 1 is not correct. I think that the percentage of sucrose converted is equal to the glucose reading divided by the original sucrose concentration as you suggested. And to calculate an “unknown” original sucrose concentration you would divide the glucose concentration found for your food sample by the percentage of sucrose converted.

As it turns out, equation 1 will give you a correct value for the percentage of sucrose converted only if the original sucrose concentration is 10% and if you don’t convert the concentrations to fractions (as you did).

I hope this helps. Please post again if you have more questions.

A. Norman

Hi, thank you so much for replying me and answering my questions! I had looked for evidence to support my idea for a long time and now finally I can re-write my results and conclusions using the right equation!
Just one more question please, so for all the calculation in this project I don't have to convert % to fractions but just consider it as the unit right?
Once again, thank you so much for helping me so that I didn't give up for finding the correct equation and answer at the last second !

Hi Zhaoxin990614 - I am very sorry for the problem you ran into with the equation in this project. You got excellent help from Expert A. Norman, and I am glad that you were able to continue with the project.

Based on your questions, the science team at Science Buddies has reviewed the project. The equation is incorrect, and this problem is being updated as soon as possible to avoid further confusion. I have some information for you from our scientist to help as you complete your work:

For all the equations, you should calculate with the numbers not including the unit - so do NOT convert 10% into 0.1 for the calculations... What should be done is actually calculating the "fraction of sucrose converted" and then multiplying by 100 to get the percentage, following this equation:

Percent of sucrose converted [%]= (Glucose amount at linear time point [%] x 100)/ Original amount of sucrose [%]

This means that in the following step (step 2.b. of "Analyzing your results" where they calculate the original sucrose concentration of their foods), you also have to calculate differently (resolve the above equation to original amount of sucrose). The correct calculation is:

Original amount of sucrose [%]= (Glucose amount at linear time point [%] x 100)/ Percent of sucrose converted [%]

Calculation example (calculate without the units):

Original amount of sucrose: 10 [%]
Glucose amount at linear time point: 2 [%]

Percent of sucrose converted = 2x100/10 = 20%

Amount of glucose in food at linear time point: 2.5 [%]

Original amount of sucrose [%] = 2.5x100/20 = 12.5

I hope this information helps. Again, I apologize for the confusion caused by this error in the procedure.

Amy
Science Buddies

Hi Amy, thank you so much for providing me the information! It really helps me to calculate my results.
Although the issue of equations in this project has confused me but I so appreciate that I could get help from all the excellent experts from Science buddies! I'm also glad that I could help with fixing the problem in this project.
Once again, it is a good experience for my first time posting questions in "Ask an Expert" and thank you for all of you!