Science Buddies: "Ask an Expert"

I need to know how to calculate the original sucrose concentration in different drinks. According to the information given in the experiment the formula is

original sucrose concentration x glucose concentration at the linear time point = percentage of sucrose converted

I determined the linear time point of a 10% sucrose concentration to be 10 minutes. The glucose concentration at that time was 0.5, therefore the the percentage of the sucrose converted was 5%.
The experiment advised to use the same linear time point to test different juices for the glucose concentration after adding invertase. I've determined the initial glucose concentration as well as final glucose concentration ( 10 minutes after adding the invertase to 15 ml of different drinks.
I am having a bit of difficulty calculating the initial sucrose concentration in the different drinks. If I use the above mentioned formula, what is the percentage of sucrose converted for different drinks? Also, should the glucose concentration considered be the difference between the initial glucose concentration after invertase and before invertase since we are not starting with zero concentration as in the case of 10% sucrose solution?

Hi Miha,

Welcome to Science Buddies. Are you doing this project?

https://www.sciencebuddies.org/science- ... #procedure

In this project, glucose is measured in the food with the glucose strips. Then the food is treated with invertase to convert the sucrose into glucose plus fructose . The initial result is the concentration of glucose in the sample, and the increase in glucose after adding invertase is proportional to the original concentration of sucrose.

You are correct in your first calculation. If the concentration of glucose was 0.5 at 10 minutes after adding invertase, then the conversion rate was 5%.

To calculate results for all of your samples, you can subtract the concentration of glucose at 10 minutes (after adding invertase) from the initial concentration of glucose to determine how much glucose was derived from sucrose. This concentration represents 5% of the original concentration of sucrose, so you would multiply this number by 20 to determine the original concentration of sucrose.

Does this help? Please let us know if you need additional information.


Donna Hardy

It is a variant from that project as we are doing the drinks and not solid food but the same principle. Thanks for your answer. It makes more sense as I expected the amount of sucrose to be higher for the drinks with higher glucose increase after adding the invertase. However, although the calculations make more sense now, I still have one question. Why sucrose 10% to determine the linear time point. Also is the linear time point independent of the initial sucrose concentration? That is, the linear time point is the same for all the drinks or it is influenced by the initial sucrose concentration of the drink?

Thank you,
Mihaela

Hi Mihaela,

I'm glad your results make sense.

Your question is a good one. Since the results depend on the activity of the enzyme, the 10% sucrose is used as a reference point to determine the initial concentration of sucrose in the test sample. Results can vary with the concentration of the sucrose, so would be a little different with 1% sucrose, for example, compared to 10%. However, this is probably not the greatest source of error in this experiment.

If the sucrose concentration in your samples if about 2%, you could improve the accuracy of the results by using 2% sucrose to determine the linear time point. However, I would not recommend repeating the experiment for this reason. The best thing to do is to include this point in the discussion section of the board.

Donna

Thank you for taking the time to answer my question. Great help.
Mihaela