Detecting starch and sugars in food

Ten test tubes, test-tube holder, test-tube rack, Nusen burner, Heat-proof mat, safety glasses

Procedures:

1. Three drops of 2cm3 of 1% starch solution was placed onto a white tile and three drops of iodine solution was added and observed the solor change.
2. 2cm3 of glucose solution was placed into a test tube and added about five frops of Benedict’s reagent and boiled gently. Observed the color change.
3. Placed small piece of ten foods in ten test tubes and added one drop of water,  three drops of 2cm3 of 1% starch to them respectively and orderly. (mushrooms, potatoes, bread piece, sweet potatoes, lemon, onion, biscuit, cucumber, tomatoes, peanuts) Oserved the color change and made records.
4. Placed small pieces of ten foods into ten different test tubes and added one drops of water, five drops of Benedict’s reagent to them in order and respectively. (lemon, lettus, tomato sauce, cucumber, tomato, dark green vegetables, milk, biscuit, cereal, sweet potatoes) And boiled each test tube gently and observed the color change.
5. Made a conclusion based on results.

Result

1.In procedure 1, the color of solution changed to blue-black when starch solution reacted with iodine solution. In procedure 2, the color of solution changed to orange-red when the glucose solution reacted with the Benedict’s reagent and was heated.

2.The diagram(fig.1) below shows what I observed directly after the procedure 3 :

3.Then,the diagram(fig.2) below shows what I observed directly after the procedure 4:

Analysis:

1. In my whole experiment, the procedure 1 and 2 were used as testing thoeries to give us a standard to testify whether certain food source contained certain kinds of carbonhydrates. If in procedure 3 the food turned color near blue or purple, the food must contain starch inside. What’s more, if in procedure 4 the solution in the test tube turned color near red or orange, the food msust contain reducing sugars glucose or fructose.
2. From the information and results I gathered from fig.1, I was able to figure out whether each food contains starch or not. The list was shown below:

Fig.3

From the diagram above we clearly see that the three foods contain starch: potatoes, bread piece, sweet potaotes and biscuit.  

3. From the information and results I gathered from fig.2, I was able to figure out whether each food contains sugars glucose and fructose or not. The list was shown below(fig.4):

From the diagram above we can clearly see that except dark green vegetables all the foods remained contain sugars glucose or fructose.

Evaluation:

During the experiment, our group made several mistakes and I listed them below to evaluate them respectively.

1. At the beginning of the experiment, during the process of adding water to the washing powder solution to 500cm3. We first ingnored the bubbles above and filled the water till the 500cm3 scale but later on we found the water was much more than 500cm3, so we repeated the experiment from the starting point once again. This time, I controled the water very carefully by pouring it softly along side the walls of the beaker and measuring cylinder to make sure there’s no bubble made during the procedures. My improvement was successful and at the second time we added water accurately to 500cm3.
2. The color of final solution in test tube “2.5mgdm-3” was as dark as that of  the solution with 10mgdm-3and it’s not consistent with the general trend of other solutions. I considered the whole procedure of our experiment thoroughly and thought of two possible errors. Firstly we could have made that firstly we might add more than 5 cm3 standard detergent solution to the test tube. Secondly, because before using the 1000cm3 beaker to heat the test tube we used 500 cm3 beaker at first then we found it too small to hold six test tubes so we removed two test tubes out of the water and then put them into the bigger container. Maybe one of the two previously heated test tube was the “2.5mgdm-3” one and it’s darker because it has been heated for seconds before others.

What’s assay ?