To test for the presence of starches and protein macromolecules, to introduce the concept of variations in chemical compounds. Hypothesis:
If biuret is blue after a test for protein, then it is a negative result because biuret reagent is blue to begin with. If a solution is pinkish purple, or purple, then the test for protein is positive. The test solution for starches is yellowish brown. If any substance that is yellowish brown when mixed with iodine, then the test for starches is negative. If the solution turns purple, or a dark purplish black, then the test for starches is positive. If DI water, and sucrose are tested for proteins using biuret reagent, then they will show a negative result. To explain, both DI water, and sucrose alone do not contain a trace of protein. In addition, deionized water is commonly used in experiments as a negative control. In both experiments, if the DI water is tested for a protein or starch, then it will come back as a negative. If milk solution, and 50% egg white solution are tested for proteins using the biuret solution, then there will be positive results because both of these are composed of protein macromolecules. The use of condensed milk brings about an even stronger positive result for protein than regular refrigerated milk, since condensed milk is higher in protein. If DI water, onion juice, and sucrose are tested for starches using iodine, then there will be a negative result for starches. This is because there are not any traces of starch in these solutions. On the other hand, if 1% starch solution is tested for starch using iodine, then there will be a positive result for starch since there is starch present. Materials and Methods:
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Test Tube Rack
4-5 Test Tubes
Transfer Pipet
Protective goggles
Deionized Water (DI water)
Evaporated Milk
50% Egg White Solution
1% Sucrose Solution
1% Starch
Onion juice
Sucrose
Biuret Reagent
Iodine
Test 1 – Protective glasses were put on at the beginning of the experiment! Test for protein molecules- The test tubes were labeled in the following order 1-4; DI water, Milk, 50% EW, 1% Sucrose, and held still in the test tube rack. 3 drops of each substance was added to the test tubes. Next. 5 drops of Biuret Reagent were added to the test tubes. The test tubes were mixed by shaking them. Results were recorded according to the color of the new solution. Test 2- For the test for starches, 4 clean test tubes were labeled as follows: DI water, 1% starch, onion juice, and sucrose. 3mL of each substance was added to each test tube. In addition, 5 drops of iodine was added to each of those. A swirling technique was used to mix the solution. Results were recorded according to the color of each end product. Results:
Table 1- Test for Proteins Using Biuret Reagent Contents
Results
1. 3 Drops DI H2O
5 Drops Biuret
Blue
2. 3 Drops Milk
5 Drops Biuret
Pinkish Purple
3. 3 Drops 50% Egg White
5 Drops Biuret
Purple
4. 3 Drops 1% Sucrose
5 Drops Biuret
Light Blue
Table 2- Test for Starch Using Iodine 1. 3mL DI Water
5 Drops Iodine
Light Yellow
2. 3mL 1% Starch
5 Drops Iodine
Light Purple
3. 3mL Onion Juice
5 Drops Iodine
Light Yellow
4. 3mL Sucrose
5 Drops Iodine
Light Yellow
Conclusion:
For the test for protein macromolecules using biuret reagent the DI water was blue after being mixed with 5 Drops of Biuret Reagent, and sucrose became light blue, both of these were determined as negative tests for proteins. This result matched the hypothesis, given that the water is a pure substance, sucrose is a non starch sugar. When milk, and 50% egg white were mixed with the biuret reagent, the solution for milk was pinkish-purple, and the 50% egg white became purple, each portraying a positive test for protein. These results were parallel to the hypothesis as well knowing that both have a significant amount of protein. One way this test could be further improved is by using whey protein isolate mixed in a water solution rather than condensed milk because though condensed milk is higher in protein concentration than regular milk, whey protein isolate has most to even sometimes all of the fat processed out of it, leaving a purer protein substance! I would hypothesize that whey protein solution would show up as a deeper purple rather than just pinkish purple.
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Proteins are complex molecules that react differently to many compounds but are also fragile and cannot withstand high temperatures or strong acids and bases without degrading. The Biuret Method, or biuret test, is used to detect the presence of peptide bonds. It has this name because it reacts positively to the biuret molecule's peptide bonds. Correctly evaluating the number of peptide bonds is a ...
