Macromolecule Lab: Lemons

This past Tuesday, I worked without a lab partner to determine the presence of different macromolecules inside lemons. We as a class used four different indicator tests to see if glucose, proteins, starch, or lipids were present in each designated food. The first test performed was for protein, and was performed by adding en drops of Biuret's solution to five mL of the food. If the result was a brown color, then the food had protein; however, my lemon juice did not change color and it was clear that no protein was present. The second test, which was for glucose, required three mL of Benedict's solution to be added to five mL of the food, and then that mixture had to be heated for five minutes. If, when the mixture ws heated, a very obvious color change to a bright orange occurred, then glucose was present. When I added the Benedict's solution to the lemon juice, it instantly started bubbling upwards, almost to the top of the test tube. This reaction was due to the acidity of lemon, which caused the solution to react. However, when I heated the mixture after the bubble had settled, no color change occurred whatsoever. With this in mind, glucose is probably present in lemon. The acidity of the lemon most likely caused inaccurate results. According to this website, the presence of glucose in lemon is not listed, but in one cup of raw lemon juice, 5.9 grams of sugars are present. Odds are that glucose makes up part of those 5.9 g. The next test performed was for starch, and called for 5 drops of Lugol's solution to be added to 5 mL of the food and if the result was a dark black color, then starch was present. I decided that my color was a dark brown, but not the dark black that starch indicates. According to the same website, starch data is also unavailable, and this may also be due to the acidity, or it could be due to a misreading on my part. I could have decided that a positive test was negative, not believing the brown to be dark enough to be a positive indicator. The final test was for lipids, and required the food to be rubbed on a brown piece of paper towel and hung up to dry. When dry, a positive read would have a shiny or glossy look. My lemon test was not shiny at all, and this makes sense, as the website I mentioned before states that no fat is present in raw lemon juice. While my results may have been tainted and not 100% reliable, I believe that glucose and starch are present in lemon, and that protein and lipids are not.