All the cells are composing of 4 different types of biological building blocks, or “macromolecules.” These are carbohydrates, lipids (fats), proteins and nucleic acids. But in this lab exercise, we will examine only three of the four macromolecules: carbohydrates, proteins and lipids. Both reducing sugar and starch fall under the category of carbohydrate, but in order to differ the both, Benedict’s and Iodine test is to be tested respectively on the onion sample. All of the experiments, except for the iodine-starch test, require that we use a sample solution. Therefore, if non-liquid sample is used, the sample must first be dissolved, mixed, or mashed in water (a few drops) before any reagent is added.
From the results, the Benedict’s test was successful. Onion juice contains glucose, and of course, glucose would test positive. Onion has both glucose (reducing sugar) and sucrose (non-reducing sugar). Meaning, onion juice contains both mono and disaccharides. All of these are carbohydrate compounds. When the Benedict’s reagent is combined with sugars at a boiling temperature, a color reaction will occur.
Our independent cariable, Benedict’s reagent color is blue. For the positive result of the test, it will change to brick red precipitate, because all monosaccharide as well as some of the disaccharide sugars posses the capacity to reduce alkaline solutions of copper. These monosaccharide and disaccharide are called reducing sugars. A reducing sugar is a sugar with actual or potential aldehyde or ketone groups. Benedict’s solution is a mixture of CuSO4, Na2CO3 and sodium citrate. During the reaction, a complex series of reactions occur, but the visible end result is that Cu2+ is reduced; causing a color change from blue to brick red (this is the positive result). The solution will remain blue if no reducing sugars are present (negative result).Often, students will be surprised that there is sugar in onions because the taste is bitter due to acids present.
The Soya bean is the seed of the leguminous Soya bean plant. Soya foods include tofu, Tempe, textured vegetable protein (chunks, mince etc), Miso, Soya sauces, Soya oil and margarine, and Soya dairy alternatives. Soybeans originate from China. In 2853 BC, Emperor Sheng-Nung of China named five sacred plants -soybeans, rice, wheat, barley, and mille. Soybean is an excellent source of high quality protein (soybeans contain on average 37% protein), is low in saturated fats (also high in polyunsaturated fatty acids), and is cholesterol free. Soy protein is the only vegetable whose protein is complete. Soybean is characterized by high lipid content: it contains about 20% fat.
Soya is the richest natural vegetable food. The dried bean is 18-22% fat, 35% carbohydrate, and one hectare of Soya beans yields 162kg/357 1b of protein (compared with 9kg/20 1b per hectare for beef). There are more than 1000 varieties. Other than that, the protein of soybean contains all the essential amino acids in adequate amounts except methionine and cystine. Since it has majority of essential amino acid it is one of the best vegetarian food item as far protein content is concerned. Main advantage if Soya bean is that the iron absorption is decreased on a Soya bean diet compared to the diet of egg albumin or casein of milk. Supplementing vitamin C can increase the iron absorption. It is also rarely causes conditions like Soya milk goiter and Chinese restaurant syndrome.
In this experiment, the Soya milk is used because the sample to be tested with the reagents needs to be in solution form. Soya milk is an alternative to dairy milk, and it is most commonly made by soaking Soya beans in water which are then strained to remove the fiber. It can also be made from Soya protein isolate or Soya flour. The purpose of this experiment was to determine the chemical composition of certain biological molecules. Four tests were performed: Iodine, Benedicts, Biuret, and grease spot test. Each test was looking for certain characteristics. In this experiment, two tests for carbohydrates will be used. The Iodine test will detect starch, and Benedict’s test will detect reducing sugars (small sugar subunits and individual sugar monomers). Biuret tested for the presence of protein. The grease spot test is tested for lipids.
From the results of this experiment, the Benedict’s, Biuret and grease spot test show positive results, while the iodine test is negative result. This shows that the reducing sugar, protein, and lipid are present, while starch does not. A positive test for iodine was to look for a purple or black precipitation. In the presence of starch the Iodine will break the glycosidic bonds to form the precipitate (an iodine-starch complex is formed causing a color change from reddish-brown to violet-black). Supposedly, the test for Iodine reagent with soybeans should produce a positive result. But unfortunately in this experiment, the soybeans do not contain much starch to proof that there’s starch present, so the reddish brown color remains unchanged. A reducing sugar is a straight chain sugar with free aldehyde or ketone at the end.
A positive test for Benedicts was an orange or brick red precipitate. In Benedict’s test, Benedict’s reagent (a solution of sodium citrate and CuSO4) will produce an insoluble brick red or orangish Cu2O precipitate in the presence of reducing sugars (aldoses and some ketoses) and heat. In the presence of reducing sugars, he benedict’s solution will cause the precipitate because it reacts with the free aldehyde or ketone present at the end of the sugar chain. To detect protein, the Biuret test will be used.
In this test, NaOH and CuSO4 are added to test solutions. In the alkali environment, the Cu2+ ions will oxidize. In the process Cu2+ ions reduced to Cu+1 producing a purple color whose intensity directly correlates with the amount of protein present. A positive test for Biuret was lavender to black precipitation, which depends on the amount of protein present. A positive test for the grease spot test was to see if there’s a translucent patch on the brown paper when it is dried.
In each of these, there were stronger test and some tests provide weaker results. Stronger corresponds to a higher amount of the substance present and weaker correspond with a lower or lowest amount substance present. For example, the potato will have a strong test towards iodine reagent \, whereas the soybean was barely positive. For the Biuret reagent, the soybean was a chunky purple, leading us to believe that it was the strongest result towards protein test. Each of these tests involved a chemical change, except for the grease spot test. In this case the change was a physical one.