Carbohydrate is generally the supply of energy for our body. Carbohydrates can be classified as monosaccharide, disaccharides, and polysaccharides. Molisch Reagent, a mixture of α-napthol dissolved in ethanol, can be used to detect the presence of carbohydrates where a purple interface will formed. Reducing sugars can be determined by using Benedict’s reagent, a solution of copper sulfate, sodium hydroxide, and tartaric acid. Reducing sugars will give a green, brick-red, or yellow precipitate while non-reducing sugar does not. Iodine Test can be used to identify carbohydrate where a formation of blue solution will appear.
The presence of protein can be detected by using Xanthoprotic, Biuret and Ninhydrin Tests. Xanthoprotic Test is used to detect amino acids and reaction of Biuret reagent, a combination of sodium hydroxide and copper (II) sulphate, will turn the blue solution to purple. Existence of protein forms a blue or purple precipitate with Ninhydrin solution, 2,2-dihydroxyproline-1,3-dione.
Fats or lipids can be identified using the grease spot test. The presence of fats will make a paper become translucent.
Detecting carbohydrates
As carbohydrate can be classified into monosaccharide, disaccharide and polysaccharide thus the reaction of the samples used with Molisch reagent should give a positive result except for distilled water. 1% glucose solution (monosaccharide), 1% sucrose solution (disaccharide), 1% lactose solution (disaccharide), and 1% starch solution (polysaccharide) should have form purple interface. This error may happen due to incomplete mixing of samples and Molisch reagent. The concentrated sulphuric acid used functioning as a dehydrating acid where samples containing carbohydrate will be dehydrate to distinguish each samples tested. The reaction of α-napthol and H2SO4 yields a purple colour ring interface.
Benedict’s test have shown that 1% sucrose solution gave positive result when react with Benedict’s reagent. However, Benedict’s Test is a test that reveals the presence of aldehydes. Usually, sucrose which also proved a non-reducing sugar will not react to this reagent because the structure of sucrose does not form an open chain which contains an aldehyde group. Benedict’s Test recognizes reducing sugars such as glucose and lactose based on their ability to lessen the copper ions to copper oxide. The brick-red precipitate observed is because of the copper oxide.
1% starch solution immediately turns to blue solution when iodine solution is added to it. This is due to the amylose in starch and basically iodide element in the presence of iodide ion will give such characteristic during reaction.
Detecting proteins
Xanthoprotic Test has shown that only 2% egg-albumin solution reacts positively where the colour of the solution has changed from yellow to orange after NaOH is added. This indicates that 2% egg-albumin solution contains amino acid. Distilled water, acts as control, turned to light purple colour due to the reaction of HNO3 and NaOH.
All the samples do not test strongly basic at first after NaOH is added. Thus, the addition of more NaOH (a strong base) will deepen the colour to orange. The yellow stains on the skin caused by nitric acid are the result of the Xanthoprotic reaction[4].
While Biuret test, a test to detect the presence of peptide bonds, shows the same where yellow colour of 2% egg-albumin solution turns to purple precipitate in the presence of proteins. The control and other samples on the other hand turned to blue and turquoise colour due to the presence of copper (II) sulphate.
Whereas Ninhydrin reagent gave positive test to 2% egg-albumin and 2% glycine solution. Ninhydrin is basically a test to detect amino acids and proteins with a –NH2 group. The presence of these in both 2% egg-albumin and 2% glycine solution has formed purple-blue complex solution.
Detecting fats
Grease spot test determine the presence of fat. After leaving the filter paper dry when some drop of oil is placed on it, fat will come out and stain on the paper. The stained left are the grease spot. Food with more fat will produce larger grease spot.
References :
[1] Denniston, K.J.; Topping, J.J.; Caret, R.L. General, Organic and Biochemistry 5th Ed. McGraw-Hill Education Asia
[2] Bobick J., Balaban N., Bobick S., and Bridges, Laurel. Biology Q&A. The Handy Biology Answer Book. 2004. Visible Ink Press
[3] http://www.madsci.org/posts/archives/2000-02/951791254.Ch.r.html
[4] http://www.uwplatt.edu/~sundin/351/351h-pro.htm
No comments:
Post a Comment