Identification of Food Constituents Essay

Method (testing for reducing sugars) 1. Add 3cm? of whole milk, by using a pipette or syringe to the test tube. 2. Add 5cm? of Benedict’s reagent and place it in the boiling water bath for 8 minutes. Do the same for semi-skimmed milk and skimmed milk. 3. Once all 3 of the test tubes are left to cool in the air, observe the colours. It will be a good idea to set up a range of colour standards from glucose concentrations of 1%, 2%, 3%, 4% and 5% so that you can relate the colours observed to these concentrations. 4. A positive result would be from green to yellow to brick-red colour. Method (testing for non-reducing sugars) 5. Make up the same solution as step 1 but this time, adding 3cm? of dilute hydrochloric acid to break the glycosidic bonds between the monosaccharides. 6. Then add 3cm? of sodium hydroxide solution to neutralise it. 7. Add 5cm? of Benedict’s reagent and place it in the water bath for 8 minutes. 8. Once it’s left to cool, it should now turn brick-red colour. 9. The concentration of a non-reducing sugar can be estimated by first adding a drop of 10% invertase (sucrase) concentrate to 2cm? of the solution to be tested and leaving for 30 minutes at room temperature. The solution is tested for the presence of a reducing sugar. This method is preferable to acid hydrolysis. Method (testing for starch) 10. On each of the three types of milk, just add a few drops of iodine which is dissolved in potassium iodide solution. 11. The sample should change from browny-orange, to a dark, blue-black colour. Method (testing for proteins) 12. Place 2cm? of the three different types of milks on each tube. 13. Then add 2cm? of Biuret reagent and you should see a purple-violet colour developing. The intensity of it is proportional to the protein content. Method (testing for fats) 14. Add 3cm? of the three different types of milk on each test tube and 3cm? of water. 15. Place 1 drop of Sudan III to each test tube and shake gently to mix. 16. Using a microscope, a slide and a cover slip, identify any emulsion of red fat droplets. 17. Alternatively, you could add a drop of each of the milk on a filter paper and see if there is a translucent stain for a positive result. Results table Solution (Milk)TestObservationsConclusion SkimmedBenedict’sLime green (lightest)A slight amount of monosaccharides or reducing sugars present Semi-skimmedBenedict’sLime green (lighter)A slight amount of monosaccharides or reducing sugars present WholeBenedict’sLime greenA slight amount of monosaccharides or reducing sugars present SkimmedInvertaseYellowish-greenHardly any monosaccharides or reducing sugars present Semi-skimmedInvertaseYellowish-greenHardly any monosaccharides or reducing sugars present WholeInvertaseYellowish-greenHardly any monosaccharides or reducing sugars present SkimmedBiuretViolet purpleProtein present. Semi-skimmedBiuretPurpleLot’s of protein present WholeBiuretLight purpleProtein present Conclusion If there were to be a fair amount of monosaccharides to be present all 3 different types of milk, then we would surely see a brick-red precipitate formed when adding the Benedict’s reagent. But according to my range of colour standards from glucose (monosaccharide) concentrations, the lime-green colour given out from each of the 3 milks shows us that it does contain a minor amount of monosaccharides (reducing sugars). Adding a drop of invertase normally should break the glycosidic bonds that are holding the disaccharides together to form monosaccharides. But my results show that it’s a yellowish-green colour instead of a brick-red colour after adding Benedict’s reagent. This shows us that there is hardly any disaccharides present which I thought there would be as lactose, a disaccharide, is mostly present in milks. But this result may have a different view on that. The fact that all 3 milks turned purple after adding Biuret reagent assures us that there is protein present. If there is protein present, that means there is starch present too because starch and proteins are polysaccharides. Evaluation It is noticeable that I haven’t done the test for starch and fats. This is simply due to the fact that I run out of time. Using a 5cm? micro syringe would be more accurate than a pipette. When a precipitate is settled, I could have used a ruler to measure it out (in mm) instead of just using my eyes. Even better, using colorimeter would have provide accurate measurements on the amount of colour present and therefore, give us an indication of how much of the food constituents were present.