Food tests for the Leaving Cert Biology course. A series of four practicals which you did when you studied the chapter on nutrition. You must know each of these four tests really well, including all the chemicals used and any colour changes.
So the first test is the test for reducing sugar and you know that glucose, fructose and galactose, those monosaccharides, are reducing sugars and maltose and lactose, disaccharides, are also reducing sugars. But sucrose is not a reducing sugar. Remember, that too is a disaccharide. So we tested glucose solution. We know it's a monosaccharide and we want to prove that it is a reducing sugar.
And the chemical that we're going to use, or the reagent, is Benedict's reagent or Benedict's solution. And it's a distinctively blue colour. The control we used in the practical was distilled water and we tested a 1% glucose solution.
Into our control test tube, we will add... a volume of distilled water and into our test solution we're going to match that with an equal volume of glucose solution. So with the control and the test all ready to go we add the reagent which is benedict solution.
An equal volume of this is added to both test tubes and the volume matches that of the solutions already there. Should end up with two test tubes with equal volume one with distilled water and benedicts and the other with glucose solution and benedicts and they are both blue. Both test tubes are then transferred to a hot water bath where they are to be heated for five minutes and the key word here is heat, not boil.
Within a few moments of heating you can see the glucose on the left has changed colour whereas the distilled water on the right has not. And the glucose has this very distinctive red colour, brick red, and the distilled water in comparison stays blue, the blue colour from the Benedict solution. A change to So brick red from blue indicates a positive for reducing sugar, whereas remaining blue is negative.
The next test is the test for starch. And remember that starch is a polysaccharide, so you could be asked what is a test for a polysaccharide. The test for starch is the iodine test, basically using iodine solution. We tested starch solution and our control was distilled water, but perhaps you used different foods.
Into the test tube labelled control we placed a volume of distilled water, which was matched in the other test tube. with an equal volume of starch solution. A few drops of iodine solution was added to both test tubes.
Very quickly a positive result was indicated in the starch solution which had turned blue-black in colour. Distilled water yielded a negative result. It had not changed colour, it had remained that sort of orangey colour associated with the iodine. The next test is the test for proteins and it's known as the burette test.
We used two solutions, sodium hydroxide and dilute copper sulfate solution. We tested milk but egg white is just as effective and once again our control was distilled water. Into the test tube label control we add distilled water about 2 cc's and an equal volume of sodium hydroxide. Then we add a few drops of copper sulfate solution.
Into the test solution we add 2 cc's of milk and an equal volume of sodium hydroxide and a few drops of copper sulfate solution and at this stage both our test tubes are blue. So the test tube that contained the milk Well, it has gone a distinctively purple colour, so milk tests positive for protein. So just think of P for purple, P for protein. Whereas when you look at the control, the distilled water, there was no colour change.
It had remained that sort of blue colour. Although the picture is not great, you can see the blue there at the bottom. The final test is the test for lipids or fats.
The easiest test for lipids is known as the brown paper test. However, there is another, it's known as the Sudan 3 test. Once again, distilled water is going to act as the control and the test food is oil, vegetable oil.
To start the test, cut two pieces of brown paper roughly the same size. To the first you're going to add two drops of water and to the second two drops of oil and then you're going to allow both pieces of paper to dry. When both had dried it was time to view the results. When you look at the piece of paper with the oil on it, even after drying there is a permanent translucent stain, a stain that allows light through. Whereas in comparison, the water, there was no stain, there was no permanent stain left at all on the paper, it had dried perfectly.
The water had tested negative for lipids, but the oil had tested positive for lipids. And we must state that a permanent translucent stain indicates a positive result for lipids. This was a very basic summary of the food tests and remember there are variations.
You can use different types of foods. One of the big mistakes that students often make is in colour changes. They forget to say what the colour was initially and what it changed to.
If you tested different food samples make sure you can explain how you prepared those samples. That's very important. So the very best of luck with your revision and with your exams.
Make sure you're doing lots of past papers, you're using your textbook and you're listening to your teacher. These videos are not intended for commercial use and were not made for monetary gain. The following icons from The Noun Project were included in this video.