Chpater 3- Food Tests Flashcards
What is the test for sugars
The Benedict’s test
How do you test for reducing sugars?
1) reducing sugars are all monosaccharides eg glucose and some disaccharides eg maltose and lactose.
2) you add Benedict’s reagent to a sample (which is blue) and heat it in a water bath that been brought to a boil.
3) if the test is positive it will Form a coloured precipitate - likely to be brick red but can range from green to brick red.
4) the higher the concentration of reducing sugar, the further the colour change.
How can you test for non-reducing sugars?
1) if the results for reducing sugars is negative, there could still be non-reducing sugars present.
2) first you have to break the sugar down into monosaccharides by adding dilute hydrochloric acid and carefully heating it in a water bath that’s been brought to a boil. You can then neutralise it with sodium hydrogen carbonate
3) then just carry out the Benedict’s test as normal.
4) if the test is positive, it will form a coloured precipitate. If the test remains blue, there are no sugars present.
Other than Benedict’s solution, how can you test for glucose?
Glucose can also be tested using test strips coated in a reagent. These test strips are dipped in a test solution and change colour is glucose is present. The colour change can be compared to a chart to see if glucose is present.
These strips are useful when testing a person’s urine for glucose, which may indicate they have diabetes
How do you test for starch?
1) add dissolved iodine in potassium iodide solution to the test sample
2) if starch is present, the sample changes from orange to a blue black colour.
3) if there’s no starch, the solution remains orange
How do you test for proteins?
1) first you add a few drops of sodium hydroxide solution to make the solution alkaline
2) then you add some copper Sulfate solution. This forms biuret solution
3) if a protein is present, the solution turns purple
4) if a protein is not present, the solution remains blue
How do you test for lipids?
1) shake the test substance with ethanol for about a minute, then pour the solution into water.
2) if lipid is present, the solution will turn milky. The more lipid there is, the more noticeable the milky colour is.
3) if there’s no lipid, the solution remains clear.
How can you get more accurate results when determining glucose concentration?
Using a colorimeter
How does a colorimeter work?
A colourimterr is a device that measures the strength of a coloured solution by seeing how much light passes through.
A colorimeter measures Absorbance. For glucose concentrations, the higher the concentration, the lower the Absorbance value. Make sure the colorimeter has a red filter.
What is a biosensor?
A biosensor is a device that uses a biological molecule such as an enzyme to detect a chemical.
The biological molecule produced a chemical signal which is converted to an electrical signal by a transducer.
The electrical signal is then processed and can be used to work out other information.
How do glucose biosensors work?
A glucose biosensor is used to determine the concentration of glucose in a solution. It does this using the enzyme oxidase and electrodes
The enzyme catalyses the oxidation of glucose at the electrodes. This creates a charge, which is then converted into an electrical signal by the electrodes (transducer).
The electrical signal is processed to work out the initial glucose concentration.
What is chromatography?
chromatography is used to seperate stuff in a mixture to identify the different compounds. For example, chromatography can be used to identify molecules such as amino acids, carbohydrates and vitamins.
What are the two types of chromatography ?
Paper chromatography
Thin-layer chromatography
What is the basic set up for chromatography?
1) a mobile phase - this is where molecules can move. In both paper and TLC, the molecule phase is the liquid solvent eg ethanol or water.
2) a stationary please - where the molecules can’t move. In paper chromatography, it is the chromatography paper. In thin layer chromatography, it is a thin layer of solid eg silica get, on a glass or plastic plate.
How can you use paper chromatography to identify amino acids?
1) draw a pencil line near the bottom of a piece of chromatography paper and put a concentrated spot of the mixture of amino acids on it.
2) add a small amount of prepared solvent (a mixture of butan-1-ol, glacial ethanoic acid and water is usually used) to a beaker and dip the bottom of the paper into it. This should be done in a fume cupboard. Cover with a lid to stop the solvent evaporating.
3) as the solvent spreads up the paper, the different amino acids move with it, but at different rates so they spread out.
4) when the solvents nearly reached the top, take the paper out and mark the solvent front with pencil. Then leave the paper out to dry before analysing.
5) Amino acids rent coloured so before you analyse, spray the paper with ninhydrin solution to turn amino acids purple. This should be done is a fume cupboard and gloves should be worn.
6) you can then use Rf values to identify seperated molecules
