ATS GCSE Biology - RP4 Qualitative reagents to test food Key words

• Biuret negative result

  no protein presentblue

  
• Biuret positive result

  proteins will turn the solution pink or purple

  
• protein test method

  1. put 2cm3 of filtrate solution into a test tube.2. add 2cm3 of Biuret solution to the solution in the test tube3. shake gently to mix4. note any colour change

  
• Sudan III negative result

  if no fat is presentcolour will spread through the whole solution

  
• Sudan III positive result

  if fat is presenta red-stained oil layer will separate out and float on the water surface

  
• lipid test method

  1. use a pestle and mortar to grind up a small sample of food2. transfer the ground up food into a small beakerthen add distilled water3. stir the mixture so that some of the food dissolves in the water DO NOT FILTER4. half fill a test tube with some of this solution. 5. add 3 drops of Sudan III stain to the solution in the test tube shake gently to mix
• carbohydrate results table headings

  name of food tested colour produced with Benedict's solutioncolour produced with iodine solution
• iodine negative result


  no starch presentorange or brown

  
• iodine positive result


  if starch is presentblack orblue-black

  
• Benedict's solution negative result

  blue

  
• Benedict's solution positive result

  if a reducing sugar (such as glucose) is present, the solution will turn greenyellowbrick-red the colour depends on the sugar concentration

  
• carbohydrate tests method

  1. half fill a test tube with filtrate 2. add 10 drops of Benedict's solution to the solution in the test tube3. put hot water (not boiling) from a kettle in a beaker 4. put the test tube in the beaker for five minutes5. note any colour change 6. take 5 ml of the filtrate into a clean test tube7. add a few drops of iodine solution and note any colour change8. record results in a table

  
• filtrate


  filtered solution

  
• preparing food method

  1. use a pestle and mortar to grind up a small sample of food2. transfer the ground up food into a small beaker. Then add distilled water3. stir the mixture so that some of the food dissolves in the water4. filter using a funnel with filter paper to obtain as clear a solution as possiblethe solution should be collected in a conical flask
• equipment


  food to be testeda pestle and mortara stirring rodfilter funnel and filter paper2 x beaker, 250 mla conical flask2 x test tubeBenedict's solutioniodine solutionkettle for boiling watera thermometersafety goggles
• risk assessment


  safety goggles should be worn when carrying out the testswash off spills on skin immediately. take care with boiling water.Biuret solution contains copper sulfate, which is poisonous, and sodium hydroxide, which is causticSudan III contains ethanol, which is highly flammable, keep the solution away from naked flamesensure that there is no eating or drinking during testing

  
• testing for carbohydrates

  use iodine and Benedict's solution

  
• water bath

  use a beaker of hot water as a water bath

  
• filtration

  fold the filter paper in half and then half againtake one piece of filter paper one way and three the other to then filter the solution

  
• high carbohydrate foods

  potato, bread, cooked noodles, biscuits, sugar, apples, flour, corn starch

  
• high lipid foods

  olive oil, sesame seed oil, grape seed oil, margarine, butter, lard, milks (full fat, semi-skimmed, skimmed), egg white solution, egg yolk solution

  
• high protein foods

  milk, yogurt, cheese, meat, tofu, apple, potato, yeast, cooked beans, eggs