Food Testing Carbohydrates
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Food Testing Carbohydrates Document Transcript

  • 1. Food Testing Carbohydrates You are going to carry out a series of tests for finding the presence, and type, of carbohydrate in food products. There are a number of stages here so be sure to read all instructions carefully and follow the procedures as they are set out. You are expected to answer the in-text questions as you go along. Preparing food stuffs You need to prepare the materials to give optimum testing conditions. Any large items of food (potato for example) should be chopped as small as possible or crushed in the pestle and mortar. Dried food stuffs may need suspending in liquid. Q1. What foods are you using and how have you had to prepare them? Food Preparation Needed Test one: The Benedictʼs test for Reducing Sugars Safety Eye protection – there is some risk of hot liquids spitting when heating test tubes Benedictʼs reagent: LOW HAZARD Take care when cutting food with a scalpel LET TRE K N O W I F Y O U H AV E A N U T ALLERGY. You should not carry out tests with nuts if you are allergic to Te s t t u b e s , t e s t t u b e Distilled water (in a rack wash bottle) Te s t t u b e h o l d e r B e n e d i c t ’s r e a g e n t i n a dropper bottle Thermostatic water bath 1% glucose solution Thermometer Freshly made 1% sucrose solution Scalpel (or a pestle and mortar) Foods Tile L a rg e p l a s t i c w a s t e beaker Spatula 250 cm3 beaker Pasteur pipettes
  • 2. Food Testing Carbohydrates suspension suggests a low concentration. Confirmation of the absence of reducing sugars or a concentration below the sensitivity of Confirmation of a reducing sugar: A colour change from a blue solution to a green/yellow/orange/brown or brick-red suspension. A green Method 1. Set up the water bath to just below boiling point 2. Label some test tubes for the substances tested 3. Add 2 cm3 of each test solution or suspension to labelled test tubes (or 1 Possible results cm depth of chopped food material and 2 cm3 of distilled water). One test tube should contain the 1% glucose solution. 4. Add ten drops of Benedict’s reagent (about 0.5 cm3) to each test solution the test for reducing sugars using Benedict’s reagent: when there is no colour change from the blue solution. (without the dropper pipette touching the inside of the test tube). 5. Use the test tube holder to transfer the test tube with glucose into the water bath. Observe the colour changes in the test tube over two minutes of heating at boiling point. Return the test tube to the test tube rack. Make a note of your observations in the spaces that are provided in Table 1. 6. Repeat the test for all of your samples. Material tested Observations Deductions 1% glucose solution A change from a blue solution Confirms that glucose is to a suspension a Distilled water (control) 1% sucrose solution Table 1 A table of observations and deductions from the test for reducing sugars using Benedict’s reagent
  • 3. Food Testing Carbohydrates Questions 1. In the test for reducing sugars using Benedict’s reagent on glucose, (a) Use pages 20 and 21 of the textbook to help you answer these questions where does the colour change begin in the blue solution and (b) why does it occur at this position first? ............................................................................................................................... .............................................................................................................................. 2. When the test is carried out by heating the test tube directly, there can be superheating of the test solution which can be ejected violently from the test tube. Why is this unlikely to occur when heating the contents of the test tube within a water bath? .............................................................................................................................. .............................................................................................................................. Discussion 1. Which of the materials tested (a) contained reducing sugars and (b) did not contain reducing sugars? .............................................................................................................................. .............................................................................................................................. 2. Suggest one reason why there could be some reducing sugar in a test solution that gives a negative result in the test for reducing sugars using Benedict’s reagent. .............................................................................................................................. .............................................................................................................................. The Details Examiners often expect you to know the following in relation to this test: 1. Why does the Benedict's reagent turn red in the presence of reducing sugar? 2. Why do you have a glucose and water sample as well as the food samples? 3. How precise is this investigation? How could you colour in these test-tubes to reflect expected results
  • 4. Food Testing Carbohydrates Test two: Serial dilution: finding the minimum sensitivity of Benedictʼs and Clinistix tests Here we are going to learn how to make very Safety Wear eye dilute solutions through a technique called serial protection – there is some risk of hot dilutions. We are going to dilute a 10% solution of glucose through a series of dilutions until we have a 0.001% glucose solution. liquids spitting You are then going to compare the sensitivity of when heating test tubes Benedict’s the Benedictʼs test with the clinical diagnostic sticks, Clinistix. This investigation is split into three parts. You will reagent: LOW need to make observations and record results HAZARD Take care with from each section before you can move onto the next. Keep your equipment set up until you have completed the comparison between the two tests. glassware Equipment and materials Eye protection White tile Labelling equipment Waste beaker Test tubes (10), bung (for test Paper towel tubes) Water Bath Test tube holder Distilled water wash bottle 10 cm3 pipette and pipette filler 1 cm3 pipette and pipette filler Clinistix test strips Step 1: Making serial tenfold dilutions of a 10% glucose solution ✴ Label four test tubes, so that you can keep track of which dilution is which ✴ Add 9.0 cm3 of distilled water to each of the four labelled test tubes. ✴ Add 1.0 cm3 of the 10% glucose solution to the first test tube. Put a clean bung into the neck of the test tube. Hold it in tightly and shake the test tube carefully ten times through 30 cm in order to get an even mixture. This process achieves a tenfold dilution. ✴ Using a clean pipette and a clean bung, repeat steps 2 to 3 for the three remaining test tubes, on each occasion transferring 1.0 cm3 of your most recently diluted glucose solution into the next test tube that contains 9.0 cm3 of distilled water.
  • 5. Food Testing Carbohydrates Step 2: Devising a standardised test For you to determine the minimum sensitivity of the test for reducing sugars with Benedictʼs reagent, you need to conduct the test in precisely controlled conditions, and you need a control. Pointers towards devising a standardised test In a trial, the mass of Benedict’s reagent that was dispensed by two Work together to discover the answer to methods was compared. Method of dispensing the Mass of the dispensed Benedict’s reagent/g Benedict’s reagent Individual values Mean 10 drops from a dropper pipette 0.34 0.37 0.39 0.35 0.39 0.37 (to 2 d.p.) 0.5 cm3 from a 1 cm3 glass pipette 0.58 0.59 0.58 0.59 0.59 0.59 (to 2 d.p.) these questions 1. From the data in the table, suggest the advantage in using measured volumes from a 1 cm3 glass pipette rather than 10 drops from a dropper pipette in a standardised Benedict’s test. ...................................................................................................................... ...................................................................................................................... 2. List other variables that you think should be kept constant for a standardised test for reducing sugars using Benedict’s reagent. .................................................................................................................................. .................................................................................................................................. 3. Suggest a control for the experiment to find the minimum sensitivity of the test for reducing sugars using Benedict’s reagent. ...................................................................................................................... Plan your standardised test here..... get the method checked before going futher...
  • 6. Food Testing Carbohydrates Glucose Observations Deductions concentration/% 10 1.0 0.1 0.01 0.001 Conclusion for the test for reducing sugars using Benedict’s reagent What was the lowest glucose concentration where reducing sugars were shown to be present (when compared with the control and against a white background)? ................................................................................................................................... Comparing the minimum sensitivity of the test using Clinistix and the test using Benedict’s reagent Now test a fresh sample of the least concentrated glucose solution that gave a positive reaction with the test using Benedict’s reagent with the Clinistix test strip. Clinistix contains glucose oxidase and peroxidase in the coloured pad and is used to detect glucose in urine as a preliminary test for diabetes. Discussion 1. Was Clinistix able to detect the same low concentration of glucose as the test using Benedict’s reagent? Could it detect lower values? .................................................................................................................................. ...................................................................................................................................
  • 7. Food Testing Carbohydrates Food test 2: biochemical tests for non-reducing sugars and starch Aims In this practical work you will carry out two Safety Wear eye qualitative food tests to find out whether certain protection – there is types of food molecules occur in various food some risk of hot samples. For example, does starch occur in liquids spitting when potatoes? You will test materials that should give heating test tubes positive results and test controls that should give Benedict’s reagent: negative results. LOW HAZARD Dilute hydrochloric acid (0.1 Equipment and materials mol dm–3): LOW As for the test for reducing sugars plus three items: HAZARD but may  Dilute hydrochloric acid (0.1 mol dm–3) cause harm if it enters  Sodium hydrogencarbonate powder the eye or a cut Sodium Method hydrogencarbonate: 1. Do this test only after a negative Benedict’s test. MINIMAL HAZARD IN 2. Label the tops of your test tubes. THIS TEST Take care 3. Use a Pasteur pipette to dispense 2 cm3 of a fresh when cutting food with sample into a test tube (or finely chopped/ground food a scalpel LET TRE to 1 cm depth and 2 cm3 of distilled water). KNOW IF YOU HAVE A 4. Use a dropper pipette to add 10 drops of dilute NUT ALLERGY. You hydrochloric acid (HCl) to the sample. should not carry out 5. Use a test tube holder to transfer the test solution to the tests with nuts if you water bath. You can save time by having up to five test tubes in the water bath. Heat to boiling point for two are allergic to them. minutes. This is the stage when acid hydrolysis may occur Confirmation of a non-reducing sugar, for example, sucrose, in the original solution: a brick-red/brown/ sugar (and the absence of reducing sugars) in the 6. Cool the test tube in cold water (in a beaker). orange/yellow or green suspension at this stage Confirmation of the absence of a non-reducing 7. Sodium hydrogencarbonate (NaHCO3) powder is needed to make the solution alkaline. With a spatula add sodium hydrogencarbonate to original solution: a blue solution remains. the solution until the fizzing stops. The solution must be alkaline for the Benedict’s reagent to work. You can confirm that the solution is alkaline by using a Pasteur pipette to take a sample and adding it to universal indicator paper on a tile; compare the colour with that in the chart. 8. Add 10 drops of Benedict's reagent, about 0.5 cm3, to the test solution. 9. Heat the test solution to boiling point in a water bath and continue heating for two minutes. Results 10. Observe the colour changes in the test tubes over two minutes of only. heating at boiling point. Record your observation in Table 1.
  • 8. Food Testing Carbohydrates Material tested Observations (after acid and heat, Deductions etc.) 1% sucrose solution Distilled water (control) 1% starch suspension The explanation of the test for non-reducing sugars using Use Pages 22 and 23 in your textbook to help you Benedictʼs reagent Sucrose is a non-reducing sugar that occurs naturally in plants. Sucrose is a disaccharide consisting of glucose and fructose units linked by a glycosidic bond. It is not a reducing sugar because the reducing groups of glucose and fructose are tied up in the glycosidic bond. When a sucrose solution is acidified and heated; the hydrogen ions of the acid catalyse the hydrolysis of sucrose to glucose and fructose, which are both reducing sugars, and cause the reduction reaction. Question Which of the materials you tested: contained reducing sugars? ……………………………………………………………...…………………………… contained non-reducing sugars? …………………………………………………………………………………………… contained neither reducing sugars nor non-reducing sugars? …………………………..……………………………………………………………… Discussion A student did not read the practical schedule carefully enough and found a brick-red suspension for the test for reducing sugars and the test for non- reducing sugars. a) What can be concluded from these results? ............................................................................................................................... ............................................................................................................................... b) What cannot be concluded from these results? ............................................................................................................................... ...............................................................................................................................
  • 9. Food Testing Carbohydrates Safety Wear eye protection Iodine solution – avoid contact The iodine test for starch with skin and clothes Take care when cutting food with a scalpel Equipment and materials  Eye protection  Spatula  Labelling equipment  Distilled water  Scalpel and tile  Iodine (in potassium iodide) solution  Pasteur pipettes  1% starch suspension  Dimple tile  Food materials, for example, potato and onion Method Results Confirmation of coloration Confirmation 1. Label individual dimples on the dimple tile with the foods to be tested. starch: a yellow iodine colour (or just no blue- 2. Use a clean Pasteur pipette to dispense a few drops of the test starch: a blue-black solution into a labelled dimple. For solid foods, use the scalpel to cut of the absence of the food sample into small pieces on a tile and use a clean spatula to transfer some of them into a labelled dimple. black colour) 3. Use the dropper pipette to add two drops of iodine solution (iodine in potassium iodide) to the food samples. Table 2 Observations and deductions from the iodine test for starch Material tested Observations Deductions 1% starch suspension The explanation of the iodine test for starch Starch contains both amylose and amylopectin molecules. The coils of its amylose molecules take up iodine molecules from the iodine solution and form the blue-black, starch–iodine (polyiodide) complex. Discussion 4. Which of the materials you tested a) contained starch? ....................................................................................................................................... b) did not contain starch? ....................................................................................................................................... 5. How could you show that a yellow liquid contains iodine? .......................................................................................................................................