Quantitative test for starch and reducing sugar present in apple and pear
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Biology TAS laboratory report - experiment 1 16th September, 2005 Name (class number): Julie Poon (25) & Fifian Chiang (4) class: F6C Title: Quantitative test for starch and reducing sugar present in apple and pear Aim: To find and compare the amount of starch and reducing sugars present in apple and pear. Principle: In testing the amount of reducing sugars, the fruits had to be grounded into juice. Since reducing sugars are soluble, the sugar present in the fruit would dissolve into the juice. We can dilute and control the volume of fruit juice when doing the test, hence it is easier to compare the quantity of reducing sugar in the same amount of apple and pear. To test for reducing sugars, Benedict's solution is used. Benedict's solution contains blue copper (II) ions that reduce to red copper (I) oxide precipitates when added to reducing sugars. By comparing the amount of precipitate present at the end of the test, we can find out which fruit contains more reducing sugars.
The number of drops of iodine solutions was recorded. 3. the piece that needed more iodine solution contains more starch. Precautions: When testing for reducing sugars, test tubes used should be uniform, so that it would not affect the observation and comparison of the amount of precipitate. Also, all solutions should be mixed thoroughly before boiling so as to ensure good results. For the starch test, the apple and pear piece must be of exactly the same size. The pieces should not be to too thick so that iodine solution can penetrate into the fruit pieces. Fruit Color of precipitate Apple Red Pear Orange Result: Benedict's test: Unfortunately, the test was not successfully done. The fruit juices were not dilute enough. Solutions in both test tubes turned into precipitate completely. No blue background was seen, thus it was impossible to tell which contains a larger amount of precipitate. Since time was limited, we were not able to repeat the experiment.
Graduated droppers were used to measure the volume instead. This might affect the accuracy of the experiment. The juices might not be pure fruit juices as the apple and the pear rusted in the air, also the juices might be contaminated while squeezing with the same filter paper (teabag). The concentration of reducing sugar then might not be true for the whole fruit. Also, the degree of rusting in apple and pear was not the same. Thus this might alter the result. In the iodine test, the size of drops of iodine solution might be different, thus the measurement might be incorrect. Conclusion: The apple was determined to contain a larger amount of reducing sugars than the pear did. Possible improvement: Use a pipette for the transferal of solutions and juices to ensure equal volume in both independent variables. Cut away the rusted surface of the fruits before grounding them into juices. Use the same dropper to add iodine droplets to the fruit, so as to minimize the error.
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