Testing for the presence of simple and complex sugars, proteins and fats in the laboratory.

Write-up

by

Cristina Zammit

Class 4.7

Step 1: IntroductionThe aim of this experiment was to establish the presence of various nutrients found in different types of food. In this way we could identify which particular nutrients predominate in each type of food tested. Thanks to these tests it became easier to establish which food has more proteins, carbohydrates and fats. Five experiments were carried out on each of 5 different types of foods, namely: crisps, pasta, milk, bread and cheese. By comparing the results it was easier to classify these food types and determine their nutrient value. These experiments have served as guidelines for choosing the right type of food when one needs a particular nutrient. Getting knowledge of which nutrients are present in different food types help us eat a balanced and healthy diet.

Testing for simple sugars using Benedict’s Solution

Step 2: Methods

Experiment 1: Test for reducing sugarsStep 1: A few drops of Benedict’s solution were added to the test substance.Step 2: The mixture was heated in a water bath (beaker of boiling water) for several minutes.

Experiment 2: Test for starchStep 1: A few drops of brown iodine solution were added to the test substance (the food may have been boiled in order to extract the starch since it does not dissolve in cold water. If this is done, allow the solution to cool before testing with iodine)Experiment 3: Biuret test (Test for Proteins)Step 1: A few drops of colourless concentrated sodium hydroxide were added.Step 2: Then a few drops of blue copper sulphate were added.Experiment 4: Test for lipidsStep 1: Ethanol (alcohol) was used to extract the lipids from the food sample (instead of water).Step 2: Water was added to the extract (an equal amount of ethanol and water were used).

Diagrams:

Testing for fats, proteins, simple sugars and starch

Step 3: Results

These are the results of the four experiments:

Type of Food

Benedict’s solution

Iodine solution

Sodium hydrox. +

copper sulphate

Alcohol + water

Crisps Orange to red Blue Black Light blue Very white cloud

Milk Orange to red Yellow Purple White cloud

Bread Orange to red Blue Black Blue White cloud

Pasta Yellow to Orange

Blue Black Light blue White cloud

Cheese Green Orange Purple White Cloud

Step 4: Conclusion and EvaluationFrom these experiments it could be concluded that all the foods have nutrients but some either lack completely particular nutrients or else have particular nutrients more than others. In the first experiment, the one with the Benedict’s solution, it is evident that all the foods used for the experiments have reducing sugars (a kind of carbohydrate) but the amount varies from food to food. The food which had the least presence of reducing sugars was cheese. In fact, its colour change stopped to the green level. Immediately after cheese, there was the pasta, with a colour change of yellow to orange. The foods with most abundant reducing sugars were: the crisps, milk and bread, with a colour change of orange to red. This proved that a large presence of reducing sugars was present here. In the Iodine solution experiment, that is, the test for starch, the milk and cheese, unlike the other three food types, did not turn blue black. So this clearly proved that milk and cheese lack the presence of starch. In the Biuret test, that is, the test for proteins, the colour change which indicates the presence of this nutrient would be such that the solution turns purple. The crisps, bread and pasta did not show this colour change, so this concludes that these foods lack proteins. Finally in the test for lipids, all foods showed the presence of lipids. All the foods in their solution had a white cloud.On considering the results obtained in these experiments, one has to put into consideration that there may have been some sources of error. These might have been caused by several factors. Namely, in the first experiment, the one with the Benedict’s solution might have undergone a difference in temperature of water, since the same beaker of water was used for all the food types. The temperature change might have influenced the colour change obtained. In the iodine experiment to test the presence of starch, the amount of iodine

drops and even the intensity of the drops of iodine might have given some imperfections in the results. Thus, the colour change might not have been one hundred per cent accurate. In the test carried out for proteins, the Biuret test, the amount of drops of concentrated sodium hydroxide and copper sulphate solution might have been inaccurate, though all the necessary care was taken. The fact that the drops were repeated for the various food types tested might have caused some margin of error. This can also be said for the test for lipids, since here drops of ethanol and water were involved for all the foods tested. All in all, for all the food types one must also note that the amount of food tested varied because the food types were not weighed beforehand. This might have given some misleading results in the experiments carried out. These food tests have clearly indicated that the different food we eat may be rich in some nutrients and may also lack completely others, thus indicating that a varied diet with the right amounts is necessary for a healthy life.