A Study On Food Tests Biology Essay

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The experiment is to use some chemical reagents to find out what is contained in various food substances, bimolecular.


To identify what is contained in the test solutions is the purpose of these experiments.

Some definitions of the test substances should be given. Firstly, starch. Starch, (C6H10O5)n, a tasteless, white solid carbohydrate, occurring in the form of minute granules in the seeds, tubers, and other parts of plants, and forming an important constituent of rice, corn, wheat, beans, potatoes, and many other vegetable foods. Also, it is made of polyglucose. (answers.com 2009)

Secondly, it is stated that “Lipids are a somewhat loosely defined group of molecules with one main chemical characteristic: They are insoluble in water (Losos, Mason, Singer, 2008, P53).” Lipid is a material which stores long-term energy, such as animal fats, and also oils, such as olive oil, and coconut oil. Besides, waxes, for example beeswax and earwax are lipids as well. (Losos, Mason, Singer, 2008)

Thirdly, glucose's definition is given. Glucose also called corn sugar, is the most widely distributed in nature and the most important, it is a simple sugar, with multi-hydroxy aldehydes ( baike baidu 2009).

The fourth one is the definition of reducing sugar. It is a monosaccharide or disaccharide sugar which can share the electrons to other molecules and therefore, it can act as a reducing agent (Encyclopedia.com 2009). Also, reducing sugars can be tested by Benedict's test, and it can compare with the non-reducing sugar (Encyclopedia.com 2009).

The fifth one is biuret reagent. It is made of copper sulfate and sodium hydroxide. The blue reagent turns violet in the presence of proteins, and changes to pink when combined with short-chain polypeptides (biosci.com 2009).

Biuret ReagentAvailable at:


The last one is protein. The base sequence of nucleotides in the DNA coding for the protein determines the specific order of the protein, a large molecule composed of one or more chains of amino acids (medterms.com 2009).

These experiments are good for understanding substances and their change in properties under chemical condition, such as its colour, states and chemical function. Besides, the material can be identified from seeing what happened after it reacts to another material.

Also, there are some backgrounds of the tests should be given:

Starch complexes with Iodine solution and the blue colour occur.

I+ KI I + K

The reason is: starch is a macromolecule which consists of glucoses. There are two types of glucose, open chain and closed ring. Glucose which form is chain will not change its color when it reacts to iodine, only the ring form will. The reason is I binds into the ring of glucose, the forces between molecules make these two materials combine. This structure makes all wavelengths of light except blue get absorbed, therefore, the blue color occurs (zhidao baidu 2009).

The principle of reducing sugar is, according to the chemical equation:

Cu + aldehyde Cu+ acid

The aldehyde is contained in glucose, therefore, the reaction occurs. The colour occurs from light blue to red precipitate.

Here is the theory of protein test. It is claimed (baike.baidu 2009) in the alkaline solution of NaOH, the biuret reacts with copper ions (Cu), a purple complex result. Therefore, the reaction between protein and biuret reagent occurs because of the combination of protein peptides (baike baidu 2009).

Besides, ethanol and lipid can dissolve, and Van Der Waals force exists in them, on the other hand, there are hydrogen bonds in water and ethanol, it is stronger than Van Der Waals force, therefore, the lipid was threw out.


There were two simple items used in these experiments.

  1. Pipette
  2. Test tubes

In this practical, starch solution, Glucose solution and protein solution (egg albumen), and lipid (peanut oil) were used as samples.

To ensure the experimental' results were accurate, the procedure was read carefully before the practical have been done, such as using a pipette to measure the 1cm of the test solution, also, careful experimental practice was performed throughout.

The first test is for starch. 3 drops of iodine solution was added to 1cm of starch solution in a test tube. The change was observed.

The Second test is for reducing sugar. 2cmof the glucose solution was warmed to 60℃ in a water bath, and 5cmof Fehlings reagent was added.

The third test is for protein. Biuret reagent A was added to 2cm of the test solution (egg albumen) until it was in excess. Then the tube was held at a 45º angle and a pipette was used to trickle in about 1 cm of Biuret reagent B.

The fourth test is for lipids, or oils. 2 cmof test solution (vegetable oil) was added to 2 cm of ethanol in a test tube. It was shaken very vigorously. Then 2 cm of cold water was added.


The table shows the observation of the experiments.

Test Samples


Iodine solution, Starch solution

From clear to blue-black colour

Fehlings Reagent, warmed Glucose solution

After a few minutes, from clear light blue to red, with some precipitate

Biuret Reagent A, egg albumen until it is in excess

Biuret Reagent B

No change, clear solution

From clear to violet colour

Ethanol, lipid

Three layers

Top is white liquid, not clear

Middle is yellow, cloudy

Bottom is not clear, white

After shaking vigorously, the layers mixed, become cloudy

Table 1


The observations of the experiments confirm the presence of the bimolecular.

The results from the test tubes show clearly that there are colour changes due to the reaction between macromolecule and the various solutions.

The equipments are good for qualitative tests and convenient; also, the tests could be made quantitative. More pipettes would be better for doing the experiment because there were a lot of solutions have to be tested. The reason would be to avoid possible contamination. Besides, the chemicals are pure and easy to see the results.


Test solution A contains starch solution, (a polymer of glucose), test solution B contains glucose solution (a reducing sugar) and the test solution C is protein solution (egg albumen). Although most of the experiments are successful, there is one experiment that could be improved. When the test for reducing sugar was being done, the test tube was not put into the 60℃ water to warm as the step suggested. As a result, the colour change can not be seen clearly. To avoid this in future, the steps of experiment have to be followed very carefully.


Answers.com (2009) Starch http://www.answers.com/topic/starch [Access date: 18/11/09]

Baidu Baike (2009a) Glucose Physical properties Available at:

http://baike.baidu.com/view/16023.htm?fromenter=glucose [Access date: 18/11/09]

Baidu Baike (2009b) Biuret Reaction The definition of biuret reaction Available at: http://baike.baidu.com/view/205762.htm [Access date: 19/11/09]

Baidu Zhidao (2009a) Fehling reagents Available at:

http://zhidao.baidu.com/question/116314514.html[Access date: 20/11/09]

Baidu Zhidao (2009b) which process does starch and Iodine react to each other? Available at: http://zhidao.baidu.com/question/49322696.html?si=6[Access date: 19/11/09]

Biosci.com (2009) Biuret Reagent Available at:

http://www.biosci.ohiou.edu/introbioslab/Bios170/170_2/biuret.htm [Access date: 02/12/09]

Encyclopedia.com (2009) Reducing sugar Available at:

http://www.encyclopedia.com/doc/1O6-reducingsugar.html [Access date: 02/12/09 ]

Losos, J. K, Mason. Sing, S (2008) Lipids: Hydrophobic Molecules Biology (8th Edition) New York: MrGraw – Hill

MedicineNet.com (2009) Definition of Protein Available at: