FOOD TESTS
This experiment is intended to conduct chemical tests for biomolecules of vitamin C, lipids, proteins and carbohydrates. These tests are performed for identifying the compositions of biochemical of the test food substances.
Large macromolecules like nucleic acids, lipids, carbohydrates and proteins are classified as biomolecules. These biomolecules have diverse structures and multiple functions. Benedict’s reagent is used for the testing of the presence of simple carbohydrates in the sample.
These biomolecules possess unique properties that ultimately contribute to the cell’s function and structure. Reducing sugars possess aaldehyde group or free ketone. When Benedict’s reagent is added to the sample solution and heated, the occurrence of a reduction reaction results in the color change of Benedict’s reagent.
The color is changed to reddish-brown, brick red, green or orange. Biuret reagent is used for testing proteins in the given biological samples. Treatment of a dilute solution of copper sulphate with biuret in basic medium results in the formation of a purple compound.
The cupric ions form a chelate complex with peptide nitrogen’s unshared pair of electron and water’s oxygen. Light is absorbed by the chelate complex at 540 nm, thereby producing a violent compound and indicating the presence of proteins.
Biochemical reagents
Benedict’s reagent
Biuret reagent
DCPIP
The procedure of this experiment is as follows:
Control solutions and test solutions A, B and C were provided. All the tubes were labelled to compare the test and control solutions. 2 ml of starch was poured into the LP4 tube using the Pasteur pipette. Iodine solution drops were added to the solution until the appearance of deep blue color.
Next, Gilson pipette was used to add 100 ul Benedict’s reagent to 2 ml glucose solution in LP4 tube. For two minutes, in the water bath, this mixture was heated. The same procedure was repeated for sucrose and fructose solutions. Absence of positive results indicates that the sample contains non-reducing sugar.
Presence of non-reducing sugar was confirmed by taking a fresh negatively tested sample solution, followed by the addition of 3 drops dilute HCL. This mixture was heated in a water bath. Few drops of sodium bicarbonate solution were added for neutralizing the acid. 100 ulBenedict’s reagent was added that resulted in a color change.
Albumin solution was added as drops to 2 ml Biuret reagent. For the lipid test, one drop sunflower oil was added to 2 ml of ethanol. Vitamin C was tested by adding it to 2 ml DCIC.
The test solutions and control solutions were examined according to the procedures mentioned in the previous section.
| Compound tested | Test prepared | Results observed |
| starch | Iodine was added | Deep blue |
| glucose | Benedict’ solution was mixed and placed in a water bath for 2 minutes | Light orange |
| fructose | Benedict’s solution was mixed and then placed in a water bath for 2 minutes | Dark orange |
| sucrose | Benedict’s solution was mixed and then placed in a water bath for 2 minutes | orange |
| Albumin (protein) | Biuret reagent was added | purple |
| Sunflower oil (lipid) | Ethanol was added followed by water (SUDHEER, 2019) | Yellow colouration accompanied by fizz |
| Vitamin C | DCPIP was added | clear |
Table 1: Tests and observations for control solutions
(Source: provided)
Table 1 illustrates the results obtained for the control samples. The tests presented above have served as a reference. These references are essential for the interpretation of the results obtained in the case of the test samples.
| solution | Test prepared | observations | inference |
| A-milk | Starch
Glucose Proteins vitamin C lipids |
Yellow
Blue Pink No change Droplets on top |
glucose, lipids and proteins are present. Vitamin C and starch are absent. |
| B- honey | Glucose
Starch Vitamin C Lipid proteins |
Bright orange
Orange No change Clear and fizzy No change |
Glucose and lipids are present. Test for proteins is negative. Vitamin C and starch are not present. |
| C- apple juice | Starch
Glucose Protein Vitamin C lipid |
Orange
Brownish orange No change Clear No change |
Glucose and vitamin C are present in sample C. Protein, lipid and starch are present. |
Table 2: Tests and observations for test solutions
(Source: provided)
Table 2 illustrates that tests for starch, glucose, vitamin C, lipids and proteins were performed for each of the three test samples (Bahugunaet al. 2017). The observations listed in table 2 are indicating the presence of various biomolecules in the test samples. However, each of this test sample does not contain all of the biomolecules.
In the case of sample A, that is milk, and glucose has tested positive for by giving a blue colour. Lipid droplets are observed on the sample’s surface (Baynes, 2018). Addition of biuret reagent gives a pink color that confirms the presence of proteins in sample A.
Sample B is honey and results have been positive as the addition of Benedict’s reagent gives a bright orange color. Addition of 2 ml ethanol to honey produced a transparent outcome accompanied by fizz. These results have confirmed the presence of lipids and glucose in the given sample B.
Sample C is apple juice. Benedict’s reagent was added to the sample that produced a brownish-orange color (Choudhary, 2016). The second test showed that the addition of DCPIP to the sample of apple juice had produced a clear sample by losing its color. The observations indicate the presence of glucose and vitamin C in sample C of apple juice.
In all these cases, the color change was observed immediately after the addition of Benedict’s reagent. There was no need to conduct any test for non-reducing sugars. Thus the common reducing sugar present in all of these samples is glucose (thestudentroom.co.uk, 2020).
As discussed in the previous section, sample A comprises of proteins and lipids.
Color change was observed immediately after the addition of Benedict’s reagent. There was no need to conduct any test for non-reducing sugars. Thus, the possible source of food for sample A is milk.
Sample B contains the biomolecules glucose, as the reducing sugar, and lipids. The possible food source for sample B is honey.
Sample C contains vitamin C and reduces sugar glucose as biomolecules. The possible food source for sample C is apple juice.
Journals
Bahuguna, Y., Kothiyal, P. and Mehra, S., 2017. Research article Bimonthly published scientific journal. Drugs, 2(3), pp.21-28.
Baynes, J.W., 2018. Carbohydrates and lipids. Medical Biochemistry E-Book, p.25.
Choudhary, S., 2016. Biochemical Tests of Different Food Products Used Frequently by the Human Population (Doctoral dissertation, MAHATMA JYOTI RAO PHOOLE UNIVERSITY).
Kumar, V. and Chandel, S., 2018. Studies on biochemical mechanism of resistance for the management of rose powdery mildew. Journal of Pharmacognosy and Phytochemistry, 7(1), pp.1234-1241.
SUDHEER, A., 2019. RANCIDITY DETECTION KIT FOR COOKING OILS.
Websites
nku.edu, 2020, benedict’s test for sugar, available at: https://www.nku.edu/~whitsonma/Bio150LSite/Lab%203%20Organic/Bio150LRevMolec.html. [Accessed on 15.2.2020]
thestudentroom.co.uk, 2020, tests for reducing and non-reducing sugar, available at: https://www.thestudentroom.co.uk/showthread.php?t=3604455. [Accessed on 15.2.2020]
