5 Organic Substances in Food
Background
Organic substances are carbon-rich and energy-rich molecules that make up cells. They are classified into four major types. Carbohydrates consist of repeated units of H-C-OH and include monosaccharides such as glucose sugar and polysaccharides such as starch. Proteins are a diverse group of large molecules consisting of chains of amino acids. They include enzymes such as catalase, carrier molecules such as hemoglobin and hormones such as insulin. Proteins are abundant in all cells, especially muscles. Lipids dissolve in organic solvents but are insoluble in water. They include fats and steroids. Nucleic acids such as DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) are organic substances made up of chains of nucleotides.
When we eat, the food that we consume consists of organic substances made by other organisms. It nourishes our body because these organic substances provide our cells energy (mainly from carbohydrates and fats) and appropriate “raw materials” (mainly from proteins) required to make specific biomolecules our body needs.
To review biomolecules, watch this video:Biomolecules by Amoeba sisters (Links to an external site.) (8 min 12 sec)
Hypothesis and prediction: If a food sample contains organic substances, then a test for the presence of those substances using specific reagents will be positive.
Purpose
- Test for the presence of two types of carbohydrates (glucose, starch) and lipids in food samples.
Part 1: Test for Carbohydrates and Protein
Materials
The following materials will be used in the demo videos:
- Benedict’s solution
- Iodine solution (diluted)
- Ninhydrin reagent (1%)
- Starch solution
- Glucose sugar solution
- Protein solution
- Beakers
- Test tube
- Test tube racks
- Stirring rods
- Disposable droppers
- Food samples (diced)
Procedure
Testing the reagents (demonstration by the instructor)
A specific reagent is used to test for the presence of a specific organic substance such as glucose, starch or protein. To determine which reagent can test for the presence of a specific organic substance, we have to show the reagent reacting in a characteristic way with that organic substance (e.g., specific color change) and not reacting in the same way with other substances.
To demonstrate this process, the instructor will test three reagents: iodine, Benedict’s solution and ninhydrin. From these tests, you will discover what organic substance (i.e. glucose sugar, starch and protein) each reagent tests for.
For a demonstration of the full lab experiment, watchOrganic substances in food lab (Links to an external site.)(watch to mark 6:08 for the first demonstration on testing reagents). Observe color changes and record in appropriate data tables. Follow procedure from the lab exercise.
Record the original color of the reagents below:
Benedict’s ______________ Iodine _______________ Ninhydrin _______________
The instructor will then add the reagents, one at a time, to sugar, starch and protein solutions to show whether a chemical reaction occurs.
Record the final color of the mixture in Table 1.
| Table 1. Final Results of Reagents with Organic Substances | |||
| Reagents/ Organics |
Benedict’s Solution | Iodine | Ninhydrin |
| Glucose sugar | |||
| Starch | |||
| Protein | |||
Interpret the previous table, determine the appropriate reagent to identify each organic substance and fill in the table below.
| Table 2. Reagent Test for Organic Substances | ||
| Organic substance | Reagent | Expected color change |
| Glucose sugar | ||
| Starch | ||
| Protein | ||
Testing for the presence of organic substances (carbohydrates and protein) in food samples
Now that you know which reagent can test for the presence of glucose sugar, starch and protein, you can determine which of those organic substances are present in food.
- Prepare a water bath (beaker half-full with tap water) on a hot plate.
- Take three samples of one type of food about the size of a small pea.
- Grind the food into fine pieces then place the ground food into two separate test tubes and one sample into a petri dish. Mark one of the tubes “B” for Benedict’s and the other tube “N” for ninhydrin. Label the petri dish “I” for iodine.
- Add Benedict’s reagent to the food sample in tube “B” and ninhydrin to the food sample in tube “N”.
- Heat tube “B” for 5 min. and tube “N” for 7 min. in the water bath. Record the color in the table below.
- Add iodine to the food sample in the petri dish. Wait 5 min. and record color in the table below.
- Choose another food sample and repeat steps 1-6.
- Pool class results. The instructor will record the results from each group on the board so each student can record data for all food samples tested in class.
- Dispose food samples in a designated tub then wash the test tubes and petri dish.
WatchOrganic substances in food lab (Links to an external site.)(mark 6:08 to end 13:14) for testing food samples. Follow procedure above and record observations in Table 3 below.
| Table 3. Observed color changes and stain pattern indicating presence or absence of organic substance in food samples | ||||||
| Reagents/Organics Food sample |
Benedict’s (final color) |
Iodine (final color) |
Ninhydrin (final color) |
Glucose sugar (+ or -) |
Starch (+ or -) |
Protein (+ or -) |
| Potato | ||||||
| Cabbage | ||||||
| Apple | ||||||
| Almond | ||||||
| Bread | ||||||
| Potato chips | ||||||
| Ground beef | ||||||
| Ham | ||||||
| Hotdog | ||||||
Part 2: Test for Lipids (DIY)
Materials
For lipid test: (Note if your group does not have these materials, please inform instructor)
- Brown paper (ie. paper bag or envelope)
- Water
- Oil
- Salt water (a pinch of table salt dissolved in ~1 Tablespooon of water)
- Sugar water (a pinch of sugar dissolved in ~1 Tablespooon of water)
- Two or more of the following:
- Preferred substances: any item in Table #3 above.
- Other liquids: for example, milk, juice, soda, salad dressing, vinegar, ketchup, mustard
Procedure
Testing the reagents
To demonstrate this process for lipids, you will use the reagent of brown paper. From this test, you will be able to determine if the substance has lipids.
Lipid test: You can perform this test on your own. You will need brown paper, such as a paper bag or envelope. Note that brown paper readily absorbs lipids. Please follow this procedure:
- Cut a piece of brown paper approximately 8.5 x 11 inches.
- Draw approximately 6 circles with ~1 inch diameter. Each circle should be spaced ~2 inches apart.
- Label one circle “water” and place one drop of water in it.
- Label one circle “oil” and place a drop of oil in it.
- Observe each circle at time 0 immediately after you placed the drop. Does the paper stain look clear, even (uniform) or uneven? Record in table below the results.
- Observe each circle after 15 minutes. Turn the paper over. Does the paper stain look clear, even (uniform) or uneven? Record in table 4 below the results.
| Table 4. Brown Paper Test for Lipids | |||
| Reagent/ Substance |
Brown Paper (stain appearance at 0 min: clear, uneven or even) |
Brown Paper (stain appearance at 15 min: clear, uneven or even) |
Lipid (fat) (+ or -) |
| water | |||
| oil | |||
| sugar water | |||
Note: Any paper with a uniformly wet-looking stain contains fat. Any paper that is relatively clear or unevenly stained does not contain fat.
Interpret the previous table, determine the appropriate stain pattern and fill in the table below.
| Table 5. Reagent Test for Organic Substances (continued) | ||
| Organic substance | Reagent | Expected color change |
| Lipid (fat) | brown paper | |
Testing for the presence of lipid (fat):
- Gather your other substances: liquids or solids with moisture (ie. milk, salad dressing, juice, soda, potato chips, etc).
- On your brown paper, place one drop of liquid in its circle or smash a piece of solid and place in circle.
- After 15 minutes, flip paper over to view and record the appearance of the stains in Table #4. Stain can be clear, uneven or even. Recall that any paper with a uniformly wet-looking contains fat similar to the oil. Any paper that is relatively clear or unevenly stained does not contain fat similar to the water.
Results
| Table 6. Observed color changes and stain pattern indicating presence or absence of organic substance (lipid) in food samples | ||
| Reagents/ Organics Food sample |
Brown Paper (final stain) |
Lipids (+ or -) |
Discussion/Conclusion
- Name example(s) for each item below and which organic substances are found in them.
| Table 5. Different Types of Food Samples and Organic Substance Content | ||
| Type of food sample | Food samples | Organic substance(s) present |
| unprocessed animal samples |
||
| unprocessed plant samples |
||
| processed food samples from animals |
||
| processed food samples from plants |
||
- Compare and contrast organic substances found in plants versus animals.
- Compare and contrast organic substances found in unprocessed versus processed foods.
- Which types of food should be avoided by an overnourished (obese) person? Briefly justify your answer.
- Which types of food should be eaten by a person who wants to build up his/her muscles? Briefly justify your answer.
