Food Analysis (Qualitative)

This is a fun little exercise that applies chemical tests from other labs to do a qualitative analysis of lipids, carbohydrates, and proteins in a piece of a cheese sandwich cracker. We are careful to use crackers that are made with real cheese and therefore have cholesterol. Students perform a solvent extraction and test for bromine decolorization and for cholesterol. A hot water extract is subsequently tested for carbohydrates by the Benedict's, Seliwanoff's, and iodine tests, and for protein by the biuret and xanthoproteic acid tests.

Solvent Extract: A quarter of a cheese sandwich cracker is crushed in a 10-mL beaker with 2 mL of methylene chloride. The mixture is filtered through dry filter paper into a 50-mL beaker using a stemless funnel. The residue in the beaker is treated with a second 2 mL of methylene chloride which is poured into the filter. Finally, after the sample has drained, a third portion of 2 mL methylene chloride is poured over the residue in the funnel. The filtrate is tested for bromine decolorization and for cholesterol as described for the Lipid Chemistry exercise. The solvent is allowed to evaporate in the hood and students are told to note the physical state of the lipid residue.

Water Extract: By now, the residue in the filter paper is dry. The next step is to do a hot water extraction using 10 mL of boiling water. Here we have had problems in the past. We have had students measure 10 mL of boiling water in a graduated cylinder and pour it over the residue, stirring for a few minutes. Not much starch or protein is extracted because the water is pretty cool by the time it gets to the sample. We have had students mix the residue with 10 mL of water in a test tube and place it in a boiling water bath for 5-10 minutes. Lots of starch is extracted, to the point where the solution is so thick it is impossible to filter. Still, they get stuff extracted, and with filtering through cheesecloth they get a cloudy but workable extract. The last time I did this, we didn't see much protein, and I'm thinking about using dilute NaOH instead of water for the extraction if I use this exercise again. At any rate, the water extract is subjected to Benedict's, Seliwanoff's, and iodine tests for carbohydrates and the biuret and xanthoproteic acid tests for protein.

All results are tabulated. In closing, students are asked (1) to comment of the degree of unsaturation of lipids in the extract and whether the physical state of the lipid residue is consistent with their conclusion, (2) what component of the cheese cracker is responsible for their (positive) cholesterol test, (3) what they can conclude about the kinds of carbohydrate in the cheese cracker, (4) whether they found any protein, and (5) what they can say about specific amino acids present.

Revised 9/1/06