Food tests Flashcards
Test for starch
Add a few drops of iodine solution (iodine dissolved in potassium iodide) to the sample.
If starch is present, the iodine solution will turn from yellow-brown to blue-black.
Test for reducing sugars
A small amount of the sample is placed in a test tube.
Benedict’s reagent is added to the sample.
The mixture is heated in a water bath or directly over a flame.
The solution is observed for a color change (Blue to Brick red)
Test for proteins
. Prepare the sample:
If the food sample is solid, break it up and mix it with distilled water. Filter the mixture to collect the liquid sample.
2. Add Biuret reagent:
Add a few drops of Biuret reagent (a solution of copper sulfate and sodium hydroxide) to the sample in a test tube.
3. Observe the color change:
If protein is present, the solution will turn from blue to violet or purple.
4. Record your results:
A color change indicates a positive result, meaning protein is present. If the color remains blue, protein is not present.
Test for Lipids
Place a small amount of the food sample in a test tube. Add 4 cm³ of ethanol and shake well to ensure the sample dissolves in the ethanol.
If the sample doesn’t fully dissolve, filter the mixture to remove any insoluble particles.
Add Water:
Pour the ethanol solution into a clean test tube. Add an equal volume (4 cm³) of cold distilled water.
Observe:
Shake the mixture gently. A positive test for lipids will result in the formation of a cloudy, milky-white emulsion.
Test for Vitamin C
A solution of DCPIP is typically used, which is a blue dye.
2. Add sample:
A sample containing a potential vitamin C source (e.g., fruit juice) is added to the DCPIP solution.
3. Observe color change:
If vitamin C is present, it will reduce the DCPIP, causing the solution to lose its blue color and become colorless.
4. Qualitative vs. Quantitative:
This test can be used qualitatively to determine if vitamin C is present, or quantitatively to estimate its concentration by measuring how much DCPIP is decolorized.
