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Question Video: Interpreting the Results of Food Tests to Determine Food Groups Present in an Egg Yolk Biology

An egg yolk was tested for different food groups and the results are provided. According to these results, which of the following is the correct conclusion about the biological molecules found in the egg yolk? [A] The egg yolk contains starch, sugars, protein, and fats. [B] The egg yolk contains fats and proteins, but neither starch nor sugars. [C] The egg yolk contains sugars and proteins, but neither starch nor fats. [D] The egg yolk contains sugars and starch, but not protein.

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Video Transcript

An egg yolk was tested for different food groups and the results are provided. According to these results, which of the following is the correct conclusion about the biological molecules found in the egg yolk? The egg yolk contains starch, sugars, protein, and fats. The egg yolk contains fats and proteins, but neither starch nor sugars. The egg yolk contains sugars and proteins, but neither starch nor fats. Or the egg yolk contains sugars and starch, but not protein.

Different biological molecules can be identified in food samples using food tests, which has been done here to get the results that we can see in this table. To answer this question, we need to know which biological molecule is identified by each food test and the positive results we would see if this molecule is present in the egg yolk.

The table tells us that four separate tests were carried out on this egg yolk. The first test was adding iodine to the egg yolk. The second test involved adding Benedict’s reagent to the egg yolk. The third test involved adding Biuret reagent to the egg yolk. And the final test was an emulsion test for the egg yolk.

We’ve removed the answer options for now. So we can first review the main biological molecules which are abundant in our food and can be identified using food tests. These molecules are carbohydrates, which are sometimes called sugars, proteins, and fats, which are sometimes called lipids.

Carbohydrates fall into two main categories: reducing or simple sugars such as glucose and more complex sugars like starch, which is made up of lots of glucose molecules joined together. Now let’s look at the food tests which can be carried out to identify the presence of each different biological molecule in a food sample.

There are two different food tests for the two main forms of carbohydrate. Reducing or simple sugars such as glucose can be tested for by adding a reagent called Benedict’s solution and heating up the mixture for a few minutes in a water bath. Positive results in food tests indicate the biological molecule they test for is present in the food, and they often involve a color change.

We’re showing these food tests being carried out in test tubes. When Benedict’s reagent is added, if glucose is present in the food, the solution will change color from blue to green, yellow, orange, or even brick red. The further along the scale the color is towards red, the more glucose the food sample contains.

Starch can be tested for by adding a reagent called iodine. If starch is present in the food sample, iodine, when added, will change color from orange to blue black. A reagent called Biuret solution can be added to a food sample to test for the presence of proteins. If protein is present in the sample, the solution will change color from blue to purple.

To test for the presence of fats in a food sample, an emulsion test can be carried out. This includes mixing the food sample with water and also with ethanol. Any fats in the food mix with the ethanol and separate into a cloudy white layer called an emulsion above a layer of water. The formation of an emulsion is therefore a positive result, indicating that fats were present in the original food sample.

Let’s look through our results to see which biological molecules were identified by each food test. When iodine was added, the solution remained orange. This is a negative result for iodine, so starch was not present in the egg yolk. When Benedict’s solution was added to the egg yolk, the solution remained blue. This is also a negative result for Benedict’s solution.

So we know that simple sugars such as glucose were not present in the egg yolk. The solution turned purple when Biuret solution was added. This is a positive result which shows that the egg yolk did contain proteins. When an emulsion test was carried out on the egg yolk, it formed a white emulsion layer. This is also a positive result which shows that fats were also present in the egg yolk.

Therefore, the results of the food tests show that the egg yolk contains fats and proteins but neither starch nor sugars.

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