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
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
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
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