Food Tests

Question 1

What are the four things the food tests are aimed to test for?

Answer 1

Carbohydrates (specifically starch), proteins, sugars, and lipids

Question 2

How do we prepare a food sample? (hint: 4 steps)

Answer 2

– Get a piece of food and break it up with a pestle and mortar
– transfer the ground up food to a beaker and add some distilled water
– give the mixture a good stir to dissolve some of the food in the sample
– filter the solution using a funnel lined with filter paper (to get rid of all the solid bits of food that could be left in the mixture)

Question 3

Why do we need food samples for these food tests?

Answer 3

For every test, we need a food sample so that we can test different kinds of food for different substances (lipids, starch, sugars, and proteins)

Question 4

What can we use in the food samples for the food test for sugar that would yield a positive result?

Answer 4

Biscuits, cereal, and bread as well. Anything sweet really.

Question 5

What reagent do we use to test for sugars?

Answer 5

Benedict’s solution

Question 6

What would Benedict’s solution test for specifically

Answer 6

SUGARS– there are two types though. Reducing and non reducing. Benedict’s only tests for REDUCING

Question 7

Define the term “positive result”

Answer 7

A positive test result is one where the test has identified the presence of what’s being tested for

Question 8

Define the term “negative result”

Answer 8

A negative test result is one where the test has not identified the presence of what we’re looking for

Question 9

What would a positive result for the Benedict’s solution look like? How does it vary? What do the diff. colors mean?

Answer 9

If the Benedict’s solution changes from cyan-blue to green/yellow/brick red, then the result is positive. Cyan means no conc. of sugar. Green means little conc. of sugar. Yellow means medium conc. of sugar. Brick red means high conc. of sugar

Question 10

What would a negative result for Benedict’s look like?

Answer 10

The solution of Benedict’s will remain cyan blue

Question 11

There are five steps in the method of testing for reducing sugars. List all of them.

Answer 11

1. Prepare a food sample with the method given earlier and transfer 5 cm3 of this into a test tube
2. Prepare a water bath set to 75 degrees Celsius
3. Add some Benedict’s solution to the test tube (about ten drops) using a pipette
4. Place the test tube into the water bath using a test tube holder and leave it in there for about five minutes
5. During time time, the food sample would either remain blue cyan or turn red, yellow, or green

Question 12

What different kinds of food could we put in the food sample for a test for starch that would yield a positive result

Answer 12

Foods like potatoes, rice and pasta contain a lot of starch

Question 13

What reagent would we use to test for starch?

Answer 13

Iodine solution tests for starch

Question 14

What does a positive test result for starch look like?

Answer 14

If your food sample contains any starch in it, the solution would turn blue-black.

Question 15

What does a negative test result for starch look like?

Answer 15

If the sample does not contain any starch then the color of the solution would remain orange-brown

Question 16

There are two steps in the method required for a test of starch. List them

Answer 16

1. Make a food sample with the method previously stated. Transfer 5 cm3 of this into a test tube
2. After you have a sample in a test tube, add a few drops of iodine solution into the sample. Gently shake the tube to mix the contents.

Question 17

What could you include in your food sample for protein that would yield a positive result?

Answer 17

Meat
Cheese
Egg (both the egg yellow and egg white)

Question 18

What reagent would we use to test for protein?

Answer 18

Biuret’s solution

Question 19

What would a positive test result for protein look like?

Answer 19

If the food smaller contains protein, the solution will change from a cyan blue to a purple (light or dark, doesn’t matter)

Question 20

What would a negative test result for protein look like?

Answer 20

If the food sample doesn’t contain any protein, then the solution will remain a cyan blue

Question 21

There are two steps in the method required to test for protein. List them

Answer 21

1. Use the method previously stated to make a food sample: take 2 cm3 of that mixture and add it to a test tube
2. Add 2 cm3 of biuret’s solution to the test tube and mix the contents by gently shaking.

Question 22

There are two reagents that we can use to test for lipids. What are they?

Answer 22

The Sudan III stain and ethanol

Question 23

What foods can we use to make a food sample for lipids that would yield a positive result?

Answer 23

Olive oil, butter/margarine/milk

Question 24

What will a negative test result from Sudan III stain look like?

Answer 24

If there are no lipids in the solution, then there will be no separate layers and no bright red color

Question 25

What will a positive test result from Sudan III stain look like?

Answer 25

Sudan III stain, stains lipids a color red, so if the food smaller contains lipids, the mixture will separate out into two layers. The top layer would be bright red and this shows that the solution has lipids in them

Question 26

What will a negative test result from the ethanol emulsion test look like?

Answer 26

If after shaking, the solution does not emulsify (turn cloudy) then there are no lipids present

Question 27

What will a positive test result from the ethanol emulsion test look like?

Answer 27

If after shaking, the solution emulsifies (turns cloudy), then there are lipids present

Question 28

The method required for the Sudan Stain III practical has three steps to it. List them

Answer 28

1. Prepare a food sample with the method previously stated, but this time you do not need to funnel out the solid bits in the mixture at the end
2. Add 5 cm3 of the food mixture into the test tube
3. Use a pipette to add three drops of Sudan III stain into the test tube and then gently shake it

Question 29

There are three steps involved in the method for the lipids emulsion test. List them

Answer 29

1. Make a food sample from the method previously instated. Transfer 5 cm3 of this sample to a test tube
2. Add equal amounts of ethanol to the test tube (so 5 cm3).
3. Cap the test tube up and shake the solution. Hard.