Biochemical testing (food testing)
Adrian Whyte and Joanna Johnson, Contributor

Action from Fame-School Rules at Camperdown High School in January this year. - Toussaint Smith

ONE PARTICULAR topic that follows you in biology is that of biochemical testing, also known as food testing. Not only do you need to know the steps of doing the various tests, but you need also to be able to perform them as well. CSEC biology SBA's are filled with food testing labs. It is normally used as a diagnostic tool to determine the absence or presence of different types of food in an unknown sample.

However, there are some mistakes that are often made by students. This week we will seek to deal with some of these problems.

The most popular tests are for:

a) Starch
b) Reducing sugar
c) Non-reducing sugar
d) Protein

TESTING FOR STARCH

1. Place 1ml of sample to be tested into a clean test tube. (Sometimes the sample is boiled then cooled before proceeding to the next step).

2. Add three drops of iodine solution to the tube contents and shake gently.

If there is starch present in the sample then there will be a distinct colour change to a dark blue-black colour. Note: if this colour is not seen then it indicates that there is no starch in the sample.

This test is one of the simplest and usually done well by most students.

TESTING FOR REDUCING SUGAR

1. Place 1ml of sample into a clean test tube. (If sample is a solid mix a small portion of it with water before going to step 2)

2. Add three drops of Benedict's solution to tube and shake well.

3. Heat tube in a hot water bath. (Approx. 1.5min.)

If there is any reducing sugar (glucose, maltose, etc.) in the sample, then there will be a brick red or orange precipitate.) Please note that if there is a small amount of reducing sugar the colour change maybe yellow instead.

TESTING FOR NON -REDUCING SUGAR

(This is similar to the reducing sugar test)

1. Test sample for reducing sugar. (If it is negative proceed to next step)

2. To a 1cm3 sample add equal amount of dilute HCl and boil in a water bath. (here is were most students mess up with this test. They normally boil for too short a time. A rule of thumb is that you boil until there is a slight yellow colour. However, this is sometimes hard to see. So boil for about three mins.)

3. Cool tube under running water.

4. Carefully add NaHCl to the mixture until the fizzing stops. (this is to neutralise any excess acid).

5. Add 1cm3 Benedict's solution and heat.

If there is any non-reducing sugar (sucrose) in the sample then there will be a brick red or orange precipitate.

Q: Why is it that we have to test for reducing sugar first?

Q: Why if the reducing sugar test is positive we should not proceed to the second step?

TESTING FOR PROTEIN (CALLED THE BIURET TEST)

1. Place 1ml of sample into a clean test tube.

2. Add 1ml of sodium hydroxide to tube and shake. (Potassium Hydroxide can also be used).

3. Add drops of Copper Sulphate solution and shake tube after each drop.

(This is a very important step. Many students tend to make all the drops then shake afterwards. They then end up with a blue gel-like mess. If you put one drop then shake vigorously then repeat a purple colour will appear if there is protein in the sample).

TESTING FOR FATS

1. Place a drop of sample or rub sample on a piece of paper. Blot excess with paper towel.

2. Let dry. Then hold paper up to light and look for a translucent spot.

OR Place 1ml of sample into a clean test tube.

Add 1ml of ethanol to the sample and shake vigorously.

Add 1ml of distilled water and observe.

A milky white appearance will be seen in the mixture if fats are present.

Complete the following table:

* Adrian Whyte and Joanna Johnson teach Biology at Ardenne High School.