Blotting paper test Tests on carbohydrate



Subject: Foods and nutrition

Topic: Scientific study of foods: © Tests for fat: (i) Action of dry heat on fat (ii) Blotting paper test. (d) carbohydrates (i) iodine test (ii) Reaction of starch paste to litmus effect of heat on carbohydrates (e) water content of food: (i) residual moisture.

Content:© Tests for fat: (i) Action of dry heat on fat (ii) Blotting paper test.

(d) Carbohydrates(i) iodine test (ii) Reaction of starch paste to litmus effect of heat on carbohydrates (e) water content of food: (i) residual moisture.


Sub topic 1: Tests for fat (i) Action of dry heat on fat (ii) Blotting paper test.

Effects of heat on fat

When fats are heated to a very high temperature, it melts to oil and lose any moisture it contains.When it is further heated, it begins to smoke. This can make the fat to catch fire.

When further heated beyond smoke point. It decomposes to produce a substance called acrolein which has an irritating effect on the digestive system.

Blotting paper test

This is done to determine the presence of fat in food substances. The procedure involves grinding a small quantity of food stuff between two pieces of blotting paper or filter paper. After grinding, if paper or some portion on it because translucent or greasy, this shows the presence of fat and oil in the foodstuff. If otherwise, there is no fat.


Sub topic 2:Tests on carbohydrate

Tests on carbohydrate

Carbohydrate test is conducted to determine the carbohydrate content of a given food substance. This can be done in the following ways:

  • iodine test:

Cut the fresh starchy food into two or three pieces to expose the flesh. Then add a few drops of iodine. If the colour turns black or blue black it confirms the presence of carbohydrate. If the colour remains brown then it shows the absence of carbohydrate.

  • Reaction of starch paste to litmus:

Peel the carbohydrate food and crush part of it into a paste. Then immerse a red litmus paper into the paste. If it turns blue then it indicates the presence of carbohydrate but if it remains red then carbohydrate is absent.

  • Effect of heat on carbohydrates:

Carbohydrate is affected by the application of moist and dry heat. Moist heat makes the starch cells to swell up and burst, making it to gelatinized and also softens the carbohydrate. The application of dry heat on the other hand turns it to brown and thereby caramelizing and dextrinising them and make them digestible.


Water content of food

Residual moisture

All food items contain water when they are fresh. In order to keep them for longer periods, they are dried to remove the excess water in them. The removal of water makes them free from the attack of enzymes and micro organisms which will normally destroy them. The little moisture that is left after processing is called residual moisture.




  1. What is the effect of iodine on starch?
  2. Explain the effect of Benedict test on carbohydrate.
  3. Describe the effect of heat on fat.

General evaluation:

Objectives test:

  1. Which of these is an effect of heat on carbohydrate foods?
    1. coagulation
    2. denaturation
    3. dextrinisation
    4. maillard reaction


  1. Which of these is not used to test for the presence of starch of food?
    1. million’s reagent
    2. Benedict’s solution
    3. felling solution
    4. iodine solution
  2. Moist heat on starch causes……..
    1. caramelization
    2. gelatinization
    3. carbonization
    4. dextrinization
  1. With Fehling’s test, simple sugar gives ……. Colour.
    1. brown
    2. orange
    3. yellow
    4. black
  2. The brown honey-like substance formed when sugar is heated is……
    1. gelatin
    2. chocolate
    3. sweet
    4. caramel


  1. Mention the three basic effects of heat on carbohydrates.
  2. How does heat affect fat?

Weekend assignment:

Read Evans food and nutrition for senior secondary school 1 by F.A. Bakare et el; (pages 46-49)

Reference texts:

  1. Evans food and nutrition for sss book 1 by F.A et al; Evans brothers Nigeria limited.
  2. Exam focus foods and nutrition for WASSCE and SSCE by J.O Olusanya et al. University press.
  3. Practical on food test
  4. Revision

12-13: Examination.