Concetti Chiave
- Heating starch, such as bread or potato, in a hot oven or toaster results in dextrinization, turning it light brown and forming dextrin.
- When corn starch paste is mixed with cold water and boiling water is added, it swells and forms a gel through the process of gelatinization.
- Gentle heating of sugar with water leads to syrup formation as the liquid evaporates, demonstrating the effect of moist heat on sugar.
- Fehling's test for simple sugars involves heating a sugar solution with Fehling's solution, resulting in an orange or brick-red precipitate indicating the presence of simple sugars.
- Benedict's test detects simple sugars similarly to Fehling's test but is more sensitive, showing a red, orange, or yellow precipitate upon boiling.
Effect of heat on starch
- obtain a slice is bread ( a slice of potato can also be used).
- place the slice in a very hot oven or a toaster.
- leave until slice turns to light brown.
Observation:
Light brown colour will be observed. The product is called dextrin. The process is known as dextrinization.
Effects of moist heat on starch
Effects of moist heat on starch
1. Place 2 table spoonfuls of corn starch paste in a bowl.
2. Add 6 table spoonfuls of cold water.
3. Mix thoroughly to obtain a slightly liquid consistency.
4. Pour boiling water rapidly into the mixture.
Observation:
The starch will swell and a jelly substance is formed. The process is gelatinization. The product is a gel.
Effect of moist heat on sugar:
- pour about 15ml of water into a sauce and add about 30g of sugar.
- heat gently.
- continue heating until syrup is formed.
Observation:
The syrup is formed as the liquid evaporates.
Food that can be used for simple sugar test are table sugar, honey, fruits for example ripe bananas.
Fehling's test for simple sugars
Fehling's Test:
1. Place about 5ml of 5% sugar solution in a test tube.
2. Prepare the Fehling's solution by adding equal amounts of Fehling's solution A (copper sulphate in water) and B ( sodium potassium tartrate and sodium hydroxide in water).
3. Add 5ml of Fehling's solution into the sugar solution.
4. Place the tube into a beaker half-filled with water.
5. Heat the beaker gently to boil.
Observation:
An orange or brick-red precipitate will be formed. This shows the preens of a simple sugar.
Benedict's test for simple sugars
Benedict's Test:
This is similar to Fehling's Test. It is however more sensitive.
- place about 5ml of 5% sugar solution in a test tube.
- add equal amount of Benedict's solution.
- boil as with the Fehling's test. Allow to stand.
- observe a red, orange or yellow precipitate that is formed. This shows the presence of a simple sugar.
Domande da interrogazione
- What happens to starch when exposed to dry heat, such as in an oven or toaster?
When starch, like a slice of bread, is exposed to dry heat, it turns light brown due to a process called dextrinization, resulting in the formation of dextrin.
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