Concetti Chiave
- Heating starch such as bread or potato results in dextrinization, turning the item light brown and forming dextrin.
- Moist heat applied to starch causes gelatinization, where the starch swells and forms a gel-like substance.
- When sugar is heated with water, a syrup forms as the liquid evaporates.
- Fehling's Test for simple sugars results in an orange or brick-red precipitate, indicating the presence of sugar.
- Benedict's Test, similar to Fehling's but more sensitive, also indicates sugar presence with a red, orange, or yellow precipitate.
Starch and Sugar - Test Effects
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
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:
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:
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
- ¿Qué ocurre cuando se aplica calor seco al almidón?
- ¿Cómo afecta el calor húmedo al almidón?
- ¿Qué indica la formación de un precipitado rojo o naranja en las pruebas de Fehling y Benedict?
Al aplicar calor seco al almidón, como en una rebanada de pan en un horno caliente, se observa un color marrón claro. Este producto se llama dextrina y el proceso se conoce como dextrinización.
Al aplicar calor húmedo al almidón, como al mezclar almidón de maíz con agua fría y luego añadir agua hirviendo, el almidón se hincha y se forma una sustancia gelatinosa. Este proceso se llama gelatinización y el producto es un gel.
La formación de un precipitado rojo, naranja o amarillo en las pruebas de Fehling y Benedict indica la presencia de un azúcar simple en la solución.
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