Saliva and starch

The idea

Saliva contains a digestive juice that breaks down starch, a carbohydrate, into sugar, which is why starchy food tastes sweet when chewed for a long time.

Chew and wait

Chew a small bite of chapati or rice for a long time.

What happens to its taste?

• It slowly starts to taste sweet
• It turns sour
• It tastes the same for ever

Test with iodine

Tube A has plain boiled rice. Tube B has chewed boiled rice. A teacher adds iodine. Watch the colours.

Starch becomes sugar

Iodine turns blue-black with starch. A turned blue-black. B stayed pale, so the starch in it had been broken down by saliva into sugar.

Saliva contains a digestive juice that breaks down starch, a carbohydrate, into sugar, which is why starchy food tastes sweet when chewed for a long time.

Notes

Saliva contains a digestive juice that breaks down starch, a carbohydrate, into sugar, which is why starchy food tastes sweet when chewed for a long time.

Check yourself

What turns blue-black with iodine?

Chewed rice stays pale with iodine. What does this show?

Which of these foods contains starch?

Why does chapati taste sweet after long chewing?

• Starch — correct. Yes!
• Sugar. Iodine shows starch. Sugar does not turn blue-black.
• Plain water. Plain water does not turn blue-black with iodine.
• A lot of starch is present. A lot of starch would give a deep blue-black colour.
• Little or no starch is left — correct. Yes!
• The rice has turned into water. The pale colour shows that the starch has been broken down into sugar.
• Plain water. Water has no starch in it.
• Common salt. Salt is not a starch food.
• Chapati — correct. Yes!
• Saliva breaks its starch into sugar — correct. Yes!
• The teeth add sugar to it. Teeth do not make sugar. Saliva breaks the starch into sugar.
• The tongue heats it. Heat has nothing to do with the sweet taste. Saliva breaks the starch into sugar.