Preserved foods

In your book, Our Wondrous World (Chapter 3), there is a table of foods and what they can be made into, so that they last longer. Roti is dried (this is called dehydration) to make khakhra. Papads are dried in the Sun. Mango becomes aam papad. Lemon becomes lemon pickle. Butter becomes ghee. Juice is put into cans as canned juice. Chillies become chilli powder, and fish becomes dried fish.

Some of these we have already met. Chillies are dried in the Sun and ground into chilli powder, as Disha's Amma and Appa do. Juice is sealed in air-tight cans in factories, to keep microbes away.

Your book asks a good question: why does butter need a fridge, but ghee does not? Here is how ghee is made, in this order: first, heat the butter; next, the water separates from the butter; then, the water evaporates (turns into vapour and goes away); last, ghee is ready.

Remember, microbes need moisture to grow. Butter still has water in it, so it needs a fridge. When butter is heated, the water is driven out, so ghee keeps well without a fridge.