Q.Why does 'Swiss cheese' have big holes?

Let’s begin with something you already know. Think of a pot of milk left out in warm weather. After a few hours, it turns sour and thickens into curd. That change is not spoilage in the usual sense — it is a controlled transformation caused by tiny living organisms called microbes. This is the heart of microbial fermentation.

Fermentation is a process in which microorganisms — bacteria, yeast, or moulds — break down organic substances (like sugars) in the absence of oxygen, producing energy for themselves and, as a by‑product, substances that change the food. When we deliberately use this process to make food, we call the result microbial fermented foods.

You have eaten many of them without thinking twice: curd (yogurt), idli and dosa batter, bread, cheese, pickles, vinegar, and even the dark chocolate you might enjoy. Each of these relies on specific microbes doing their work.

Why does this matter? Because fermentation does three things that are valuable for us:

• Preserves food — The acids or alcohol produced by microbes prevent spoilage by harmful bacteria. Pickles stay edible for months because of lactic acid from fermentation.
• Improves digestibility — Microbes break down complex molecules. Lactose in milk becomes easier to digest in curd; the proteins in soy become more digestible in tempeh.
• Enhances flavour and texture — The tang of yogurt, the airy holes in bread, the umami of soy sauce — all come from fermentation.

Let’s look at a few examples from your daily life, as the NCERT would describe them:

• Curd (yogurt): Milk is boiled and cooled, then a small amount of previous curd (containing Lactobacillus bacteria) is added. The bacteria convert lactose into lactic acid, which thickens the milk and gives it a sour taste.
• Idli and dosa batter: Rice and urad dal are soaked, ground, and left to ferment overnight. Naturally occurring Leuconostoc and other bacteria produce carbon dioxide gas, which makes the batter rise and gives idlis their spongy texture.
• Bread: Baker’s yeast (Saccharomyces cerevisiae) ferments the sugars in dough, releasing carbon dioxide that makes the dough rise. The alcohol produced evaporates during baking.
• Cheese: Milk is curdled using rennet (an enzyme) or acid, then specific bacteria and moulds are added to ripen it. Different microbes give different cheeses their distinct flavours — for example, Penicillium roqueforti gives blue cheese its veins and sharp taste. …