Rising of dough is due to: (a) Multiplication of yeast (b) Production of CO2 (c) Emulsification (d) Hydrolysis

NCERT Exemplar · Q2

Q. Rising of dough is due to:

(a) Multiplication of yeast

(b) Production of CO2

(d) Hydrolysis of wheat flour starch into sugars.

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Concept understanding — Microbial Fermentation Foods

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.

Note

The NCERT textbook (Class XII, Biology, Chapter 10: Microbes in Human Welfare) introduces fermented foods under the topic “Microbes in Household Products.” It lists curd, cheese, idli, dosa, and bread as common examples. The key point is that these are not modern inventions — they have been part of Indian and global diets for centuries.

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.

Important

The NCERT emphasises that these processes are not random. They require specific conditions — temperature, pH, moisture, and the right starter culture. If the conditions go wrong, harmful microbes can grow instead. That is why traditional methods (like adding a spoonful of previous curd) are actually precise microbial techniques passed down through generations.

For a commerce or humanities student, the relevance goes beyond the kitchen. Fermented foods are a multi‑billion‑dollar industry. They involve supply chains (milk, grains, fruits), processing technology, quality control, and marketing. Understanding the basic science helps you see why a product like yogurt has a “use by” date, why some cheeses are expensive, and why traditional foods like kimchi or kombucha have become global trends.

In short: microbial fermentation is nature’s way of transforming food using invisible helpers. It is a bridge between biology and everyday life — and it has been feeding humanity long before anyone knew what a microbe was.

Microbial fermentation is a well-established topic in the NCERT Class 12 Biology curriculum, commonly explored through searches like "Microbial Fermentation Foods: definition, examples and applications" and "list microbes used in food production" ahead of CBSE board exams. Students preparing for NEET and other competitive exams often revise this alongside "Microbes in Human Welfare important questions," since it links directly to biotechnology and industrial microbiology.

The rising of dough is primarily due to the production of carbon dioxide (CO₂) gas by yeast during fermentation, which gets trapped in the dough's gluten network, causing it to expand.

When you mix flour, water, and yeast to make dough, you are setting the stage for a fascinating biological and chemical process. The yeast, a living fungus, is the key actor here. It begins to feed on the sugars present in the flour, breaking them down through a process called fermentation. This is not a quick event; it takes time for the yeast to become active and multiply.

The most important outcome of this fermentation is the release of carbon dioxide gas. As the yeast consumes the sugars, it produces CO₂ as a byproduct. This gas forms tiny bubbles within the dough. The dough's structure, made of gluten proteins from the flour, is elastic and stretchy. These gas bubbles get trapped in the gluten network, much like air being blown into a balloon. As more and more CO₂ is produced, the dough inflates and rises. This is why you see the dough double in size after it is left to rest.

Note

While yeast does multiply during this process, its multiplication is a slower, longer-term effect. The immediate and direct cause of the dough rising is the gas produced by the existing yeast cells, not the increase in their number.

Now, let's look at the other options to see why they are incorrect. Option (A), "Multiplication of yeast," is a part of the overall process but not the direct cause of the rising. The yeast cells do divide and increase in number, but this population growth does not itself create the physical expansion. The rising happens because of the gas they release.

Option (C), "Emulsification," is a process where two immiscible liquids (like oil and water) are mixed together. This has nothing to do with dough rising. Option (D), "Hydrolysis of wheat flour starch into sugars," is actually the first step that provides food for the yeast. Enzymes in the flour and yeast break down the complex starch into simple sugars. This is necessary for fermentation to occur, but it is the consequence of that fermentation—the CO₂—that makes the dough rise.

Important

The NCERT textbook clearly states that the rising of dough is due to the production of carbon dioxide gas by yeast. This is a fundamental concept in the chapter on Microbes in Human Welfare.

So, while yeast multiplication and starch hydrolysis are essential supporting acts, the star of the show is the carbon dioxide gas. It is the physical force that puffs up the dough, creating the light, airy texture we associate with bread and idli.

✓Final answer

In short, the rising of dough is directly caused by the production of carbon dioxide (CO₂) gas during yeast fermentation, which gets trapped in the dough's gluten network and causes it to expand.

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Q.Rising of dough is due to:

Concept understanding — Microbial Fermentation Foods

Alternative Approach: A Cause-and-Effect Chain (Not a Definition)

Q.Rising of dough is due to:

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