Give examples to prove that microbes release gases during metabolism.

Exercises · Q2

Q. Give examples to prove that microbes release gases during metabolism.

Mahe DhseTextbookSubjective· 2mImportance★★★★★

3% · 2/80 Questions

✓ Free question

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.

Microbes release gases as byproducts of their metabolic processes, particularly during anaerobic respiration or fermentation, which is evident in bread making, alcoholic beverage production, and biogas generation.

Microbes, the microscopic organisms ubiquitous in our environment, are constantly engaged in a myriad of biochemical reactions collectively known as metabolism. These processes are fundamental to their survival, allowing them to extract energy from nutrients, synthesize cellular components, and reproduce. During these metabolic activities, microbes break down complex organic substances into simpler ones. Often, as a consequence of these transformations, various gases are released as byproducts.

The release of gases by microbes is particularly prominent during anaerobic metabolism, a process that occurs in the absence of oxygen. When oxygen is unavailable, many microbes resort to fermentation or anaerobic respiration to generate energy. In these pathways, organic compounds serve as electron acceptors, and their breakdown often yields gaseous end products. The specific type of gas produced depends on the microbe, the substrate it is metabolizing, and the particular metabolic pathway it employs.

Here are some prominent examples illustrating how microbes release gases during their metabolism:

Bread Making: The soft, porous texture of bread is a direct result of microbial gas production. Yeast, specifically Saccharomyces cerevisiae, is commonly used in baking. When mixed with flour and water, the yeast ferments the sugars present in the dough. During this fermentation, yeast metabolizes glucose and other simple sugars, producing ethanol and, crucially, carbon dioxide gas. The carbon dioxide gets trapped within the dough, causing it to rise and giving bread its characteristic airy structure. Baking then evaporates the ethanol, leaving behind the gas pockets.
Alcoholic Beverages: The production of alcoholic drinks like wine, beer, and whisky also relies on the gas-releasing metabolism of yeast. In wine production, yeast ferments the sugars in grape juice. For beer, yeast ferments the malted cereals. In both cases, the yeast converts sugars into ethanol and carbon dioxide. While ethanol is the desired product, the carbon dioxide is also a significant byproduct. In some beverages, like sparkling wine or champagne, this dissolved carbon dioxide is intentionally retained to create effervescence.
Biogas Production: A powerful example of microbial gas release for human benefit is the generation of biogas. Biogas plants utilize a consortium of anaerobic microbes, including methanogens (methane-producing bacteria like Methanobacterium), to break down organic waste materials such as animal dung, agricultural residues, and sewage. These microbes anaerobically digest the complex organic matter, converting it into a gaseous mixture known as biogas. Biogas primarily consists of methane (CH4), along with carbon dioxide (CO2) and some hydrogen (H2). Methane, being combustible, serves as an excellent fuel for cooking, lighting, and generating electricity.

Important

Methanogens are obligate anaerobes, meaning they cannot survive in the presence of oxygen. Their metabolic pathways are specifically adapted to produce methane in oxygen-free environments.
Sewage Treatment: In the secondary treatment stage of wastewater, after primary sedimentation, the effluent is passed into large aeration tanks where aerobic microbes consume organic matter. However, in the subsequent anaerobic sludge digesters, different types of anaerobic bacteria come into play. These microbes digest the activated sludge (the microbial flocs from the aerobic stage) and other organic components, producing biogas. This process not only reduces the volume of the sludge but also generates a valuable energy source.

Note

The gases produced during sewage treatment, primarily methane, carbon dioxide, and hydrogen, are collectively known as biogas and can be used as an energy source.

The diverse metabolic capabilities of microbes, particularly their ability to thrive in anaerobic conditions, lead to the production of various gases. These gaseous byproducts are not merely waste; they are often harnessed for industrial processes, food production, and energy generation, showcasing the profound impact of microbial metabolism on our daily lives and environment.

✓Final answer

Microbes release gases such as carbon dioxide, methane, and hydrogen as byproducts of their metabolic activities, especially during anaerobic respiration or fermentation, which is exemplified by the rising of bread dough, the effervescence in alcoholic beverages, and the production of biogas from organic waste.

←Previous questionQ1 — Bacteria cannot be seen with the naked eyes, but these can be seen wit…→Next questionQ3 — In which food would you find lactic acid bacteria? Mention some of the…

Q.Give examples to prove that microbes release gases during metabolism.

🎓Unlock everything free for 14 days

Unlock everything free for 14 days