Microbes at home: food processing
- Curd: Lactobacillus and other lactic acid bacteria (LAB) turn milk sugar into lactic acid. The acid makes milk proteins clump, so milk sets. A little curd added to milk is called the starter (inoculum) because it has millions of LAB. LAB also increase vitamin B12, and in our stomach they help to keep harmful germs down.
- Dosa and idli batter: bacteria ferment it and the CO2 gas makes it puff up.
- Bread: baker's yeast Saccharomyces cerevisiae ferments the dough; CO2 makes it rise.
- Toddy: a traditional drink from fermented palm sap.
- Cheese: different microbes give different taste and texture. The big holes in Swiss cheese come from CO2 made by Propionibacterium sharmanii. Roquefort cheese is ripened with a specific fungus for its flavour.
- Fish, soybean and bamboo shoots are also fermented to make foods.
Microbes in industry
In industry, microbes are grown in huge vessels called fermenters. They are fed sugar and kept at the right temperature. They give many products.
Drinks
Yeast (Saccharomyces cerevisiae, also called brewer's yeast) ferments malted grain and fruit juice to make ethanol. Wine and beer are made without distillation. Whisky, brandy and rum are made by distilling the fermented broth, so they are much stronger.
Organic acids
| Product | Microbe |
|---|---|
| Citric acid | Aspergillus niger (fungus) |
| Acetic acid (vinegar) | Acetobacter aceti (bacterium) |
| Butyric acid | Clostridium butylicum (bacterium) |
| Lactic acid | Lactobacillus (bacterium) |
Enzymes
- Lipases in detergents remove oily stains.
- Pectinases and proteases make bottled fruit juice clear.
- Streptokinase from Streptococcus (now also made by genetic engineering) dissolves blood clots in heart-attack patients: a 'clot buster'.
Other bioactive molecules
- Cyclosporin A from the fungus Trichoderma polysporum: an immunosuppressant, it stops the body rejecting a transplanted organ.
- Statins from the yeast Monascus purpureus: they lower blood cholesterol by blocking an enzyme that makes it.
Sewage treatment
Sewage is the dirty water from homes and cities. It has lots of organic matter and germs. It is cleaned in a sewage treatment plant (STP) in two main stages.
Primary treatment (physical)
Big objects are removed by filtering. Grit (soil and small stones) settles. Then solids settle at the bottom as primary sludge. The liquid above is the effluent.
Secondary treatment (biological)
- The effluent goes to large aeration tanks. Air is pumped in and it is stirred.
- Useful aerobic microbes grow into flocs: groups of bacteria tied together with fungal threads, like tiny nets.
- Flocs eat the organic matter. This lowers the BOD.
- When BOD is low enough, the water goes to a settling tank. The flocs settle as activated sludge. A small part is sent back to the aeration tank as a starter (inoculum).
- The rest of the sludge goes to anaerobic sludge digesters. Here other bacteria digest it without air and make biogas (methane, H2S, CO2).
- The cleaned water is released into rivers and streams.
What is BOD?
BOD (Biochemical Oxygen Demand) is the amount of oxygen that bacteria use up to break down all the organic matter in one litre of water. More dirt → more food for bacteria → more oxygen used → higher BOD. So BOD tells us how polluted water is. Clean water has a low BOD.
India's Ministry of Environment and Forests started the Ganga Action Plan and Yamuna Action Plan to build many STPs so that only treated sewage enters these rivers.
Biogas
Biogas is a gas fuel made by microbes. It is mostly methane, with CO2 and a little H2S and H2.
The bacteria that make methane are called methanogens, for example Methanobacterium. They live only where there is no oxygen (anaerobic): in sewage sludge digesters and in the rumen (first stomach part) of cattle, where they help digest cellulose. So cow dung (gobar) is full of methanogens.
How a biogas plant works:
- A concrete tank (about 10–15 feet deep) is filled with a slurry of dung and water.
- A floating cover sits on top. As gas forms, the cover rises.
- A pipe takes the gas to homes for cooking and lighting.
- Used slurry comes out from another outlet and is used as fertiliser.
In India this technology was developed mainly through the efforts of IARI (Indian Agricultural Research Institute) and KVIC (Khadi and Village Industries Commission).
Microbes as biocontrol agents
Biocontrol means using living things to control pests and plant diseases, instead of chemical poisons. Poisons also kill useful insects, pollute soil and water and enter our food.
Organic farmers try to keep a balance: they do not kill every insect, they let predators keep pests low.
- Ladybird beetle eats aphids; dragonflies eat mosquitoes.
- Bacillus thuringiensis (Bt): its spores are sprayed on plants. Caterpillars eat them, the toxin is released in their gut and they die. Other insects are not harmed. Bt toxin genes have also been put into plants, such as Bt cotton.
- Trichoderma: a free-living fungus in roots; it fights many fungi that cause plant disease.
- Baculoviruses (genus Nucleopolyhedrovirus): attack insects and some arthropods only. They are narrow-spectrum, so they do not harm plants, birds, mammals or useful insects. Good for integrated pest management (IPM).
Microbes as biofertilisers
Biofertilisers are living organisms that make soil richer in nutrients. They cut the use of chemical fertilisers.
- Rhizobium: lives in root nodules of legume plants (pea, gram, beans) and fixes nitrogen from the air into forms the plant can use (symbiotic).
- Azospirillum and Azotobacter: free-living in soil; also fix nitrogen.
- Mycorrhiza: fungi (like Glomus) living with plant roots. They take up phosphorus from soil for the plant, and also help against root germs, salt and drought.
- Cyanobacteria (blue-green algae) like Anabaena, Nostoc, Oscillatoria: fix nitrogen, especially in paddy fields, and add organic matter.
In India, many biofertilisers are sold in markets and farmers use them regularly.
Antibiotics and their careful use
Antibiotics ('against life') are chemicals made by some microbes that kill or stop the growth of disease-causing microbes (mainly bacteria).
Penicillin was the first. Alexander Fleming was growing Staphylococcus bacteria. He saw that a mould, Penicillium notatum, had grown on one plate, and around it the bacteria could not grow. He named the chemical penicillin. Later Ernst Chain and Howard Florey made it into a working medicine; it saved many soldiers in World War II. The three shared the Nobel Prize in 1945.
Antibiotics have greatly reduced deaths from plague, whooping cough, diphtheria and leprosy.
Use them carefully
- Antibiotics do not work on viruses (cold, flu, dengue).
- Take them only on a doctor's advice, in the right dose.
- Complete the full course. If you stop early, the toughest bacteria survive, multiply and pass on resistance.
- Overuse and misuse make antibiotic-resistant bacteria ('superbugs'), which are very hard to treat.
Try it: make curd and watch the change
Take one cup of warm milk (about body temperature). Add one spoon of curd and stir. Cover and keep it in a warm place. Look at it and taste a drop after 2, 4 and 8 hours. Write down when it becomes thick and sour. Then try with cold milk: it takes much longer, because LAB grow slowly in the cold.
Key formulas and definitions
- Milk --(LAB)--> curd (lactic acid; more vitamin B12)
- Sugar --(yeast)--> ethanol + CO₂
- High BOD = more organic pollution; secondary treatment lowers BOD
- Biogas = CH₄ (mostly) + CO₂ + H₂S + H₂; made by methanogens without O₂
- Rhizobium, Azotobacter, Azospirillum, cyanobacteria → nitrogen; mycorrhiza (Glomus) → phosphorus
- Penicillin: Fleming (Penicillium notatum); Chain and Florey made it a medicine
Worked examples
1. Why does warm milk turn to curd faster than cold milk after adding a starter?
LAB are living cells. In warm milk (about 30–40 °C) they grow and divide fast, so they make lactic acid quickly. In cold milk they grow slowly, so curd takes much longer.
2. Sample A has a BOD of 20 mg/L and sample B has a BOD of 400 mg/L. Which is more polluted and why?
Sample B. A higher BOD means there is more organic matter for bacteria to eat, so they use more oxygen. B has 20 times the BOD of A, so it is far more polluted.
3. A plant has 300 mg/L BOD in sewage. After aeration it is 30 mg/L. By what percent did BOD fall?
Fall = 300 − 30 = 270 mg/L. Percent = 270 / 300 × 100 = 90%. The flocs ate about nine-tenths of the organic matter.
4. Why must a biogas plant be kept airtight?
Methanogens are anaerobic. Oxygen stops them working. Also, if air mixes with methane, it can become an explosive mixture. So the tank must be sealed.
5. A farmer grows pea and then wheat on the same land. Why does the wheat need less urea?
Pea is a legume. Its root nodules hold Rhizobium, which fixes nitrogen from air into the soil. Some of this nitrogen stays in the soil for the next crop, so wheat needs less chemical fertiliser.
6. A patient stops an antibiotic on day 2 because he feels better. A week later the infection returns and the same medicine does not work. Explain.
Two days killed the weak bacteria but not the tough ones. The survivors multiplied. They are now resistant to that antibiotic, so it no longer works. The full course would have killed them too.
Common mistakes
- Saying antibiotics kill viruses. They act on bacteria; cold and flu are viral.
- Thinking high BOD means clean water. High BOD = more organic pollution.
- Mixing up products: citric acid – Aspergillus niger; acetic acid – Acetobacter aceti; statins – Monascus purpureus; cyclosporin A – Trichoderma polysporum.
- Saying wine and beer are distilled. They are not; whisky, brandy and rum are distilled.