Food microbiology studies the microbes that spoil food, cause food-borne illness, and help make fermented foods. This topic covers factors affecting microbial growth in food, types of spoilage, food poisoning and food infection, methods of food preservation, and useful fermented foods including Indian examples.
Food poisoning (intoxication): illness from a toxin already formed in food — Staphylococcus aureus (heat-stable enterotoxin; vomiting within a few hours), Clostridium botulinum (botulinum neurotoxin; paralysis; linked to poorly canned low-acid foods), Bacillus cereus (reheated rice).
Food infection: live pathogens eaten multiply in the gut — Salmonella (eggs, poultry), Shigella, E. coli O157:H7, Vibrio parahaemolyticus (seafood), Listeria monocytogenes (grows even at refrigerator temperature).
Preservation methods: high temperature (pasteurization, canning, boiling), low temperature (refrigeration about 4 °C, freezing −18 °C or lower), drying/dehydration, high salt or sugar (lowers aw: pickles, jams), chemical preservatives (sodium benzoate, potassium metabisulphite, sorbic acid), irradiation, modified-atmosphere packaging, fermentation, bacteriocins like nisin (from Lactococcus lactis).
Canning uses the 12-D concept: heat enough to reduce C. botulinum spores by 12 log cycles.
Fermented foods: curd/dahi, idli and dosa batter (lactic acid bacteria such as Leuconostoc mesenteroides and yeasts), bread (Saccharomyces cerevisiae), vinegar (Acetobacter), sauerkraut, soy sauce, cheese.
Probiotics: live beneficial microbes (e.g. Lactobacillus, Bifidobacterium) that improve health when eaten in enough amounts.
Important terms
Water activity (aw) – free water available for microbes (pure water = 1.0).
Intoxication – illness due to a pre-formed toxin.
Food infection – illness due to growth of live pathogens in the body.
Bacteriocin – antibacterial protein made by bacteria.
Hurdle technology – combining several mild preservation methods.
Key facts
Disease
Organism
Typical food
Staphylococcal food poisoning
Staphylococcus aureus
Milk products, sweets handled by people
Botulism
Clostridium botulinum
Improperly canned foods
Salmonellosis
Salmonella spp.
Eggs, poultry, meat
Emetic food poisoning
Bacillus cereus
Reheated fried rice
Listeriosis
Listeria monocytogenes
Soft cheese, cold meats
5 important questions
Q1. What is water activity?
The ratio of the vapour pressure of water in a food to that of pure water at the same temperature. It shows how much free water is available for microbial growth; lowering it (drying, salting) preserves food.
Q2. Differentiate food intoxication and food infection.
Intoxication is caused by eating toxin already formed in food (e.g. S. aureus, C. botulinum); symptoms start quickly. Infection is caused by live microbes multiplying in the body (e.g. Salmonella); symptoms take longer.
Q3. Explain intrinsic and extrinsic factors affecting microbial growth in food.
Fermented foods are foods changed by the controlled action of microorganisms, which improves taste, texture, shelf life and nutrition. India has a rich tradition of them — curd, idli, dosa, dhokla, jalebi batter, pickles. Probiotics are live beneficial microbes, prebiotics are the food that feeds them, and synbiotics combine both. This topic covers important fermented foods and the health role of probiotics.
Key points
Lactic acid bacteria (LAB) — Lactobacillus, Lactococcus, Leuconostoc, Pediococcus, Streptococcus thermophilus — are the main fermenting organisms. They are Gram-positive, non-spore-forming, catalase-negative and make lactic acid, which lowers pH and stops spoilage organisms.
Yeasts (Saccharomyces cerevisiae) make CO₂ and ethanol (bread, beverages); moulds (Aspergillus oryzae, Rhizopus) are used in Asian foods like soy sauce and tempeh.
Idli and dosa: soaked rice and black gram (urad dal) are ground and fermented overnight. Leuconostoc mesenteroides starts the fermentation and produces CO₂ that raises the batter; Enterococcus faecalis, Pediococcus and yeasts join later. The fermentation increases B vitamins and digestibility.
Dhokla (Bengal gram batter), jalebi batter, kanji (black carrot drink), bhatura dough and dosa are other Indian fermented foods.
Sauerkraut (fermented cabbage): heterofermentative Leuconostoc mesenteroides first, then homofermentative Lactobacillus plantarum; 2–3% salt draws out juice.
Pickles (cucumber, mango) and olives are also LAB fermentations under salt.
Bread: baker's yeast ferments sugars to CO₂ (raises dough) and ethanol (evaporates on baking).
Vinegar: two steps — yeast changes sugar to ethanol, then Acetobacter (Gluconobacter) oxidizes ethanol to acetic acid (about 4–8%).
Soy sauce (Aspergillus oryzae koji, then LAB and yeasts), tempeh (Rhizopus oligosporus), kimchi (Korean, LAB).
Probiotics: "live microorganisms which, when administered in adequate amounts, confer a health benefit on the host" (FAO/WHO, 2001). Examples: Lactobacillus acidophilus, L. rhamnosus GG, Bifidobacterium spp., Saccharomyces boulardii (a yeast).
Qualities of a good probiotic: survives stomach acid and bile, sticks to the gut lining, is safe (GRAS), has proven benefit, stays alive during storage.
Benefits: prevent and treat diarrhoea (including antibiotic-associated diarrhoea), help lactose digestion, compete with pathogens, strengthen gut barrier and immunity, may lower cholesterol.
Prebiotics: non-digestible food ingredients that feed good gut bacteria — inulin, fructo-oligosaccharides (FOS), galacto-oligosaccharides (GOS); found in onion, garlic, banana, chicory.
Synbiotics: probiotic + prebiotic together. Postbiotics: non-living microbial products or cell parts that give health benefits.
Important terms
Starter culture – known microorganisms added to start a controlled fermentation.
Homofermentative – LAB that make mainly lactic acid from glucose.
Heterofermentative – LAB that make lactic acid plus CO₂ and ethanol or acetic acid.
Probiotic – live microbe that gives health benefit in adequate amounts.
Prebiotic – non-digestible food that selectively feeds beneficial gut microbes.
Prebiotics are non-digestible food ingredients that pass to the colon and selectively feed beneficial bacteria like bifidobacteria. Examples: inulin and fructo-oligosaccharides.
Q2. Why are lactic acid bacteria important in food fermentation?
They produce lactic acid that lowers pH, preserves the food, gives sour flavour, and some make bacteriocins (e.g. nisin) and vitamins. They are safe (GRAS).
Q3. Describe the microbiology of idli fermentation.
Ingredients and soaking, grinding, overnight fermentation at room temperature.
Role of Leuconostoc mesenteroides (CO₂, acid), later Enterococcus, Pediococcus, yeasts.
Changes: leavening, pH fall, increased B vitamins and protein digestibility.
Flow chart.
Q4. Explain probiotics: definition, properties and health benefits.
FAO/WHO definition and examples.
Selection criteria.
Mechanisms: competitive exclusion, acid and bacteriocin production, immune modulation.
Health benefits and products in market (probiotic curd, capsules).
Q5. Write notes on vinegar and sauerkraut production.
Vinegar: alcoholic fermentation then acetic fermentation; Orleans (slow) and generator (quick) methods.
Sauerkraut: shredded cabbage, salt, anaerobic, sequence of LAB.
Exam tip
Learn the organism for each fermented food in table form; "idli fermentation" and "probiotics vs prebiotics" are frequent short notes in Maharashtra universities.
Milk is a highly nutritious food and so a good medium for microbes. This topic covers the composition of milk, sources of contamination, types of milk spoilage, pasteurization and its tests, milk quality tests like the MBRT, and fermented dairy products such as curd, yoghurt, butter and cheese.
Key points
Composition of cow milk (approximate): water about 87%, lactose about 4.5–5%, fat about 3.5–4%, proteins about 3.3% (mainly casein, plus whey proteins), minerals about 0.7%.
Milk inside a healthy udder is nearly sterile, but it gets contaminated from the teat canal, udder skin, milker's hands, utensils, air, water and fodder.
Normal milk flora: Lactococcus, Lactobacillus, Micrococcus, coliforms, yeasts and moulds.
Gassy fermentation – coliforms, Clostridium produce CO₂ and H₂.
Proteolysis – protein breakdown, bitter taste (Bacillus, Pseudomonas).
Lipolysis – fat breakdown → rancid odour.
Ropy (slimy) milk – capsule-forming bacteria like Alcaligenes viscolactis.
Colour changes – blue (Pseudomonas syncyanea), red (Serratia marcescens), yellow (Pseudomonas synxantha).
Types of spoilage:
Pasteurization: LTLT/holding method — 63 °C for 30 min; HTST — 72 °C for 15 s; UHT — about 135–150 °C for a few seconds (long shelf life in sealed packs).
Coxiella burnetii is used as the index organism for pasteurization, as it is among the most heat-resistant non-spore-forming pathogens in milk.
Phosphatase test: alkaline phosphatase is destroyed by proper pasteurization; its presence shows under-heating or mixing with raw milk.
MBRT (methylene blue reduction test): methylene blue is decolourized by microbes as they use oxygen; faster decolourization = poorer quality.
Other tests: standard plate count, coliform count, resazurin test, clot-on-boiling test.
Fermented products: curd/dahi (Lactococcus lactis, Lactobacillus), yoghurt (Streptococcus thermophilus + Lactobacillus delbrueckii subsp. bulgaricus), cheese (rennet + starter culture; ripening with moulds like Penicillium roqueforti in blue cheese, P. camemberti in Camembert), butter and buttermilk, kefir.
Important terms
Starter culture – selected microbes added to start a fermentation.
Rennet – enzyme (chymosin) that coagulates milk protein in cheese making.
Pasteurization – heat treatment that kills pathogens but not all microbes.
Mastitis – infection of the udder (Staphylococcus aureus, Streptococcus agalactiae).
Index organism – organism used to set a process standard.
Key facts
MBRT grading (common textbook scale):
Decolourization time
Milk quality
5 hours or more
Very good
3–4 hours
Good
1–2 hours
Fair
30 minutes or less
Poor
(Exact time bands vary slightly between textbooks and standards.)
5 important questions
Q1. What is the phosphatase test?
A test for proper pasteurization. Alkaline phosphatase, a natural milk enzyme, is destroyed at pasteurization temperatures; if it is still active, the milk was under-pasteurized or mixed with raw milk.
Q2. What is ropy milk?
Milk that becomes slimy and forms threads because of capsule-producing bacteria such as Alcaligenes viscolactis.
Q3. Describe the types of spoilage of milk.
Souring by lactic acid bacteria.
Gassy fermentation by coliforms, clostridia.
Proteolysis (bitter milk).
Lipolysis (rancid milk).
Ropiness.
Abnormal colours (blue, red, yellow) with organisms.
Q4. Explain the methods of pasteurization and their testing.
LTLT: 63 °C, 30 min.
HTST: 72 °C, 15 s, followed by rapid cooling.
UHT: very high temperature for seconds.
Purpose: kill pathogens (index Coxiella burnetii), increase shelf life.
Tests: phosphatase test, standard plate count, coliform test.
Q5. Describe the MBRT and its significance.
10 mL milk + 1 mL methylene blue solution in a sterile tube.
Incubate at 37 °C in a water bath.
Note time taken for the blue colour to disappear.
Microbes use dissolved oxygen and reduce the dye.
Shorter time → more microbes → poor milk.
Quick, cheap test used at collection centres.
Exam tip
Remember the yoghurt pair (S. thermophilus + L. bulgaricus) and both pasteurization temperatures — they are asked often.
Soil is full of microorganisms — bacteria, actinomycetes, fungi, algae and protozoa — that keep it fertile by breaking down organic matter and recycling nutrients. This topic covers soil microflora, the rhizosphere, the nitrogen cycle, nitrogen fixation, phosphate solubilization, and biofertilizers used in Indian agriculture.
Key points
Soil microbes: bacteria (most numerous), actinomycetes (e.g. Streptomyces, give the earthy smell due to geosmin), fungi, algae (including cyanobacteria) and protozoa.
Rhizosphere: narrow zone of soil around plant roots, rich in microbes because roots release sugars and amino acids (root exudates). R:S ratio compares microbial numbers in rhizosphere and non-rhizosphere soil (usually more than 1).
Denitrification: NO₃⁻ → N₂ (Pseudomonas, Paracoccus) under anaerobic conditions.
Nitrogen cycle steps:
Symbiotic: Rhizobium in root nodules of legumes (gram, soybean, groundnut); Frankia in non-legumes like Casuarina; Anabaena azollae in the water fern Azolla.
Free-living (non-symbiotic): Azotobacter (aerobic), Clostridium (anaerobic), Beijerinckia, cyanobacteria like Nostoc and Anabaena.
Associative: Azospirillum with roots of cereals and grasses.
Biological nitrogen fixation uses the enzyme nitrogenase, which is damaged by oxygen and needs much ATP.
Leghaemoglobin in nodules (pink colour) binds oxygen and protects nitrogenase.
Phosphate-solubilizing microbes (PSM) like Bacillus megaterium, Pseudomonas, Aspergillus niger release organic acids that dissolve insoluble phosphates.
Mycorrhiza: fungus–root partnership; VAM/AM fungi (e.g. Glomus) increase phosphorus and water uptake.
Biofertilizers: preparations of living microbes that improve nutrient supply to plants. Types: nitrogen-fixing (Rhizobium, Azotobacter, Azospirillum, blue-green algae, Azolla), phosphate-solubilizing, potassium-mobilizing, mycorrhizal.
Carrier: lignite or peat powder is commonly used to carry the microbes.
Advantages: cheap, renewable, eco-friendly, improve soil health; reduce chemical fertilizer use. Limitations: slower action, short shelf life, sensitive to heat and chemicals, need specific strains for specific crops.
Composting and vermicomposting use microbes (and earthworms) to turn organic waste into manure.
Important terms
Rhizosphere – soil zone influenced by roots.
Nitrogenase – enzyme complex that reduces N₂ to NH₃.
Root nodule – swelling on legume roots where Rhizobium fixes nitrogen.
Inoculant – biofertilizer culture applied to seed or soil.
Humus – dark, stable organic matter formed by decomposition.
The increase in the number and activity of microorganisms in the soil around roots, because roots release nutrients (exudates). It is expressed as the R:S ratio.
Q2. Name two symbiotic and two free-living nitrogen fixers.
Plants suffer from diseases caused by fungi, bacteria, viruses, phytoplasmas and nematodes, and these cause huge crop losses in India every year. Plant pathology (phytopathology) studies the causes, symptoms, spread and control of these diseases. This topic covers how pathogens attack plants, the main symptoms, important bacterial, fungal and viral diseases of Maharashtra and Indian crops, and methods of control.
Key points
Disease triangle: a disease needs a susceptible host, a virulent pathogen and a favourable environment (humidity, temperature, rain).
Fungi cause most plant diseases (roughly 70–80%), then viruses, bacteria and others.
How pathogens enter: directly through the cuticle (fungi with appressoria), through natural openings (stomata, hydathodes, lenticels) and through wounds (most bacteria).
Weapons of pathogens: cell wall-degrading enzymes (pectinases, cellulases – cause soft rot), toxins (e.g. victorin, tabtoxin), growth regulators (auxin in crown gall), and blocking of xylem (wilts).
Late blight of potato – Phytophthora infestans (an oomycete, a fungus-like organism); caused the Irish famine of 1845–47.
Blast of rice – Magnaporthe oryzae (Pyricularia oryzae).
Wilt of cotton and pigeon pea (tur) – Fusarium spp.
Powdery mildew (grapes, peas) and downy mildew (grapes, bajra).
Red rot of sugarcane – Colletotrichum falcatum.
Fungal diseases:
Viral diseases: tobacco mosaic (TMV, mechanical spread), leaf curl of tomato and papaya ring spot (whitefly or aphid vectors), banana bunchy top (aphid), yellow vein mosaic of okra (whitefly).
Phytoplasma diseases: grassy shoot of sugarcane, little leaf of brinjal, sandal spike (spread by leafhoppers).
It shows that a plant disease develops only when three factors meet: a susceptible host, a virulent pathogen and a favourable environment. Removing any one prevents the disease.
Q2. Name the causal organism and one symptom of citrus canker.
Xanthomonas citri (Gram-negative rod). It forms raised, brown, corky lesions with a yellow halo on leaves, twigs and fruits.
Q3. Describe the mechanisms by which pathogens attack plants.
Entry: direct, natural openings, wounds.
Enzymes degrading cuticle and cell wall.
Toxins and growth regulators.
Vascular blocking in wilts.
Defence of plants (brief).
Q4. Describe two bacterial diseases of plants with symptoms and control.
Citrus canker and bacterial blight of rice (or crown gall).
Pathogen, symptoms, spread, control (copper sprays, streptocycline, resistant varieties, pruning).
Q5. Explain the methods of controlling plant diseases.
Learn one bacterial, one fungal and one viral disease fully (pathogen, symptoms, control). Crown gall links to genetic engineering — mention the Ti plasmid.
Water can carry disease-causing microbes, so testing and purifying drinking water is a key public-health job. This topic covers water-borne diseases, indicator organisms, bacteriological tests of water (MPN and membrane filter), water purification, and how sewage is treated by primary, secondary and tertiary methods.
Key points
Water-borne diseases: cholera (Vibrio cholerae), typhoid (Salmonella Typhi), bacillary dysentery (Shigella), hepatitis A and E (viruses), polio, amoebic dysentery (Entamoeba histolytica), giardiasis (Giardia).
Testing for every pathogen is not practical, so we test for indicator organisms that show faecal pollution.
Coliforms (Gram-negative, non-spore-forming rods that ferment lactose with acid and gas within 48 h at 35–37 °C) are the usual indicators. Faecal (thermotolerant) coliforms, especially E. coli, grow at 44.5 °C and show recent faecal contamination.
Other indicators: faecal streptococci (enterococci), Clostridium perfringens (shows older or intermittent pollution, as spores survive long).
An ideal indicator: present in faeces in large numbers, absent from clean water, survives slightly longer than pathogens, easy and quick to detect, does not multiply in water.
Presumptive test: water inoculated into lactose broth (or MacConkey broth) tubes with Durham's tubes; acid and gas = positive.
Confirmed test: positive tubes subcultured onto EMB or Endo agar (or BGLB broth); E. coli gives colonies with a green metallic sheen on EMB.
Completed test: colonies checked by Gram stain (Gram-negative rods) and gas in lactose broth.
MPN (multiple tube fermentation) test — three stages:
The MPN (most probable number) of coliforms per 100 mL is read from statistical MPN tables.
Membrane filter technique: known volume (e.g. 100 mL) filtered through a 0.45 µm membrane; membrane placed on selective medium (e.g. m-Endo); colonies counted directly. Fast and precise.
IMViC tests (Indole, Methyl red, Voges–Proskauer, Citrate) separate E. coli (+ + − −) from Enterobacter aerogenes (− − + +).
Water purification (municipal): sedimentation → coagulation/flocculation (alum) → filtration (slow or rapid sand filters) → disinfection (chlorination, ozone, UV).
BOD (biochemical oxygen demand): oxygen used by microbes to oxidize organic matter in water in 5 days at 20 °C (BOD₅). High BOD = more pollution. COD uses a chemical oxidant (dichromate) and is usually higher than BOD.
Q1. Why is E. coli used as an indicator of faecal pollution?
It lives only in the intestine of humans and warm-blooded animals, is passed in large numbers in faeces, does not normally multiply in clean water and is easy to detect. So its presence shows recent faecal contamination.
Q2. Define BOD.
Biochemical oxygen demand — the amount of dissolved oxygen used by microorganisms to break down organic matter in a water sample during 5 days at 20 °C. It measures organic pollution.
Q3. Describe the MPN test for coliforms.
Presumptive: lactose broth tubes with Durham's tubes (e.g. 3 × 10 mL, 3 × 1 mL, 3 × 0.1 mL sample), 35–37 °C, 24–48 h; acid + gas = positive.
Confirmed: streak on EMB/Endo; typical colonies (metallic sheen).
Completed: Gram stain, lactose fermentation.
Read MPN/100 mL from tables.
IMViC to identify E. coli.
Q4. Explain secondary treatment of sewage.
Biological oxidation of organic matter by aerobic microbes.
Activated sludge: aeration tank, flocs of bacteria and protozoa, settling, sludge recycled.
Trickling filter: sewage sprinkled over stones covered with microbial film.
Industrial microbiology uses microbes on a large scale to make useful products — alcohol, antibiotics, organic acids, enzymes, vitamins and vaccines. This topic covers the fermenter (bioreactor) and its parts, types of fermentation, strain improvement, media, downstream processing, and production of key products like penicillin, ethanol and citric acid.
Key points
In industry, "fermentation" means any large-scale culture of microbes to make a product, aerobic or anaerobic.
Primary metabolites are made during the growth (log) phase and are essential for growth (ethanol, amino acids, organic acids). Secondary metabolites are made mostly in the stationary phase (antibiotics, pigments).
Fermenter (bioreactor) — usually stainless steel. Parts: vessel, impeller/agitator (mixing), sparger (air bubbles), baffles (prevent vortex), cooling jacket or coils, probes for pH, temperature, dissolved oxygen and foam, inlet/outlet ports, sampling port.
Types of fermenters: stirred tank (most common), airlift, bubble column, packed bed.
Batch, fed-batch (nutrients added gradually, e.g. penicillin, baker's yeast) and continuous fermentation.
Submerged (liquid) and solid-state fermentation (on moist solids like wheat bran, e.g. some enzymes).
Upstream processing: strain selection and improvement (mutation, recombination, genetic engineering), media formulation, sterilization, inoculum development.
Industrial media use cheap raw materials: molasses (sugar), corn steep liquor (nitrogen, used in penicillin production), whey, starch.
Downstream processing: separation of cells (filtration, centrifugation), cell disruption (if product is inside), extraction, purification (precipitation, chromatography), concentration, drying, packaging.
Antifoam agents (silicone oils, vegetable oils) control foam.
Penicillin: Penicillium chrysogenum (high-yield strains); fed-batch, aerobic, 25–26 °C; precursor phenylacetic acid added to get penicillin G; corn steep liquor in medium.
Ethanol: Saccharomyces cerevisiae on molasses (sugarcane industry, important in Maharashtra); anaerobic; product distilled. Zymomonas mobilis is another producer.
Citric acid: Aspergillus niger, aerobic, on molasses or sucrose, with low iron/manganese and low pH.
Q1. Differentiate primary and secondary metabolites.
Primary metabolites are formed during active growth and are essential for the cell (ethanol, amino acids). Secondary metabolites are formed mainly in the stationary phase and are not essential for growth (antibiotics, toxins).
Q2. What is the role of the sparger and baffles in a fermenter?
The sparger releases sterile air as fine bubbles for aeration. Baffles are metal strips on the wall that break the vortex and improve mixing.
Q3. Describe the design of a typical stirred-tank fermenter.
Vessel of stainless steel with headspace.
Agitator and impellers; motor.
Sparger; air filter.
Baffles.
Cooling jacket/coils.
Probes: pH, temperature, DO, foam.
Ports: inoculation, sampling, harvest, acid/base and antifoam addition.
Besides antibiotics and alcohol, microorganisms are used to manufacture enzymes, amino acids and vitamins on an industrial scale. Microbes are preferred because they grow fast on cheap media, can be improved genetically and give products in large amounts. This topic covers the production of amylase and protease, glutamic acid and lysine, and vitamin B₁₂ and riboflavin, along with strain improvement.
Key points
Why microbial enzymes? Fast growth, easy scale-up, no seasonal dependence, easy genetic improvement, and many are extracellular (secreted into the medium), making recovery simple.
α-Amylase – Bacillus amyloliquefaciens, B. licheniformis (heat-stable, used in starch liquefaction), Aspergillus oryzae.
Proteases – Bacillus licheniformis (subtilisin; alkaline protease used in detergents), Aspergillus.
Glucose isomerase – Streptomyces spp.; converts glucose to fructose for high-fructose syrup.
Pectinase – Aspergillus niger; clears fruit juices.
Lipase, cellulase (Trichoderma reesei), rennet substitute (microbial chymosin made in recombinant microbes).
Main industrial enzymes and producers:
Production steps for an enzyme: strain selection → medium (starch, corn steep liquor, soybean meal) → submerged or solid-state fermentation (wheat bran for fungal enzymes) → remove cells (filtration/centrifugation) → concentrate (ultrafiltration) → precipitate (ammonium sulphate or alcohol) → dry or formulate → immobilization for repeated use.
L-Glutamic acid (sold as monosodium glutamate, MSG) – Corynebacterium glutamicum (discovered by Kinoshita, Japan, 1957).
Key trick: biotin limitation (or adding penicillin or surfactants) makes the membrane leaky so glutamate is excreted.
Aerobic, pH about 7–8, 30–37 °C; ammonium supplied as nitrogen source.
L-Lysine – auxotrophic or regulatory mutants of C. glutamicum (Brevibacterium flavum) that escape feedback inhibition; used in animal feed.
Amino acid production:
Vitamin B₁₂ (cyanocobalamin) – made only by microbes; Pseudomonas denitrificans and Propionibacterium shermanii; needs cobalt and DMB (5,6-dimethylbenzimidazole); cyanide added during recovery converts it to cyanocobalamin.
Riboflavin (vitamin B₂) – fungi Ashbya gossypii and Eremothecium ashbyii, and Bacillus subtilis.
Vitamin C – partly microbial (Reichstein process step using Gluconobacter, or two-step fermentation).
Vitamin production:
Regulation tricks: use of auxotrophic mutants and feedback-resistant mutants so that cells overproduce the product.
Uses: detergents (proteases, lipases), food (amylase in baking, pectinase in juice), textiles (amylase for desizing), leather (protease for dehairing), medicine (streptokinase, asparaginase), feed (lysine), flavour (MSG).
Important terms
Extracellular enzyme – enzyme secreted out of the cell.
Immobilized enzyme – enzyme fixed on a support so it can be reused.
Auxotroph – mutant that cannot make a nutrient and needs it in the medium.
Feedback inhibition – end product inhibits the first enzyme of its pathway.
Solid-state fermentation – growth of microbes on moist solid substrate with little free water.
Key facts
Product
Organism
Main use
α-Amylase
Bacillus licheniformis, Aspergillus oryzae
Starch processing, baking, textiles
Alkaline protease
Bacillus licheniformis
Detergents, leather
Glucose isomerase
Streptomyces spp.
High-fructose syrup
Glutamic acid
Corynebacterium glutamicum
MSG flavour enhancer
Lysine
C. glutamicum mutants
Animal feed
Vitamin B₁₂
Pseudomonas denitrificans, Propionibacterium
Medicine, feed
Riboflavin
Ashbya gossypii
Food and feed supplement
5 important questions
Q1. Why is biotin kept limited in glutamic acid fermentation?
Biotin is needed for fatty acid synthesis. Under biotin limitation the cell membrane becomes weak and leaky, so glutamic acid leaks out of the cell into the medium instead of being used inside, giving a high yield.
Q2. Name two microorganisms used for vitamin B₁₂ production.
Pseudomonas denitrificans and Propionibacterium shermanii (P. freudenreichii). Cobalt must be added to the medium.
Q3. Describe the industrial production of amylase.
Organism and its features.
Medium, fermentation conditions (submerged or solid-state).
Downstream recovery and formulation.
Applications in starch, baking, textile and brewing industries.
Q4. Explain the production of L-glutamic acid by fermentation.
Corynebacterium glutamicum; medium (glucose/molasses, ammonia, minerals, limiting biotin).
Conditions: aerobic, pH, temperature, duration about 2–3 days.
Mechanism of excretion.
Recovery as MSG by crystallization.
Flow chart.
Q5. Write a note on microbial production of vitamins.
B₁₂: organisms, cobalt, DMB, recovery as cyanocobalamin.
Riboflavin: Ashbya gossypii, flavinogenic fungi.
Uses and importance.
Exam tip
Remember organism–product pairs from the table; examiners often ask "name the organism used for …". Draw one simple flow chart for any production process.
Single cell protein (SCP) is the dried cells (biomass) of microorganisms such as algae, yeasts, fungi and bacteria, used as a protein source for humans or animals. Mushrooms are edible fleshy fruiting bodies of certain fungi grown on agricultural waste. Both turn cheap or waste materials into protein-rich food, which matters for a country like India with protein malnutrition and plenty of crop residue.
Key points
The term single cell protein was coined by Carroll L. Wilson (MIT) in 1966.
Advantages of SCP organisms: very fast growth (bacteria double in 20–120 min, yeasts in about 1–3 h), high protein content, grow on cheap or waste substrates, need little land and water, not affected by season, and can be genetically improved.
Problems / limitations: high nucleic acid content (bacteria up to about 15%) — excess raises uric acid and can cause gout and kidney stones (reduced by heat treatment that activates the cells' own RNases); possible toxins or contaminants; poor taste and digestibility of cell walls; high cost; public acceptance.
Mushroom biology: mushrooms are mostly Basidiomycetes. The fruiting body has a stalk (stipe), cap (pileus) and gills bearing basidiospores. The underground mycelium is the vegetative part.
Button mushroom – Agaricus bisporus; temperate (about 14–18 °C for fruiting); grown on composted straw with a casing layer. Main producer areas include Himachal Pradesh and Haryana; also grown in climate-controlled units.
Oyster mushroom (dhingri) – Pleurotus spp.; grows at about 20–30 °C; easiest for Maharashtra farmers; grown on chopped, soaked and pasteurized paddy or wheat straw in polythene bags.
Paddy straw mushroom – Volvariella volvacea; tropical, 30–35 °C.
Milky mushroom – Calocybe indica; suited to hot regions.
Common cultivated mushrooms in India:
Steps of oyster mushroom cultivation: spawn preparation (mycelium on boiled wheat/sorghum grains) → substrate (straw) chopped, soaked and pasteurized (hot water about 80 °C or chemical) → spawning (layering spawn in bags, about 2–5% of wet substrate weight) → spawn run in dark (about 2–3 weeks, white mycelium covers straw) → bags cut, high humidity (80–90%), light and ventilation → cropping of fruiting bodies in flushes.
Nutritional value: mushrooms are rich in protein (about 20–35% dry weight), B vitamins, minerals and fibre, and low in fat; they are good for diabetics and heart patients. Wild mushrooms can be poisonous (e.g. Amanita phalloides).
ICAR–Directorate of Mushroom Research, Solan (Himachal Pradesh), leads mushroom research in India.
Important terms
Single cell protein – dried microbial biomass used as protein food or feed.
Mycoprotein – protein-rich food made from fungal mycelium.
Spawn – pure mushroom mycelium grown on grain, used as "seed".
Spawn run – period when mycelium spreads through the substrate.
Casing – layer of soil-like material placed on compost to induce fruiting of button mushrooms.
Key facts
Mushroom
Scientific name
Fruiting temperature
Substrate
Button
Agaricus bisporus
About 14–18 °C
Compost + casing
Oyster
Pleurotus spp.
About 20–30 °C
Paddy/wheat straw
Paddy straw
Volvariella volvacea
About 30–35 °C
Paddy straw
Milky
Calocybe indica
About 30–35 °C
Straw + casing
5 important questions
Q1. Why is the high nucleic acid content of SCP a problem?
Humans break nucleic acids down to uric acid. Too much uric acid in blood can cause gout and kidney stones, so the RNA content of SCP for human food must be lowered.
Q2. What is spawn?
Spawn is the pure culture of mushroom mycelium grown on sterilized cereal grains (wheat, sorghum). It is used like seed to inoculate the substrate.
Q3. What is SCP? Give its advantages and limitations.
Definition and term origin.
Organisms with substrates (table).
Advantages: fast growth, high protein, waste use, no land.