Fermented milk products

In short

Fermented milk products are made by letting selected lactic acid bacteria (and sometimes yeasts or moulds) grow in milk, converting lactose into lactic acid so that the pH falls, casein coagulates at its isoelectric point and the milk sets into a gel with a clean acid flavour and longer keeping quality. In India they are the single largest value-added dairy category — dahi, lassi, chhach, shrikhand, misti dahi, yoghurt and probiotic drinks — sold by Amul (GCMMF), Mother Dairy, Nandini (KMF), Hatsun, Heritage, Britannia, Danone, Yakult and thousands of local halwais. The topic is examined in every B.Tech Dairy Technology course (Dairy Microbiology, Traditional Indian Dairy Products, Dairy Products Technology), in ICAR/ASRB NET and JRF papers, and in technical interviews for QA, production and R&D roles in dairy plants. Standards come from the Food Safety and Standards (Food Products Standards and Food Additives) Regulations, 2011 under the FSS Act, 2006, with BIS standards for yoghurt and for starter cultures.

Explained

Definitions, principle and classification

• Fermented (cultured) milk: milk product obtained by fermentation of milk with specific micro-organisms, in which the micro-organisms must be viable, active and abundant in the product to the end of shelf life.
• Principle: starter bacteria ferment lactose → lactic acid (homofermentative, Embden–Meyerhof pathway) → pH drops from ~6.6 to 4.6, the isoelectric point of casein → colloidal calcium phosphate dissolves, casein micelles aggregate into a three-dimensional gel holding whey.
• Benefits of fermentation: preservation (low pH inhibits spoilage and pathogens), flavour and texture development, partial lactose hydrolysis (helps lactose-intolerant consumers), probiotic and therapeutic effects, better digestibility of protein, synthesis of B vitamins (folate, B12 by some strains).
• Classification by metabolism:
  1. Lactic fermentation — dahi, yoghurt, acidophilus milk, cultured buttermilk, bulgarian buttermilk.
  2. Lactic + alcoholic (yeast) — kefir (~0.5–1% alcohol), koumiss (mare's milk, up to ~2.5%).
  3. Lactic + mould — viili (Finland, *Geotrichum candidum*).
• Classification by temperature of incubation: mesophilic (20–30 °C; *Lactococcus lactis* subsp. *lactis*/*cremoris*, *Leuconostoc*) — dahi, cultured buttermilk, taetta; thermophilic (37–45 °C; *Streptococcus thermophilus*, *Lactobacillus delbrueckii* subsp. *bulgaricus*, *L. helveticus*) — yoghurt, Swiss-type cultures.
• Classification by body: set type (fermented in the retail cup, undisturbed gel), stirred type (fermented in bulk, gel broken, then packed), drinking type (lassi, chhach, Yakult-style), concentrated/strained (shrikhand, chakka, Greek yoghurt, labneh), frozen (frozen yoghurt).

Starter cultures

• Starter culture: a selected, active preparation of micro-organisms added to milk to bring about desired fermentation. Forms: liquid/mother culture, deep-frozen concentrate, DVS (direct vat set) freeze-dried concentrate — DVS dominates organised Indian plants because it avoids daily propagation and phage build-up.
• Single-strain / multiple-strain / mixed-strain cultures; defined cultures give reproducible acidity, undefined (traditional back-slopping with previous day's dahi) gives variable results.
• Yoghurt culture = *S. thermophilus* + *L. bulgaricus*, ideally 1:1 by cell count. Classic protocooperation: *L. bulgaricus* proteinase releases peptides and free amino acids (valine, glycine, histidine) that stimulate the *Streptococcus*; the *Streptococcus* produces formate, folate, pyruvate and CO₂ that stimulate the *Lactobacillus*. Together they acidify faster than either alone.
• Dahi cultures: mesophilic *Lactococcus lactis* subsp. *lactis*, subsp. *cremoris* and subsp. *lactis* biovar *diacetylactis* with *Leuconostoc mesenteroides* subsp. *cremoris*; for misti dahi (sweetened Bengali dahi) thermophilic cultures plus caramelised sugar; for dahi for lassi the diacetyl-producing strains are essential for aroma.
• Flavour compounds: lactic acid (acid taste), acetaldehyde (the characteristic "green apple" yoghurt note, 10–40 ppm desirable), diacetyl (buttery note of dahi/buttermilk, from citrate by *Lc. lactis* biovar *diacetylactis* and *Leuconostoc*), acetoin, acetic acid, CO₂; EPS (exopolysaccharide) producing "ropy" strains give body without added stabiliser.
• Starter activity tests: activity test (acid produced in 3–3.5 h at set temperature), Horrall–Elliker test, pH/titratable acidity curve, phosphatase-free milk check, coliform count.
• Starter failures: bacteriophage attack (most important; countered by phage-unrelated culture rotation, phage-resistant strains, phage-inhibitory medium, strict hygiene and aerosol control), antibiotic residues in milk, sanitiser residues (chlorine, QAC), agglutinins in raw milk, natural inhibitors, high-fat or high-solids milk, mastitic milk, wrong incubation temperature.
• Probiotics: live micro-organisms which, when administered in adequate amounts, confer a health benefit — *Lactobacillus acidophilus*, *L. rhamnosus* GG, *L. casei* Shirota, *Bifidobacterium bifidum*/*lactis*. Usual target ≥10⁶–10⁷ CFU/g at consumption; ICMR–DBT guidelines for evaluating probiotics in food (2011) and FSSAI's list of approved strains govern claims. Prebiotic = non-digestible substrate (inulin, FOS, GOS); synbiotic = probiotic + prebiotic.

Manufacture of dahi and yoghurt (step-by-step)

1. Milk selection: fresh, sweet, low-acidity milk, free of antibiotics and sanitisers, SPC low, no mastitic milk. Standardise fat and SNF — dahi commonly 3.0–6.0% fat; yoghurt 0.5–3.5% fat.
2. Fortification: raise total solids to 14–16% (yoghurt) with skim milk powder (1–3%), condensed skim milk, evaporation or ultrafiltration; higher SNF gives firmer gel and less whey separation. Sugar (6–8%) for sweet dahi/yoghurt; stabilisers (gelatin, pectin, starch, guar) within permitted limits.
3. Homogenisation: 15–20 MPa (150–200 bar) at 60–70 °C — prevents creaming, improves viscosity and whiteness.
4. Heat treatment: 85 °C/30 min or 90–95 °C for 5–10 min (not mere pasteurisation) — destroys competing flora, expels oxygen (lowers redox potential), and above all denatures whey proteins (β-lactoglobulin) which complex with κ-casein, giving a firmer gel with higher water-holding capacity and less syneresis.
5. Cooling to incubation temperature: 22–25 °C for mesophilic dahi, 37 °C (long set, 6–8 h) or 42–43 °C (short set, 3–4 h) for yoghurt.
6. Inoculation: 1–2% bulk (mother) starter, or DVS at the supplier's dose — far lower, typically about 0.01–0.02% (a sachet per vat) — uniformly mixed; avoid air incorporation.
7. Packaging then incubation for set type (cups/sachets), or incubate in bulk then stir, cool and pack for stirred type.
8. Incubation until pH 4.5–4.6 (TA ~0.8–0.9% lactic acid for yoghurt, 0.7–0.9% for dahi). Do not judge by clock alone — judge by pH.
9. Cooling (break): rapid cooling to <10 °C, finally 4–5 °C, to arrest acid development; for stirred type cool through a plate cooler with gentle pumping to avoid gel damage.
10. Storage and distribution at ≤5 °C; shelf life typically 7–15 days (dahi in cups) and up to 21–30 days for aseptically filled/high-barrier packs. Post-fermentation heat-treated (long-life) fermented drinks exist but cannot be called "fermented milk with live cultures".

Indian traditional fermented products

• Dahi: curd obtained by souring of boiled/pasteurised milk by harmless lactic cultures; sweet/sour, mesophilic or thermophilic. India's largest fermented product by volume; most still made in homes.
• Misti dahi / payodhi / lal dahi: milk concentrated and sweetened with sugar or jaggery (caramelised to give the brown colour), fermented at 40–45 °C in earthen pots; high solids give firm body.
• Lassi: dahi blended with sugar (10–15%) and water, flavoured (mango, rose, saffron, cardamom); sweet lassi TS ~15–20%. Chhach / mattha: salted-spiced buttermilk, low in fat, often from makhan churning residue.
• Shrikhand: dahi is broken, whey drained in muslin (or by quarg separator/basket centrifuge) to give chakka, then blended with sugar (~40–45% of final product), ghee, nuts, cardamom, saffron; FSSAI specifies milk fat (min 8.5% on dry basis norms as per the standard for shrikhand — check the current regulation), milk protein and TS requirements. Amrakhand = shrikhand + mango pulp.
• Raita, kadhi (dahi + besan), dahi-vada — culinary uses; Churpi/Chhurpi, Dahi of Himalayas, Philu, Somar, Mohi — ethnic Himalayan fermented milks.
• Industrial relevance: Amul Masti Dahi, Amul Lassi, Mother Dairy Classic Dahi, Nandini Curd, Hatsun Arun Curd, Epigamia Greek yoghurt, Yakult (*L. casei* Shirota), Amul Prolife probiotic dahi.

Other fermented milks of the world

• Yoghurt: thermophilic; set, stirred, fruit, drinking, frozen, strained (Greek) types.
• Acidophilus milk: *L. acidophilus* at 37 °C, TA up to 1.5–2.0%; sweet acidophilus milk is unfermented milk with added viable *L. acidophilus* (no acid produced) — a favourite exam distinction.
• Cultured buttermilk: skim/low-fat milk with *Lc. lactis* biovar *diacetylactis* + *Leuconostoc*, diacetyl-rich; distinct from traditional buttermilk, the aqueous residue of churning cream/dahi for butter.
• Bifidus milk / bio-yoghurt: *Bifidobacterium* spp., anaerobic, needs growth promoters and oxygen-barrier packs.
• Kefir: kefir grains (LAB + lactose-fermenting yeasts in a kefiran matrix); mildly alcoholic, effervescent. Koumiss: mare's milk, *L. bulgaricus* + *Kluyveromyces*. Yakult: fermented skimmed milk drink with *L. casei* Shirota, ≥6.5 × 10⁹ per 65 mL bottle.
• Cheeses and butter/ghee also use starters, but belong to the other topics in this subject.

Quality, defects and standards

• Quality tests: pH and titratable acidity (% lactic acid), TS/fat/protein, curd firmness and whey separation, viscosity (Brookfield), syneresis (centrifugation/drainage), sensory score, viable LAB count on MRS/M17 agar, yeast and mould count, coliforms, pathogens (*Salmonella*, *L. monocytogenes*, *S. aureus*).
• Common defects and causes: *lack of firmness / weak body* — low TS, inadequate heat treatment, wrong temperature, stirring during set; *whey separation (syneresis)* — over-acidification, low TS, vibration during incubation, inadequate heat treatment; *too acid / bitter* — over-incubation, slow cooling, proteolytic strains; *yeasty/fruity* — yeast contamination, poor hygiene; *ropy/slimy* — EPS strains or *Alcaligenes*; *gassy* — coliforms or heterofermentative organisms; *flat flavour* — weak starter, no diacetyl/acetaldehyde; *post-acidification in the market chain* — temperature abuse, *L. bulgaricus*-dominant culture.
• Legal frame (India): FSS Act, 2006 with the Food Safety and Standards (Food Products Standards and Food Additives) Regulations, 2011 — standards for fermented milk products (dahi/curd, yoghurt, cream/sour cream, acidophilus milk, kefir, shrikhand, chakka, lassi); Food Safety and Standards (Food Products Standards and Food Additives) Amendment notifications update these periodically, so verify the latest gazette. Labelling under the Food Safety and Standards (Labelling and Display) Regulations, 2020 — milk fat, live-culture declaration, "best before", veg logo. Microbiological requirements in the Food Safety and Standards (Food Products Standards and Food Additives) Regulations, 2011 and the microbiological standards notification (coliform, yeast and mould, pathogen criteria).
• Indicative FSSAI compositional points: dahi/curd must be prepared from pasteurised/boiled milk by harmless lactic acid cultures, with milk fat and milk SNF as for the milk used; titratable acidity not less than 0.5% as lactic acid for dahi, and total solids/fat declared on the label (exact figures vary by product and by amendment — confirm against the current regulation). Yoghurt: fermented by *S. thermophilus* and *L. bulgaricus*, cultures viable in the final product.
• BIS: IS 9617 (yoghurt), IS 13334 (skim milk powder for cultures) and IS standards for starter cultures exist; some older numbers have been revised or withdrawn, so quote the current edition. Codex STAN 243 is the international standard for fermented milks (yoghurt: min 2.7% milk protein, min 0.6% titratable acidity, ≥10⁷ CFU/g starter).
• GST: curd, lassi and buttermilk sold other than pre-packaged and labelled are exempt; pre-packaged and labelled dahi/lassi/buttermilk attract 5%. Flavoured milk and yoghurt-based beverages are taxed differently — slabs were restructured to 5%/18%/40% from 22 September 2025, so verify the current rate notification for the exact product.

Key points

• Fermentation principle — lactose → lactic acid lowers pH to casein's isoelectric point 4.6, causing gelation.
• Mesophilic vs thermophilic — 20–30 °C *Lactococcus*/*Leuconostoc* (dahi, buttermilk) vs 37–45 °C *S. thermophilus* + *L. bulgaricus* (yoghurt).
• Protocooperation — *L. bulgaricus* supplies amino acids/peptides, *S. thermophilus* supplies formate and CO₂; combined acidification is faster.
• Heat treatment 90–95 °C/5–10 min — denatures whey proteins which bind κ-casein, giving firm gel and low syneresis; this is why yoghurt milk is heated far beyond pasteurisation.
• Total solids 14–16% — the single biggest lever on body, viscosity and whey separation.
• Homogenisation at 150–200 bar, 60–70 °C — stops creaming, raises viscosity and whiteness.
• Break point pH 4.5–4.6 — cooling must start here; delay causes over-acid, bitter product.
• Acetaldehyde — yoghurt's key flavour compound; diacetyl — dahi and cultured buttermilk's buttery aroma, from citrate.
• DVS culture — freeze-dried concentrate added straight to the vat; avoids propagation errors and phage build-up.
• Bacteriophage — the classic cause of starter failure; remedy is rotation of phage-unrelated cultures and hygiene, not more inoculum.
• Set vs stirred — set is incubated in the retail pack (firm, cut gel); stirred is incubated in bulk, gel broken, viscous and pourable.
• Chakka and shrikhand — chakka is drained dahi solids; shrikhand is chakka + ~40–45% sugar + fat, nuts and flavour.
• Sweet acidophilus milk — unfermented milk with added live *L. acidophilus*; no acid development, no curd.
• Probiotic dose — typically ≥10⁶–10⁷ CFU/g viable at the end of shelf life; FSSAI-approved strains and ICMR–DBT guidelines govern claims.
• Kefir and koumiss — lactic + alcoholic fermentations with yeasts; kefir ~0.5–1% alcohol.
• Syneresis — whey separation, controlled by TS, heat treatment, stabilisers, avoiding vibration and over-acidification.
• Legal basis — FSS Act 2006 and FSSAI Food Products Standards Regulations, 2011; Codex STAN 243 internationally.
• Shelf life — 7–15 days at ≤5 °C for cup dahi; cold-chain failure causes post-acidification, the commonest market complaint.

Formulas and examples

Example 1 — Titratable acidity of dahi. 10 g dahi is titrated with 0.1 N NaOH; 9.5 mL is needed to reach the phenolphthalein end point.

• % LA = (9.5 × 0.1 × 9) / 10 = 8.55/10 = 0.855% lactic acid.
• This is the typical break point for yoghurt/dahi (0.8–0.9%); cooling should begin now. If the fresh milk had 0.14% LA (as lactic acid, natural acidity), the developed acidity = 0.855 − 0.14 = 0.715%.

Example 2 — Fortifying yoghurt milk with SMP. 1,000 kg of standardised milk has 12.0% TS; target is 15.0% TS using SMP of 96% TS.

• M_SMP = 1000 × (15.0 − 12.0) / (96 − 15.0) = 3000/81 = 37.04 kg SMP.
• Check: total mass = 1,037.04 kg; TS = 120 + (37.04 × 0.96) = 120 + 35.56 = 155.56 kg; 155.56/1037.04 = 15.0%. Correct.

Example 3 — Sugar for sweet lassi. A plant makes 500 kg of sweet lassi at 12% added sugar from dahi (TS 14%) + sugar + water, with lassi TS target 18%.

• Sugar needed = 500 × 0.12 = 60 kg.
• Solids from sugar = 60 kg; required total solids = 500 × 0.18 = 90 kg; so milk solids needed = 90 − 60 = 30 kg.
• Dahi required = 30/0.14 = 214.3 kg; water = 500 − 214.3 − 60 = 225.7 kg.

Example 4 — Lactose conversion and pH logic. Milk contains 4.8% lactose. If 0.75% lactic acid is developed in dahi:

• Lactic acid formed per 100 g = 0.75 g → lactose consumed = 0.75/1.05 ≈ 0.71 g, i.e. only about 15% of the lactose is fermented.
• This is why fermented milk still contains substantial lactose, yet is better tolerated — bacterial β-galactosidase continues to act in the gut.

Example 5 — Chakka and shrikhand balance. 100 kg dahi (TS 15%, i.e. 15 kg solids) is drained; chakka obtained is 32 kg with TS 40%.

• Solids in chakka = 32 × 0.40 = 12.8 kg → solids recovery = 12.8/15 = 85.3%; whey = 68 kg carrying 2.2 kg solids.
• Shrikhand made by adding sugar at 40% of final product weight: if all 32 kg chakka is used and sugar is S, with final weight W = 32 + S and S = 0.40 W → S = 0.40(32 + S) → 0.6 S = 12.8 → S = 21.33 kg, final shrikhand = 53.33 kg, TS = (12.8 + 21.33)/53.33 = 64.0%.

Where it is used

• Organised dairy plants: curd and lassi lines at Amul/GCMMF unions, Mother Dairy (Delhi NCR), KMF Nandini, Hatsun, Heritage, Dodla, Milma, Verka, Saras — cup filling, sachet lassi, Tetra/aseptic buttermilk.
• Job roles: production/shift officer on the dahi line, QA/QC chemist running acidity, pH and LAB counts, starter-room microbiologist managing DVS and culture rotation, R&D executive on Greek yoghurt and probiotic launches, NPD and sensory panels.
• Probiotic and functional foods: Yakult India, Amul Prolife, Epigamia, Danone — strain selection, viability through shelf life, FSSAI claim compliance.
• Ingredient and equipment suppliers: Chr. Hansen, DuPont/IFF Danisco (cultures), GEA, Tetra Pak, SPX/APV, Alfa Laval (homogenisers, incubation tanks, quarg separators), stabiliser houses.
• Public and cooperative sector: NDDB and state federations on village-level dahi and lassi units; FSSAI/State Food Safety Officers on standards enforcement; NDRI Karnal and SMC College Anand for research and testing.
• Entrepreneurship: small chakka/shrikhand and misti dahi units, hotel and QSR supply chains, PMFME and NABARD-financed mini dairy plants.

Interview questions

• Q: Why is yoghurt milk heated to 90–95 °C instead of just pasteurised? A: To denature whey proteins so they complex with κ-casein, which raises water-holding capacity and gives a firmer gel with less whey separation, besides destroying competitors and removing dissolved oxygen.
• Q: At what pH does milk gel in fermentation and why? A: Around pH 4.6, the isoelectric point of casein, where net charge is zero, colloidal calcium phosphate has dissolved and micelles aggregate into a gel.
• Q: What is protocooperation in yoghurt culture? A: *L. bulgaricus* proteolyses casein to supply peptides and amino acids to *S. thermophilus*, which returns formate, folate and CO₂ — so the pair acidifies faster than either strain alone.
• Q: Difference between set and stirred yoghurt? A: Set yoghurt is incubated in the retail container so the gel stays intact; stirred yoghurt is incubated in bulk, the gel is broken, cooled and then packed, giving a viscous pourable product.
• Q: Difference between traditional buttermilk and cultured buttermilk? A: Traditional buttermilk is the aqueous by-product left after churning cream or dahi for butter; cultured buttermilk is skim or low-fat milk deliberately fermented with diacetyl-producing mesophilic cultures.
• Q: What is sweet acidophilus milk? A: Pasteurised milk to which viable *L. acidophilus* is added without allowing fermentation — it stays sweet and fluid but delivers the organism.
• Q: Which compound gives yoghurt its typical flavour, and which gives dahi its buttery note? A: Acetaldehyde in yoghurt (about 10–40 ppm); diacetyl, produced from citrate by *Lc. lactis* biovar *diacetylactis* and *Leuconostoc*, in dahi and cultured buttermilk.
• Q: Starter fails to set the milk — how do you troubleshoot? A: Check culture activity and inoculum level, incubation temperature, then test milk for antibiotic residues, sanitiser carry-over and high initial acidity, and check for bacteriophage; rotate to a phage-unrelated culture and review CIP and starter-room hygiene.
• Q: What causes whey separation in cup dahi and how do you fix it? A: Low total solids, inadequate heat treatment, over-acidification, or vibration/movement during setting; fix by fortifying to 14–16% TS, heating to 90–95 °C, cooling at pH 4.6 and keeping trays undisturbed.
• Q: What is post-acidification and why does it matter commercially? A: Continued acid production by *L. bulgaricus* during storage, which makes the product too sour and drives consumer complaints; controlled by rapid cooling, cold chain below 5 °C and low-post-acidifying culture blends.
• Q: Minimum probiotic count and the Indian regulatory position? A: Commonly 10⁶–10⁷ CFU/g viable at the end of shelf life; in India, strains must be from FSSAI's approved list and health claims follow the FSS Act framework and ICMR–DBT probiotic guidelines — the approved list is revised, so the current notification should be checked.
• Q: How is shrikhand made and what sets its texture? A: Dahi is broken and whey drained to chakka (by muslin, basket centrifuge or quarg separator), then kneaded with about 40–45% sugar plus ghee, nuts and cardamom; texture depends on chakka TS, sugar dissolution and fineness of grinding.

Common mistakes

• Writing the gelation pH as 6.6 or 5.2 instead of 4.6, or calling it the "isoelectric point of lactose".
• Saying yoghurt milk is "pasteurised at 72 °C/15 s" — the whole point of the 90–95 °C hold is whey protein denaturation.
• Forgetting the 90 g/mol equivalent weight of lactic acid in the titratable acidity formula, or reporting total acidity as developed acidity without subtracting natural acidity.
• Confusing mesophilic dahi cultures with thermophilic yoghurt cultures, and quoting 42 °C for traditional dahi.
• Treating traditional (churned) buttermilk and cultured buttermilk as the same product; likewise dahi and yoghurt as identical.
• Blaming "bad milk" for every starter failure and ignoring bacteriophage, antibiotic residues and sanitiser carry-over.
• Judging the end of incubation by time on the clock rather than by pH/acidity, which causes over-acid, bitter and syneretic product.
• Claiming fermentation removes all lactose — only roughly 15–30% is fermented in dahi and yoghurt.

Related topics

Dairy microbiology: starter cultures and bacteriophage · Butter, ghee and cheese · Ice cream and frozen desserts · Market milk: pasteurisation, homogenisation and standardisation · Dairy chemistry: milk proteins and casein micelle · Food safety: FSSAI standards and HACCP in dairy plants