Curd making is one of the oldest home-scale biotechnologies. A spoon of previous day’s curd added to warm milk triggers a living cascade: lactic acid bacteria (LAB) ferment milk sugar (lactose) into lactic acid, the pH drops, casein proteins coagulate, and milk transforms into a semi-solid, pleasantly sour food—curd (dahi) or, with different cultures and temperatures, yogurt. This guide unpacks the microbiology, chemistry, process parameters, nutrition, troubleshooting, and industrial perspectives—paired with exam-friendly facts from the history of lactic acid and a quick table of naturally occurring acids.
The Core Science in One Picture
Fresh milk is heated to kill unwanted microbes and to denature whey proteins that later improve gel strength. When the milk cools to incubation temperature and a little curd (starter) is added, LAB multiply and convert lactose into lactic acid. The rising acidity neutralizes the negative charges on casein micelles, calcium bridges form, and a continuous gel traps water and fat: curd. Whey is the liquid that sometimes separates as the gel contracts.
Key reaction (homolactic fermentation):
Lactose → (β-galactosidase) → glucose + galactose → 2 lactic acid + energy (ATP)
Meet the Microbes: LAB Workhorses
Household dahi is typically a mixed-culture product dominated by Lactococcus, Lactobacillus and Streptococcus species (collectively LAB). Commercial yogurt uses a defined symbiotic pair—Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus—that cross-feed and accelerate acidification.
Why LAB are safe and useful:
- They are acid-tolerant, outcompeting many spoilage organisms.
- They produce bacteriocins and organic acids that suppress pathogens.
- Several strains are probiotic, associated with gut and immune benefits.
Temperature, Time, and Texture: Getting Parameters Right
| Parameter | Home curd (dahi) | Set yogurt (thermophilic) | Why it matters |
|---|---|---|---|
| Heat treatment | Bring milk to a gentle boil (≈ 95–100 °C) for 2–5 min; cool | Same | Kills competitors; denatures whey proteins → stronger gel |
| Inoculation temp | ~35–40 °C (warm to the touch) | 42–45 °C | Matches growth optima of LAB used |
| Starter dose | 1–2 tsp per 500 mL milk (well mixed) | 2–3% w/w of active culture | Too much starter can cause grainy/over-sour curd |
| Incubation time | 4–8 h (season-dependent) | 3–5 h | Acidification rate controls set time |
| End-pH / titratable acidity | ≈ 4.5–4.6 pH | ≈ 4.5 pH | Casein coagulates near its isoelectric point |
| Holding after set | Refrigerate | Refrigerate | Cold halts over-souring and whey separation |
Pro tip (GEO-India): In warm climates, reduce starter or shorten incubation; in winter, pre-warm the container (pour hot water, discard, then fill with inoculated milk).
Milk Choices and Their Effects
- Buffalo milk (higher fat and casein) → thicker, creamier curd.
- Cow milk → softer gel, slightly more whey.
- Toned/skim milk → firmer but less creamy; add 2–3% skim milk powder to improve body.
- Plant “milks” lack casein; they need different coagulants or starter blends (outside classic dahi scope).
From Lactose to Lactic Acid: What the Chemistry Changes
As LAB form lactic acid, pH falls from ~6.6 to ~4.5. Calcium phosphate dissolves from casein micelles; κ-casein stabilizing “hairs” collapse, and micelles aggregate into a gel. The acid also gives the tangy taste, improves mineral availability, and acts as a natural preservative.
Lactic acid facts (exam-ready):
- Identified from sour milk by Scheele (1789); Pasteur (1857) linked microbes to its formation.
- First commercial fermentation plant (1881, Clinton, Iowa, USA).
- Chemical routes proved uneconomical/impure for food; fermentation dominates.
Hygiene & Safety
- Boil milk and use clean ladles/containers.
- Use fresh, active starter; replace periodically to avoid off-flavors or contamination.
- Refrigerate as soon as the curd sets.
- Discard curd with gas pockets, sliminess, or bad odor—signs of coliforms/yeasts.
Nutrition & Health
Curd delivers high-quality protein, calcium, B-vitamins, and bioactive peptides released during fermentation. Acidification can improve lactose tolerance for some individuals because LAB consume part of the lactose and provide β-galactosidase. Probiotic strains may aid gut health, immunity, and antibiotic-associated diarrhea reduction (strain-specific).
Troubleshooting at Home
| Problem | Likely cause | Fix |
|---|---|---|
| Curd didn’t set | Inactive starter; milk too cold; very short time | Use fresh starter; inoculate at ~38 °C; incubate longer |
| Excess whey on top | Over-incubation; mechanical shock; low solids | Chill at set; move gently; add skim milk powder next time |
| Too sour | Too much starter or long incubation | Reduce inoculum/time; refrigerate at set |
| Grainy/curdled texture | Very high temp at inoculation; heavy shear after gel starts | Cool properly before adding starter; avoid stirring once set |
Industrial Perspective: From Dahi to Lactic Acid Market
Modern dairies run pasteurization, homogenization, and starter dosing under HACCP. Texture is engineered with solids standardization, heat profiles, and stabilizers (pectin, guar). Lactic acid from fermentation (often using Lactobacillus on glucose/corn syrup) is purified and used in foods (acidity regulator), pharmaceuticals, and as a monomer for polylactic acid (PLA) bioplastics.
Some Naturally Occurring Acids (Quick Reference)
| Natural source | Acid | Natural source | Acid |
|---|---|---|---|
| Vinegar | Acetic acid | Sour milk (curd) | Lactic acid |
| Orange/Lemon | Citric acid | Tamarind/Grapes | Tartaric acid |
| Tomato/Spinach | Oxalic acid | Ant/Nettle sting | Methanoic (formic) acid |
Curd vs. Yogurt vs. Probiotic Yogurt
| Feature | Traditional curd (dahi) | Set/strained yogurt | Probiotic yogurt |
|---|---|---|---|
| Cultures | Natural mixed LAB (region-specific) | Defined pair: S. thermophilus + L. bulgaricus | Yogurt cultures + added probiotic strains (e.g., L. rhamnosus) |
| Texture | Variable; depends on milk type and climate | Consistent; can be strained (Greek) | Similar to yogurt; functional claims depend on strain counts |
| Flavor | Regional, complex | Mildly tart | Mild; sometimes fortified |
| Regulation | Traditional food | Codified standards | Requires viable counts at end of shelf-life |
Step-by-Step: Reliable Home Method
- Heat: Bring 1 litre milk to a gentle boil; hold 2–3 minutes.
- Cool: Let it reach ~38 °C (comfortably warm finger test).
- Inoculate: Whisk 1–2 teaspoons fresh curd into a small portion; mix back.
- Incubate: Cover and keep undisturbed 4–6 hours (warm spot).
- Stop acidification: Move to refrigerator once set.
- Store: Consume within 2–3 days for best taste.
FAQs
Is curd lactose-free?
No. Lactose is reduced but not eliminated; many lactose-sensitive people tolerate curd better than milk due to partial lactose breakdown and active β-galactosidase.
Why is my curd sticky or ropey?
Some LAB produce exopolysaccharides (EPS) that add viscosity; minor EPS is desirable for body, but pronounced “ropy” texture may indicate contaminated culture. Refresh the starter.
Can I make curd with UHT milk?
Yes. UHT milk sets, but texture can be softer. Adding 2–3% skim milk powder improves firmness.
What is whey and can I use it?
Whey is the nutrient-rich liquid expelled. Use it in kneading dough, smoothies, or as a souring agent in curries.
How is lactic acid produced commercially?
Primarily by fermenting sugars with Lactobacillus spp., followed by purification; optical purity (L- or D-) is controlled by strain and process.
Is Greek yogurt just “thick curd”?
It is yogurt that has been strained to remove whey, increasing protein. “Hung curd” is the Indian analogue used for kebabs and desserts.
What temperature kills my starter?
Above ~55 °C (very hot to touch), many LAB die. Always cool milk to ~35–45 °C before inoculation.
Can I reuse yesterday’s curd forever as starter?
Rotate periodically. Over many transfers, wild microbes can drift flavor and safety. Refresh with a clean, active starter.
