Milk fermentation is one of the oldest and most universal methods of food preservation and flavor development. It relies primarily on lactic acid bacteria (LAB) and, in some cases, yeasts and molds, to convert lactose into lactic acid and other metabolites, lowering pH, inhibiting pathogens, and creating distinctive textures and flavors.
1. Basic Biochemical Mechanism (Common to All Fermented Milks)
- Lactose → Glucose + Galactose (by β-galactosidase)
- Glucose → Lactic acid via homolactic or heterolactic pathways
- Homolactic fermentation (e.g., Lactobacillus delbrueckii subsp. bulgaricus, Streptococcus thermophilus): ~95–98 % lactic acid
- Heterolactic fermentation (e.g., Leuconostoc mesenteroides, Lb. brevis): lactic acid + ethanol + CO₂ + acetic acid / diacetyl / acetaldehyde
The drop in pH (usually to 4.0–4.6) causes casein micelles to coagulate, forming the characteristic gel or curd.
2. Major Fermented Milk Products and Their Processes
| Product | Primary Microorganisms | Temperature | Key Features & End Products | Typical pH |
|---|---|---|---|---|
| Yogurt | Streptococcus thermophilus + Lb. delbrueckii subsp. bulgaricus (1:1) | 40–45 °C | Symbiotic growth; acetaldehyde main flavor | 4.0–4.6 |
| Kefir | Kefir grains (Lactobacillus, Leuconostoc, yeasts, acetic bacteria) | 20–25 °C | Lactic + alcoholic (0.5–2 % ethanol), effervescent, kefiran polysaccharide | 4.2–4.6 |
| Koumiss (mare/donkey) | Lactobacillus, Lactococcus + yeasts | 20–30 °C | Higher alcohol (1–3 %), fizzy, traditionally back-slopped | 3.9–4.4 |
| Viili (Finnish) | Lactococcus lactis subsp. cremoris + Geotrichum candidum | 18–20 °C | Ropy texture, velvety mold surface | ~4.5 |
| Dahi (Indian curd) | Mixed natural or yogurt-like starters | 30–40 °C | Often buffalo milk → very thick | 4.2–4.8 |
| Cheese (general) | Starter LAB + rennet + secondary flora | Varies | Acid + Rennet enzymatic coagulation, ripening. | Varies |
3. Influence of Milk Source on Fermentation
- Buffalo milk (high fat & casein) → very firm gel, excellent for yogurt and cheese
- Goat milk (smaller fat globules, low αs1-casein) → softer curd, faster fermentation
- Camel milk (low β-lactoglobulin, high antimicrobials) → poor coagulation; needs modified starters
- Donkey / Mare milk (very low casein, high whey) → almost no gel; texture from CO₂ and alcohol in koumiss
- Sheep milk (highest solids) → extremely thick yogurt, highest cheese yield
- Human milk → not traditionally fermented (high oligosaccharides inhibit many LAB)
4. Modern Industrial Yogurt Process (Example)
- Milk standardization (fat/protein adjustment)
- Homogenization
- Heat treatment (90–95 °C, 5 min) → whey protein denaturation
- Cooling & inoculation with starter culture
- Incubation until pH ≈ 4.6
- Cooling to 4 °C → fermentation stop
Milk fermentation is a beautiful interplay of milk chemistry, microbial ecosystems, temperature, and time—producing an astonishing global diversity of textures, flavors, and functional foods from the same fundamental biochemical foundation.