Knowledge Base

Yogurt

Dairy ferment · Traditional · peer-reviewed

Milk set thick by two bacteria working in partnership at blood heat — the most reliable home ferment there is, done in hours, not days.

Easy to make
live-cultures, protein, calcium supplied

Ingredients

  • Whole milk1 L
  • Live plain yogurt (as starter)2 tbsp

How to make it

  1. Step 1

    Heat milk to 82 °C to denature whey proteins (this sets a thicker gel), then cool to 43 °C.

  2. Step 2

    Whisk in 2 tbsp live yogurt as your starter culture.

  3. Step 3

    Hold at 42–45 °C — an oven with the light on, a flask, or a yogurt maker.

  4. Step 4

    Incubate 4–8 hours undisturbed until set and tangy.

  5. Step 5

    Refrigerate to stop the ferment. Save 2 tbsp to start the next batch.

What it is

Yogurt is milk fermented to a soft gel by a defined pair of thermophilic (heat-loving) bacteria. Codex Alimentarius defines yogurt specifically as milk cultured with Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus (FAO/WHO CXS 243-2003). They convert lactose to lactic acid, dropping the pH to about 4.6, where milk proteins coagulate into the familiar set.

The science

The two species grow far better together than alone — a textbook case of protocooperation. S. thermophilus grows first, consuming oxygen and producing formic acid and CO₂ that stimulate L. bulgaricus; in return, L. bulgaricus breaks milk proteins into peptides and amino acids that feed S. thermophilus (Sieuwerts et al., 2008). This mutual feeding is why a small spoon of live yogurt reliably seeds a whole litre.

Heating the milk to ~82 °C before culturing denatures whey proteins so they bind into the casein gel, giving a thicker, less weepy set — a step supported by decades of dairy-science practice.

Health

A systematic review of the evidence links yogurt and cultured milks to improved lactose digestion and modest benefits for cardiometabolic and gut health, though the authors note more controlled trials are needed (Savaiano & Hutkins, 2021).

Signs it worked / troubleshooting

  • ✅ Good: set within 4–8 h, clean tang, glossy surface.
  • ⚠️ Thin or runny = incubation too cool, or old/weak starter → hold nearer 44 °C, use fresh live culture.
  • 🚫 Yeasty, bitter, or gassy = contamination → discard and re-start from a fresh commercial culture.

How to store

Refrigerate promptly; it keeps 1–2 weeks. Re-culturing from your own batch works for several generations before the strain balance drifts and you should re-seed from fresh culture.

References

  • Sieuwerts S, de Bok FAM, Hugenholtz J, van Hylckama Vlieg JET (2008). Unraveling microbial interactions in food fermentations: from classical to genomics approaches. Applied and Environmental Microbiology 74(16):4997–5007. doi:10.1128/AEM.00113-08
  • Savaiano DA, Hutkins RW (2021). Yogurt, cultured fermented milk, and health: a systematic review. Nutrition Reviews 79(5):599–614. doi:10.1093/nutrit/nuaa013
  • FAO/WHO Codex Alimentarius (2003, rev. 2018). Standard for Fermented Milks (CXS 243-2003).

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