Starter cultures and fermented milks
How lactic acid bacteria make yoghurt, dahi and kefir: mesophilic and thermophilic starters, yoghurt making, probiotics, phage, and a growth sum.
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Long before refrigeration, people in hot climates kept milk by letting it sour. Lactic acid bacteria, naturally present or carried over from a previous batch, turned milk sugar into lactic acid, thickening the milk and protecting it from many spoilage and disease-causing microbes. The results, yoghurt, dahi, kefir, koumiss, laban, skyr and many others, remain among the world's most popular dairy foods. Modern dairies make them with defined starter cultures under carefully controlled conditions, giving products of consistent flavour, texture and safety.
Starter cultures and fermented milk technology are core topics in dairy microbiology and technology exams.
What starter cultures do
A starter is a culture of selected microorganisms added to milk to begin fermentation. Its main jobs:
- Acid production: converting lactose to lactic acid, lowering the pH, coagulating casein at about pH 4.6, and inhibiting pathogens and spoilage bacteria.
- Flavour: compounds such as acetaldehyde (yoghurt's typical flavour) and diacetyl (buttery flavour).
- Texture: some strains produce exopolysaccharides that thicken products naturally.
- In cheese, proteolysis and flavour development during ripening.
Types of starters
Mesophilic starters (optimum about 20 to 30 °C)
Lactococcus lactis (subspecies lactis and cremoris), citrate-fermenting strains (such as L. lactis biovar diacetylactis) and Leuconostoc species, which produce diacetyl and carbon dioxide. Used for cultured buttermilk, sour cream, cultured butter, many cheeses and dahi made at ambient temperatures.
Thermophilic starters (optimum about 37 to 45 °C)
Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus, the two bacteria that, under the Codex standard, define yoghurt, and thermophilic lactobacilli used in hard Italian and Swiss cheeses.
Mixed and special cultures
- Kefir grains: a complex community of lactic acid bacteria and yeasts in a polysaccharide matrix, producing a slightly alcoholic, fizzy drink.
- Koumiss: mare's milk fermented by lactobacilli and yeasts in Central Asia.
- Probiotic cultures, such as Lactobacillus acidophilus, Lacticaseibacillus rhamnosus and Bifidobacterium species, added for health benefits supported by evidence for particular strains.
Making yoghurt
- Standardise fat and raise solids, for example with skim milk powder, to give body.
- Homogenise, to prevent cream separation and improve texture.
- Heat to about 85 to 95 °C for 5 to 10 minutes: kills competitors and denatures whey proteins, which then bind water and give a firmer gel.
- Cool to about 42 to 45 °C and inoculate with starter.
- Incubate for about 4 to 6 hours until the pH reaches about 4.4 to 4.6.
- Cool quickly to stop fermentation, then store chilled.
Set yoghurt is incubated in the retail pot; stirred yoghurt is fermented in a tank, then the gel is broken, cooled, and filled, often with fruit. Strained yoghurts (Greek-style, labneh, skyr) have whey removed for a thicker, higher-protein product. Dahi, the traditional fermented milk of South Asia, is often made with mixed mesophilic and thermophilic cultures carried over from the previous batch.
Fermented milks around the world
- Yoghurt (Balkans, Türkiye, the Middle East) and its strained forms, labneh and Greek-style yoghurt.
- Dahi and lassi (South Asia), and shrikhand, made from strained dahi with sugar.
- Kefir (Caucasus) and koumiss (Central Asia).
- Skyr (Iceland), technically a fresh acid-curd cheese eaten like yoghurt.
- Ayran and doogh, salted yoghurt drinks of Türkiye and Iran.
- Amasi and mursik in Southern and East Africa, and villi in Finland, a ropy fermented milk.
Problems in fermentation
- Bacteriophages: viruses that attack starter bacteria, stopping acid production. Controlled by plant hygiene, phage-resistant strains and culture rotation.
- Antibiotic residues in milk inhibit starters.
- Post-acidification during storage makes products too sour; strains with low post-acidification are chosen.
- Contamination by yeasts and moulds, causing gas, yeasty off-flavours and swollen packs.
Starter forms
Traditional bulk starter was propagated daily in the dairy, a skilled and phage-prone task. Most large plants now use direct vat set (DVS) or direct vat inoculation cultures: concentrated frozen or freeze-dried cultures added straight to the milk, giving consistent results.
How long does fermentation take?
A yoghurt milk is inoculated to give about 107 colony-forming units (cfu) per mL, and the culture must reach about 109 cfu per mL to acidify the milk properly. Under good conditions at 43 °C, the mixed culture doubles about every 35 minutes.
- Increase needed = 109 ÷ 107 = 100 times.
- Doublings = log2 100 ≈ 6.6.
- Time ≈ 6.6 × 35 ≈ 230 minutes, about 3.9 hours, consistent with the usual 4 to 6 hours once the lag phase is included.
Inoculum. Using a bulk starter at 2.5 per cent for 1,000 L of milk needs 25 L of active starter; a DVS culture replaces this with a pack of frozen concentrate.
Key points
- Starters ferment lactose to lactic acid, coagulate casein near pH 4.6, add flavour (acetaldehyde, diacetyl) and texture.
- Mesophilic (20–30 °C): Lactococcus, Leuconostoc; thermophilic (37–45 °C): Streptococcus thermophilus, Lactobacillus bulgaricus.
- Yoghurt: standardise, homogenise, heat 85–95 °C for 5–10 min, inoculate at 42–45 °C, ferment to pH 4.4–4.6, cool.
- Kefir and koumiss use mixed bacteria and yeasts; probiotics add specific health-promoting strains.
- Bacteriophages, antibiotics and post-acidification are key problems; DVS cultures improve consistency.
What the exams ask
- School-leaving and entrance papers: what yoghurt and other fermented milks are, and the bacteria involved.
- Degree and research-fellowship entrances: types of starters and their functions, protocooperation, yoghurt technology, set versus stirred, phage problems and probiotics.
- Dairy technology examinations: starter propagation and DVS, process control, defects and their causes, and standards for fermented milks.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Why heat milk to 90 °C if the bacteria are added afterwards?
The heat kills competing microbes and, importantly, denatures whey proteins so they join the casein network and hold water, giving a thicker yoghurt with less whey separation.
Is all yoghurt probiotic?
Yoghurt contains live cultures unless heat-treated after fermentation, but the term probiotic is reserved for specific strains with demonstrated health benefits at adequate doses.
Why does homemade yoghurt sometimes fail?
Common causes are milk too hot or too cold at inoculation, weak or contaminated starter, antibiotic residues in the milk, or incubation that is too short or too cool.
Sources
- Tamime, A. Y. and Robinson, R. K. Tamime and Robinson's Yoghurt: Science and Technology, 3rd edition. Woodhead Publishing, 2007.
- Codex Alimentarius Commission. Standard for Fermented Milks (CXS 243-2003).
- Walstra, P., Wouters, J. T. M. and Geurts, T. J. Dairy Science and Technology, 2nd edition. CRC Press, 2006.
- Hill, C. et al. "The International Scientific Association for Probiotics and Prebiotics consensus statement on the scope and appropriate use of the term probiotic." Nature Reviews Gastroenterology and Hepatology 11, 506–514 (2014).
- Marco, M. L. et al. "Health benefits of fermented foods: microbiota and beyond." Current Opinion in Biotechnology 44, 94–102 (2017).
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