How Is Yogurt Made? Explained
How is yogurt made? The short answer
How is yogurt made? Milk is heated, cooled to about 110°F (43°C), and mixed with two bacteria, Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus. Over several hours the bacteria ferment lactose, the milk sugar, into lactic acid. The rising acidity makes the milk proteins set into a soft, tangy gel: yogurt.
What happens inside the milk
The two starter species work as a team. When they grow together in milk, S. thermophilus grows quickly at first while L. bulgaricus grows more slowly, and the pair show synergy, according to a 2014 review in Foods. As they turn lactose into lactic acid, the pH of the milk falls.
That drop in pH is what thickens the milk. Coagulation begins at about pH 4.6 to 4.7, the isoelectric point of casein, and the casein micelles aggregate into a network. That network is the gel you spoon out of the cup.
What US law calls “yogurt”
In the US, yogurt has a federal standard of identity, 21 CFR 131.200. To use the name, a product must have:
- A characterizing culture of L. bulgaricus and S. thermophilus
- Dairy ingredients that were pasteurized or ultra-pasteurized before the culture was added
- At least 8.25% milk solids not fat and at least 3.25% milkfat (yogurt with 2.44% to under 3.25% milkfat must state its percent milkfat on the label)
- A pH of 4.6 or lower, measured within 24 hours after filling
The standard also allows extra cultures, sweeteners, flavorings, stabilizers and other optional ingredients.
Why the process matters for gut health
Live cultures depend on what happens after fermentation
The bacteria are alive when the yogurt sets, but they don’t have to stay that way. The federal standard lets manufacturers treat yogurt after culturing to inactivate the microorganisms and extend shelf life. If they do, the label must say “does not contain live and active cultures.”
A label may say “contains live and active cultures” if the yogurt has at least 10 million colony-forming units (CFU) per gram at manufacture, with a reasonable expectation of 1 million CFU per gram through its shelf life. The International Dairy Foods Association’s voluntary Live & Active Cultures seal sets a higher bar: at least 100 million cultures per gram at the time of manufacture for yogurt and other cultured dairy, and 10 million for frozen yogurt.
Live starter cultures aren’t automatically probiotics. The ISAPP consensus statement on fermented foods says a “contains probiotics” label belongs only on foods whose strains are defined to the strain level, with evidence that the live microbes provide a health benefit. Our probiotic yogurt buyer’s guide covers how to read those labels.
Fermentation lowers lactose
During fermentation, lactose is reduced by about 30% as it’s converted to lactic acid, according to the 2014 Foods review. The cultures also carry lactase, the enzyme that breaks down lactose. On that basis, European regulators at EFSA have approved a health claim linking live yogurt cultures to improved lactose digestion, as the ISAPP consensus notes.
In the US, the National Institute of Diabetes and Digestive and Kidney Diseases describes yogurt as lower in lactose than other milk products, and says some people find it easier to tolerate.
What the fermented-food research shows so far
Yogurt has more human research behind it than most fermented foods. The ISAPP consensus counted more than 20 randomized controlled trials with yogurt and cultured milk products, while noting that much of the evidence comes from prospective or epidemiological studies.
In a 2021 Stanford trial published in Cell (Wastyk and colleagues), 36 healthy adults followed either a high-fermented-food diet, including yogurt, kefir and kimchi, or a high-fiber diet for 10 weeks. According to Stanford Medicine, the fermented-food diet led to an increase in overall gut microbial diversity, and blood levels of 19 inflammatory proteins decreased. It was a small, short study of mixed fermented foods, so it can’t tell you what yogurt alone does.
How yogurt is made step by step, from dairy plant to home kitchen
Commercial plants follow the same basic sequence at a larger scale, with tighter control at each stage. A typical run looks like this:
- Standardize the milk. Raw milk is filtered and centrifuged to a standardized fat and solids content. Milk solids not fat in yogurt typically range from 9% to 16%, above the 8.25% legal minimum.
- Homogenize. At about 122–149°F (50–65°C) and high pressure, homogenization shrinks fat globules to 2 microns or less so the fat stays evenly mixed.
- Heat-treat. Milk is typically held at 185°F (85°C) for 30 minutes or 203–210°F (95–99°C) for 7–10 minutes to destroy pathogens. The heat also denatures whey proteins, which favors a firmer, more viscous yogurt.
- Cool to incubation temperature. The hot milk is brought down to fermentation temperature; fermentation tanks are kept at 109°F (43°C).
- Inoculate. The S. thermophilus and L. bulgaricus starter goes in, sometimes with additional cultures.
- Incubate. In one 2022 Greek-yogurt study, milk incubated at 109°F (43°C) reached pH 4.6 in about 4 hours.
- Cool. Once the pH falls to about 4.3 to 4.7, yogurt is cooled to around 41°F (5°C), which slows the cultures and keeps acidity from climbing.
Set vs. stirred. Set yogurt is incubated in its retail container so the gel stays intact. Stirred yogurt is incubated in large tanks and agitated into a smooth, viscous product before packing.
Greek (strained) yogurt. Greek-style yogurt has some of its acid whey removed by straining or centrifuging. Researchers report that making 1 kg of Greek strained yogurt leaves 2–3 kg of acid whey behind (the same ratio holds in pounds). Learn more in how Greek yogurt is made.
How home yogurt making mirrors the plant
Home recipes compress the same steps into one pot. These are the guidelines from the University of Alaska Fairbanks Cooperative Extension Service:
| Step | Home method |
|---|---|
| Choose milk | Use commercially pasteurized milk, or pasteurize raw milk first. Added nonfat dry milk powder gives a firmer set. |
| Heat | 185°F (85°C), or up to 200°F (93°C) for a firmer set; hold 10–20 minutes. This step builds texture and doesn’t replace pasteurization. |
| Cool | 112–115°F (44–46°C) |
| Inoculate | ¼ cup plain yogurt with live cultures per quart of milk, or a dry starter |
| Incubate | 110°F ±5°F (43°C ±3°C) for 4–7 hours |
| Chill | Refrigerate at 40°F (4°C) or below |
Temperature control matters most. The same guide notes that cultures can be killed above 130°F (54°C) and won’t grow well below 98°F (37°C). Incubating longer makes yogurt tarter and can cause whey to separate.
Any setup that keeps the milk steadily in that range will work. A yogurt maker with an adjustable thermostat does it automatically, which matters most for long ferments.
Related topics
- How Is Greek Yogurt Made? Explained: how straining changes texture and yield.
- Plain Yogurt: What You Need to Know: what to look for in unsweetened, unflavored yogurt.
- Probiotic Yogurt: A Buyer’s Guide: how to tell live cultures from labeled probiotic strains.
- Best Yogurt for Gut Health (2026): a side-by-side look at yogurts through a gut-health lens.
Sources
- Electronic Code of Federal Regulations. 21 CFR 131.200, Yogurt.
- Allam A, Shafik N, Zayed A, et al. Plain set and stirred yogurt with different additives: implementation of food safety system as quality checkpoints. PeerJ. 2023;11:e14648.
- Sfakianakis P, Tzia C. Conventional and innovative processing of milk for yogurt manufacture; development of texture and flavor: a review. Foods. 2014;3(1):176–193.
- Karastamatis S, Zoidou E, Moatsou G, Moschopoulou E. Effect of modified manufacturing conditions on the composition of Greek strained yogurt and the quantity and composition of generated acid whey. Foods. 2022;11(24):3953.
- Cascio J, Dinstel RR. Making Yogurt at Home (FNH-00062). University of Alaska Fairbanks Cooperative Extension Service.
- International Dairy Foods Association. Live & Active Cultures Seal.
- Marco ML, Sanders ME, Gänzle M, et al. The International Scientific Association for Probiotics and Prebiotics (ISAPP) consensus statement on fermented foods. Nature Reviews Gastroenterology & Hepatology. 2021;18:196–208.
- Weaver J. Fermented-food diet increases microbiome diversity, decreases inflammatory proteins, study finds. Stanford Medicine News Center. July 12, 2021. (Reports on Wastyk HC, et al., Cell, 2021.)
- National Institute of Diabetes and Digestive and Kidney Diseases. Eating, Diet, & Nutrition for Lactose Intolerance.
