Yogurt: The Complete Guide

What yogurt is

Yogurt is milk that has been soured and thickened by two bacteria, Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus. They turn milk sugar (lactose) into lactic acid, and the acid sets the milk proteins into a soft gel. In the United States, that pairing is part of the legal definition of yogurt (21 CFR 131.200).

The same federal standard sets a few more rules. The dairy ingredients must be pasteurized or ultra-pasteurized before the culture goes in, and the finished yogurt must reach a pH of 4.6 or lower within 24 hours of filling. Regular yogurt has at least 3.25% milkfat and 8.25% milk solids other than fat. Lower-fat versions must state the milkfat percentage on the front of the label (21 CFR 131.200).

Everything else you see in the dairy case, from thin drinkable yogurt to thick Greek and Icelandic styles, starts from that same base. The differences come from the milk, the cultures added on top of the two required ones, how long the yogurt ferments, and whether it is strained.

How milk turns into yogurt

Yogurt making has three stages: heat, culture and chill. Each one changes the milk in a specific way. Our explainer on how yogurt is made goes deeper; here is the short version.

Heating the milk

Milk is heated well above pasteurization temperature before culturing. The University of Alaska Fairbanks Cooperative Extension Service tells home cooks to hold milk at 185 to 200°F for 10 minutes for a thinner yogurt or up to 20 minutes for a thicker one, because heating helps “the proteins bind together instead of forming curds and whey” (UAF Extension, Making Yogurt at Home). Heat also clears out most competing bacteria so the culture has the milk to itself.

Fermenting

After the milk cools to about 110°F, the culture is stirred in. The two yogurt bacteria work as a team. S. thermophilus usually starts the acid production, and L. bulgaricus takes over as the milk gets more sour. As lactic acid builds, the pH falls. When it reaches about 4.6, the milk proteins (mostly casein) come out of solution and link into a gel (UAF Extension). The UAF guide incubates at 110°F, give or take 5°F, for 4 to 7 hours.

Chilling and straining

Moving yogurt to the refrigerator slows the bacteria down and firms the gel. Straining is optional. Draining off the liquid whey leaves a thicker, higher-protein yogurt, which is how Greek and Icelandic styles are made. Some manufacturers reach a similar texture by concentrating the milk or adding milk proteins before fermentation instead.

Types of yogurt

Most yogurt falls into a handful of styles. The differences are mostly about texture, protein and how much whey is left in.

Style	What makes it different	Texture
Regular (set or stirred)	Fermented and sold with its whey; set yogurt ferments in the cup, stirred yogurt is fermented in a tank and then mixed	Spoonable, from soft to firm
Greek or Greek-style	Strained to remove much of the whey, or made from concentrated milk	Thick and dense, more protein per cup
Icelandic-style (skyr)	Strained even further than most Greek yogurt	Very thick, mild to tangy
Drinkable yogurt	Thinner base, often blended with fruit or sweeteners	Pourable
Plant-based “yogurt”	Made from soy, coconut, almond, oat or other bases, usually with added thickeners	Varies widely
Kefir (a cousin, not yogurt)	Fermented with kefir grains that contain many bacteria and yeasts	Thin and drinkable, sometimes fizzy

If you want more detail on a style, see what Greek yogurt is. The federal standard covers dairy yogurt only. Plant-based versions vary much more in protein and sugar, so their labels are worth a closer look.

Yogurt nutrition at a glance

The table below uses USDA FoodData Central values, converted to a 6-ounce (170 g) serving so the styles can be compared directly. Brands vary, so treat these as typical values and check your own label.

Yogurt (6 oz, 170 g)	Calories	Protein	Fat	Total sugars	Calcium
Plain, whole milk (USDA)	104	5.9 g	5.5 g	7.9 g	206 mg
Plain, low fat (USDA)	107	8.9 g	2.6 g	12.0 g	311 mg
Plain, skim milk (USDA)	95	9.7 g	0.3 g	13.1 g	338 mg
Greek, plain, whole milk (USDA)	165	15.3 g	8.5 g	6.8 g	170 mg
Greek, plain, nonfat (USDA)	100	17.3 g	0.7 g	5.5 g	187 mg
Low fat, fruit-flavored (USDA)	158	7.8 g	2.3 g	21.8 g	272 mg

A few patterns stand out. Greek yogurt has roughly twice the protein of regular plain yogurt because straining concentrates the protein, but it also carries away some of the calcium with the whey. Regular low-fat and skim yogurts often have more calcium than Greek. The sugar in plain yogurt is natural milk sugar, while a fruit-flavored cup carries about 14 grams more sugar per serving than plain low-fat, most of it added.

Fermentation itself uses up part of the lactose. USDA lists about 5 g of lactose per 100 g of whole milk and about 3.4 g per 100 g of plain whole-milk yogurt. Straining lowers it further, since lactose leaves with the whey.

MyPlate counts 1 cup of yogurt, dairy or fortified soy, as 1 cup from the dairy group, and lists 3 cups a day for adults (MyPlate, Dairy).

Live cultures and probiotics

All yogurt starts with live bacteria, but not all yogurt still has them when you eat it. The federal standard allows yogurt to be heat-treated after culturing to extend shelf life. If that happens, the label must say “does not contain live and active cultures.” A label may say “contains live and active cultures” only if the yogurt has at least 10 million colony-forming units (CFU) per gram at the time of manufacture, with a reasonable expectation of 1 million CFU per gram through its shelf life (21 CFR 131.200).

“Live cultures” and “probiotic” are not quite the same thing. The National Center for Complementary and Integrative Health defines probiotics as live microorganisms intended to have a health benefit, and notes that yogurt is one common source (NCCIH). Many yogurts add extra strains, such as Lactobacillus acidophilus or Bifidobacterium species, on top of the two required cultures. Whether a particular strain does anything for you depends on the strain and the amount, which most labels do not spell out. Our article on whether all yogurt has probiotics covers how to read those labels.

Do the yogurt bacteria survive the trip through the gut? Two small studies say many do. In one, 13 healthy adults ate fresh yogurt for 12 days, and live S. thermophilus turned up in 32 of 39 stool samples and L. bulgaricus in 37 (Mater et al., 2005). A second study fed commercial yogurt to 20 volunteers and also recovered yogurt bacteria in their stool (Elli et al., 2006). Surviving the trip is not the same as settling in, and neither study measured health outcomes.

What the research says about yogurt and health

Yogurt has been studied a lot, but most of the research is observational: it tracks what people eat and what happens to them over years. That kind of study can show a link but cannot prove that yogurt caused the outcome. People who eat yogurt regularly may differ in other ways too. The table sums up where the evidence stands.

Claim	Evidence strength	Source
Lactose in yogurt is easier to digest than lactose in milk	Strong for this specific effect; consistent small trials and a clear mechanism	Kolars, 1984; Savaiano, 2014
Yogurt bacteria survive digestion	Moderate; small studies of healthy adults	Mater, 2005; Elli, 2006
Regular yogurt eaters have a lower risk of type 2 diabetes	Limited; observational, and the FDA calls the evidence limited	Chen, 2014; FDA, 2024
Yogurt eaters gain less weight over time	Weak to moderate; observational only	Mozaffarian, 2011
A diet high in fermented foods raises gut microbiome diversity	Early; one small randomized trial using many fermented foods, not yogurt alone	Wastyk, 2021
Eating yogurt changes the gut microbiome of healthy people	Weak; very small study with mixed results	Lisko, 2017

Lactose digestion

This is the best-supported benefit. In a breath-hydrogen study of people who do not make enough lactase, 18 g of lactose eaten in yogurt produced only about one third as much hydrogen as the same amount in milk or water, a sign that more of it was digested, and caused fewer reports of diarrhea and gas (Kolars et al., New England Journal of Medicine, 1984). A later review explains why: the yogurt bacteria carry their own lactase enzyme, protected inside their cells through the stomach, which then breaks down lactose in the small intestine (Savaiano, American Journal of Clinical Nutrition, 2014). The NIDDK also suggests that people with lactose intolerance try yogurt and hard cheeses, which some find easier to tolerate (NIDDK).

Type 2 diabetes

A Harvard analysis followed nearly 195,000 U.S. health professionals and nurses for up to 30 years. Total dairy was not linked to type 2 diabetes, but each daily serving of yogurt was linked to about 17% lower risk across the three cohorts. A pooled analysis of 14 cohorts found a similar 18% lower risk per daily serving (Chen et al., BMC Medicine, 2014). In 2024 the FDA allowed a qualified health claim that eating at least 2 cups (3 servings) of yogurt a week “may reduce the risk of type 2 diabetes,” but only with wording that the evidence is limited (FDA, 2024). That is a careful, middle-of-the-road position, and a fair way to read the research.

Weight

In three long-running cohorts with 120,877 U.S. adults, each added daily serving of yogurt was linked to 0.82 lb less weight gain over each 4-year period, the strongest inverse link of any single food studied (Mozaffarian et al., New England Journal of Medicine, 2011). Again, this is an association, not proof. For practical ideas, see the best time to eat yogurt for weight loss.

The gut microbiome

The most-cited trial here did not test yogurt alone. Stanford researchers randomly assigned 36 healthy adults to a high-fiber diet or a high-fermented-food diet that included yogurt, kefir, kombucha, fermented vegetables and similar foods. Over the 17-week study, the fermented-food group showed a steady rise in gut microbiome diversity and a drop in several inflammatory markers (Wastyk et al., Cell, 2021). It was small, and it cannot say how much of the effect came from yogurt. A separate study of 8 volunteers eating 250 g of yogurt a day for 42 days found changes that differed from person to person, and two people dropped out (Lisko et al., Microorganisms, 2017).

How to choose a yogurt

The label tells you most of what you need. These checks cover the main differences between yogurts on the shelf.

• Start with plain. Plain yogurt has only natural milk sugar. You can add fruit, a little honey or cinnamon yourself and control the amount.
• Read “added sugars.” The Nutrition Facts panel lists added sugars separately. In the USDA values above, a fruit-flavored low-fat yogurt had about 14 g more sugar per 6 ounces than plain low-fat.
• Look for live cultures. “Contains live and active cultures” means the yogurt met the 10 million CFU per gram threshold at manufacture. “Does not contain live and active cultures” means it was heat-treated (21 CFR 131.200).
• Check named strains if you want probiotics. Extra strains are listed in the ingredients. A strain name with letters or numbers after the species (for example, a code like “BB-12”) makes it possible to look up research.
• Pick the fat level you enjoy. Whole-milk yogurt is richer and more filling; nonfat has more protein and calcium per calorie. MyPlate suggests low-fat or fat-free options for most people.
• Choose Greek or skyr for protein. They give roughly 15 to 17 g of protein per 6 ounces, compared with about 6 to 10 g for regular plain yogurt.
• Watch the extras. Thickeners like pectin or starch are common and harmless for most people, but they can mask a thin product. A short ingredient list usually means milk and cultures do the work.

For brand-by-brand picks, see our guide to the best yogurt for gut health.

Making yogurt at home

Homemade yogurt takes about 15 minutes of hands-on time and costs roughly what the milk costs. It also lets you choose the milk, the culture and how long it ferments. This short method follows the UAF Extension guide (UAF Extension). Our full walkthrough on how to make yogurt at home covers each step in more detail, and our comparison of the best milk for making yogurt helps you pick a base.

1. Heat 1 quart of milk to 185°F. Hold it at 185 to 200°F for 10 minutes for a softer yogurt or up to 20 minutes for a firmer one. Stir so it does not scorch. For extra body, whisk in 1/3 cup of nonfat dry milk before heating, not after.
2. Cool to 112 to 115°F. Set the pot in a sink of cold water and stir. Starter added to milk that is too hot can be killed.
3. Add the starter. Mix 1/4 cup of plain yogurt with live cultures, or a freeze-dried starter packet, into a cup of the warm milk, then stir it back into the pot.
4. Incubate at about 110°F for 4 to 7 hours. Keep it still. Longer times make tangier yogurt; very long times can cause extra whey to separate.
5. Chill. Refrigerate at 40°F or below. The UAF guide says homemade yogurt keeps about 10 to 21 days.

Use pasteurized milk. If you have raw milk, UAF says to heat it to 165°F first, and the CDC warns that raw milk and products made from it can carry germs such as Salmonella, Listeria and E. coli (CDC).

Storing yogurt safely

Keep yogurt in the refrigerator at 40°F (4°C) or colder. Homemade yogurt keeps about 10 to 21 days at that temperature (UAF Extension). Store-bought yogurt should be eaten by the date on the container and within a reasonable time after opening. Use a clean spoon each time so you do not add new bacteria.

A thin layer of liquid on top is whey, and it is safe to stir back in or pour off. Throw yogurt out if you see fuzzy mold, pink or orange spots, or if it smells yeasty, cheesy or rotten rather than clean and tart. For details on homemade batches, see how long homemade yogurt lasts.

Who should be careful with yogurt

• People with a milk allergy. Yogurt contains the same milk proteins as milk. Lactose intolerance and milk allergy are different conditions.
• People with lactose intolerance. Many tolerate yogurt well, especially plain or strained yogurt, but start with small amounts and see how you feel (NIDDK).
• People with weakened immune systems. The NCCIH says the risk of harmful effects from probiotics is greater in people with severe illnesses or compromised immune systems, and severe infections have been reported in premature infants given probiotics. If that describes you, ask your doctor before adding high-dose probiotic products (NCCIH).
• Anyone watching sugar. Flavored yogurts and yogurt drinks can carry as much sugar as a dessert.

This article is general information, not medical advice.

Frequently asked questions

Related topics

• How to Make Yogurt at Home: the full step-by-step method.
• Best Yogurt for Gut Health: store-bought options compared.
• What Is Greek Yogurt?: how straining changes texture and nutrition.
• Probiotic vs Yogurt: when a supplement and a food differ.
• Is Greek Yogurt Lactose Free?: how much lactose is left.
• Fermented Foods for Gut Health: yogurt alongside kefir, kimchi and more.

Sources

• 21 CFR 131.200 Yogurt (Electronic Code of Federal Regulations)
• Making Yogurt at Home (FNH-00062) (University of Alaska Fairbanks Cooperative Extension Service)
• FDA Announces Qualified Health Claim for Yogurt and Reduced Risk of Type 2 Diabetes (U.S. Food and Drug Administration, 2024)
• Eating, Diet, & Nutrition for Lactose Intolerance (National Institute of Diabetes and Digestive and Kidney Diseases)
• Probiotics: Usefulness and Safety (National Center for Complementary and Integrative Health)
• Dairy (MyPlate, U.S. Department of Agriculture)
• Raw Milk (Centers for Disease Control and Prevention)
• Yogurt, plain, whole milk, Yogurt, plain, low fat, Yogurt, plain, skim milk, Yogurt, Greek, plain, whole milk, Yogurt, Greek, plain, nonfat, Yogurt, low fat milk, fruit, Yogurt, plain, whole milk (Foundation) and Milk, whole, 3.25% milkfat (USDA FoodData Central)
• Kolars JC, et al. Yogurt: an autodigesting source of lactose (New England Journal of Medicine, 1984)
• Savaiano DA. Lactose digestion from yogurt: mechanism and relevance (American Journal of Clinical Nutrition, 2014)
• Mater DD, et al. Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus survive gastrointestinal transit of healthy volunteers consuming yogurt (FEMS Microbiology Letters, 2005)
• Elli M, et al. Survival of yogurt bacteria in the human gut (Applied and Environmental Microbiology, 2006)
• Chen M, et al. Dairy consumption and risk of type 2 diabetes: 3 cohorts of US adults and an updated meta-analysis (BMC Medicine, 2014)
• Mozaffarian D, et al. Changes in diet and lifestyle and long-term weight gain in women and men (New England Journal of Medicine, 2011)
• Wastyk HC, et al. Gut-microbiota-targeted diets modulate human immune status (Cell, 2021)
• Lisko DJ, et al. Effects of dietary yogurt on the healthy human gastrointestinal (GI) microbiome (Microorganisms, 2017)