A quart of homemade yogurt costs about $2 if you make it from regular whole milk and use a couple of tablespoons of last batch’s yogurt as the starter. The equivalent quart of commercial Greek-style yogurt is $7 to $9 depending on brand. The technique that produces homemade yogurt has not changed in 4,000 years (warm milk, bacterial culture, time, cool storage) and requires no equipment that is not already in most kitchens. What it requires is paying attention to temperature.
Yogurt that fails at home almost always fails for one of three reasons. The milk was not heated enough to set up the protein structure. The incubation temperature drifted out of the bacteria’s active range. Or the incubation time was cut short. All three are temperature-and-time problems, which means a kitchen thermometer is the most useful piece of yogurt equipment a home cook can buy.
What is happening in a jar of yogurt
Yogurt is milk plus two bacterial strains, Lactobacillus bulgaricus and Streptococcus thermophilus. The bacteria consume lactose (milk sugar) and excrete lactic acid as a byproduct. The lactic acid lowers the pH of the milk from its starting point of about 6.7 down toward 4.6, at which the milk proteins (mainly casein) lose their dispersed structure and tangle together into a soft solid gel. That gel is yogurt.
The bacteria work best between 105 and 115 F. Below 100 F they slow significantly. Above 120 F they start to die. The window for productive fermentation is narrow, which is the whole reason yogurt makers exist and the reason home cooks need an insulated incubation method.
The other key process is heating the milk to 180 F (just below boiling) before introducing the culture. This step denatures the whey proteins in the milk and unfolds them so they cross-link with the casein during the gel formation. Skipping the 180 F heat produces a thinner softer yogurt because the whey proteins stay folded and do not contribute to structure. Commercial yogurt manufacturers always pasteurize at high temperatures for this reason, even when starting with already-pasteurized milk.
Equipment
Heavy-bottomed saucepan or pot, large enough to hold the volume of milk plus headspace.
Instant-read or candy thermometer. Accuracy within 2 degrees is enough.
Whisk for incorporating the starter.
Incubation container. Options: a thermos, glass jars in an oven with the light on, jars in a slow cooker on warm wrapped in towels, jars in a cooler with a separate jar of warm water for heat ballast, or a dedicated yogurt maker.
Clean sealable containers for storage. Glass or BPA-free plastic, both work fine.
Step by step method
Heat the milk. Pour one quart of whole milk into the saucepan. Place over medium-low heat. Stir occasionally to prevent scorching on the bottom. Bring the milk slowly to 180 F. This takes 15 to 20 minutes on a typical home burner. Hold at 180 F for 5 minutes by adjusting the heat. The held time is what denatures the whey proteins.
Cool the milk to 110 F. Move the pot off the heat. Let it sit on the counter for 25 to 35 minutes, or speed the cool by setting the pot in a sink of cold water and stirring occasionally. The target is 110 F, which is hot to the touch but not painful when you dip a clean finger in for two seconds.
Whisk in the starter. Combine 2 tablespoons of plain yogurt (with live cultures, no sweetener or flavor) with about half a cup of the warm milk in a small bowl. Whisk until uniformly mixed. Pour the mixture back into the pot of warm milk and whisk to distribute. This pre-mixing step prevents the starter from clumping when it hits the larger volume.
Transfer to incubation. Pour the inoculated milk into your incubation container. Set the incubation temperature to hold at 105 to 115 F.
Incubate undisturbed. Six hours for a mild yogurt with light tang. Eight to ten hours for a medium tang and firmer set. Twelve hours for a thick assertive yogurt. The longer the incubation, the more lactic acid the bacteria produce, and the tarter the result. Do not stir or jiggle the container during incubation. Movement disrupts the gel formation.
Check the set. Tilt the container gently. The yogurt should pull away from the side as a single mass rather than slosh. If it still looks liquid, replace the lid and incubate another hour. Older starter cultures or cooler incubation temperatures need more time.
Cool and store. Once set, transfer to the refrigerator and chill for at least 4 hours before serving. The chill firms the gel further. Yogurt can be eaten within 24 hours of making but is fully developed in texture and flavor after the overnight chill.
Incubation methods that work
Oven with light on. Most home ovens with the oven light turned on hold an interior temperature of about 100 to 105 F from the bulb’s heat. Place jars inside, leave the light on, close the door, and check temperature with a thermometer placed inside. If the oven is too cool, add a pan of just-boiled water on the lower rack. If too warm, prop the door open slightly.
Slow cooker on warm. Set the slow cooker to its warm setting, wrap the entire unit in a heavy towel, and place jars inside. Most slow cookers on warm hold about 110 F. Verify with a thermometer.
Thermos. Pour the inoculated milk directly into a 1-quart insulated thermos. Pre-warm the thermos by filling it with hot water for 5 minutes and draining before adding the milk. The thermos holds temperature within 2 to 3 degrees over 8 hours.
Cooler with warm water jar. Place jars in a small cooler along with a separate jar filled with 130 F water. Close the cooler. The water jar provides a heat reservoir. Check temperature at the 4-hour mark and add another warm jar if cooling.
Greek yogurt and labneh
Standard homemade yogurt is comparable to American-style yogurt in thickness, somewhere between drinkable and spoonable. To thicken to Greek style, strain.
Line a fine-mesh sieve with cheesecloth or a clean cotton tea towel. Set over a bowl. Pour finished yogurt into the lined sieve. Refrigerate.
Two hours produces a softly thickened yogurt. Four hours produces commercial Greek-style thickness. Six to eight hours produces labneh, which is spreadable like cream cheese.
The strained-out liquid (yellowish, slightly tangy) is whey. Save it. Whey is useful as a substitute for water in bread doughs (the extra protein produces a softer crumb), in smoothies, or in lacto-fermented vegetables.
Reusing yogurt as starter
Save 2 tablespoons of finished yogurt in a clean jar to use as starter for the next batch. The cultures stay viable for about 2 weeks refrigerated. After that the bacteria population thins and the set gets weaker.
Most home yogurt cooks reach 8 to 10 batches off one purchased commercial starter before the culture noticeably weakens, at which point they buy a fresh container of commercial yogurt and start over. The cycle costs almost nothing and produces consistently better yogurt than the cheapest dairy-aisle options.
Frequently asked questions
Can I make yogurt without a yogurt maker?+
Yes. Any insulated container that holds milk at 110 F for 6 to 12 hours works. A thermos, an oven with only the light on, a slow cooker on the warm setting wrapped in a towel, or a cooler holding a jar of warm water all incubate successfully. The yogurt maker just automates temperature control. The result from a thermos is identical to the result from a $80 machine if both hold the same temperature.
Why is my homemade yogurt runny?+
Three common causes. The milk was not heated to 180 F before cooling, so the whey proteins did not denature and the structure stayed loose. The incubation temperature dropped below 100 F, which slows the culture and prevents full setting. Or the incubation time was too short, less than 6 hours. Heat the milk fully, hold incubation steady at 110 F, and give it at least 8 hours for a firm set.
What starter culture should I use?+
Plain commercial yogurt with live active cultures works fine as a starter. Choose a brand you like the taste of because the cultures carry over. Use 2 tablespoons of starter per quart of milk. Greek-style commercial yogurt makes thicker homemade yogurt. Avoid sweetened or flavored yogurt as starter, the sugar interferes with the culture. Powdered freeze-dried starter cultures from a fermentation supplier work too and last months in the freezer.
How long does homemade yogurt last?+
Two to three weeks refrigerated in a clean sealed container. The tartness increases over time as the cultures continue to convert lactose to lactic acid at fridge temperatures, just much more slowly. After three weeks the yogurt is still safe but the flavor becomes assertive. Save 2 tablespoons from each batch within the first week to start the next batch.
Can I make Greek yogurt at home?+
Yes, by straining. Line a fine sieve with cheesecloth or a clean cotton towel. Pour finished yogurt in and refrigerate over a bowl for 2 to 4 hours for Greek-style thickness, or 6 to 8 hours for labneh (a thick spreadable strained yogurt). The yellowish liquid that drains off is whey, which is useful in baking or smoothies and not waste.
Priya Sharma
Priya Sharma writes for The Tested Hub.