How is Yogurt Made?

If you look at the ingredients label on the cartons of yogurt lining store shelves, you’ll see Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus listed (usually as S. thermophilus and L. bulgaricus) at the head of the list. These two bacteria work symbiotically when they ferment milk — each helping the other, and each essential to the final product.

Once introduced to warm milk, these microbes go to work breaking down lactose (milk sugar) into two simple sugars, glucose and galactose. These are then further broken down into energy for the microbes, and into byproducts. In yogurt, the most important byproduct is lactic acid. When the right amount of acid is produced, it coagulates the milk proteins, creating a custard-like curd.

Beyond the basic fermentation microbes, a plethora of lesser-known microbes might be employed to impart subtle aromas, textures, flavors, and probiotic benefits. Not all yogurt microbes are probiotic, by the way! (The term “probiotic” can only be used when a microbe imparts a known health benefit, as defined by the World Health Organization.) If you’re eating or making yogurt for the sake of consuming probiotics, be sure these types of microbes are included on the ingredients list or in the starter culture you’re using to make your own.

How is Yogurt Made?

Heat plays two roles in the making of yogurt — it sets the stage for proper fermentation, and it provides the optimal temperature for fermentation to occur.

Most yogurt recipes include a step in which the milk is heated to at least 180 degrees and often held at that temperature for 10 minutes or more. Lovers of raw milk might initially balk at this process, which takes the milk beyond even regular pasteurization temperatures! So why include it? For two primary reasons. First, it almost completely clears the milk of other microbes, creating a blank canvas for yogurt microbes to grow quickly and produce the maximum byproducts. Second, the high heat denatures some of the proteins in the milk, causing them to create a thicker final texture. These proteins, known as “whey proteins,” are normally not incorporated into the curd and instead remain in the whey, the watery portion of the coagulation process.When exposed to high heat, however, the whey proteins change and stick to the other proteins (the “caseins”), thus becoming a part of the curd. Even if the yogurt isn’t drained, the denatured whey proteins help increase the thickness of the yogurt. They also help increase the protein content, particularly if the whey is subsequently drained from the yogurt.

Incubation, when the yogurt milk is held at a warm temperature until coagulation is complete and the desired flavor profile is achieved, might vary in both time and temperature. In my book Homemade Yogurt & Kefir, you’ll find several recipes that are examples of this, as well as tips for choosing the right fit for you. But basically, the cultured or inoculated milk needs a spot where the temperature can be maintained at between 100 and 125 degrees, with 110 degrees being a good fit for most makes. The lower the temperature, the longer the fermentation might take. For example, at 125 degrees, it might take 4 hours. At 100 degrees, it might take 12 to 18 hours.

Subtlety in flavor and texture comes from tweaking the preheat step, the incubation time, and the incubation temperature. That said, if you decide to make yogurt at home, don’t get too focused on this fact. Remember, fermentation happens. Provide a warm spot (a heating pad, an oven with a pilot light, an ice chest with a hot water bottle, or a fancy yogurt-maker), wait, test for taste, and learn.

Benefits of Yogurt

The first, but sometimes least-appreciated, benefit of yogurt is its digestibility. Fermentation leads to the breakdown of a good portion of lactose into more easily digested molecules. It’s this result that helped our ancestors, who were all lactose-intolerant, benefit from eating fermented milk. In fact, yogurt-like residues have been detected on ancient pottery shards found in parts of the world where lactose intolerance continues to be the norm today. Even for those who can digest milk sugar, the work of the microbes makes yogurt easy for our systems to process. When yogurt is drained of all or part of the whey (such as in labneh and Greek yogurt), even more lactose is removed.

Probiotics have most recently given yogurt its boost in popularity. But as I mentioned earlier, not all yogurts contain probiotics. When they do, however, these gut-friendly microbes offer myriad known and suggested benefits. They assist with immune function by helping occupy space in our intestinal tract, creating acidic conditions that are directly inhibiting to many pathogens; they’re known to produce natural bacteriocins that target pathogens for destruction; they work as a thermostat of sorts, boosting the body’s immune activity during infection; and more. Studies show they also have a positive effect on stress management, gastrointestinal problems, antibiotic-induced diarrhea, and vaginal flora insufficiency. Even the gastric-ulcer-producing microbe Helicobacter pylori might be limited by regular probiotic intake. These positives all make sense when we remind ourselves that for our forebears, fermented foods were a part of regular meals, meaning our own microbiome has a sort of dependency upon them. So don’t think of yogurt, or any fermented food, as a trendy product to be eaten only when one feels ill.

Storing and Using Yogurt

Yogurt, whether homemade or purchased, doesn’t spoil quickly, thanks to its acidic conditions. If the yogurt is drained, it will last even longer. However, the bacteria count starts to drop soon after the yogurt has been made. If you’re looking for high numbers of probiotic microbes, use it within a week or so. The number of bacteria still active over time will have to do with how many were present after the yogurt was made, as well as if there are any remaining sugars for the bacteria to survive on. (If there’s residual sugar, more fermentation will take place, even in the fridge — leading to a tarter yogurt.)

Easy Yogurt-Making at Home

Here’s a super-simple approach to making great yogurt at home. For more options, including unique recipes from other countries, I hope you’ll refer to my book.

Ingredients:

• 1/2 gallon milk (I use organic whole milk)
• 1/4 cup plain yogurt with live active cultures, or 1/4 teaspoon powdered yogurt culture

Directions:

1. Pour milk into a stainless steel pot and place on stove. Stir gently, scraping the bottom, and heat to boiling. Remove from heat.
2. Set pot in cool water bath in sink, and stir until milk reaches 120 to 125 degrees Fahrenheit.
3. Put fresh yogurt in a small bowl, and add enough milk to stir into a thin, smooth liquid.
4. Add thinned yogurt to warm milk, and stir well. Or, if using powdered culture, sprinkle on top of milk, wait a few minutes, and then stir in.
5. Pour the mixture into two quart-sized jars, and cover snugly with lids.
6. Line an insulated cooler with a towel, and set yogurt jars inside.
7. Fill another quart jar with 150-degree water (or any water that’s almost too hot to touch), cover, and add to cooler.
8. Tuck towel over tops of jars and close cooler lid. Set in a fairly warm spot. If you don’t have a warm spot, or if the weather is cold, simply refill the hot water jar after about 2 hours.
9. Wait 8 hours, and then check for thickness. (When the yogurt is warm, it will be a bit thinner than once it’s chilled.) You’re looking for a custard-like texture.
10. Chill in the refrigerator overnight.
11. The next day, taste. If the yogurt isn’t tart enough, incubate it longer next time, or allow it to cool on the counter overnight and then refrigerate. Use within 2 weeks.

Buying the Best

When choosing commercially made yogurt, read the label! Very few yogurts are made without added thickeners, unless they’re drained. Some larger modern makers are turning to a water removal step, which occurs before the milk is made into yogurt. (This makes the milk more concentrated with proteins and fats, creating a thicker yogurt.) Thickeners might include guar gum, tapioca starch, or nonfat milk (as a dry powder). None of these are truly bad for you, but they do change the way the yogurt tastes.

Beyond thickeners, you might see a variety of microbes listed as cultures. Even if one yogurt lists identical microbes as another, they might be quite different — the genus and species are only the beginning of the diversity! So, taste several brands and see what you and your family like.

One caution I’d like to add is regarding sugar. In particular, added sugar to yogurt. Interestingly, lactose is nature’s least sweet sugar. But it’s still identified as sugar by our bodies the same way a sweeter sugar would be. If you’re trying to reduce your consumption of sugar (which is unnaturally high in most modern Western diets), then avoid buying yogurt with added sugar. If you want your yogurt to be sweeter, try adding low glycemic fruits (most berries fit this bill), or add vanilla extract, cinnamon, or other unsweetened flavoring. You can also make or buy lactose-free yogurt. This product will actually taste a bit sweeter thanks to any remaining lactose having been broken down into the simple sugars of glucose and galactose — which are naturally much sweeter than lactose.

Gianaclis Caldwell is the author of author of six books on cheesemaking, small dairying, and goat care, including the award-winning Mastering Artisan Cheesemaking. Along with her husband, she milks goats and makes cheese at Pholia Farm Creamery in southern Oregon. This is an excerpt from Homemade Yogurt & Kefir (Storey Publishing).