Foolproof Homemade Yogurt: Science, Techniques, and Troubleshooting

Homemade yogurt, including Greek-style, is one of the easiest things to make, not to mention that it is 50% cheaper and incomparably fresher and tastier than any store-shelf brand. Yet, for those who haven’t tried it, the process can often be shrouded in mystery and fear. In reality, however, making yogurt is really straightforward if you understand what it actually is and what makes it succeed or fail.

What is Yogurt?

Yogurt is made by fermenting milk with the help of particular lactose-loving bacteria. These bacteria consume lactose – the sugar in the milk – and excrete lactic acid as waste product, which is responsible for both the tart flavor and the thickening by acting on the casein protein in the milk.

Different compositions of lactic-acid bacteria are used to make yogurt-like products in different parts of the world, each varying by thickness and acidity. What we came to call “yogurt” in the United States is made with lactobacillus bulgaricus and streptococcus thermophilus. While other types of lactic bacteria, such as lactobacillus acidophilus, may also be added to commercial American yogurt for their probiotic properties, only the first two are really needed for the fermentation.

As the name indicates, l. bulgaricus/s. thermophilus culture is thermophilic, which means that in order for the organisms in this culture to function, the milk needs to be fairly warm, which in the case means between 105 and 115 degrees (as opposed to the so-called “mesophilic” cultures, which thrive between room temperature and 104 degrees).

Why is American Store-Bought Yogurt Thicker than Homemade?

This is why:

Most of store-bought yogurt in the United States is additionally thickened with pectin – a fruit-derived component used to thicken jams and jellies. While pectin itself is a natural product, it renders yogurt far stiffer than it would get if thickened with yogurt bacteria alone. This is done because, under normal circumstances, custard-like yogurt structure is not very stable, and allows the whey (water in the milk) to seep out and collect on the surface. While there is nothing wrong with this, uninitiated American consumers could mistake this for a sign of spoilage, and the addition of pectin minimizes this separation. As a result, Americans now expect their yogurt to be thick and spoonable, rather than tender and pourable as nature intended. To achieve the same results at home, American cooks will sometimes add gelatin or powder milk to yogurt to make it thicker. Having grown up in a traditional Old-World culture, however, and a yogurt-centric one at that, I don’t like the unnaturally firm consistency of commercially produced yogurt and don’t add any thickeners in mine.

Choosing Milk for Homemade Yogurt

• Either whole, 2%, low-fat or skim milk can be used successfully.
• Non-homogenized, pasteurized milk sometimes sold at food cooperatives also works great.
• Contrary to what you may have heard, ultra-pasteurized milk works just fine for making yogurt.
• Raw milk, which still contains live bacteria of its own, makes a poor medium for the yogurt starter to grow, as the milk’s native bacteria can interfere with the yogurt bacteria and lead to thin or even failed yogurt. While die-hard raw milk enthusiast will swear off any cooking of milk under any circumstances, my recommendation is to go ahead and pasteurize the raw milk anyway by heating it to 180 degrees – if you want thick, consistent results, that is (more about this below).

Yogurt-Making Equipment and Starter

Contrary to what product marketers would have you believe, making yogurt does not require any particular yogurt-making device or special mail-order culture. In fact, I recommend that you stay away from yogurt makers altogether, since they tend to be over-priced, are only capable of producing a limited, usually small amount at a time, and tend to take up storage space while not being really useful for any other purpose.

Instead, what you really need is this:

• a pot (enameled cast-iron or stainless steel) for heating and incubating the milk
• finished yogurt containing the above live culture, to serve as starter
• a kitchen thermometer capable of registering temperatures as low as 100 degrees (meat thermometers work great for this)
• a way to keep the milk at above 105 degrees Fahrenheit during the 6 hours or more that it takes for yogurt to set (more about this below)

In other words, when choosing commercial yogurt as your starter, make sure that it contains the following:

Look for the smallest container you can find since you will only be needing 1-3 tablespoons, depending on the amount of yogurt you intend to make.

Note that it does not matter whether your starter yogurt is low or whole-fat, or whether it contains pectin and/or sweeteners. All that matters is that the live culture of l. bulgaricus and s. thermophilus be listed among the ingredients.

As long as you make yogurt at least every couple of weeks, you need not purchase new container of commercial yogurt every time you want to make some. Instead, use the last of your homemade yogurt as the starter for a new batch as it already contains the bacteria you need.

The Method

Whether you are starting with raw or pasteurized milk, you will need to heat your milk to 180 degrees first to kill off any organisms that may be present in it, thus creating a sterile medium for the yogurt bacteria to grow by eliminating any competition. When dealing with raw milk in particular, heating the milk to just below boiling serves a secondary purpose as well – the heat treatment denatures the proteins in the milk, resulting in their increased ability to absorb water and leading to thicker yogurt. This is another reason raw-milk yogurt tends to be thin.

Allow your milk to cool to 110 degrees. If you started out with raw or non-homogenized milk, where the milk and the cream have not been artificially integrated, you will notice a thick, dry skin of cream floating on the surface (note that you will not see this if you are using homogenized milk). I do not stir it back in since I want my yogurt perfectly smooth and will not tolerate any dry chunks in the texture. Instead, I skim the film off and discard it prior to inoculation (i.e. the addition of the starter).

Remove approximately 1/2 C of warm milk and mix it with a small amount of finished yogurt. It does not matter whether your starter is cold or room-temperature at the time of addition, as it will be brought to the right temperature in the course of this operation. The exact amount of starter does not matter as long as it is kept small – and I do mean small. Yogurt bacteria do not do well when crowded, and adding more starter than necessary will lead to thinner, not thicker yogurt. 

From time to time, I am shocked to see recipes that call for as much as 1/4 C of starter to a quart of milk, which is far too much. Here are the approximate proportions you really need:

• 1 to 2 quarts of milk – 1 T starter
• 1 gallon of milk – 2 T starter
• 2 gallons of milk – 3 T starter

Stir the milk-yogurt mixture into the rest of the milk. Take care to stir your starter in thoroughly and for at least 10 seconds to assure that it is evenly distributed. Do not stir in a circle like your would other liquids. When stirring any kind of starter into milk, be it for yogurt or cheese, always stir up and down and from side to side for best results.

Next, you will need to find a way to keep the milk between 105 and 110 degrees for the next 6 or more hours necessary for the yogurt to set.

While there are different ways to do it, nothing is easier, neater, and more effective than placing it in a turned OFF oven with the oven light left ON. Though it might not seem so, oven light, provided your oven is good at holding heat, is a powerful way to keep things like bread dough and yogurt warm for as long as needed. If you are incubating 1 to 2 gallons, you need not do anything other than place your pot in there overnight. If you are dealing with a smaller amount, such as 1/2 gallon, you will need to additionally wrap your pot in a thick bath towel for extra insulation before putting it in (since the oven itself remains off, and the light doesn’t make it that hot, this is entirely safe).

Alternatively, you could incubate your yogurt in the following ways:

• in your gas oven with a pilot light on (if you have that kind of model)
• partially submerged in a pot with 110-degree warm water (replace the water after it cools)
• by wrapping it in a thick towel and/or a blanket and setting it in a warm place, such as near a wood stove (just don’t put it too close to the wood stove)
• by placing it an insulated cooler with a few jars of hot water to keep the cooler warm
• by placing it on top of a heating pad

Not every one of these approaches will work for every home and every kind of weather, so play with different methods to see what works best for you and your environment.

Be sure that your milk remains undisturbed during the incubation, as agitation interferes with the setting. This is the reason I prefer to incubate mine overnight, when everyone is in bed.

Your finished yogurt will look something like this:

Note that, some of the time, though not always, your finished yogurt may be covered with a layer of blueish-yellow whey. This is perfectly normal and does NOT mean your yogurt didn’t set, since thicker curd will lie underneath. You can either pour the whey carefully off or stir it back in, but keep in mind that any stirring will break down the yogurt’s structure and thin it.

 Crock-Pot Yogurt

While the crock-pot method is relatively popular, know that a crock-pot does not provide any kind of special magic beyond the fact that its inset is ceramic, and, as such, holds the heat longer than stainless steel or glass, which is useful when you are dealing with a smaller amount of milk, such as 1/2 gallon. If you come across a crock-pot yogurt tutorial somewhere, you will often see people recommending to heat and cool it for a certain amount of time, but this is grossly inaccurate, because different crock-pot models vary wildly by the time it takes for them to heat and cool. The only factor you need to watch is the temperature – heat it to 180 and cool it to 110, regardless of how long it takes in your particular crock. Now it just so happens that I am the author of one of the most detailed and reliable crock-pot yogurt tutorials on the web. Here it is:

Making Yogurt in a Crock-Pot

24-Hour Yogurt

Many people are surprised to discover that homemade yogurt that has been ripened in the course of 6 hours to overnight tastes sweet and mild, rather than sharp and tart like store-bought. This is because acid production continues beyond this time frame, increasing over time. If you place your yogurt in the fridge, you will notice that by next day it will have grown both tarter and thicker (for thickening, if you remember, is caused by the acid), which is exactly what happens to manufactured yogurt by the time it reaches your table.

If you find the flavor of fresh yogurt too mild, you can leave it to ferment for 24 hours instead of overnight to accelerate the acid production, which will happen faster at warm temperature than it will in the fridge. Do not leave your yogurt out any longer, however, as it will begin to spoil.

Homemade Greek Yogurt

Greek yogurt is nothing other than regular yogurt that has been strained, which removes some of the whey and concentrates the curd.

Note: do not use store-bought cheesecloth, as it is not dense enough for this purpose even if you layer it. Use a flour sack or another kind of kitchen towel (or a large cloth napkin) instead.

To make Greek yogurt, set a colander over a large bowl (or in a sink if you plan to discard the whey), line the colander with a kitchen towel, pour in the yogurt, and allow to drip until the desired consistency has been achieved. Check on your yogurt regularly to make sure that it doesn’t turn into yogurt cheese (a cream-cheese like substance).

Sweetened Homemade Yogurt

I generally like my yogurt plain, as sweetened yogurt is a distinctly Western phenomenon and was not a part of the culture where I grew up, but since my kids like it a little sweeter, I sweeten their individual portions with a small amount of maple syrup after the fermentation is complete. It is however possible to sweeten your milk with sugar or a honey and such when you are heating it up, which would give the sweetener the chance to dissolve. I have not tried this myself since I use my yogurt mainly as an ingredient in savory dishes.

If you decide to try sweetening your milk before the inoculation, I don’t recommend trying it until you become confident with your plain yogurt so you can at least rule out excessive sweetener if it fails to set.

Troubleshooting

If you wake up in the morning and your yogurt is still thin, the first thing to do is to put it back and wait a few more hours, since acid content, which is responsible for the thickening, will continue to increase with time. If after 12 hours your yogurt is still the consistency of milk, this means that it wasn’t kept warm enough during the incubation. Consider improving your incubation method or trying a different one.

Meanwhile, failed yogurt can be salvaged by re-heating it carefully over low heat back to 110 degrees and incubating it for the second time.

 If All is Lost: Turn Failed Yogurt into Homemade Ricotta

Another way to deal with failed yogurt is to turn it into homemade ricotta. While true ricotta was originally made by reheating the whey left over from making other cheeses and straining it again (which I have done this successfully myself), present-day commercial ricotta is made by adding acid to hot milk. As our failed yogurt already contains acid from the addition of yogurt starter, all you need to do is heat it until it curdles (to approximately 165 degrees) and then strain it like Greek yogurt above. Click here and scroll down to the end for a step-by-step photo tutorial.