Based on what I've written on the subject of fresh-fermented dairy so far, you might be tempted to think that bacteria are the only microbes that can ferment milk. This isn't so. There are a few oddball varieties of yeast that can do it too. What difference does yeast make in a yogurt culture? Quite a lot as it happens. For while the waste product of lactose-eating bacteria is acid, the waste product of lactose-eating yeast is...anyone? Anyone? Yes you at the back with the hangover. Right: alcohol.

And in fact there are quite a few Central Asian fresh-fermented yogurt drinks that are alcoholic. Kumis is probably the most famous of these, a mildly carbonated, reasonably alcoholic drink made from horse milk. Why horse milk? Probably because back in the day, it was the only thing available to Central Asian nomads like the Mongols (famous and fearsome horse riders). Though by a very interesting coincidence horse milk also has more lactose (sugar) in it than the milk of other ruminants like sheep or goats. And more sugar in the milk means more alcohol in the kumis. Humans...so predictable.

Kumis is still extremely popular in Central Asia, in Russia and places like Kyrgyzstan, Kazakhstan and of course Mongolia. Though these days most of it is made with cow's milk instead of horse milk. That's partly because the Central Asian horse herd is small relative to the demand for kumis, partly because a cow is far less likely to kick your brains out your left ear when you try to milk it.

I confess I've always been very curious about real kumis, though long ago gave up hope that I'd ever taste the real thing. Who knows, though? I live in the heart of horse country nowadays. Maybe I should go buy a football helmet and give the horse-milking thing a try.

UPDATE: Reader Bronwyn from New Zealand adds:

The udder of the average nursing mare is also a great deal smaller than that of a dairy cow; the cow having been bred for centuries to give much more milk than needed by a single offspring.

Reader Thames asked me to comment on kefir, which is a yogurt beverage that originated in the Caucasus region (essentially the dividing point between Europe and Asia, roughly the area of extreme southern Russia, Georgia and Azerbaijan). That part of the world is what you might call a hotbed of milk fermentation, and brother do those people ever get into it. They'll ferment the milk of cows, sheep, goats, you name it. Kefir can be made from any of them.

The thing that makes kefir different is that it's fermented from "grains", little semi-solid, bud-like structures that look like clumps of tapioca, but which are actually amazingly complex colonies of yeasts and bacteria combined with proteins, fats and starches. Add them to a quantity of milk and the result is a beverage that's tangy but also alcoholic and, interestingly, carbonated. These characteristics reflect the by-products of the various microbes, those being acid, alcohol and CO2. Oh, and more kefir grains, which are handy for culturing more kefir.

Here I should note that most of the kefirs that are available commercially have no alcohol in them, though most do have at least a little carbonated "bubble". Usually the ones that you find for sale in stores are also sweetened and/or flavored with fruit. If you're curious about yogurt drinks, kefir is a good place to start. You can make your own by ordering the grains themselves through the mail.

What an anticlimax this is going to be after all this talk — just a lot of shots of white things. But that's the reality of fermentation: all the really sexy stuff is happening on scale that's far too small to see. Hmm...maybe I should buy a microscope and become a lactic acid bacteria voyeur. Or are there laws against that? While I ponder, combine your liquid and powdered milks...

...give them a whisk...

...then start heating the mixture. I suggest taking it up to 195 and letting it cool back down to 120, but simply warming the mixture to 120 will also do.

Add your store-bought yogurt or packaged starter (and any flavoring you might want at this point, a little honey or vanilla extract let's say)...

...whisk again...

...and pour the yogurt mix into a container. I use this very tall one for reasons that should be obvious when you get to the next photo...

...because my yogurt-making rig consists of heating pads. Now, I've received quite a lot of feedback on this front. Some people use thermos bottles, some use pots placed on top of heating pads, some use pots wrapped in blankets placed under beds. Whatever method you choose, be it one of those or something else (like an empty oven with a pilot light), do your best to keep the mixture around 110 degrees, but not more that 125.

Between two and eight hours later, you should have something that looks like this:

Pretty darn easy, yes? Yes.

...sent to me last night by the folks at the Bowling Green tourism bureau, of the Duncan Hines museum there. Don't you wish grocery stores still looked like this?

Reader Gerhard in Vienna did a little research and dug up this clip on the German and/or European myth that lightning and yogurt are connected.

"This folk wisdom has a basis in reality. In agrarian societies it was customary to place a jug of milk in a warm place [to culture yogurt]. In the evening it was then eaten as a junket with bread. Since the odds of thunderstorms with lightning is quite high on warm summer afternoons, people most probably noticed that the milk thickens very well on those days," explains Gerhard Kielwein, former professor of hygiene and technology of milk from Giessen. "Before a storm, the air is often moist and warm. These are favorable conditions for the lactic acid bacteria that are present in the milk. They convert lactose into lactic acid when the milk is not refrigerated. The sour milk coagulates."

There's that word "junket" again. For those of you who aren't familiar with it — or only know it as the term for those shady fundraising trips politicians take — "junket" is a mixture of milk, sugar (or honey) and other flavorings that's quick-coagulated with chymosin-rich animal rennet. It's curds and whey, in other words, the stuff Little Miss Muffet ate. Junket isn't around much anymore, though it was once a very popular food item with children and/or sick people because it's sweet, mild, silky and easy to digest. You might think of it as sort of an instant, sweet cheese. That said, I've never heard the term applied to yogurt before, but I suppose the word mostly fits. On the subject of lightning, reader Laura offers this:

This is just a guess, but I think that pre-storm weather is characterized by increased positive ions in the air, and I think that positive ions are associated with increased bacterial activity. Maybe this is why yogurt sets up better before a storm.

Thanks to both of you for helping me out in this!

Where the word "yogurt" comes from? It's Turkish and it means "thick." No surprises there. Except, why do we use the Turkish word for yogurt when there were so many other yogurt-eating peoples so much closer to Europe that the Turks? The Greeks, for example, and the Scandinavians. The answer is because yogurt first entered Western Europe via the Balkans, where they used the Turkish word. Oh.

With all the talk this week of microbes, dairy and culturing, some of you out there are probably wondering what the difference is between yogurt making and cheese making. Oh who am I kidding? No one's probably wondering that. But I have no other ideas for an intro and it's mid-afternoon already.

As it happens, the process of yogurt making is almost identical to the process of cheese making, save for one critical thing: chymosin. Chymosin is the protein-digesting enzyme that's responsible for curdling milk into solids that can be formed, salted and aged. Yeast and bacteria don't have the muscle to curdle proteins into those kinds of big, firm curds, however they can make itty bitty yogurt curds of a kind that can be strained to make a kind of pseudo-cheese.

All you need is some cheese cloth, a wooden spoon and pot. You scoop the yogurt into the center of a double-layer of cheesecloth, tie it up into a bundle, and suspend the bundle inside the pot, tied to the spoon. A few hours later (up to 24 depending on how runny the yogurt is) enough of the liquid whey will have dripped out and something that looks and tastes very much like cream cheese will be left (Labna is what's it's called in Lebanon, and in fact when you mix it with little chopped mint, basil or thyme, it's fantastic shaped into balls and drizzled with olive oil).

This is as close to a cheese as you'll get with yogurt as your starting point, but it ain't half bad. And now for an overly detailed and long-winded discussion of the chemistry of cheese...

Just kidding. Some other time. (Maybe).

UPDATE: Reader Bronwyn from New Zealand adds:

You can, however, make perfectly good cheese with yoghurt as a starter culture if you add rennet, which contains chymosin, is available from the supermarket, and is what my mother used to use to make junket. All you need is a way to control the temperature of the curds for a few hours. I've made cheddar, provolone, mozzarella, and parmesan using yoghurt as my starter culture and it's just fine. More than fine, in fact. Much nicer than Mum's junket. Cheesemaking is a hell of a lot less complicated than people would have you think - so long as it's not important to you that it turns out exactly the same each time you make it.

The missus had a good question about probiotic critters last night. She wanted to know whether when you make yogurt at home using a commercial probiotic yogurt as a starter, you grow more of those same probiotic bugs in your batch. My guess was yes, if only because everything that's in those factory-made cups probably has to be able to thrive under the same environmental conditions. Curious, I decided to go over to the Stonyfield Farm (my go-to starter yogurt) web site to see if I could discover what kind of microbial zoo they were keeping. I found the usual suspects there (L. bulgaricus and S. thermophilus) as well as some of the more typical probiotic bugs L. acidophilus, Bifidus and L. casei. The sole suprise was a bacterium by the name of L. reuteri. This is a microbe that isn't typically found in the guts of humans, but rather in barnyard animals like goats and pigs. The fact that it's found in yogurt isn't a big deal at all (in fact for Stonyfield Farm it's a selling point), since L. bulgaricus and S. thermophilus themselves probably originated in the guts of cows. And anyway there's evidence to show that L. reuteri offers significant benefits to humans when ingested regularly. Still, there's something a little unsettlling about eating bacteria that are more common to the intestines of mice than of people. Like I said, when it comes to probiotic bacteria, it's probably better not to know.

But to answer the question my feeling is that if these creatures are alive in the starter culture, they'll be alive in a finished homemade yogurt, though whether they'll remain in numbers that will do a body any good, I can't say.

UPDATE: Reader Aaron suggests Liberté yogurt as a starter (if you live in North America).

Priobiotics is the term for edible microbes that are supposed do good things in our insides. For nearly 100 years now, yogurt ambassadors in the West have maintained that yogurt has beneficial, even youth-sustaining properties. Science is now beginning to validate some of their claims.

Microbes like Lactobacillus fermentum, L. plantarum, L. Casei and L. brevis — found in fermented milk products in Eurasia — coat our intestinal walls, producing acid and other compounds that help keep other undesirable microbes from entering our systems. Other bugs actually do things like take apart cholesterol molecules and/or help boost the body's response to disease.

Unfortunately, the microbes that have traditionally been employed by commercial yogurt makers (Streptococcus salivarius subsp. thermophilus and Lactobacillus delbrueckii subsp. bulgaricus) do none of these things, for the simple reason that they can't survive inside our bodies. Which is why virtually every yogurt manufacturer has, over the last decade or so, begun inoculating their products with a combination of good-guy Eurasian micro-flora and L.acidophilus (which is native to the human gut).

More recently, yogurt makers have begun adding fun-to-say Bifidobacteria to the mix. These critters are commonly found in human breast milk, where they work to acidify and in other ways defend the intestinal tract of babies. God only knows what other things yogurt manufacturers are going to think to put in your yogurt next, but odds are you won't want to ask where they come from. Just eat it son, it's good for you.

Several years ago I had occasion to hear a very unusual story about yogurt from one of the oldest of Chicago's old-school German chefs. I was working on an article on food preservation at the time, and nobody but nobody preserves food like a old German, my friends. We were poring over the technical details of his grandmother's fruit preserves, pickles, sausages, cheeses and the like, when suddenly he digressed onto the subject of yogurt. He told me that German housewives once believed that the trick to good yogurt was to leave milk out under the house eaves on a warm spring evening when there was a storm rolling in. Somehow, it was thought, the combination of milk, moisture, temperature and lightning all combined to make yogurt cultures grow.

I've looked high and low for someone else who's heard this story before, and can give me some clue as to where/how it evolved. I mean, lightning? What the heck does that have to do with the price of Acetobacter in orientalis? If anyone out there can shed any light on this, please do send me an email.

A couple of emails over the weekend from readers who asked that I clarify the fact that yogurt is not 100% lactose free. Indeed that's true. The most you can say about yogurt is that, like cheese, it's a dairy product in which the lactose has been drastically reduced (usually by half, frequently by 60% or more). It's this significant reduction in lactose that makes yogurt acceptable to the majority of lactose intolerant people. The reason — at least as I understand it — is because lactose intolerance is more about a threshold of tolerance than an allergy-like sensitivity. Many doctors maintain that even a lactose intolerant person can digest about a cup of milk per day, or the equivalent amount of lactose in the form of cheese or yogurt. Some people are more sensitive even than that, in which case they can't eat yogurt, either.

Those who looked at the previous post carefully may have found themselves wondering: Well now if the bacteria eat most of the lactose in the milk, does that mean people who are lactose intolerant can eat yogurt? Why yes it does. And in fact most people who are lactose intolerant do indulge freely in it. For yogurt is, in a sense, "pre-digested", with lactic acid bacteria preemptively applying the lactase enzyme that's absent in many humans.

Today many of us tend to think of lactose intolerance as just one more malady-du-jour, something fussy people who aren't really sick get to complain about over brunch. But the inability to digest milk as an adult is real, and is in fact normal if you consider the human race as a whole. Among the vast majority of humans, lactase production falls off sharply around the time we're weaned from our mother's milk, then vanishes entirely after the age of four. Only a very few people (say one or two in ten) continue to produce lactase into adulthood.

Yet somehow, suddenly, around 7,000 years ago, something happened — at least among animal herding peoples in Eurasia. Some of them spontaneously evolved the ability to produce lactase on an ongoing basis, which meant they could go on consuming milk throughout their lives. In time this genetic mutation spread to the point where now it's only about 10% of Eurasian adults (or people of Eurasian descent) who can't digest milk. It's all a pretty amazing coincidence when you stop to think about it: a random genetic mutation that allows adults to digest milk just happened to occur among people who herded milk-producing animals. Or maybe it wasn't an accident at all. Maybe that "random" mutation is in fact concrete evidence of evolution in humans. Many scientists believe just that.

But the neat thing about yogurt is that you need not be genetically disposed to digesting dairy in order to digest it. Any human can do it. Which is why you find (and have always found, at least on the Asian continent) peoples who eat yogurt but who don't otherwise consume much dairy.

This is where the fun happens, at least for me. The milk is 120 or so degrees and the starter (room-temperature commercial yogurt) goes in. The cooler starter brings the temperature of the milk down to about 115 degrees Fahrenheit, at which point the lactic acid bacteria start gorging themselves. Seizing the closest available lactose molecules, the bacteria hit them with enzymes that break the lactose into its constituent parts (galactose and glucose), which the bacteria can then convert into energy.

First to the buffet are the Streptococcus salivarius subsp. thermophilus (that last part of their name literally means "heat loving") which thrive in warm, low-acid environments. But here's the rub: they produce lactic acid as a by-product of their metabolism. So as they chow down and reproduce the acid level in their environment rises, until it reaches roughly half a percent of the volume of the milk. At that point the Streptococcus essentially begin to poison themselves with their own waste. And that, as it happens, is just the cue that the Lactobacillus delbrueckii are waiting for. They thrive in higher-acid environments, and so pick up where the Streptococcus left off: eating, reproducing, and eating some more until they too succumb to a combination of acidity (which by now is about 1%) and lack of food.

While this tiny drama is playing itself out, something else that's very interesting is happening. The rising acidity causes the proteins in the mixture not only to uncoil, but to become increasingly attracted to one another. This has the effect of creating the tangly mesh I spoke about earlier, and a gel forms.

The whole process can take as little as two hours or as many as 18 depending on the kind of microbes involved and the temperature. The standard Streptococcus/Lactobacillus tag team will get the job done in just a few hours when the temperature is at the top of their preferred range, about 115 degrees. At 90 it'll take a heck of a lot longer, though interestingly, the longer gelation will produce a different texture. Whereas as rapid gelling will yield the much coveted Jell-O-like consistency, the protein network will be coarser and more prone to breaking (commercial yogurt makers compensate for this by adding starch or other "stabilizers" to prevent this from happening after their products have been shipped). The long-gelling yogurt by contrast will be runnier, but since its protein network is finer, it will be a smoother and more uniform.

Just about everyone who reads a yogurt recipe through for the first time has the same reaction: Wait, you mean I have to BUY yogurt in order to MAKE it from scratch? The answer is yes. BUT...a little of the commercial stuff can be used to make a whole lot of homemade stuff. Up to 20 times more depending on the recipe and how good you are at the yogurt-making process. It works, for all intents and purposes, like a bread starter.

So great then — you might say — if yogurt works like a bread starter then all I need to keep making yogurt forever is to just keep it going from batch to batch. Um, no, not really. The reason for that is because commercial yogurt is more like a mail order San Francisco sourdough starter than a homemade bread starter. Which is to say it will lose its potency and flavor every time you re-use it. Why? Because just like a mail order sourdough starter, commercial yogurt contains microbes that aren't adapted to living in climates that aren't their own. After a few batches they'll be out-competed by whatever happens to live in your area, and your yogurt will get soupier (and probably stranger tasting). So yes, you have to buy and keep buying every so often, or just go pro and buy powdered starter, which works just as well.

All of which begs the question: what's so special about these creatures? If lactic acid bacteria can be found anywhere, what's wrong with culturing my own? The answer is that in the same way few (if any) microbes outside of San Francisco can produce enough acid to make a sourdough bread truly sour, few outside of the Asian steppes are capable of making a really good gel. Leave milk out to sour in North America, and the result will be a slightly thick peudo-yogurt that our forebears knew as clabber. And while the previous generations consumed clabber in much the same way Eurasian peoples consume yogurt (as a drink, with fruit, with honey or sugar sprinkled on top), it isn't really the same thing.

Thus the American yogurt industry relies on cultures grown from Eurasian stock. Historically they've only used one or two: Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus salivarius subsp. thermophilus which have been judged by Western consumers as offering the most appealing combination of flavor and texture. Yogurt makers still rely heavily on these bugs, though have steadily been introducing more variety into the mix as a result of the probiotics craze (which I'll blog more about later).

The truth is that the world abounds with interesting bugs that make yogurts of all kinds. The Japanese, for example, treasure something called Caspian Sea Yogurt, which is derived from a culture called Matsoni, a combination of Lactococcus lactis subsp. cremoris and Acetobacter orientalis. This tag team combo produces a yogurt that is at once milder than our yogurt and a bit runnier. Their key benefit, however, is that they grow happily at room temperature, which means the Japanese need no clever strategies to maintain a 110-degree growing environment. They just stir it into a container of milk, set it on the kitchen counter and the next morning they have yogurt. Lucky sods.

But look around. If you're interested you can find all sorts of interesting cultures available from online sources. In no time at all you might become a lactic acid bacteria epicure.

There are several competing theories when comes to heating milk for yogurt. One holds that you needn't heat the milk much at all. Another, that it must be heated to boiling and kept that way for hours. Still another, that the milk should get very hot but only for a little while. To help us evaluate which of these methods is the best, we'll employ the handy tool of science.

Long-time readers of joepastry.com will know that proteins are long chain-like molecules. They're made of amino acids, and are generally found balled up in little clumps, especially in things like milk. Heat them (or expose them to acid) and they "denature", which is to say the chemical bonds that keep them clumped break. The molecules then uncoil into long strings that get tangled up with one another, forming a mesh. This mesh traps and holds other types of molecules like fish in a net, forming what is known in the world of high-end basketball shoe commercials as a gel.

It stands to reason then that the more protein you have the firmer your gel. Which is why yogurt makers of old would boil milk for hours. The long boiling evaporated some of the water and concentrated the proteins. The milk was cooled, the culture added and bingo: yogurt. Nowadays we don't need to do that since we have protein-rich powdered milk available to us. This lets the modern yogurt maker add all the extra protein he or she wants without having to endure the laborious step of long-term boiling (then cooling).

This opens the door to the low-heat approach to preparing your milk, since if you've got plenty of protein, the only thing you really need to do is warm the milk to the point that it promotes microbial growth, about 120 degrees or so. The bugs start digesting the lactose and creating acid, which denatures the proteins and there you go: gel.

The only trouble is that not all gels are created equal, which is where the middle approach comes in: heating the milk to 195 for ten minutes, then letting it cool down 120 before introducing the culture. This produces a finer gel that won't break as easily into curds and liquid whey when you insert a spoon. The operative question is: why?

The reason is because milk contains many types of proteins (caseins and wheys), not all of which behave the same way. Some denature when they're exposed acid, others will only uncoil when heat is applied. One of them in particular, a protein called lactoglobulin, is especially useful in a gel as it prevents other proteins (notably caseins) from clumping too tightly together as the gel forms. Unlock its potential and you get a smoother textured yogurt with more stability that gives up less free water when the gel is disturbed.

But guess what it takes to get lactoglobulin into the milk gel game? That's right: heat. So we raise the temperature of the milk to just below boiling, the temperature at which it uncoils. I should emphasize that it's important to be careful here, because much more heat than that and the sensitive lactoglobulin will clench back up irrevocably and its stabilizing abilities will be lost.

You may well ask: do you really need denatured lactoglobulin to make a good yogurt? If people used to boil their milk, they clearly destroyed their lactoglobulin, but got along just fine anyway. Can't I? You certainly can. I recommend that heating step because I think it yields a better product. Feel free to skip it if you want, I wanted to include it in the discussion so you had another arrow in your yogurt-making quiver, as it were.

This week is going to be geeky. Those of you with weak constitutions should leave the building now.

Since yogurt is almost all milk, this step matters, as it will have big impact on your yogurt's consistency and flavor. Contrary to what you might think, whole milk, while it does deliver more fat, doesn't make home-made yogurt thicker. In fact it makes it thinner, or perhaps I should say "soupier". For reasons I'll discuss in detail later, it's protein that makes yogurt thick, not fat. The more protein you have, the better the gel you get. Since most American dairies supplement their skim and low-fat milks with extra protein to improve "mouthfeel", lower fat milks are preferred.

This is not to say that soupy yogurts are a bad thing. I quite like a good whole milk yogurt even though it wants to run off the spoon instead of stand upright on it in a quivering slice. It's luxurious, like cream, though with much less fat. So it's really all up to you. As you'll see as I march through the process, the recipe below is just a guide.

I also want to mention that when it comes to your milk, you need not feel limited to what you find in the market dairy case. For that matter, you need not feel limited to cows. Depending on where you live, you might be able to procure something a bit more exotic. Most health food stores carry goat's milk these days, and if you're really enterprising and live in a rural area, you can sometimes find it fresh. The same goes for cow's milk. Laws concerning the sale of raw milk vary from state to state in America. Here in Kentucky it's not legal to sell "raw" milk even in a health food store, though it's not illegal for health food store owners to connect you with a farmer who'll sell you a "share" in a cow for the price of a gallon of its milk (the legal loophole being that it's not milk that 's actually being sold, but rather livestock, the milk from which an owner is free to drink if he or she pleases).

So look around. You might be surprised at the milk products you might find. For those of you in the US a partial list of raw milk sellers can be found here. Those not living in the US can try their luck here.

Pretty much everywhere between, say, Scandinavia south to Turkey and from there east to India or so. Anywhere, in other words, where you historically find herding peoples who consume the milk of ruminants, be they cattle, yaks, goats, sheep, water buffalo...camels even. Just when and where yogurt first appeared no one knows, but there's no question that it happened several millennia ago, when humans first began to domesticate animals.

The first fermentation of milk into yogurt was undoubtedly an accident, for just like a bowl of grain slurry or fruit pulp left out at room temperature, milk ferments all by itself without any additional help from people. Opportunistic lactic acid bacteria move in and start disassembling milk sugar molecules (lactose) into simple sugars they can eat. The side effect of this micro pig-out (or one of them, at least), is acid. That acid makes the environment extremely unfriendly to other types of microbes, notably most of the kinds that are dangerous to humans. Which makes it a pretty good deal all the way around.

Another nice thing about fermentation is that in addition to making foods taste good, it preserves them, for days, weeks or even longer. Which would have been a very, very good thing for Eurasian herding peoples living several millennia ago, since Maytag had yet to invent a portable refrigerator light enough to transport on yak back. No wonder yogurts are found in so many places and in so many styles. They just make too much sense.

In less than a day I've been inundated with yogurt-making methods from around the world. It's been great, but daunting too. I had a feeling something like this might happen, since putting up just one recipe for yogurt is like putting up just one recipe for bread. There are lots and lots of different ways to make it. I put up the one that gives me the best results, and I'll say this to all you aspiring yogurt makers out there: yogurt, like bread, takes practice. Every attempt will get you a little closer to the taste and texture you prefer. So if you don't care for my method — and you may well not — there's a whole world of yogurt-making techniques out there for you to explore. The important thing is to have a good time, yes? Deepest thanks to everyone who sent me a recipe.

This is essentially the yogurt making process I saw Alton Brown do on one of his shows. The thing I love about it is that it dispenses with those silly and expensive yogurt making devices you see in kitchen gadget shops. Everything you need for this you probably have between your kitchen cabinets and your bathroom closet. So, to make home-made yogurt you’ll need:

1 quart low fat milk
1/2 cup powdered milk
1/4 cup room-temperature yogurt (plain)
Optional: 2 - 4 tablespoons honey, maple syrup or refiner's syrup
Optional: A teaspoon or so of flavoring like vanilla, lemon or coffee extract

To start simply pour the milk into a saucepan with the powdered milk and sweetener (if using). Whisk the mixture gently over medium heat until it registers 195 degrees, maintain the temperature there, taking the pan on and off the heat as needed, for ten minutes (you can skip this heating step if you wish and simply bring the mixture to 120, though you'll get a better texture if you apply the higher heat). Then, pour it into a taller-than-it-is-long container, either a wide-mouthed jar or piece of miscellaneous tupperware. Allow it to cool to 120, then add the yogurt and stir until blended.

After that you simply need to keep the culture as close to 110 degrees as you can for the next 4-10 hours...which is easier said than done since most of our modern-day kitchen devices are designed to prevent microbial growth, not encourage it. You’ll need an instant-read or probe thermometer to take regular readings.

Some people like an electric oven for this job, assuming it can be set to 110 degrees or so, though most ovens do a very poor job of maintaining steady low temperatures and sudden spikes in temperature (when the oven turns on) can be disastrous. Other people like an unlit gas oven (assuming it has a pilot light) and a warm water bath, though the water needs to be changed periodically to keep the temperature up.

Me, I like the hi-tech approach that Mr. Brown suggested on one of his show, whereby you wrap your culture jar in one or two electric heating pads using rubber bands (or a larger container that you can tightly stuff the whole works in). Careful regulation of heat is critical, since the bacterial cultures that make yogurt are extremely fussy. They’ll die if they’re heated much over 120, and slow down considerably if they cool much below 105. More on this in the tutorial.