I’ve taken a good number of food hygiene courses in my life, and I think maybe I love fermenting things because it means disobeying everything I’ve been taught about food safety. So today, we’re going to take a dairy product, heat it up, let it cool down right into the middle of the danger zone where bacteria thrive, introduce some bacteria into it, and then leave it in a warm place for 8-12 hours. And when it has changed form and smells funny, we’ll eat it.
I am, of course, talking about making yoghurt.
Yoghurt, as with most fermented products, can feel a bit overwhelming to approach. But once you wrap your mind around the basic principles of what you’re trying to do, and once you get past the idea that a difference of a few degrees one way or another is going to kill you, it’s really not a big deal.
The first step is to bring some milk (I use whole, but you could use semi-skimmed or even nonfat milk if you don’t mind your yoghurt being runnier) slowly and gently to a temperature in the neighbourhood of 180-190°F (82-88°C). You want to do this slowly and stir occasionally because milk can be a bit temperamental and doesn’t respond well to boiling. If you don’t have a thermometer, what you’re aiming for is to bring the milk to the point just before it boils and then remove it from the heat. A skin will form on the top of your milk as it heats; this is normal and harmless, but you can pull it off and throw it away if it offends you.
Once your milk is hot enough, you’ll remove it from the heat and set it aside to cool. I generally pour it into another container at this point to cool faster, but you can leave it in the pan if you’re hoping to minimize dishes. The milk needs to reach a temperature of between 110-115°F (43-47°C) for the next step.
You may be noticing that my temperature readings are not exactly on the numbers I’m giving you. This tells you two things. 1: It is difficult to take pictures and do science at the same time. 2: This is a pretty forgiving process.
While your milk is cooling, you’ll place a bit of yoghurt (a couple of tablespoons, definitely no more than a half cup) in a small bowl. Note: this yoghurt needs to be the real deal, with live and active cultures. That “real-style” yoghurt you got for 20p at Aldi isn’t going to cut it. I usually buy the tiny, single-serving cup that’s just about to expire at the Co-op and is therefore 57p, and then once I’m in the yoghurt-making cycle I just hang onto the last few tablespoons of each batch and use it to feed my next batch.
So anyway, you’ll put a bit of real yoghurt in a bowl…
And then you’ll pour about a cup of the warm (not hot!) milk into the yoghurt and give it a good whisk. This, by the way and if you like words, is called inoculating. You are inoculating the yoghurt cultures into the milk. In layman’s terms, you are putting live bacteria into a swimming pool of food that they love. Gross. You don’t want the milk too hot at this stage because it would kill the little guys.
Once you have inoculated that cup of milk, you’ll pour it back into the rest of the milk and whisk it all together to make sure the cultures disperse evenly.
At this point I usually put the bacteria-milk into whatever container is going to be its long-term home. I find that this helps it to set (and stay set) a bit more solidly. When I let my yoghurt set in one container and then move it to another container for storage, it never seems to hold its shape quite as well.
Okay now, if you’ve spent any time working in the food industry, here’s the really counterintuitive part. We need this bacteria-milk to stay in that 110°F (43°C) region for about 8-12 hours. This temperature, which is right in the middle of the danger zone they warned you about in food handler’s class, will allow the bacteria to multiply at an alarming rate until they’ve completely taken over and turned your milk into yoghurt. The reason you don’t need to be afraid of food poisoning here is that the good bacteria basically crowd out anything else that might wander in and take an interest in your milk.
There are a few different ways to keep your yoghurt warm for the duration. Actually, there are probably an infinite number of creative solutions– coolers and hot water bottles, slow cookers, piles of towels, and warm windowsills, to name a few. I usually just pop the jar of bacteria-milk in the oven and switch the oven light on, and that little bit of heat is enough to work the yoghurt-magic.
You can check your yoghurt after as few as 4 hours, giving it a little wiggle to see if it has set, but I usually give it a good 8 hours (overnight, while I’m asleep and not using the oven). I think I’ve even left it as long as 16 hours, though I wouldn’t necessarily recommend that. The longer you leave it, the tarter and stronger the flavour will be. Once it seems to have set, you’ll move it to the fridge to really solidify, and then voila!
Makes about 1 kilogram yoghurt
1 litre/quart milk (I use organic, whole milk)
2-3 tablespoons yoghurt (look for live & active cultures listed on the label)
Glass or ceramic container with lid
- In a pan over medium-low heat, bring your milk to 180-190°F (82-88°C), or just shy of boiling. Stir occasionally and discard skin as it forms.
- Allow milk to cool to 110-115°F (43-47°C).
- Place 2-3 tablespoons yoghurt in a small bowl. Whisk 1 cup of warm milk into yoghurt, combining thoroughly.
- Stir yoghurt-milk mixture back into remaining milk, whisking thoroughly to combine.
- Pour mixture into glass or ceramic container with lid. Place in oven, with oven light on, for 8-12 hours.
- When mixture has set, transfer to refrigerator to cool completely.
– Recipe can be adapted to make more or less yoghurt. If making less, use less starter yoghurt (down to 1 tablespoon). If making more, use slightly more yoghurt, but you should never need more than 1/2 cup of starter yoghurt. The yoghurt cultures need room to grow, and too many starter cultures will actually prevent your yoghurt from setting properly.
– If your yoghurt smells or tastes questionable, trust your judgment and throw it out! It’s possible for other bacteria to creep in and spoil your yoghurt, and it’s not worth the stomachache.
– You can use some of each batch of homemade yoghurt as the starter culture for your next batch, but if it starts to taste off or have a hard time setting up, it may be that your cultures are weakening. I usually buy new yoghurt about once a month just to keep things lively.
– Homemade yoghurt will naturally separate, and you’ll notice a yellowy-clear liquid in your yoghurt container. This perfectly harmless stuff is called whey, and you can either pour it off (making your yoghurt thicker) or stir it back in.
– I make a batch of this about every 2 weeks, paying £1 for a litre of milk. This makes 1 kg of yoghurt, which would have cost me £3 at the shop. So I’m saving about £1/week and enjoying organic yoghurt with zero questionable ingredients.
I’ve used quite a few great sites for reference over the years, but this how-to from The Kitchn is my go-to.