Yogourt is a traditional way of preserving milk and enhancing nutrition–and by making it I have avoided also making a literal wall of empty plastic yogourt containers. I have been making yogourt for many years now, and I think I finally understand it well enough to be able to laugh at my mistakes. So this is going to be a pile of trivia, some links, some science, and a summary of my current method.
How about we laugh first? Or skip straight to the instructions if you like.
Back in the day, I could not get my yogourt to be as firm as I would like so I started adding powdered milk. By increasing the milk solids, I did get firmer yogourt.
This is stretching my memory of the chain of events, but I think I then had the bright idea that I could make yogourt entirely from powdered milk, which would allow me to buy a bulk sack of dried milk and save money.
I also used to not pasteurize my milk, reasoning the bottling plant or the drying plant had already done that. This saved me a lot of electricity, and also eliminated the fussiest parts of yogourt-making, getting the right temperature at each step. Knowing what I do about food poisoning in the industrial food system, I am horrified I published this and I dearly hope nobody died after following my instructions.
But I understand fermentation a lot better now,1 google has more stuff in it, and also, we saw Sandor Katz speak.
Heat the milk up, and hold it at temperature for a period of time.
After years of googling and silliness with powdered milk I finally came across a paper on industrial yogourt production that talked about denaturing whey proteins by heating the milk to a temperature and holding it for a set time. Hotter is a shorter time, and cooler is a longer time.
“The temperature/time combinations for the batch heat treatments that are commonly used in the yogurt industry include 85°C for 30 min or 90-95°C for 5 min”
This step may be adequately met in the traditional scalding stage, which may be why many people make good yogourt without knowing this info–whereas my newfangled low-temperature-powdered-milk yogourt was never very firm.
Here is a very readable article which has an interesting temperature method. They mention the scientists Lee and Lucey, which, if you google them, you will find a ton of papers on yogourt science, including the quote about temperature above.
If you want thick Greek-style yogourt, you can leave the yogourt in a fine mesh strainer so some of the whey drips out.
Reusing yogourt as a starter
The next mystery of yogourt is that when you make a batch using grocery store yogourt as the starter, it always seems to poop out after about a half dozen lifecycles. We saw Sandor Katz, Fermenting Guru, speak,2 and he said that is simply a consequence of commercial cultures.
And when you think of it, of course that makes sense. Our ancestors in yogourt-eating societies did not have labs and sterile workspaces to replicate and freeze-dry cultures. So either it was being refreshed from the environment, or the culture itself did not weaken after a few lifecycles. Katz says the latter. Some heritage cultures reproduce indefinitely, and some have additional properties favoured by different cultures, like the challengingly-textured Finnish Viili, or “ropy yogourt”.
I find I usually go out of town, or clean out the yogourt jar and eat my starter, or have some other life event interrupt me before my starter loses performance so I like to keep a few pouches of Yogourmet around. Yogourmet is a common starter culture available in better health food stores, often in the refrigeration section. I would expect you would find it in “ethnic” stores as well, like Greek, and maybe Indian. It comes in a box with several pouches of freeze-dried culture.
The basic rule is to heat milk slowly and cool it quickly. This prevents burnt flavours, and reduces the chances of pathogens infecting the milk as it cools. At this stage, milk is warm, wet, and full of protein, so it is a bacterial playground. Ice or cool baths might be used in cheesemaking, but for yogourt I just wait until it has cooled down naturally. The milk will be inoculated with billions of acid-producing bacteria that create an inhospitable environment for spoilage causing bacteria.
I have fermented yogourt in all sorts of ways. I have a giant Korean cooking thermos–a thermal cooker3–which does a great job, I have wrapped blankets around pots, I have tried a crockpot, I have made a giant bain-marie, some people use a sous-vide. I had a two litre electric yogourt maker–the multiple tiny pots do not suit our eating style–and it worked great until it died.
But the best by far is our current house, which has an old gas stove with a pilot light.4 It holds a wonderful incubating temperature inside the oven, so after I have inoculated the milk, I just put the pot inside the oven and walk away.
Lastly, here is an amazingly useful tool for making yogourt and cheese. This thermometer’s alarm will sound for both High and Low temperature settings, so it will tell you when your milk is hot enough, and then it will tell you when your milk has cooled enough for inoculation. This just saves a ton of fussing and allows me to do other things without boiling over or forgetting milk until it is too cold.
How to make (firm) yogourt
I typically make two litres of yogourt at a time, as it keeps nicely.
Over low to medium heat, warm milk to 95°Celsius (205°F) and hold it there for five minutes.5
Let the milk cool to 45°C (115°F), then inoculate it with a starter. For starter, use one pouch of Yogourmet or a couple of teaspoons of yogourt per litre of milk, and whisk thoroughly or blend with a stick blender.6 If you are using a heritage culture, experiment.
Incubate the inoculated milk in stainless steel or glass container at 45°C for four hours. Incubating yogourt for a longer time makes it more acid, not more firm. I like sharp yogourt, so I often make my yogourt at night and take it out of the warm oven in the morning.
Congratulations! You have made yogourt–just chill and enjoy. I use it most in smoothies, and as a sour cream substitute for perogies. I love to drink Lassi, with cardomom and rosewater. And labneh is seriously easy and wondrously delicious.