I'm going to be experimenting with dairy products as the Viking Age Scandinavians would have made/used/consumed them over the coming months. I've been making a soft fresh cheese curdled with vinegar for years, but I think that's probably not the most accurate.
For the first experiment, I made my first batch of skyr this week. Skyr was once purportedly made all over the Viking world, but has only survived to the present day in Iceland, where it has remained daily fare. We don't know how closely modern Icelandic skyr resembles Viking skyr, but it's such a simple process that I don't imagine it's changed all that much.
To make skyr, you bring non-fat milk up to around 185 deg. and hold it there for 5-10 minutes, then let it cool down to slightly warmer than body temperature, around 108. Take your culture (I've seen references to using sour cream or buttermilk, or yogurt of various kinds. The Vikings would have used some skyr from the last batch. I read a couple of references to the use of s. thermophilus and l. bulgaricus, which happen to both be in "Greek" or "Bulgarian" style yogurt, so that's what I used. Greek Gods brand to be specific.) and mix it with a little of the warm milk, then add the result to the rest of the milk, along with some rennet. I used Junket brand from the grocery store, but will soon be trying cheese-making-grade rennet, and I'll report on the differences. Then let the milk sit for something between 6 and 24 hours. I've seen various suggestions. I let mine go about 24 hours.
The milk-mass should start to pull away from the sides of the container, and you'll see clear-yellowish whey around the sides and over the top of the curd. That's good. Scoop out the curd with a ladle or spoon into a sieve or colander lined with several layers of cheesecloth, or better still, a nice clean piece of muslin fabric. Let it sit until most of the whey has drained out, and it starts to firm up to somewhere between firm yogurt and soft-serve ice cream.
Store it in the fridge when it's done. The result I got was not very sour, and has a very pleasant texture. I've used it in crepes, and with granola so far with great success.
Save the whey, which you can use in soups of porridge. I have more whey experiments to try too. The 16th Century Icelanders let the whey ferment until quite sour, and then used it as a refreshing drink, and also as a medium for pickling meat, eggs, and vegetables for long storage.