Check the rennet package for specific instructions on how much rennet to use. This will vary depending on whether you are using vegetable rennet or not, and whether it is liquid, granular, or tablets. If you are not using liquid rennet, you will need to dissolve the rennet beforehand in a little tepid water. Ideally this should be done in a small measuring cup which has been pre-warmed using hot water.
Heat the milk to 185-195°F (85-90°C) and hold it at that temperature for about 10 minutes. Be careful not to boil or scorch the milk. Cool down to 100-102°F (38-39°C). It is important that you allow the milk to cool properly, or else the rennet may not work. Check the rennet package instructions for heat tolerance guidelines.
Stir the sour cream (or skyr, if you're lucky enough to have the Icelandic variety) into a tablespoon of milk until well mixed. Pour into the warm milk and mix well. Add the rennet. You now need to allow the rennet to work its magic. For best results, the skyr needs to cool down gradually. I sometimes use a crockpot for making skyr, because the insulated cooker and heavy stoneware vessel cool very slowly. Allow the skyr to cool about 6 hours.
You will be ready to proceed to the next step when you can make a cut in the skyr which will not close immediately. Line a sieve or colander with cheesecloth or a fine linen cloth and pour in the skyr. Tie the ends of the cloth together over the top and hang over a bucket or other container so the whey can drip off. Be sure to retain the whey -- it can be used to pickle foods, and adds lots of flavor to recipes when substituted for part or all of the water.
Allow the skyr to drain until it is fairly firm. The consistency should be like ice cream. Before serving, whip the skyr with a whisk until smooth. Skyr should not be lumpy or grainy. Skyr may be served with cream and honey, and goes very well with fruit such as bilberries or lingonberries. Skyr may instead be flavored with garlic, chives or caraway seeds.
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