A friend of mine started doing this on the CDT and I have tried it at home and plan to do this on an upcoming trip to Glacier. I'm amazed at how good it works. You can make yogurt or buttermilk in your sleeping bag.
1. Mix your Nido or other powdered milk as per the instructions. The instructions for a single-serving (8oz glass) makes an 8oz serving of yogurt. I have found that it doesn't require warm water to mix the milk as it says on the instructions for Nido. Mix the milk inside a container with a secure, screw-on lid such as a cleaned plastic peanut butter jar or inside a secure ziploc bag (as my friend does.)
2. Add spoonful of plain yogurt with active cultures. You will have to do this in civilization your first time. Out on the trail, you will reuse a portion of yesterday's yogurt. My friend found that whatever yogurt that still clung to the ziploc after eating the yogurt was enough to make the next batch.
3. Shake or stir then screw the lid on tight and sleep with your yogurt baby.
4. In the morning, you will have delicious yogurt! If the whey separates, forming a yellowish watery fluid, just give it a good stir.
5. If you prefer your yogurt cold, you may cool it down by setting it out for a while on a cool morning or setting it in a cool creek.
6. Don't eat all of it! Save a spoonful for your next batch. Store your precious starter somewhere safe in your pack.
This also works with buttermilk just as well and has a slightly different flavor. Buttermilk also cultures at lower temperatures (70F – 85F) so it may be possible to make it during the day in your pack of you are hiking somewhere warm enough.
Here is a picture of the yogurt:
