Thermos hack: yogurt-maker

I have a large plastic thermos, and inexpensive item that is world-weary and not particularly water-tight. It has a capacity of about 3 Litres, and is perfect for making a large batch of yogurt for our family. Since we've been freezing the local summer fruit bounty, the kids are making a lot of smoothies and subsequently we go through a lot of yogurt. This is so cheap and easy.

A Thicker Yogurt

4 cups boiled water
4 cups tap water
4 2/3 cups instant skim milk powder
1/2 cup fresh yogourt with active bacterial culture, or two packets of yogourt starter

Combine boiled and tap water in a large thermos. In my house the resulting temperature is about 115ºF, which is ideal for starting a yogourt fermentation. I can trust this temperature, but if you're trying this for the first time, definitely check the temperature of the mixture and adjust as necessary. Whisk in skim milk powder, then whisk in the fresh yogurt. Using skim milk powder allows you to get a thicker yogurt by getting more milk solids in less liquid volume, and it means you don't have to go the fussy process of heating the milk to kill any lingering thermophils.

Place lid on thermos. Place in unheated oven with the light turned on to provide a bit of warmth. Leave undisturbed for 8 hours. (Put a sticky-note on the oven to remind yourself and others not to turn it on! Ask me why my thermos is world-weary with bubbly plastic on the bottom...) With luck you should have a nice thick yogurt. It will firm up a bit more in the fridge.

Yield: 8.5 cups of yogurt