Electric -v- Thermos Style Yoghurt Makers
Yoghurt cultures have quite a wide ideal fermentation temperature range of 37 to 43 °C, so you will need to use a yoghurt maker to help out. You now get to pick between two types of yoghurt maker, an electric, or a thermos-style yoghurt maker.
Electric yoghurt makers are designed to maintain the correct temperature range for yoghurt fermentation. Being low-powered they are not designed to be used to heat the milk to the correct temperature, as the heating process will be very slow.
Some electric yoghurt makers will slowly creep up in temperature over time and reach temperatures that will kill the culture. This will extend the life of the yoghurt in your fridge, but any probiotic cultures will be killed too.
GLA's Yoghurt Maker has been designed to hold the temperature in the correct range indefinitely and is ideal for extended fermentation times.
Thermos-style yoghurt makers will help maintain the temperature of your milk, so the milk should be at 40 to 43 °C when placed in the yoghurt maker. Any hot water placed in your system to act as a heat reservoir should not touch the internal yoghurt container, as this can cause the milk to heat rapidly, and excessively. In the cold months, or if you wish extended inoculation times, you may need to change the hot water used as the heat reservoir, and keep the yoghurt maker well wrapped and in a warm place. Testing the temperature of your yoghurt with a thermometer is okay, just don't stir the thickening yoghurt with the thermometer shaft.
If using the EasiYo system, do not fill the external container so high with boiling water as to have it come into direct contact with the internal yoghurt container, as this will cause the temperature to rise too high, scalding and killing the culture. Just fill it to just below the hole in the baffle (red shelf). This will give you the benefit of a heat reservoir, without risking killing the culture. Monitor the temperature of the fermenting yoghurt and replace the boiling water as required to maintain the temperature between 37º to 43ºC.
Both types of yoghurt makers have advantages. Electric yoghurt makers are low-powered so are cheap to run and require no monitoring or attention. Thermos-style systems only use the power it takes to boil the water for the heat reservoir and are ideal for use off-grid or camping and are pretty cheap to buy, but may require periodic attention during fermentation.